029_HANDB OF FRUITS_AND PROSESSING_2006_688

504 Part III: Commodity Processing

Preliminary operations
Harvesting and transportation
Leaf removal and washing

Paste preparation
Crushing

Termo-mixing (25-30°C)

Liquid-solid separation Centrifugation
Water (25-30°C) 2-phase centrifugation

3-phase centrifugation

Solid phase Aqueous phase Liquid phase Solid phase
olive-pomace Residual vegetable Virgin Olive Oil 2-phase olive-pomace

“orujo water “Alpeorujo

Liquid-liquid separation
Filtration

Centrifugation

Packing
Figure 26.3. Diagram showing olive oil extraction by centrifugation methods.

olive processing to obtain virgin olive oil (Boskou, maximum oil content and best characteristics. The
1996; Alba, 1997; Giovacchino, 2000). factors, which have an impact on the optimum time
of harvesting, are (Porras, 1997):
Processing Description
Preliminary Operations – Resistance to the pull of the stalk of the olive.
– Oil content of the fruit.
Harvesting. The time of year in which this op- – Changes in the oil quality.
eration is carried out, as well as the system used, – The falling of the fruit.
influences considerably the characteristics of the – Date of the previous harvest.
olive oil obtained. The fruit needs to be harvested at
the optimum degree of ripeness taking into account The resistance to stalk detachment is determined
by the force necessary to break the pull of the
fruit stalk, which can vary enormously throughout

26 Olive Processing 505

ripening and has a bearing on the performance of the the olive (leaves and twigs) using an adjustable flow
operation, if the harvest is manual. The harvest must of air, and water is used for the wash that eliminates
coincide with the time when the green fruit disappears heavier objects such as mud, stones, etc. There are
from the tree, which is also when the maximum oil washers that operate by density, adding common salt
content is reached. The organoleptic characteristics to the water until the olives start to float, and others
of the oil deteriorate when the harvest of the fruit that work by trawling, adjusting the circulation rate
is delayed. If more fruity and aromatic oils are de- of the water in such a way that it only carries the fruit.
sirable, the harvest can be advanced by a few days.
With a substantial percentage of green fruit, a better Paste Preparation
quality product is obtained, although the production
yield of the oil is decreased. A paste is produced by means of crushing and mixing
operations with the objective of separating the olive
The natural fall of the fruit basically depends on oil as a continuous oily liquid phase from the rest of
the cultivar, although it can be influenced by climac- the components of the olive, without changing the
tic conditions and the health of the fruit. The end of composition and organoleptic characteristics.
the harvest should coincide with the time when the
natural fall of the olive starts and should reach a per- Crushing. This operation breaks down cellular
centage that would have a significant effect on the membranes to liberate oil droplets. These droplets
cost of the harvest. group together in variable sized drops that come into
contact with the aqueous phase present in the paste,
Lastly, a delay in the time of the harvest also pro- which comes from the vegetation water and the re-
duces certain inhibition in the floral differentiation of mains of the washing water. This operation is fun-
the leaf buds that can result in losses in the harvest damental for the rest of the extraction process and
the following year. influences the industrial yield and quality of the oil.
Currently, the traditional mills with their cone-shaped
Harvesting that spoils the fruit and produces dam- millstones have been replaced by metallic crushers,
age, with bruises, broken branches, or twigs should be generally of the hammer type.
avoided. In general, the fruit is brought down manu-
ally from the tree, known as handpicking, or mechan- Thermo-Mixing. The olive paste obtained after
ically, using a trunk shaker. The fruit is collected in crushing is mixed slowly with the objective of gath-
nets of cloth or plastic material that extend below the ering together the larger sized drops of oil and
olive trees, occupying an area larger than the drop combining into a continuous oily phase, with the
zone of the tree. Finally, the contents of these nets separation from the solid and the aqueous phase
are poured into boxes or trailers for transferring to (Mart´ınez–Moreno et al., 1957). To improve the yield
the olive-oil mill. The daily capacity of oil extraction of this operation, the mixers have a heating system,
in the mill is normally lower than the rate of receiv- which helps to decrease the viscosity of the oil and
ing the harvested fruit, requiring fruit to be stored in makes outflow easier. The temperature in the ther-
the so-called trojes (or silos) during a variable pe- momixer should not exceed 25–30◦C, to prevent loss
riod of time. During this time, the olives can undergo of aromatic compounds and an increase in the oxida-
various changes such as fermentation, enzymatic and tion process.
microbial lipolysis, autoxidation of fatty acids, etc.;
these changes will damage the quality of the oil by in- Liquid–Solid Separation
creasing acidity, decreasing stability, and degrading
organoleptic characteristics. The fundamental objec- This stage constitutes the fundamental part of obtain-
tive during this preservation period is to maintain ing oil, with the objective of separating liquids con-
the fruit without changing the oil characteristics and tained in the olive paste, that is, the oil and residual
without significantly increasing the cost of the pro- water (liquid phase), and the solid mixture (skin, pulp
cess. and broken pits), which is known as “olive-pomace.”
Three separation systems are used in the industry:
Leaf Removal and Washing. This operation is selective filtration, extraction by pressure, and ex-
necessary to prevent contamination from impurities traction by centrifugation.
that affect the organoleptic characteristics of the oil,
as well to reduce the wear and tear on the mills. The
winnowing machine is used as a cleaning system,
which separates the impurities of lesser weight than

506 Part III: Commodity Processing

Selective Filtration. After mixing, it is possible to This system produces a final aqueous phase (resid-
obtain an important part of the freed olive oil con- ual vegetable water) of approximately 0.7–1.1 l/kg of
tained in the olive paste, by means of cold extraction olives. This effluent contains a high load of contami-
by selective filtration. In this way, the first oil con- nants, as assessed by the chemical demand for oxygen
tained in the fruit is obtained, which has superior and spillage into public waterways negatively affects
characteristics to that obtained later by mechanical the flora and fauna (Alba et al., 1995). Government
extraction. This system is recommended for obtain- measures compel the olive mills to put a purification
ing high-quality oil but is used little due to high cost system in place. The first system recommended as an
(Hermoso et al., 1991). emergency measure was the storage of the residual
water in pools for natural evaporation. At the same
Extraction by Pressure. The hydraulic press sys- time, exploitation and purification techniques were
tem was used traditionally. It consists, in essence, of developed but were not generally accepted by the
placing the paste, once mixed, over some filters made sector, due to the level of efficiency and installation
of esparto or synthetic fiber (or a mixture of both), and operational costs.
known as pulp mat or olive mat, making up the press-
ing or load unit. This load, located over a trolley with Due to this situation, nowadays, the consumption
a central spike, is installed in a container in the press, of water tends to be lower to minimize the volume
in such a way that by pressing the load over this con- of residual vegetable water produced and thus reduc-
tainer, the filtrate of the liquids contained in the paste ing the environmental problem caused by lack of pu-
is obtained. These are fundamental factors in the ex- rification. A variation of the process has been devel-
ecution of the pressing: the paste preparation, its dis- oped called a two-phase centrifugation system, which
tribution and thickness on the olive mats, the state of does not need the fluidity by the addition of extra wa-
the olive mat, specific pressure of the press, and the ter. The separation of the olive paste is achieved in
speed and time of pressing (Mart´ınez-Moreno et al., only two phases, a solid-olive-pomace and the other
1964). liquid–oil. In this system, the vegetable water of the
olive is incorporated into the solid phase, which is
Extraction by Centrifugation. The use of cen- distinguished from that obtained by the three-phase
trifugal force is the modern method to carry out the process for it is a more moist and pliable consistency
liquid–solid separation. It is carried out in dynamic and is now being called two-phase olive-pomace or
phase systems, where the solids are displaced what “alpeorujo” (Hermoso et al., 1995). The final effluent
is long of the draft shaft and continuously removed. in this production system is reduced to 0.25 l/kg of
The mixed paste is introduced (partially diluted in olives.
water) into a horizontal centrifuge called a decanter.
This consists, essentially, of a conical-cylindrical re- Liquid–Liquid Separation
volving rotor and a hollow bore helicoidal scraper,
which revolves coaxially in its interior and at a dif- The oil obtained in the previous stage contains some
ferent speed. By the action of the centrifugal force, residual water and a few solids, in the same way that
the solids are pushed to the interior wall of the ro- the residual water contains some oil and a large per-
tor and are drawn to the furthest point from the axis centage of solids. It is necessary, therefore, to purify
of spin, that is to say, the space nearest to the wall the oil (by freeing it from solids and the residual wa-
of the warm gear motor, continuing in this manner ter) as well as the residual water to obtain the little
until it runs out of liquid. The liquid (oil and aqueous oil that it contains. Before separating the liquids, the
phase) forms concentric rings, which depending on solids are eliminated, also known as “fines,” by us-
their density move further toward the middle. Two ing vibratory sieves or filters, to separate the highest
outlets, situated at a different level in the periphery possible quantity of “fines.” Finally, the oil separation
of the decanter, are used as exits for the residual wa- from the aqueous water is carried out by natural de-
ter and oil (Giovacchino and Mascolo, 1988). There- cantation, centrifugation, or a combination of both.
fore, three phases are formed: a solid-olive-pomace
or “orujo”—and two liquids–oil and residual veg- Decantation. The difference in density between
etable water—and the system is known as three-phase the oil (0.915–0.918) and the residual water (1.015–
centrifugation. 1.086) enables separation by natural decanting into
vats or settling pits, which are linked together. It is the

26 Olive Processing 507

oldest method and even today it is used in some oil Composition of Olive Oil
mills. However, it requires a large space and a long
time, during which, the contact of the oil with the The composition of olive oil basically depends on the
aqueous phase can cause fermentation and changes factors that influence the composition of the fruit,
in the oil quality. such as latitude, climate, state of ripeness, etc., al-
though oils are also influenced by the variables asso-
Centrifugation. The use of a vertical centrifuge ciated with processing and storage conditions.
enables continuous and rapid liquid–liquid separa-
tion. By the addition of a specific quantity of water, The components in olive oil, as in all veg-
the separation of the oil from the rest of the resid- etable oils, can be subdivided into saponifiable (e.g.,
ual water is achieved and in the same way, but inde- triglycerides, free fatty acids, waxes, mono- and
pendently, the recovery of the oil fraction from the diglycerides, sterol esters and terpene alcohols, and
aqueous phase. The factors to be considered in this phospholipids) and nonsaponifiable (e.g., hydrocar-
operation are the homogeneity and volume of the liq- bons, tocopherols, sterols, terpenes, alcohols, pig-
uid to be centrifuged, temperature, volume of added ments, polyphenols, and volatile compounds) frac-
water, and time taken between unloading. The oil tions. The nonsaponifiable constituents are minority
that comes out of the centrifuge passes into small de- compounds (0.5–1.5% of the total oil weight) but
canters for degassing and continues to the recipients, contain the “fingerprint” of the oil; and also play an
where its quality classification is carried out. important part in the stability and organoleptic char-
acteristics of taste, aroma and color (Boskou, 1996;
Harwood and Aparicio, 2000).

Storage Saponifiable Fraction

The storage period is limited to a season or part of the Most of the fatty acids of olive oil are present as
following one. Longer periods are only anticipated in triglycerides. The proportion of free fatty acids is
industrial mills, which include bottling and market- small and is directly related to the level of triglyc-
ing operations, as well as the extraction process. Dur- eride hydrolysis, normally associated with the de-
ing storage, every possible precaution should be taken terioration process.The major olive oil triglycerides
to prevent three causes of oil spoilage: contact with are OOO (43.5%), POO (18.4%), OOL (6.8%), POL
unsuitable materials, prolonged contact with aqueous (5.9%), and SOO (5.1%) (O, oleic; P, palmitic;
impurities, and oxidation. It is advisable to use vats of L, linoleic, and S, stearic). The waxes are esters of
a capacity of approximately 50 tons, for easier clas- fatty acids and primary fatty alcohols. The princi-
sification and marketing. Vats must be constructed pal waxes found in virgin olive oil are esters C36 and
with impermeable and inert materials, so oil will not C38, whereas in olive-pomace oil and in refined olive
penetrate or react with the surface since any absorbed oil the majority of esters are longer (C40, C42, C44,
oil, which cannot be removed by cleaning, will be al- and C46). The quality regulations of the Economic
tered and will compromise on the further use of the European Community (EEC) establish a maximum
vat. The storage cellar must have minimum luminos- content of waxes of 250 mg/kg for virgin olive oil
ity with walls and floors that are easy to clean and are (nonlampante) and 350 mg/kg for refined oil.
maintained at a temperature between 15˚C and 18˚C,
without abrupt changes in temperature. Nonsaponifiable Fraction

Nowadays, the vats which best accomplish these Hydrocarbons. Squalene (C30H50) is the major
conditions are called “trujales” or traditional under- hydrocarbon (30–60%) of olive oil and its con-
ground vats which, thanks to an appropriate cover centration (2500–9250 mg/kg) is much higher than
of vitrified refractory tiles, ensure the preservation that in the majority of other vegetable oils (16–
of the oils. If surface vats are used, they must be 370 mg/kg)(Gutfinger and Letan, 1974).
covered and protected from atmospheric agents and
variations in temperature. The most ideal material is Tocopherols. The tocopherol content can vary be-
stainless steel although other materials can be used, tween 5 and 300 mg/kg, although in good quality
provided an inert inner covering is used. Vats must oils it is usually above 100 mg/kg (Maestro-Dura´n
have a conical bottom or a flat incline and a purge tap
to facilitate sediment removal.

508 Part III: Commodity Processing

Table 26.8. Quality Parameters Established by the ECC for Olive Oil and Olive-Pomace Oil

Acidity PIa(mEq Organoleptic
(%) O2/kg) Evaluationc

Categories K 232 K 270 K 270 b K Md Mf

Extra virgin olive ≤1.0 ≤20 ≤2.50 ≤0.20 ≤0.10 ≤0.01 0 >0
oil ≤2.0 ≤20 ≤2.60 ≤0.25 ≤0.10 ≤2.5 >0
≤3.3 ≤20 ≤2.60 ≤0.25 ≤0.10 ≤0.01 ≤6.0d ≥0
Virgin olive oil >6.0
(fine) ≤0.01

Ordinary virgin –

olive oil >3.3 >20 ≤3.70 >0.25 ≤0.11 ≤0.16
Lampante virgin ≤0.13
≤0.5 ≤5 ≤3.40 ≤1.20 – ≤0.25
olive oil ≤1.5 ≤15 ≤3.30 ≤1.00 –
Refined olive oil ≤0.5 ≤5 ≤5.50 ≤2.50 – ≤0.20
Olive oil
Refined

olive-pomace oil ≤1.5 ≤15 ≤5.30 ≤2.00 –
Olive-pomace oil

Source: Regulation number 2568/91 of the European Communities (EC, 1991)

Note: Commercial categories.
aPI, Peroxide index.
bValues of K270 after alumina treatment.
cMedian values of defects (Md) and fruity aroma (Mf).
dIf median of fruity aroma is 0, Md must be ≤2.5.

and Borja-Padilla, 1990; Gutie´rrez et al., 1999; 5-avenasterol and campasterol. Sterols are directly
Psomiadou et al., 2000); ␣-tocopherol (vitamin E) related to olive oil quality and are used to detect adul-
represents 90–95% of the total tocopherols, ␤- and terations with seed oil as well as to characterize the
␥ -tocopherols are less than 10%,and ␦-tocopherol is monovarietal virgin olive oils according to geograph-
found in very low amounts. Consumption of 25 g/day ical origin (Aparicio and Luna, 2002). Table 26.8
of virgin olive oil meets 50% of the recommended shows the limits established by the IOOC (2001) for
daily requirement of vitamin E. Because tocopherols each one of the olive sterols.
function as antioxidants, they make an important con-
tribution to the stability of virgin olive oil, although Olive oil contains triterpene alcohols (1000–3000
are very unstable compounds to thermal treatment mg/kg), generally tetracyclic or pentacyclic in struc-
(Baldioli et al., 1996; Aparicio et al., 1999). Also, ture, which are markers of certain monovarietal olive
refined olive oils have less protection against oxida- oils. Two hydroxytriterpenes, erythrodiol and uvaol,
tion due to the disappearance of practically all the have been identified; the presence of these triterpene
tocopherols during the deodorizing process in the re- alcohols together with the wax content allows the de-
fining. tection of adulteration of olive oil with olive-pomace
oil (Morales and Leo´n-Camacho, 2000). The Com-
Aliphatic Alcohols. The most representative of mission of the European Communities (EC, 2002)
olive oil is linear alcohols with even-numbered car- establishes a relative maximum value of 4.5% for
bon atoms: hexacosanol, octacosanol, and tetra- these alcohols. To distinguish lampante olive oil
cosanol. The profile of these compounds is used to from olive-pomace oil, regulations establish that for
detect adulteration with seed oils, which contain a a content of wax between 300 and 350 mg/kg, it is
greater proportion of chains with odd-numbered car- considered as olive-pomace oil only if the erythrodiol
bon atoms (Morales and Leo´n-Camacho, 2000). and uvaol is greater than 3.5% or if the total aliphatic
alcohol content is more than 350 mg/kg.
Sterols. The total content of sterols is in the or-
der of 800–2600 mg/kg, ␤-sitosterol being found Phenolic Compounds. Although water-soluble
in the highest proportion (75–90%), followed by compounds, a small percentage is transferred from
the fruit to the oil during the extraction process.

26 Olive Processing 509

These phenolic compounds increase the oxidative spontaneously by the action of atmospheric oxygen,
stability of the olive oil and improve the flavor con- producing a series of chain reactions by a free radi-
siderably (Gutie´rrez et al., 2001; Tsimidou, 1998). cal mechanism. The products of this oxidative change
The phenolic profile can also be used to authenti- are responsible for rancidity, as well as a significant
cate virgin olive oils of the Picual variety (Brenes deterioration in nutritional quality.
et al., 2002). The average content (expressed as caf-
feic acid) is between 50 and 800 mg/kg. The principal When the intensity of these sensory defects is high,
polyphenols of olive oil are tyrosol and hydroxyty- the oils are classified as “lampante” and according to
rosol, both derived from the hydrolysis of oleuropein. current regulations (EC, 2001), require further refin-
Also found are smaller proportions of caffeic, vanil- ing and are often rejected by the consumer.
lic, siringic, p-coumaric, o-coumaric, protocatechic,
synapic, p-hydroxybenzoic, p-hydroxyphenylacetic, Pigments. Virgin olive oil contains two types of
and homovanillic acids. pigments, chlorophylls, responsible for the green
color and the yellow-colored carotenoids. Due to
Volatile Compounds. In this group are found all their lipid soluble nature, these constituents of the
compounds responsible for the characteristic aroma olive are transferred to the oil during the extraction
of virgin olive oil. More than a hundred volatile com- process at a time when they are undergoing certain
pounds have been identified such as hydrocarbons, transformation reactions.
alcohols, aldehydes, ketones, esters, acids, phenolic
compounds, terpene compounds, and furan deriva- The chlorophyll and carotenoid composition of
tives. But there is still no objective method to mea- virgin olive oil is subject to wide variations due
sure the aroma, which requires evaluation by a panel to the differences between cultivars and the degree
of specialist tasters (Panel Test). of ripeness of the olive fruit, latitude, environmen-
tal conditions, processing techniques, and storage
In good quality oils, the retained volatile com- conditions (Gandul-Rojas and M´ınguez-Mosquera,
pounds come from the actual fruit and are generated 1996; Gandul-Rojas et al., 2000; Morello et al., 2003;
by biogenesis mechanisms. Some compounds are Salvador et al., 2003). The average content of chloro-
originally found in the intact flesh of the olive, while phyll pigments can vary between 1 and 40 mg/kg
others, called secondary or technological volatiles, and of carotenoids between 2 and 20 mg/kg (Gandul-
are formed during the breaking down of cellular Rojas et al., 2000; Psomiadou and Tsimidou, 2001).
structure as a result of enzyme reactions that take
place in the presence of oxygen. The principal bio- These differences are mainly quantitative, since
chemistry pathway is by the enzyme lipoxygenase, the qualitative composition of pigments is basically
precursors being the 18-carbon atom polyunsatu- the same in all the olive cultivars and is not modified
rated fatty acids, linoleic and linolenic (Ol´ıas et al., with the advance in ripening of the fruit. The color
1993; Sa´nchez and Salas, 2000). When the har- intensity of the olive oil is directly related to the total
vest conditions of fruit and production or storage content of chlorophylls and carotenoids, whereas the
of the virgin olive oil are defective, a whole series color tone depends on the relative proportion between
of disagreeable volatile compounds responsible for the two pigment fractions. The relationship main-
sensory defects (off-flavors) are generated. Very dif- tained between the chlorophyll and the carotenoids
ferent mechanisms are implicated in the genesis of in the fruit decreases with the ripening process due to
these undesirable compounds. The majority are re- the greater rate of chlorophyll disappearance as op-
lated microorganisms (yeast, bacteria, and molds) posed to carotenoids. The oils obtained from green
during storage of fruit before oil extraction, gener- and mottled olives at the beginning of the season have
ating volatile compounds responsible for sensory de- a greenish color. As the harvesting season of the
fects such as fusty, musty, or winey-vinegary. During fruit progresses and the percentage of black olives
the storage of unfiltered virgin olive oil, sediments increases, the color of the olive oils is less intense
are deposited, which can also produce fermentation due to the decrease in the total pigment content of
that generate volatile metabolites responsible for the the fruit. At the same time, the color tone becomes
sensory defect known as muddy sediment. more yellowish as a result of an increase in the rel-
ative proportion of carotenoids (M´ınguez-Mosquera
There are also other chemical mechanisms that et al., 1991; Gandul-Rojas et al., 2000).
produce undesirable volatiles. The main one is au-
toxidation of fatty acids. This process is generated It has been shown that in general, in the ripeness
stage at which the fruit is harvested for olive oil, the
chlorophyll-to-carotenoid ratio tends to be more or

510 Part III: Commodity Processing

less constant, around 3, independent of the olive cul- since the presence of chlorophyll and carotenoid pig-
tivar and different pigment content of the fruit. How- ment different from those just described, or the detec-
ever, this ratio is lowered to 1 with processing into tion of a high level of pigment transformation is in-
corresponding oils (Roca and M´ınguez-Mosquera, dicative of incorrect or fraudulent practices (Gandul-
2001b). This change occurs, despite the lipophilic Rojas et al., 1999c).
character of these pigments, during the extraction
process where only 20% of the chlorophyll content of Two quality parameters are provided to authenti-
the fruit is transferred to the oil, while the carotenoid cate oils based on the composition of chlorophylls
fraction is 50% (M´ınguez-Mosquera et al., 1990a). and carotenoids of monovarietal virgin olive oils of
The balance of pigment substances among fruit, oil, Spanish origin, as well as defining the pigment pro-
and alperujo shows that a large part of the pigmen- file specific for virgin olive oil (Gandul-Rojas et al.,
tation stays behind in the vegetable material, which 2000). On the one hand, it has been shown that there
forms part of the alperujo, estimating a retention of is a constant ratio, of around 1, between the chloro-
60% chlorophyll and 25% carotenoids. The rest of the phyll and carotenoid pigment fractions, independent
pigments (20–25%) are degraded to colorless prod- of the cultivar and degree of ripeness. This ratio is
ucts (Gallardo-Guerrero et al., 2002). maintained within the narrow limits, that is, between
0.5, when the oils are processed from ripe fruits, and
The changes that the components undergo dur- 1.4, when oils are obtained from unripe fruits. On
ing olive crushing and paste mixing trigger the re- the other hand, the carotenoid fraction has a constant
lease of cytoplasmic components that come into con- relationship between lutein, which is the principal
tact with different enzymes and substrates isolated component of this fraction, and the rest of the minor-
under normal physiological conditions. The modi- ity carotenoids. In all virgin olive oils, the minority
fication of pigments is influenced by the release of carotenoid–lutein ratio is around 0.5 and, exception-
acid compounds. For the chlorophyll pigment frac- ally, greater than 1 in the specific case of oils pro-
tion, the most common reaction is pheophytinization duced from the Arbequina cultivar in which this pa-
and for the carotenoids, the isomerization of the xan- rameter, as well as authenticating the “virgin olive
thophylls with epoxide groups, such as violaxanthin, oil” category, allow the differentiation of this variety.
neoxanthin, and antheraxanthin, which are trans- In the same way as occurs with the chlorophyll-to-
formed into 5,8 furanoids, luteoxanthin + auroxan- carotenoid ratio, in oils obtained from very ripe fruits
thin, and neochrome + mutatoxanthin, respectively. the relationship between minor carotenoids and lutein
␤-carotene, lutein, and ␤-cryptoxanthin are trans- decreases, falling as low as 0.2.
ferred to the oil without transformation (M´ınguez-
Mosquera et al., 1990a). On the other hand, the percentage of lutein, the per-
centage of violaxanthin, and the total pigment con-
Traces of metabolites of chlorophyll catabolism tent allow distinguishing between monovarietal vir-
can also be detected, such as 13-OH-pheophytin gin olive oils of Spanish origin. For this reason, they
and 151-OH-lactone pheophytin among the oxida- have been proposed as suitable qualifying variables
tion metabolites and chlorophyllides and pheophor- in establishing a prediction model, which could indi-
bides as intermediaries in the deesterification path- cate the varietal origin of the Spanish virgin olive oil
way, initiated by the enzyme chlorophyllase. In the (Gandul-Rojas et al., 2000).
case of cultivars with high chlorophyllase activity,
such as Arbequina, a significant increase in deesteri- These quality parameters based on relationships
fied chlorophyll derivatives are found (Gandul-Rojas between pigments remain stable after 12 months of
and M´ınguez-Mosquera, 1996; Gandul-Rojas et al., oil storage at 15˚C in the dark, conditions generally
2000). used in the industry to store oils that are not going
to be marketed immediately after production (Roca
The small, but always important differences found et al., 2003). However, it has been shown that dur-
in chlorophyll and carotenoid metabolism during the ing this period, the chlorophyll molecule undergoes
ripening of the fruit, depending on cultivar and influ- specific changes, which implicate alterations in the
enced by endogenous enzyme activities, are going to pigment profile, as compared to recently extracted
be reflected in the pigment composition of the cor- virgin olive oil. From the quantitative point of view
responding oils. Consequently, the chlorophyll and no pigments are lost, although there is an increase in
carotenoid profile of virgin olive oil is used as a pa- the reactions catalyzed by acids, which started dur-
rameter of quality and authenticity for this product, ing the extraction of the oil: the pheophytinization

26 Olive Processing 511

reaction of chlorophyll and the isomerization of the index of the alterations experienced by the fruit and
5, 6-epoxide groups of the minority xanthophylls. the fermentations that could have taken place during
The color change of the oil during storage is mainly production and storage.
due to the transformation of the chlorophyll (green)
into pheophytins (gray brown). Possibly, to a greater The peroxide index evaluates the state of primary
or lesser extent the acidic substances that could be oxidation of oil. The most important causes of a high
transferred from the fruit to the oil will regulate the value of this index are the history of the fruit (i.e.,
rate of these reactions. collection from soil, injuries by frosts, etc.) and ex-
posure to factors, which cause oxidation (high tem-
Likewise, there is a slight increase in the hydrox- peratures, aeration, and the presence of trace metals)
ylation of the C-132 of pheophytin a, as well as a during production and storage.
small amount of pyropheophytin a, a pigment not
present in recently extracted oil. It has been shown The k232 is a spectrophotometric measurement of
that in the previously mentioned storage conditions, the conjugated dienes that is used along with the per-
the maximum amount of pyropheophytin found in oxide index to evaluate the primary oxidation of the
oils is around 3% of the total chlorophyll compounds, oil. This parameter is included only in the EC regu-
and that the ratio of pheophytin a to pyropheophytin lation.
a is always more than 20. In this way, these small
structural changes of pigment, which are not inher- The k270 is used to evaluate, spectrophotometri-
ent to the extraction process of the virgin olive oil and cally, the conjugated trienes and to indicate the state
therefore not desirable, are indicators that the oil has of secondary oxidation of the oil. The resulting prod-
been stored. Inadequate storage conditions, with high ucts of this oxidation (aldehydes and ketones) also
temperatures and/or light, cause marked increases in absorb at wavelengths of 262, 268, and 274 nm. The
the content of these compounds (Gallardo-Guerrero
et al., 2004). During physical refining of oils through k coefficient takes into account these absorbencies
deodorization with nitrogen, the most widespread re- and is defined as
action is pyropheophytinization (Gandul-Rojas et al.,
1999a). On the other hand, during classical chem- k = k268 − [(k262 + k274)/2]
ical refining processes to which poor-quality olive
oils are subjected, pigments are mostly eliminated Organoleptic characteristics of virgin olive oils are
by adsorption onto decolorizing earth and/or deodor- determined by a group of trained tasters, accord-
izing treatment at high temperatures (Usuki et al., ing to the rules of sensory analysis (IOOC, 2001).
1984). When no defects exist in the flavor (visual together
with olfactory-taste-tactile), the tasters evaluate the
Quality of Olive Oil presence and intensity of positive attributes. The
sensory attribute “fruity,” responsible for a mixture
Different definitions of quality exist depending on the of olfactory–taste sensations attributed to fresh and
use of the olive oil from the established regulations healthy fruit at an optimal degree of ripeness is the
for quality, to the nutritional and therapeutic quality, one which plays an important part in the sensory eval-
commercial, culinary, sensory, etc. uation since the absence of this attribute excludes the
oil from the virgin grade. However, increased inten-
The regulated quality is the easiest to define as sity of fruity is often accompanied by a perception
it is clearly defined by EC Regulation 2568/91 and of high intensities of bitterness and spiciness which
its later modifications (EC, 1991). Currently two makes the oil unacceptable for direct consumption
rules exist, which regulate the denominations of olive and requires mixing with other virgin oils of less in-
oils and olive-pomace oil (EC, 2001; IOOC, 2001). tense flavor. On the new sensory evaluation form,
In both regulations, there are parameters, known as proposed in 1996 by IOOC and adopted by the EC in
quality parameters, to evaluate the quality of the oils May 2002, the most frequent negative attributes are
and these are free acidity, peroxide index, k232, k270, scored (“fusty, musty, muddy sediment, and winey-
vinegary”). Also included in the “Others” section
k, and organoleptic characteristics. is all other well-defined negative attributes such as
The degree of free acidity measures the percentage “earthy, esparto, pungent, cucumber, grubby, etc.”
of free fatty acids, expressed as oleic acid. It is an Only fruity, bitter, and pungent are considered pos-
itive attributes. Depending on the median value of
the defect perceived with greater intensity and the

512 Part III: Commodity Processing

median value of the fruity attribute, the virgin olive According to the International Olive Oil Council,
oil is classified into distinct categories. olive oil is “the oil obtained solely from the fruit
of the olive tree (O. europaea sativa Hoffm. Link) to
Color is another organoleptic quality characteris- the exclusion of oils obtained using solvents or by re-
tic fundamental to virgin olive oil as it is one of the esterification processes and of any mixture with oils
attributes that most affects the consumer at the time from other sources.” This oil can be produced com-
of purchase. The first evaluations of color were based mercially under different denominations, virgin olive
on the visual comparison of oil with the standard so- oil being “the oil obtained from the fruit of the olive
lutions of bromothymol blue (BTB), using an adapta- tree solely by mechanical or other physical means un-
tion of methods developed for seed oils (Gutierrez et der conditions particularly thermal that do not lead
al., 1986; AOCS, 1977, 1988). Photometric evalua- to alteration in the oil and which has not undergone
tions at two wavelengths have been proposed for olive any treatment other than washing, decantation, cen-
oil (Papaseit et al., 1986) and numerical correlations trifugation, and filtration.”
between the chromatic coordinates and the chloro-
phyll and carotenoid content (M´ınguez-Mosquera et All the oils obtained in an olive oil mill are consid-
al., 1991), as well as with the BTB indices (Moyano ered virgin olive oils and, depending on their qual-
et al., 1999). Despite the importance of this quality ity parameters, are classified into different categories
attribute, none of the current regulations include the in accordance with the EC and IOOC regulations.
organoleptic quality, color. In Figure 26.4, the aforementioned classification is

Olive fruits
Oil mill

Virgin Olive oil Olive-pomace

Quality parameters Oil extraction by solvents

Extra virgin olive oil
Virgin olive oil (fine)
Ordinary virgin olive oil

Lampante virgin olive oil Crude olive-pomace oil

Refinig Refinig

Refined olive oil Refined Olive-pomace oil

Olive oil Olive-pomace oil

Figure 26.4. Scheme of the categories established by the EC for olive oil and olive-pomace oil.

26 Olive Processing 513

Table 26.9. Sterol Composition Established Aparicio R., Alonso V. 1994. Characterization of
by the IOOC for Olive Oil and Olive-Pomace virgin olive oils by SEXIA expert system. Prog.
Oil Lipid Res. 33:29–38.

Sterol Percentage of Aparicio R., Luna G. 2002. Characterization of
Total Sterol Content monovarietal virgin olive Oils. Eur. J. Lipid Sci.
Technol. 104:614–627.
␤-sitosterola ≥93
Cholesterol ≤0.5 Aparicio R., Roda L., Albi M.A., Gutierrez F. 1999.
≤0.1b Effect of various compounds on virgin olive oil
Brasicasterol ≤4.0 stability measured by Rancimat. J. Agric. Food
Campesterol <Campesterol Chem. 47:4150–4155.
Stigmasterol
≤0.5 Balatsouras G., Tsibri A., Dalles T., Doutsias G. 1983.
7-Stigmasterol Effects of fermentation and its control on the
sensory characteristic of Conservolea cultivar green
Source: International Olive Oil Council: Trade standard ap- olives. Appl. Environ. Microbiol. 46:68–74.

plying to olive oil and olive-pomace oil. COI/T.15 No2/Rev Baldioli M., Servili M., Perretti G., Montedoro G.F.
1996. Antioxidant activity of tocopherols and
10 (IOOC, 2001). phenolic compounds of virgin olive oil. J. Am. Oil
a␤-sitosterol + 5-avenasterol + 5,23stigmastadienol + Chem. Soc. 73:1589–1593.

clerosterol + sitostanol + 5,24stigmastadienol. Barranco D. 1997. Variedades y patrones in El cultivo
b<0.2 for olive-pomace oil. del Olivo edited by D. Barranco, R.
Ferna´ndez-Escolar, L. Rallo. Madrid: Junta de
shown, and in Table 26.9, the limits established by Andaluc´ıa and Ediciones Mundi-Prensa, pp 59–60.
the EEC for each one of the quality parameters ac-
cording to the category of the oil is recorded. Bendich A., Olson J.A. 1989. Biological actions of
carotenoids. FASEB J. 3:1927–1932.
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olive oil composition. Food Chem. 81:547–553. of chlorophyllase in chlorophyll metabolism in olive
Moyano M.J., Melgosa M., Alba J., Hita E., Heredia varieties with high and low chlorophyll content.
F.J. 1999 Reliability of the bromothymol blue Physiol. Plant 117:459–466.
method for color in virgin olive oils. J. Am. Oil Romero C., Brenes M., Garc´ıa P., Garrido A. 2002.
Chem. Soc. 76:687–692. Hydroxytyrosol 4-beta-D-glucoside, and important
Nosti-Vega M., Castro-Ramos R. de. 1985. phenolic compound in olive fruits and derived
Composicio´n y valor nutritivo de algunas variedades products. J. Agric. Food Chem. 50:3835–3839.

26 Olive Processing 517

Romero C., Brenes M., Garcia P., Garcia A., Garrido Tsimidou M. 1998 Polyphenols and quality of virgin
A. 2004. Polyphenol Changes during fermentation olive oil in retrospect. Ital. J. Food Sci. 10:99–116.
of naturally black olives. J. Agric. Food Chem.
52:1973–1979. Usuki R., Suzuki T., Endo Y., Kaneda T. 1984.
Residual amounts of chlorophylls and pheophytins
Ruiz-Barba J.L., Cathcarth D.P., Warner P.J., in refined edible oils. J. Am. Oil. Chem. Soc.
Jime´nez-D´ıaz R. 1994. Use of lactobacillus 61:785–788.
plantarum LPCOIO, a bacteriocin producer, as
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Environ. Microbiol. 60:2059–2064. Maestro-Dura´n R. 1974. Componentes feno´licos de
la aceituna I. Polifenoles de la pulpa. Grasas y
Salvador M.D., Aranda F., Gomez-Alonso S., Aceites 25:269–279.
Fregapane G. 2003. Influence of extraction system,
production year and area on Cornicabra virgin olive Vincenzo M., Campestre C., Lanza B. 2001. Phenolic
oil: A study of five crop seasons. Food Chem. compounds change during Californian-style ripe
80:359–366. olive processing. Food Chem. 74:55–60.

Sa´nchez J., Salas J.J. 2000. Bioge´nesis of olive oil Vlahov G. 1976. Gli acidi organici delle olive: II
aroma in handbook of olive oil: Analysis and rapportto malico/c´ıtrico quelle indice de
properties edited by J. Harwood, R. Aparicio. maturazione. Ann. Ist. Sper. Elaiot. II:93–112.
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123:7–11.

Handbook of Fruits and Fruit Processing
Edited by Y. H. Hui

Copyright © 2006 by Blackwell Publishing

27
Peach and Nectarine

Muhammad Siddiq

Introduction to remove most of the pubescence (Magness et al.,
Production and Postharvest Physiology 1971).

Fruit Classification and Maturity World production of peach and nectarine has been
Harvesting and Storage on a steady rise since 1997, as shown in Figure 27.1;
Chilling Injury between 1997 and 2003, peach production saw an in-
Processed Products crease of about 30%. China is the leading peach and
Canned Peaches nectarine producing country with about 37% share
Frozen Peaches of the total world production in 2003 (Table 27.1),
Dried Peaches followed by the United States and Italy. China also
Peach Jam and Jelly leads in the area harvested, followed by Italy and the
Peach Juice United States (Table 27.2). However, on a per hectare
Minimally Processed Products yield basis (Table 27.3), France is the leader with
Fresh-Cut Peaches and Nectarines 18.6 metric tons and the United States a close second
Chemical Composition and Nutrient Profile (18.5 metric tons). China ranks ninth on per hectare
References yield basis at 9.2 metric tons, which is just half of
both France and the United States. The U.S. commer-
INTRODUCTION cial peach production is mostly centered in California
and in the Southern Atlantic states (South Carolina,
Peaches (Prunus persica), like apricots, belong to Georgia), followed by Michigan, New Jersey, and
the genus Prunus of Rosaceae (rose) family hav- Pennsylvania. Elsewhere in the world, peaches are
ing decorative pink blossoms and a juicy, sweet cultivated in Southern Europe, Africa, Japan, and
drupe fruit. They are categorized as “stone fruit,” Australia. Purple-leaved and double-flowering forms
their seed being enclosed in a hard, stone-like endo- of peaches are cultivated as ornamentals. In China,
carp. Peach originated in China, later introduced into where the peach flower is largely used in decoration,
Persia, and spread by the Romans throughout Europe. it is considered a symbol of longevity.
Early Spaniards brought several varieties of peaches
to North America. Commercially grown peaches are The per capita consumption of fresh and processed
generally distinguished as clingstone (pit adheres to peach products in the United States in 2002 was 9.8 lb
flesh) or freestone (pit relatively free of the flesh); the (Table 27.4). More than half of this consumption was
famous Elberta peaches belong to the latter type. Nec- as fresh; this is likely to increase because of the pop-
tarine is a smooth-skinned fuzz-less peach; the lack ularity of fresh-cut fruits in recent years.
of fuzz is due to a single gene (Anon, 2004a; Rieger,
2004). Peach and nectarine fruits are relatively large PRODUCTION AND
in size, ranging from 2 to 3.5 in. in diameter. Peach POSTHARVEST PHYSIOLOGY
fruit is pubescent throughout the growing season, and
is usually brushed by machine prior to marketing, The number of peach cultivars commercially har-
vested far exceeds apple and pear. One of the main

519

520 Part III: Commodity Processing

15000 13177 13294 14022 14739 14788

Thousand MT 12000 11385 11429

9000

6000

3000

Figure 27.1. World peach and 0
nectarine production (1997–2003). 1997 1998 1999 2000 2001 2002 2003
(Source: Adapted from
FAO (2004), data.) Year

Table 27.1. Peach and Nectarine: Production in Leading Countries (1997–2003)

Production (1000 metric tons)

Countries 1997 1998 1999 2000 2001 2002 2003

China 3,177.1 3,237.2 3,983.4 3,851.9 4,586.2 5,259.8 5,529.4
United States 1,429.0 1,292.3 1,394.2 1,412.4 1,353.2 1,440.2 1,396.7
Italy 1,158.0 1,425.6 1,766.7 1,655.2 1,708.4 1,586.6 1,357.4
Spain 1,129.8 1,082.3 1,247.4 1,284.8
Greece 961.9 907.4 982.3
Turkey 588.6 527.6 884.2 920.3 927.1 739.6 800.0
Iran 355.0 410.0 400.0 430.0 460.0 455.0 460.0
France 217.8 267.2 317.5 350.0 380.0 385.0 385.0
Chile 464.0 341.3 477.5 480.7 458.1 464.6 355.8
Egypt 242.6 206.3 252.6 260.0 290.0 274.0 275.0
Mexico 377.0 4 29.9 301.2 240.2 247.3 257.0 257.0
Argentina 128.6 116.0 126.1 147.2 175.8 197.9 223.9
230.3 261.6 240.0 211.5 259.1 213.3 215.0

Source: FAO (2004).

Table 27.2. Peach and Nectarine: Area Harvested in Leading Countries and World (1997–2003)

Area Harvested (1000 ha)

Countries 1997 1998 1999 2000 2001 2002 2003

China 960.4 1,139.9 1,182.5 1,268.5 454.9 549.8 602.7
93.8 93.5 93.0 92.8 92.7 97.5
Italy 95.1 79.2 78.1 77.2 76.1 76.8 75.5
71.2 70.3 72.2 69.1 71.6 70.6
United States 78.4 52.5 52.5 52.5 52.5 52.5 52.5
39.5 37.3 40.9 39.2 38.6 43.4
Spain 70.5 34.7 36.1 32.7 33.0 34.0 34.0
23.9 24.1 24.5 25.4 25.4 25.4
Greece 52.0 21.2 24.0 24.5 24.5 25.0 25.0
24.6 23.0 24.0 23.5 23.0 23.0
Mexico 36.9 24.0 23.7 22.8 21.8 20.4 19.1
18.2 18.0 17.8 18.1 18.2 18.5
Egypt 35.6 1,259.1 1,355.2 1,421.2
1,936.7 1,985.6 2,078.8
Turkey 21.9

Iran 19.5

Argentina 25.3

France 26.7

Chile 17.9

World 1,774.5

Source: FAO (2004).

27 Peach and Nectarine 521

Table 27.3. Peach and Nectarine: Yield per Hectare, Leading Countries and World Average
(1997–2003)

Yield (metric tons/Ha)

Countries 1997 1998 1999 2000 2001 2002 2003

France 17.4 14.2 20.1 21.1 21.0 22.7 18.6

United States 18.2 16.3 17.9 18.3 17.8 18.8 18.5

Spain 13.6 12.7 14.0 15.6 15.7 17.4 18.2

Turkey 16.2 17.2 16.6 17.5 18.1 17.9 18.1

Iran 11.2 12.6 13.2 14.3 15.5 15.4 15.4

Greece 11.3 10.0 16.8 17.5 17.7 14.1 15.2

Chile 13.6 11.3 14.0 14.6 16.0 15.1 14.9

Italy 12.2 15.2 18.9 17.8 18.4 17.1 13.9

Argentina 9.1 10.6 10.4 8.8 11.0 9.3 9.3

China 3.3 2.8 3.4 3.0 10.1 9.6 9.2

Egypt 10.6 12.4 8.3 7.3 7.5 7.6 7.6

Mexico 3.5 2.9 3.4 3.6 4.5 5.1 5.2

World (Average) 6.4 5.9 6.6 6.4 11.1 10.9 10.4

Source: FAO (2004).

Table 27.4. Peaches: U.S. Per Capita Consumption (in lb), 1998–2002

Processed

Year Total Fresh Total Canned Frozen Dried

1998 8.8 4.8 4.0 3.5 0.5 0.1
4.2 3.6 0.6 0.1
1999 9.6 5.4 4.5 3.8 0.6 0.1
4.2 3.5 0.6 0.1
2000 9.9 5.4 4.5 3.8 0.6 0.1

2001 9.4 5.2

2002 9.8 5.3

Source: Adapted from USDA-ERS (2004).

reasons for this is the ease with which peaches can the apples. Frost can be a problem in most peach
be bred; in contrast, apple and pear cultivars result growing areas of the world as open flowers and
mostly from chance selection. Almost all regions of young fruits can be seriously damaged or killed by
the world, including the United States, have their own brief exposure to −2◦C or below. The U.S. peach
breeding programs to produce peach cultivars that production in the southeastern states has suffered a
adapt to a particular region. Therefore, there is no decline in the past several decades due to severe late
single cultivar that is dominant worldwide, or in in- spring frosts (Rieger, 2004; Logan et al., 2000).
dividual peach growing countries (Rieger, 2004).
The pruning of peach trees is done extensively to
The peach tree grows better in well-drained, produce quality fruit. Pruning is either done manually
loamy soils (pH 6–7). Peach and nectarine cultivars or by mechanical hedgers. However, proper manual
do not require cross pollination and set satisfactory pruning is widely recommended since it has proven
crops with their own pollen. Peaches have been bred to be more effective (Westwood, 1993). Peaches are
to perform in climates from Canada to the tropics, heavily thinned for proper size development; this is
and have the widest range of chilling sensitivity generally done manually, leaving one fruit per 6 in. of
requirements of any tree crop. Peaches bloom shoot length, which makes it an expensive operation.
relatively early (e.g., mid-March in Georgia), and In some cases, chemical blossom desiccants could be
are less cold-tolerant than apple or pear. This is used for flower thinning, but no chemicals are widely
the principal reason that they are generally grown used for peach thinning. Miranda-Jimenez and Royo-
in more southern states in the United States than Diaz (2002) reported that peach tree productivity is

522 Part III: Commodity Processing

improved if trees are pruned early; either in full bloom from sugar beet pulp cellulose as a hydrophilic poly-
or soon after the fruit has been set. Chemical thinning mer, in an emulsion coating containing beeswax, tri-
could reduce the high cost of manual pruning; how- ethanolamine, and oleic acid was shown to extend the
ever, it distributes the fruit irregularly on the shoots. shelf life of peaches from 12 to 16 days at 25◦C and
75% RH.
Fruit Classification and Maturity
Chilling Injury
Peaches can be classified according to appearance
and sensory characteristics as: round, flat, or beaked; Peaches (most mid- and late-season) and nectarines
pubescent or smooth-skinned; freestone or cling- (some mid- and late-season cultivars) are susceptible
stone; white-, yellow-, or red-fleshed; sweet, sour, to chilling injury during storage. Chilling injury de-
or astringent; and melting-fleshed or non-melting- velops faster and more intensely in fruit that is stored
fleshed (Brovelli et al., 1999). In most cultivars, at 2.2–7.6◦C (peaches) and 2.2–7.8◦C (nectarines)
horticultural maturity for harvesting both peaches than that stored at 0◦C or below (Crisosto and Kader,
and nectarines is determined based on changes in 2004). Brovelli et al. (1998) showed that based on the
skin ground color from green to yellow. In Califor- development of flesh mealiness, the quality of non-
nia, a color chip guide to determine maturity and a melting flesh peaches was not as severely affected as
two-tier system are used: (1) U.S. Mature, which cor- a result of chilling exposure as was the case with melt-
responds to minimum maturity and (2) Well-Mature ing flesh peaches. This quality of non-melting flesh
and/or tree-ripe. Where skin color is masked with a peaches can be a major advantage for refrigerated
red blush, fruit firmness can be measured to know storage and long-distance transportation. Modified
fruit maturity. Maturity index is the minimum flesh atmosphere storage (12% CO2 and 4% O2) of peaches
firmness, measured with an 8-mm tip penetrometer, in unperforated polypropylene bags could be useful
at which stage the fruit can be handled without any as it is associated with lower weight loss, less senes-
damage from bruising (Crisosto and Kader, 2004). cence and chilling injury, reduced incidence of de-
cay, and delayed ripening of the fruit beyond normal
Harvesting and Storage shelf-life period (Fernandez-Trujilio et al., 1998).

Harvesting of peaches and nectarines is done manu- PROCESSED PRODUCTS
ally into bags, baskets, or totes. Picked fruit is then
dumped into bins on trailers, transported from or- More than 50% of peaches and almost all of nec-
chards to packinghouse, and cooled as soon as possi- tarines produced are consumed fresh. Canned and
ble after harvest. At the packinghouse, the fruit is frozen peaches are the two major processed peach
cleaned and sorted. Attention to details in sorting products. Other products, such as dried peaches,
line efficiency is especially important with peaches, peach jam, jelly, and juice are processed on a much
where a range of colors, sizes, and shapes of fruits smaller scale (Rieger, 2004). With increasingly new
are encountered. Sizing segregates the fruit by ei- scientific claims of health benefits of phytochemicals,
ther weight or size. Optimum storage conditions yellow-flesh peaches, which are rich in many such
for peaches and nectarines are a temperature of phytochemicals (Gil et al., 2002), have great poten-
−1−0◦C and relative humidity of 90–95% with air- tial for processing into a variety of new products.
flow of approximately 50 ft3/min. Fruit tissue soft-
ening is accelerated at elevated temperatures where Canned Peaches
respiration rate can be as high as 10 times at 20◦C
than at 0◦C. At elevated temperatures, depending on About 40% of peaches produced in the United
fruit maturity, ethylene production rate increases sig- States are processed into canned products, mostly
nificantly from <5.0 ␮l/kg/h at 0◦C to as high as halves and slices. Clingstone peaches are the type
160 ␮l/kg/h at 20◦C (Crisosto and Kader, 2004). most commonly used for commercial canning, as
Polygalacturonase enzyme is responsible for the soft- they are exceptional in their ability to retain flavor
ening of fruit tissue as a result of depolymerization of and consistency. In most cases, peaches are canned
pectic polysaccharide chains during postharvest stor- within 24 h of delivery to the processing plant,
age (Tijskens et al., 1998). Togrul and Arslan (2004) which ensures that the peaches maintain nutritional
reported that the use of carboxymethylcellulose, value and flavor (Anon, 2004b; Rieger, 2004). The

27 Peach and Nectarine 523

optimum maturity of peaches for canning purposes 7. Processing and Cooling: It is recommended to
is when the color is orange-yellow and the fruit is process cans immediately after exhausting and
still firm. A brief description of processing steps for closing of cans so that the heat of the exhaust
canning peaches is given below (Lopez, 1981). is not lost before processing begins. Clingstone
peaches are processed using continuous reel type
1. Grading: Since peaches received at the cannery cookers at 100◦C (212◦F) for 20–35 min depend-
usually include a wide range of sizes, it is nec- ing on the diameter of the cans and the type of
essary to grade fruits using a mechanical grader. product—halves or slices. Some processors also
Where fruits are to be stored before canning, a use commercial processes of 14–18 min at 116◦C
pre-grading is done before the fruits have reached (240◦F). After processing, the cans are water-
canning maturity to minimize bruising. cooled immediately to 35–41◦C (95–105◦F).

2. Halving and Pitting: Peaches are usually canned Firmness of canned peaches is an important qual-
as halves or slices. The halving and pitting of ity attribute. In an attempt to improve the firmness
peaches is accomplished using automatic ma- of canned peaches, Wang (1994) treated peach slices
chines. by submerging them in pectinase solution contain-
ing 100 mg/l CaCl2 under vacuum for 0.5–2 h. This
3. Peeling and Washing: Clingstone peaches are lye- treatment was effective in improving the firmness of
peeled either by spray or by immersion method. peach slices from 7 to 25 J/kg; also, calcium content
Conditions used for lye-peeling are 5–11% lye so- increased from 280 to 430 mg/kg. Canning is shown
lution at a temperature of 102–104◦C (215–220◦F) to have negative effect on the retention of procyani-
for 45–60 s. The treatment time, strength, and tem- dins in canned Ross clingstone peaches as well as
perature of the lye solution depend on the maturity in the syrup used in the canning (Table 27.5, Hong
of the fruit. Following the hot lye treatment, fruit et al., 2004).
pieces are thoroughly sprayed with cold water for
complete removal of the peel and the lye residue. Styles of Canned Clingstone Peaches
Freestone peaches are peeled by (a) steaming,
(b) scalding in water, (c) lye-peeling, or (d) a com- The styles of canned clingstone peaches classified as
bination of steam and lye. per the U.S. standards are (a) “Halves or Halved”
canned peaches—peeled and pitted, cut approxi-
4. Quality Grading and Slicing: An inspection belt mately in half along the suture from stem to apex;
is provided for the grading and sorting of poorly (b) “Quarters or Quartered” canned peaches—halved
sized, off-color, partially peeled, and otherwise peaches cut into two approximately equal parts; (c)
imperfect fruit. It is recommended to grade and “Slices or Sliced” canned peaches—peeled and pit-
fill the halves directly from the inspection belt into ted peaches cut into sectors smaller than quarters; (d)
cans as mechanical grading/filling often results in “Dices or Diced” canned peaches—peeled and pit-
additional injury to the fruit. Fruits that are not ted peaches cut into approximate cubes; (e) “Whole”
canned as halves go directly from the belt to the canned peaches—peeled, unpitted, whole peaches
slicing machine. with or without stems removed; and (f) “Mixed pieces
of irregular sizes and shapes” are peeled, pitted, and
5. Filling and Syruping: The peach halves should be cut units of canned peaches that are predominantly
filled as rapidly as possible after peeling and grad- irregular in size and shape, which do not conform
ing as extended exposure to the air results in dis- to a single style of halves, quarters, slices, or dices
coloration. Slices should go over an inspection belt (USDA-AMS, 1985).
for removal of defective pieces. The slices are then
discharged to hand-pack fillers. The Standards of Liquid Media and Brix Measurements
Identity for canned peaches specify the cut-out
Brix of the syrup (Table 27.6) that correspond to Cut-out requirements for liquid media in canned free-
“heavy syrup” or “light syrup,” etc. Syruping is stone peaches are not incorporated in the grades of
accomplished by rotary and straight-line syrupers the finished product since syrup or any other liquid
or by pre-vacuumizing syrupers. medium is not a factor of quality for the purpose of
these grades. The cut-out Brix measurements for the
6. Exhausting and Closing: The cans are given respective designations are shown in Table 27.6.
a steam exhaust for about 6 min at 88–91◦C
(190–195◦F). Cans are closed immediately after
exhausting; a gross headspace of 5/16 of an inch
is recommended.

524 Part III: Commodity Processing

Table 27.5. Content of Procyanidin Oligomers in Frozen and Canned
Peaches (mg/kg)a

Oligomer Frozen 0 month Canned Peach 3 month
Fraction Peach 1 month 2 month

P1 19.59 17.50 17.18 15.77 15.85
P2 39.59 36.50 34.84 29.03 30.55
P3 38.81 34.77 31.33 19.23 19.06
P4 17.81 16.97 10.07
P5 12.43 11.85 6.26 3.61
P6 10.62 7.61 2.83 –
P7 3.94 7.87 3.52 –
P8 1.75 2.76 – –
– – –
Source: Hong et al. (2004). – – –
aOn wet-weight basis.

Frozen Peaches Air-blast tunnel freezing is the most commonly
used system by most processors. In this method, the
About 6–8% of peaches produced are processed as prepared product ready to be frozen is placed on wire
frozen peaches. Processors usually need three differ- mesh trays and loaded on to racks. The tray racks are
ent types of peaches: (1) for retail packages of slices, moved into the freezing tunnel. Cold air is usually in-
varieties with red color around the pit cavity, such as troduced into the tunnel at the opposite end from the
Rio Oso Gem, are desirable; (2) peaches frozen in one where the product to be frozen enters. The tem-
bulk for later processing into preserves should have perature and velocity of the air are of critical impor-
good flavor and should not have red color around tance in the freezing process. The temperature of the
the pit cavity for good color/appearance of the jam or air is usually between 0◦F and −30◦F (−18◦C and
marmalade. Fay Elberta peaches picked on the imma- −34◦C, respectively). Air velocity can range from
ture side are ideal for preserves; and (3) for the insti- 100 to 3500 ft/min. The length of time a product is
tutional market, mainly pies, a highly flavored, firm- subjected to cold air blast in the tunnel depends on
textured variety with resistance to browning is best the product size (Boyle et al., 1977).
suited. For freezing peaches, an ideal variety prefer-
ably should have the shape, bright color, and firmness Otero et al. (2000) compared classical methods
of Rio Oso Gem, the flavor of Elberta, and the non- of freezing and high-pressure-shift freezing (HPSF)
browning characteristic of Sunbeam (Boyle et al., with respect to their effect on modification to the mi-
1977). Fruit preparation steps of washing, peeling, crostructure of peach and mango using a histochem-
and slicing are the same as discussed under “Canned ical technique. In the HPSF method, samples were
Peaches.” cooled under pressure (200 MPa) to −20◦C with-
out ice formation, and then pressure was released to

Table 27.6. Cut-Out Brix Levels of Syrups Used in Canned Peaches

Designations Brix Measurements

“Extra heavy syrup;” or “Extra heavily sweetened 22◦ or more but less than 35◦
fruit juice(s) and water;” or “Extra heavily
sweetened fruit juice(s).” 18◦ or more but less than 22◦
14◦ or more but less than 18◦
“Heavy syrup”; or “Heavily sweetened fruit juice(s) 10◦ or more but less than 14◦
and water”; or “Heavily sweetened fruit juice(s).”

“Light syrup;” or “Lightly sweetened fruit juice(s)
and water;” or “Lightly sweetened fruit juice(s).”

“Slightly sweetened water;” or “Extra light syrup;”
or “Slightly sweetened fruit juice(s) and water;”
or “Slightly sweetened fruit juice(s).”

Source: USDA-AMS (1985).

27 Peach and Nectarine 525

atmospheric level (0.1 MPa). The super-cooling un- be sulfured sufficiently to retain a characteristic color.
der high pressure led to uniform and rapid ice Federal inspection certificates shall indicate the mois-
nucleation throughout the volume of fruits. The ture content of the finished product, which shall be
HPSF method prevented quality losses due to freeze- not more than 25% by weight. Dried peaches may be
cracking or large ice crystal presence, thus, maintain- processed from freestone or clingstone peach types
ing the original tissue structure. (USDA-AMS, 1967).

Hong et al. (2004) used normal-phase LC-MS Only 1–2% of peaches produced are processed
to determine the levels and fate of procyanidins in as dried halves, quarters, or slices. Fruit preparation
frozen and canned Ross clingstone peaches, as well steps of washing, peeling, and cutting are the same as
as in the syrup used in the canning over a 3-month discussed under “Canned Peaches.” To minimize dis-
storage period. Retention of these health beneficial coloration, cut peaches are dipped in ascorbic acid or
compounds was better in frozen peaches as compared other anti-browning solution for 5–10 min (see more
to canned peaches (Table 27.5). Storage of canned detail on anti-browning agents under “Fresh-Cut
peaches for 3 months demonstrated a time-related Peaches and Nectarines”). Prepared fruit is spread
loss in high molecular weight oligomers (P5–P8) and in single-layers on trays, usually 3 lb/sq. ft. Peaches
that by 3 months, oligomers larger than tetramers are dried to moisture content of about 25%. Gen-
were not observed. After 3 months of post-canning erally, forced-draft tunnel dehydrators are used for
storage, levels of monomers had decreased by 10%, drying peaches. For drying in a countercurrent tun-
dimers by 16%, trimers by 45%, and tetramers by nel, temperature should not exceed 68◦C (155◦F). To-
80%. tal drying time (24–30 h) depends on the size of the
product and the temperature used. Blanched peaches
Varietal Types and Styles of Frozen Peaches dry at a faster rate, requiring about 18 h to reach
25% moisture (Brekke and Nury, 1964). In peach–
Varietal types of peaches that are processed as frozen producing developing countries, sun-drying is still
include: (a) “Yellow freestone”—freestone peaches the most widely used method due to its low cost;
of the yellow-fleshed varieties, which may have or- however, quality is not as good as those dried in con-
ange or red pigments emanating from the pit cav- trolled and sanitary environment.
ity, (b) “White freestone”—freestone peaches that are
predominately white-fleshed, (c) “Red freestone”— Hansmann et al. (1998) investigated the drying be-
freestone peaches that have substantial red coloring havior of clingstone peach halves dehydrated with-
in the flesh, and (d) “Yellow clingstone”—clingstone out sulfites, and suggested that enzymatic browning
peaches of the yellow or orange-fleshed varieties. reactions can be controlled during dehydration by
Styles of frozen peaches include: (a) “Halved or selecting dehydration conditions favoring low super-
halves”—the peaches are cut approximately in half ficial product temperature and lower water activity
along the suture from stem to apex, (b) “Quar- (aw). Also, a peeled fruit dried faster than an unpeeled
tered or quarters”—halved peaches cut into two ap- one. Wang et al. (1996) dried yellow peach fruits cut
proximately equal parts, (c) “Sliced or slices”—the into halves to a moisture content of 18% with mi-
peaches are cut into sectors smaller than quarters, crowaves of 2350 MHz at 0.3–0.45 m/s air velocity
(d) “Diced”—the peaches are cut into approximate and hot air and far ultra-red waves. Microwave dried
cube-shaped units, and (e) “Mixed pieces of irregu- peaches had better color than those dried by the other
lar sizes and shapes”—means peaches cut or broken two methods.
into pieces of irregular sizes and shapes, and which
do not conform to a single style of halves, quarters, Many researchers have shown the benefits of os-
or slices (USDA-AMS, 1961). motic drying before traditional dehydration. Lerici
et al. (1988) reported that the osmotically dehydrated
Dried Peaches products had very good texture and good retention
of aroma and color; the aw was reduced sufficiently
As per the U.S. Standards, dried peaches are the to improve shelf life but further processing (e.g.,
halved and pitted fruit from which greater portion freezing, drying, and pasteurization) was necessary
of moisture has been removed. Before packing, the to ensure shelf-stable products. Erba et al. (1994) de-
dried fruit is processed to cleanse the fruit and may scribed “osmodehydro-freezing” as a combined pro-
cess where osmotic drying is followed by air drying
and freezing to prepare reduced-moisture fruit ingre-
dients, free of preservatives, with a natural flavor,

526 Part III: Commodity Processing

color, and texture, and with functional properties suit- cleaning and washing, peaches are heated to about
able for different food applications. Souti et al. (2003) 45◦C in large kettles. Stirring with a propeller-type
studied suitability of osmotic drying as a method for blender and addition of pectinase and hemicullulase
pre-drying of peaches, and showed that the use of os- enzymes aid in extraction of juice, higher yields, and
motic pre-drying treatment produced dried peaches higher soluble solids contents. Other steps such as fil-
of better sensory quality than traditionally solar-dried tration, clarification, and pasteurization are similar to
peaches, as judged by a sensory panel. those for other juices. Pagan et al. (1997) extracted
juice from Caterino variety of peaches by treating
Peach Jam and Jelly pulp with liquefying enzymes at temperatures be-
tween 30◦C and 70◦C. Yield and soluble solids con-
About 2–3% of peaches are processed into jam, jelly, tents of juice were higher with the enzymatic liq-
and juice combined (Rieger, 2004). Jam, jelly, pre- uefaction method (up to 55◦C) than by simple ex-
serves, and marmalades are similar products (all are traction. At temperatures over 55◦C, the enzymes
made from fruit, preserved by sugar and thickened used were not effective due possibly to their inac-
or gelled to some extent). Jam is made from crushed tivation. Dogan and Erkmen (2003) investigated the
or chopped fruit, holds its shape, but is less firm than inactivation of Escherichia coli by ultra high hydro-
jelly. Jelly is a mixture of fruit juice and sugar that is static pressure ranging from 300 MPa to 700 MPa in
clear and firm enough to hold its shape. Granulated peach juice and reported that a 12-min treatment at
white sugar is most often used to make jelly or jam. 600 MPa was sufficient to produce a commercially
Jams and jellies can be made with or without added sterile peach juice.
pectin (Willenberg and Hughes, 2004).
MINIMALLY PROCESSED
Type of sugar or sweetener and pectin added in jam PRODUCTS
can affect its textural qualities. Costell et al. (1993)
reported that differences in formulations influenced Fresh-Cut Peaches and Nectarines
rheological properties of peach jam. Raphaelides
et al. (1996) investigated the effects of sugars as Fresh-cut produce is one of the fastest growing seg-
present in commercial mixtures on mechanical prop- ments of the food industry in the United States. Re-
erties and texture of peach jam. A series of jam sam- tail sales of fresh-cut produce have grown from $5
ples was prepared using commercial glucose syrups billion in 1994 to over $10 billion. These figures are
of 38 and 44 dextrose equivalents, isoglucose, mal- projected to reach $15 billion by 2005. Fresh-cut pro-
tose syrup, and their mixtures with sucrose. Jam tex- duce is defined as any fresh fruit or vegetable, or any
ture was markedly affected by composition of the combination thereof that has been physically altered
syrups. Sugar type was important, e.g., monosaccha- from its original form, but remains in a fresh state
rides and their mixtures with sucrose formed more (IFPA, 2004). Currently, <1% of peaches and nec-
rigid gels than disaccharides. These researchers con- tarines are processed as fresh-cut; however, both of
cluded that, by carefully selecting blends of glu- these fruits have great potential to capture a sizeable
cose syrups with or without sucrose, a range of jam share of fresh-cut market owing to the fact that more
consistencies could be derived. Grigelmo-Miguel than half of peaches (Table 27.4) and almost all of
and Martin-Belloso (2000) compared the quality nectarines produced are consumed fresh. Fresh-cut
of conventional peach jam with those made by to- produce falls under “Minimal Processing” that has
tal or partial substitution of pectin with added di- two main purposes: (1) keeping the produce fresh,
etary fiber (DF) as a thickener. Peach jam with without losing its nutritional quality and (2) ensuring
added DF had similar color but was more viscous a product shelf life that is sufficient to make its distri-
than conventional jam. The sensory characteristics bution feasible within the region of its consumption
of high peach DF jams were similar to conventional (Laurila and Ahvenainen, 2002).
jams.
For processing of fresh-cut peach slices, the op-
Peach Juice timal ripeness is when flesh firmness reaches 13–
27 N (3–6 lb-force). Depending on the cultivar, peach
Peach juice is produced on a much smaller scale as slices retain good eating quality for 2–8 days at 5◦C
compared to other processed peach products. After and 90–95% RH. The optimal ripeness for prepar-
ing fresh-cut nectarines slices is the partially ripe

27 Peach and Nectarine 527

(27–49 N or 6–11 lb-force) or ripe (13–27 N or 3–6 honey, and natural fruit juices (e.g., lemon juice)
lb-force); depending on the cultivar and variety, these have been tried with varying success. Chitosan coat-
slices keep good eating quality at 0◦C and 90–95% ing (Huaqiang et al., 2004), sodium hexametaphos-
RH for 2–12 days (Crisosto and Kader, 2004). Gorny phate (Pilizota and Sapers, 2004), oxalic acid (Yoruk
et al. (1998) investigated the effects of fruit ripeness and Marshall, 2003), and NatureSealTM, a commer-
and post-cutting storage temperature on the deterio- cially available product containing calcium ascorbate
ration rate of fresh-cut Flavorcrest peaches and Zee (Arvind et al., 2004), are the other alternatives tried
Grand nectarines. They reported that for preparing more recently either alone or in conjunction with
fresh-cut slices, the optimal ripeness was the ripe other inhibitors. In the case of peach and nectarine,
stages for peaches and partially ripe or ripe stages post-cutting dips in ascorbate and calcium lactate, or
for nectarines. While retaining good eating quality, use of modified atmosphere packaging has shown to
peach and nectarine slices had a shelf life of 6 and prolong the shelf life of fresh-cut slices (Gorny et al.,
8 days, respectively, at 0◦C and 90–95% RH. Gorny 1998).
et al. (1999) processed slices from 13 cultivars of
peaches and 8 cultivars of nectarines using a 2% (w/v) CHEMICAL COMPOSITION AND
ascorbic acid +1% (w/v) calcium lactate post-cutting NUTRIENT PROFILE
dip and reported acceptable sensory color and shelf
life of 2–12 days at 0◦C for slices from all cultivars, Peaches and nectarines contain significant amounts
except Cardinal. Controlled atmosphere (CA) stor- of some major nutrients as shown in Table 27.7.
age extended the shelf life by additional 1–2 days for Peaches and nectarines are good sources of beta-
slices from some cultivars. carotene, which is thought to be a powerful anti-
aging agent. Palmer-Wright and Kader (1997) stud-
Some important attributes to be considered for ied changes in quality, retinol equivalents, and in-
the sensory quality of a fresh-cut product, such as dividual provitamin A carotenoids in fresh-cut Fay
peaches and nectarines, are: (1) color or appearance, Elberta peaches held for 7 days at 5◦C in air or CA.
(2) texture, (3) flavor, (4) taste, and (5) overall accep- They concluded that the limit of shelf life was reached
tance. However, from consumers’ perspective, color before major losses of carotenoids were observed.
or appearance of fresh-cut produce is the single most Dried peaches, though high in calories, are an excel-
important factor among all the quality attributes men- lent source of fiber, most vitamins and minerals. Beta-
tioned above (Siddiq et al., 2004). If the color of carotene, along with vitamin C, is important for our
a fresh-cut product is not acceptable or attractive, immune system as it helps to prevent damage from
the consumer is least likely to purchase it regardless free radicals. Vitamin C boosts the immune system,
of its excellent texture, flavor, taste, or other quality promotes healing, and helps prevent cancer, heart dis-
attributes. ease, and stroke. In addition, peaches are rich in B
vitamins, vitamin C, folic acid, calcium, and many
Two enzymes, polyphenol oxidase and peroxi- other nutrients essential to good health. Peach and its
dase, have been implicated in color deterioration processed products are very low in fat and have no
in cut fruits and vegetables. In addition to visible cholesterol.
color changes, these enzymes not only impair the
other sensory properties and, hence, the marketabil- Peaches and nectarines, especially unpeeled, are
ity of the product, but also often lower its nutritive a good source of dietary Fibre (DF), which is im-
value (Vamos-Vigyazo, 1981). Both these enzymes portant to a healthy diet and can help control weight
have been widely studied in different fruits and veg- and lower cholesterol levels. Grigelmo-Miguel et al.
etables with special emphasis on their inactivation (1999) investigated insoluble and soluble DF frac-
(Escribano et al., 2002; Nicolas et al., 1994; Siddiq tions in peach DF concentrates prepared from dried
and Cash, 2000; Weemaes et al., 1998). Traditionally, washed peach bagasse or pomace remaining after
sulfites were used extensively in the food industry to juice extraction and showed that such concentrates
control enzymatic browning. However, since the late which had low energy value could be an adequate
1980s, there has been an effort to avoid the use of sul- source of DF with an insoluble to soluble DF ratio of
fiting agents in foods due to safety, regulatory, and 66–34.
labeling issues (Lambrecht, 1995). A number of al-
ternatives to sulfites such as ascorbic acid, citric acid, Gil et al. (2002) investigated the concentration of
4-hexylresorcinol, erythorbic acid, and sodium ery- total phenolics, total ascorbic acid, beta-carotene,
thorbate (stereoisomers of ascorbates), benzoic acid, and ascorbic acid equivalent antioxidant capacity

528 Part III: Commodity Processing

Table 27.7. Composition of Peaches, Their Processed Products and Nectarines (per 100 g
Edible Portion)

Raw Canned Frozen Dried Raw

Nutrient Unit Peaches Peachesa Peachesb Peaches Nectarines

Proximate g 88.87 84.72 74.73 31.80 87.59
Water kcal 39 54 94 239 44
Energy 0.91 0.45 0.63 3.61 1.06
Protein g 0.25 0.03 0.13 0.76 0.32
Total lipid (fat) g 0 0.082
Fatty acids, total saturated g 0.019 0.014 61.33 0.025
Carbohydrate, by g 9.54 14.55 23.98 10.55
8.2
difference g 1.5 1.3 1.8 41.74 1.7
Fiber, total dietary g 8.39 13.25 22.18 7.89
Sugars, total 2163
Vitamins IU 326 354 284 4.8 332
Vitamin A, IU mg 6.6 2.4 94.2 5.4
Vitamin C, total ascorbic 0.002
mg 0.024 0.009 0.013 0.212 0.034
acid mg 0.031 0.025 0.035 4.375 0.027
Thiamin mg 0.806 0.593 0.653 0.564 1.125
Riboflavin mg 0.153 0.05 0.132 0.067 0.185
Niacin mg 0.025 0.019 0.018 0.25
Pantothenic acid mcg 0
Vitamin B-6 mg 4 3 3 0.19 5
Folate, total 0.73 0.49 0.62 0.77
Vitamin E mcg 15.7
2.6 0 2.2 2.2
(alpha-tocopherol) mg 28
Vitamin K mg 6 3 3 4.06 6
mg 0.25 0.36 0.37 42 0.28
(phylloquinone) mg 119
Minerals mg 9 5 5 996 9
Calcium mg 20 11 11 26
Iron mg 190 97 130 7 201
Magnesium mg 0 5 6 0.57 0
Phosphorus mg 0.17 0.09 0.05 0.364 0.17
Potassium 0.068 0.052 0.024 0.305 0.086
Sodium 0.061 0.046 0.029 0.054
Zinc
Copper
Manganese

Source: USDA (2004).
aIn light syrup (solids and liquids).
bSweetened.

(AEAC) in a number of cultivars of both yellow-flesh Peach bark has been used as a herbal remedy for a
and white-flesh peaches and nectarine; and found a wide variety of ailments. It is said to be “one of the
strong correlation (0.93–0.96) between total pheno- stronger blood moving herbs,” and therefore has use
lics and antioxidant activities. Yellow-flesh fruit had in encouraging menstruation in females with delayed
significantly higher amounts of total phenolics than menses or congested blood. It also relieves bladder
white-flesh fruits, and so did peel as compared to flesh inflammation and urinary tract problems; functions
regardless of the fruit flesh color (Table 27.8). Asami as a mild laxative; has expectorant activity for the
et al. (2003) reported that lye-peeling of peaches re- lungs, nose, and throat; and relieves chest pain and
sulted in 21% less loss of total phenolics than manual spasms. The ancient Chinese considered the peach a
peeling. symbol of long life and immortality (Rieger, 2004).

Table 27.8. Total Phenolics, Total Ascorbic Acid, Beta-Carotene, and Antioxidant Capacity in the Peel and Flesh Tissue of Peaches and
Nectarines

Total Phenolics Total Ascorbic Acid Beta-carotenes AEAC
(mg/kg) (mg/kg) (μg/kg) (mg/kg)a

529 Fruit Peel Flesh Peel Flesh Peel Flesh Peel Flesh

Yellow-flesh peaches 485–1,202 172–547 72–181 31–126 2,650–3,350 530–1,680 313–1,107 93–432
White-flesh peaches 670–1,836 228–1,042 112–202 48–65 110–430 40–80 530–1,789 146–1,006
Yellow-flesh nectarine 427–1,403 138–415 53–61 277–981 62–317
White-flesh nectarines 418–2,020 91–901 78–130 42–122 1,870–3,070 580–1,310 230–1,447 46–837
93–200 50–570 20–100

Source: Gil et al. (2002).
aAscorbic Acid Equivalent Antioxidant Capacity.

530 Part III: Commodity Processing

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Handbook of Fruits and Fruit Processing
Edited by Y. H. Hui

Copyright © 2006 by Blackwell Publishing

28
Pear Drying

Raquel de Pinho Ferreira Guine´

Introduction climates. Despite being a slow drying process and re-
Shrinkage Characteristics quiring much handwork, this is the cheapest of drying
Sorption Isotherms methods. However, it has some important disadvan-
Drying Kinetics tages, like the dependence on natural factors and the
Moisture Diffusivity need of great exposure areas.
Commercial Drying
Acknowledgments The factors that determine the end of the drying
References process are mainly the high concentration of sugar,
the low moisture content, and the energetic optimiza-
INTRODUCTION tion of the process. The desirable amount of water
present at the end of the drying varies according to
The drying of fruits allows their better preservation the type of fruit (the smaller the fruit, the less shall
by reducing water content thus inhibiting microbial be its final moisture content). However, the choice of
growth and enzymatic modifications. One of the most the final moisture content must take into considera-
important advantages of foodstuff drying is the reduc- tion not only the stability of the fruit, but also the final
tion in size and weight, which facilitates transporta- physical and chemical properties that characterize its
tion and reduces storage space, and more importantly, quality (Guine´ and Castro, 2002b).
avoids the need of using expensive cooling systems
and preservation. Finally, it increases food diver- The fruits that have been traditionally dried are es-
sity, opening alternatives to the consumption of fresh sentially grapes, figs, plums, apricots, and peaches,
products, and improving quality of life in rural areas. but some other species have recently gained increased
importance, like apples, mango, pawpaw, pineapple,
The dehydration of foods generally involves a banana, and pears. The production of dried pears
series of interdependent unit operations, such as has some relevance in countries like Australia, South
blanching, pasteurization, and preconcentration, all Africa, Chile, Argentina, and Portugal (Guine´ and
of which contribute to the overall quality of the final Castro, 2003).
product and to improve the efficiency of the process.
Ideally, the process of water removal in dehydration The knowledge of the phenomena taking place dur-
of food products should be reversible. This is not so ing drying is crucial to optimize this process, render-
due to the inevitable loss of nutritional and functional ing it a more profitable and competitive production
attributes, which in turn depends on the type and ex- method, and offering the consumer products of un-
tension of dehydration and on the sensitivity of the questionable quality. For instance, the study of the
specific food components (Rizvi, 1986). drying kinetics is of utmost importance in the design
and optimization of dryers (Kiranoudis et al., 1997;
Solar drying has been used for centuries, but it is Saravia and Passamai, 1997). On the other hand,
restricted to countries with tropical and semitropical in the case of fruits, and of pears in particular, the
sugar concentrations are relatively high (60–75%, dry
To the memory of Professor Jose´ Almiro Castro. basis) and greatly increase as the water evapo-
ration proceeds, offering an additional resistance

533

534 Part III: Commodity Processing

to moisture transfer from the fruit (Guine´ and well as the apparent density. Therefore, the knowl-
Castro, 2002b). In addition, success in food drying
and storing (e.g., conditioning and packing) strongly edge of the shrinkage behavior assumes an impor-
depends on the isothermal relationship between the
water activity and water content of the food material tant role in understanding and modeling drying pro-
being processed. Therefore, the knowledge of such
relations, often represented in the form of sorption cesses and in controlling the characteristics of the
isotherms, both at the drying and at the storage tem-
peratures, is of unquestionable relevance for the food food (Zogzas et al., 1994b). Various models have
industry (Weisser, 1986).
been proposed in the literature to describe shrink-
This chapter is an overview and presents the per-
sonal assessments of the author’s work. Dried pears age of the foods in drying, and it has been shown that
are produced in Portugal in the summer by a tradi-
tional solar drying process, but recently many efforts either volumetric shrinkage or dimensional shrink-
have been made to study pear drying, not only lim-
ited to this type of drying. As a matter of fact, many age has a strong dependency on the moisture content
aspects related to the drying process have been inves-
tigated to develop reliable process models and new (Khraisheh et al., 1997).
drying techniques applied to this traditional product
that is much appreciated and has many economic po- When studying the physical properties of foods,
tentialities.
namely of fruits, it is very important to know with
This work was organized in sections, the first one
concerning the shrinking behavior of the pears during some accuracy estimations for geometrical param-
drying, namely the evolution of size and density with
moisture content. Sorption isotherms of pears are also eters like shape, size, volume, and surface area, as
presented and the experimental data were used to test
the suitability of different sorption models. Drying ki- well as density. Methods based on longitudinal and
netics was also investigated for its importance in the
design of alternative driers. From the drying curves, it transversal sections of the material have been devel-
was possible to examine the dependence of moisture
diffusivity on pear composition (in particular, water oped to assess shape and size, and the results are
and sugar concentrations). Finally, commercial dry-
ing is also mentioned, the main emphasis being on compiled in the form of charts for some food prod-
solar dryers.
ucts, namely apples, peaches, and potatoes (Mohs-
SHRINKAGE CHARACTERISTICS
enin, 1986). The experimental measurements are
Recently, much attention has been paid to the qual-
ity of food products, commonly characterized by a usually obtained by taking pictures of the object to-
significant number of parameters like texture, taste,
color, porosity, and other physical properties includ- gether with a millimeter scale. However, other more
ing density and specific volume, which are strongly
influenced by the processing conditions. In particu- sophisticated methods based on computer-aided im-
lar during drying, the water present evaporates in a
rather important extension, and phenomena such as age processing have also been used (Sabliov et al.,
shrinkage significantly affect physical structure and
properties of the food, contributing for its final qual- 2002). In most cases, fruit shape can be approx-
ity standards (Krokida and Maroulis, 1997).
imated to known geometric forms, allowing the
Shrinking of biological products, as foods, is very
much dependent on the internal pressure of the water calculation of volume and surface area. In addi-
vapor resulting from the evaporation process, affect-
ing the moisture diffusion and water removal rate, as tion, it has been possible to quantify other geomet-

rical parameters associated to shape, like spheric-

ity and roundness (Mohsenin, 1986; Ochoa et al.,

2002).

In the present study, the shape of the pears was

considered as a combination of a semisphere and a

cone, as depicted in Figure 28.1, and thus it was pos-

sible, assuming constant shape throughout drying, to

estimate their volume and surface area as:

Volume = 1 Volume of sphere + Volume of cone
2

V = 1 4 ␲r 3 + 1 ␲r 2h = ␲ r 2(2r + h) (28.1)
23 3 3

Surfac area = 1 Area of sphere + Area of cone −
2

Area of cone base

As = 1 (4␲r 2) + ␲r( r2 + h2 + r) − ␲r2
2

= ␲r (2r + r 2 + h2) (28.2)

where r is the radius of the semisphere and h the
height of the cone.

The dimensions r and h were evaluated for pears
of the variety D. Joaquina during the drying process
from direct readings of pear photographs taken over

28 Pear Drying 535

Inc., 2002) is relatively good, as shown by the values
of R2.

r = 0.0129 + 0.0268[1 − exp(−0.1281W )],

R2 = 0.923 (28.3)

h = 0.0339 + 0.0180[1 − exp(−0.3598W )],

h R2 = 0.951 (28.4)

r Pear surface area, As, was calculated for the as-
sumed geometrical shape [Eq. 28.2] and its evolution
r throughout drying was also found to follow an expo-
nential law, as a function of dry basis water content
Figure 28.1. Approximation of the pear shape to (W ), expressed by:
known geometries.
As = 0.0017 + 0.0057[1 − exp(−0.3081W )],
a millimeter scale paper. Figure 28.2 shows the evo-
lution of the pear dimensions as drying proceeds and R2 = 0.987 (28.5)
consequently the moisture content diminishes. Wa-
ter content was determined with a Halogen Moisture where As is expressed in m2. Figure 28.3 shows the
Analyser (Mettler Toledo HG53). variation of surface area with W .

Both variables (r and h) obey an exponential raise The volume of the pears was determined not only
law described by Equations 28.3 and 28.4, where W by using the mean values of the pear dimensions
is the dry basis moisture content (kg of water/kg of [Eq. 28.1], but also by using liquid displacement (liq-
dry solids), and r and h are in meters. The quality uid picnometry is currently used for fruits since it
of the fitting (performed with Sigma Plot 8.0, SPSS, is independent of the irregularity of these materials;
Mohsenin, 1986). In this work, two different solvents
were used (water and toluene) but the results obtained
were analogous.

Figure 28.4 shows the plot of the values obtained
for pear volume using the two different methodolo-
gies against dry basis moisture content, W . The re-
sults highlight the good agreement between the two
sets of data which were both used to compute the

0.07 h
0.06 r
0.05
Pear dimensions (m) 0.04
0.03
0.02 12345 6 Figure 28.2. Evolution of pear
0.01 Moisture content (kg/kg dry basis) dimensions during drying.
0.00

0

536 Part III: Commodity Processing

Superficial area (m2) 0.010
0.009
Figure 28.3. Variation of surface area 0.008 12345 6
with moisture content. 0.007 Moisture content (kg/kg dry basis)
0.006
0.005
0.004
0.003
0.002
0.001

0

Volume (m3) 1e-4
9e-5
8e-5 Volume calculated from pear dimensions
7e-5 Volume determined from liquid displacement
6e-5
Figure 28.4. Variation of pear volume 5e-5 12345 6
with moisture content. 4e-5 Moisture content (kg/kg dry basis)
3e-5
2e-5
1e-5

0

following linear relationship between V and W : To express the shrinking behavior during drying,
a bulk shrinkage coefficient, Sb, has been defined as
V = 1.5711e − 5 + 1.7617e − 5W, the ratio of the sample volume at any stage to the
initial volume (V /V0) (Khraisheh et al., 1997). This
R2 = 0.972 (28.6) was determined following two different ways:

where V is the volume (m3). a. as the ratio of V /V0 with both V and V0 given by
In addition, and since both pear volume and weight Equation 28.6, deriving

were known, specific gravity of the pears, dr , was 0.89 + W (28.8)
calculated, being its variation with moisture content Sb = 0.89 + W0
presented in Figure 28.5. This variation is an expo-
nential function of the form:

dr = 0.2247 exp(−2.7168W ) + 1.1527 (28.7) where W0 is the initial dry basis water content;
× exp(−0.0255W ), R2 = 0.898, b. by linear regression applied to the experimen-

where dr is dimensionless. tal values of V /V0 when plotted against the

28 Pear Drying 537

Relative density 1.6
1.5
1.4 12345 6 Figure 28.5. Variation of specific
1.3 Moisture content (kg/kg dry basis) gravity with moisture content.
1.2
1.1
1.0
0.9
0.8

0

1.2

1.0

0.8

Sb 0.6

0.4

Experimental data

0.2 Model A: Sb = (0.89 + W)/(0.89 + W0)

Model B: Sb = 0.1694 + 0.8651 W/W0

0.0 Figure 28.6. Evolution of shrinkage
0.0 0.2 0.4 0.6 0.8 1.0 1.2 coefficient, Sb, along the drying

W/W0 process.

dimensionless moisture content (W /W0) (Fig. W0)] (Khraisheh et al., 1997), whereas Equation 28.9
28.6), derives is quite similar to the one presented by Raghvan and
Silveira (2001) for the microwave drying of straw-
Sb = 0.1694 + 0.8651 W , R2 = 0.990 (28.9) berries [Sb = 0.0741 + 0.8121(W/W0)].
W0
The good agreement between the experimental
Both types of equations have been reported points and Equations 28.8 and 28.9, apparent in
in the literature to compute Sb. Indeed, Equa- Figure 28.6, indicates that they are both adequate
tion 28.8 is analogous to the model suggested by to correlate the shrinkage coefficient with moisture
Kilpatrick et al. (1955) [Sb = (0.8 + W )/(0.8 + content.

538 Part III: Commodity Processing

SORPTION ISOTHERMS sigmoidal, as type II in Figure 28.7. However, for
foods rich in soluble components, such as sugars, the
All food materials are characterized by a particular type that best seems to describe the sorption behavior
sorption isotherm at a constant temperature. At the is type III (Rizvi, 1986; Va´zquez-Un˜a et al., 2001).
end of the drying process, the moisture content of
the food becomes stationary, reaching an equilibrium Many different models can be found in the liter-
moisture content with the surrounding atmosphere. ature to describe the sorption of water on capillary
The moisture sorption isotherms are obtained by plot- materials (more than 200; Viswanathan et al., 2003),
ting the equilibrium moisture content against the cor- most of them represented by semiempirical equations
responding water activity of the food product over a with a variable number of parameters. As water is
certain range of values, for a constant temperature. associated with the food matrix in different ways for
Their knowledge is very important for the determina- different activity regions, it is not surprising that one
tion of the optimal storage conditions as well as for sorption model might not be enough to represent the
the prediction of thermodynamic equilibrium mod- sorption behavior over the entire water activity range.
els. Besides, such isotherms also give information on
how strongly the water is bound to the material and Table 28.1 presents eight of the most commonly
thus are essential for the prediction of the drying and used sorption isotherm models, where We is the equi-
rehydration rates (Va´zquez-Un˜a et al., 2001; Guine´ librium moisture content (dry basis), Wm the mono-
and Castro, 2002a). layer moisture content (dry basis), aw the water activ-
ity, R the gas constant (R = 8.31451 J/mol K), and
The final water content of the food determines the T the absolute temperature.
endpoint of the drying process, which should corre-
spond to the optimum residual moisture content of the Three of these models exhibit an explicit tem-
final product. In fact, a high water content obtained perature dependency (Chung–Pfost, Halsey, and
with a short drying time leads to reduced stability, Henderson), other three include temperature-
whereas drying the material to a moisture content be- dependent parameters [Chen, Brunauer–Emmett–
low a given optimum value leads to energy wasting Teller (BET), and Guggenheim–Andersen—de Boer
with no apparent benefit. (GAB)], and in the remaining (Iglesias–Chirife and
Oswin), the temperature dependency has not been
Brunauer et al. (1940) classified adsorption considered. Iglesias–Chirife model, corresponding
isotherms in terms of the Van der Walls adsorption to a type III curve (Fig. 28.7), proved to be quite
of gases into five different types (Fig. 28.7). For adequate to describe the sorption behavior of many
most foods, sorption isotherms are nonlinear, usually sugar-rich foods, as most fruits, where the monolayer
is completed at a very low moisture content and the
I V dissolution of sugars takes place (Rizvi, 1986).
W IV
Generally, higher temperatures result in a reduc-
II tion of adsorbed molecules and therefore adsorption
decreases with increasing temperature, although this
III dependency is usually small. However, this behavior
is not universal as temperature affects many differ-
0 P/Po 1.0 ent phenomena and an increase in temperature can
in fact increase the rates of adsorption, hydrolysis,
Figure 28.7. Types of adsorption isotherms (adapted and recrystallization processes (Rizvi, 1986). Food
from Brunauer et al., 1940). rich in soluble solids, such as sugars, seem to exhibit
antithetical temperature effects for higher values of
aw due to their increased solubility in water (Rizvi,
1986).

The sorption behavior of pears has been inves-
tigated using circular slices of D. Joaquina pears
(30 mm diameter and 3 mm thick) dried in a
ventilated chamber for constant drying tempera-
tures of 20◦C, 25◦C, and 30◦C (Guine´ and Castro,
2002a). The sorptions isotherms were obtained from
the measurements of the equilibrium moisture con-
tent of the pears (Mettler Toledo, HG53 Halogen

28 Pear Drying 539

Table 28.1. Most Common Models Found in Literature to Describe Food Sorption Isotherms

Model Equation Parameters References

BET aw = 1 + C −1 Wm, C(T ) Brunauer et al. (1938)
Chen We(1 − aw) WmC WmC aw A(T), B(T) Chen and Clayton (1971)
Chung and Pfost (1967)
aw = exp[− A exp(−BWe)]
Guggenheim (1966),
aw = exp A Anderson (1946), de Boer (1953)
Chung–Pfost − exp(−C We) A, C Halsey (1948)
RT Henderson (1952)
Iglesias and Chirife (1978)
GAB We = (1 − WmC K aw + C K aw) Wm, C(T), Oswin (1946)
K aw)(1 − K aw K(T)

Halsey A Wm, A, b
Henderson aw = exp − RT (We/ Wm)b C, b
Iglesias–Chirife A, B
1 − aw = exp(−C T Web)
Oswin C, b
ln[W + (W 2 + W0.5)1/2] = Aaw + B

aw b
1 − aw
We = C

BET, Brunauer–Emmett–Teller; GAB, Guggenheim–Andersen–de Boer.

Moisture Analyser) and the corresponding water ac- corresponding standard deviation are presented for
tivity (Rotronic Hygrometer). Figures 28.8 and 28.9 the models that do not have an explicit dependency
confront these data with the models of Table 28.1 on temperature (Chen, BET, GAB, Iglesas–Chirife,
for 30◦C, as an example. From Figure 28.8, it is and Oswin). In these cases, the experimental data
clear that the similarity between the BET and GAB in the form of sets (W , aw) were treated separately
models, both quite good in predicting the sorption for the three different temperatures studied, and the
behavior of the pears for this temperature, contrary number of observations varied from 40 to 56.
to the Chen and Chung–Pfost models that although
also similar, are apparently not so adequate to the Similar results are summarized in Table 28.3 for
present case. With regard to Figure 28.9, it can be seen the models that have an explicit dependency on tem-
that all models approximately follow the experimen- perature (Chung–Pfost, Halsey, and Henderson). In
tal points, with the exception of the Iglesias–Chirife this case, the experimental data in the form of sets
model, that seems to be quite inadequate to reproduce (W , aw, T ) were treated all together, and the number
the experimental data in this case, contradicting what of observations was 138.
would be expected according to Rizvi (1986). How-
ever, the quality of fitting of a determined sorption To evaluate the influence of temperature on the
model to the experimental data does not necessarily sorption isotherms, they were predicted by the Halsey
reveal the nature of the sorption process. model for three different temperatures, and the re-
sults are presented in Figure 28.10. The tempera-
The parameters of the most adequate models were tures selected were 20◦C, 50◦C, and 80◦C, as with
estimated from the experimental data (Guine´ and these intervals the visualization of the differences be-
Castro, 2002a) using an orthogonal distance re- comes easier. The selection of the model was based
gression algorithm (ODR) with the derivatives ap- on its nature and its performance. In fact, it should
proximated by central finite differences, where the be an explicit temperature-dependent model to al-
unknown errors are taken into account in both depen- low the easy extrapolation for different temperatures
dent and independent variables. The software pack- than those studied experimentally, and from the three
age used to compute the parameters was ODRPACK, models available in the present study correspond-
developed by the Center for Computing and Ap- ing to that characteristic (Chung–Pfost, Halsey, and
plied Mathematics of the National Institute of Stan- Henderson), the Halsey model is apparently the best
dards and Technology, USA (Boggs, 1992). In Table one to fit the experimental data (Figs 28.8 and 28.9).
28.2, the values of the parameters estimated and the
From Figure 28.10, it is apparent that, at con-
stant moisture content, an increase in temperature

Figure 28.8. Desorption isotherms of
pears at 30◦C: fitting to BET, Chen,

Chung–Pfost, and GAB models.

Figure 28.9. Desorption isotherms of
pears at 30◦C: fitting to Halsey,

Henderson, Iglesias–Chirife, and Oswin

models.

540

28 Pear Drying 541

Table 28.2. Parameter Estimation for the Non-explicit Temperature-Dependent Models

Temperature

Model Parameters 20◦C 25◦C 30◦C

BET Wm 0.1770 (±0.0182) 0.1819 (±0.0215) 0.1887 (±0.0189)
Chen C 0.3440 (±0.0757) 0.3434 (±0.0884) 0.4653 (±0.1004)
GAB A 0.4555 (±0.0251) 0.4433 (±0.0293) 0.8030 (±0.0450)
B 1.0850 (±0.0931) 1.0942 (±0.1225) 1.7641 (±0.1450)
Iglesias–Chirife Wm 0.1668 (±0.0502) 0.2226 (±0.0994) 0.2372 (±0.0857)
Oswin C 0.3736 (±0.1819) 0.2578 (±0.1596) 0.3379 (±0.1580)
K 1.0034 (±0.0143) 0.9906 (±0.0192) 0.9880 (±0.0197)
A 9.6110 (±0.5629) 9.9273 (±0.6010) 10.2884 (±0.3662)
B −7.3664 (±0.4824) −7.6327 (±0.5082) −7.8718 (±0.3009)
C 0.0960 (±0.0067) 0.0995 (±0.0080) 0.1228 (±0.0079)
b 1.1941 (±0.0490) 1.1912 (±0.0579) 1.1441 (±0.0468)

BET, Brunauer–Emmett–Teller; GAB, Guggenheim–Andersen–de Boer.

enhances water activity, in agreement to the find- condensation effects, surface diffusion, and liquid
ings of other authors for most solid fruits (Kiranoudis transport due to gravity (Rizvi, 1986; Mulet et al.,
et al., 1997; Johnson and Brennan, 2000). 1989).

DRYING KINETICS When developing process models, of utmost im-
portance is the knowledge of the drying kinetics,
Dehydration is the removal of food moisture which which accounts for the mechanisms of moisture re-
results in an unsaturated gas phase by evaporation moval and for the influence of certain variables dur-
due to heat penetration. Thus, this process involves ing the process (Kiranoudis et al., 1997). In air-drying
simultaneously heat, mass, and momentum transfer, processes, two drying stages are usually present: an
in a rather complex way, difficult to predict with initial constant rate period corresponding to pure wa-
accuracy. Moreover, in foods such as fruits, with a ter evaporation and a falling rate period where the
capillary-porous structure, the food matrix has inter- moisture transfer is essentially limited by internal re-
stitial spaces, capillaries, and cavities filled with gas, sistances.
and many different mechanisms of mass transfer have
to be considered which, in turn, may act in different Figures 28.11 and 28.12 illustrate the different
combinations. These mechanisms include liquid dif- ways of representing the drying behavior of foods
fusion due to concentration gradients, liquid trans- and show respectively, a moisture content versus time
port due to capillary forces, vapor diffusion due to plot (commonly referred to as batch drying curve) and
partial vapor pressure gradients, liquid or vapor trans- the plots of the drying rates versus time and moisture
port due to the difference in total pressure caused by content (i.e., last known as the Krisher rate–moisture
external pressure and temperature, evaporation and curve).

Table 28.3. Parameter Estimation for the However, it is a common procedure to intercon-
Temperature-Dependent Models vert between the different types of drying curves, as
usually the data recorded from experiments (moisture
Model Parameters Estimation vs time) are not in the most adequate form for math-
Chung–Pfost ematical fitting. In fact, the Krischer rate–moisture
Halsey A 1717.61 (±81.02) curve, which is obtained from the batch drying curve
C 1.7633 (±0.1117) and its differentiation, is actually the most suitable
Henderson Wm 0.1281 (±0.0522) form of treating the drying data in terms of the char-
A 1638.17 (±51.25) acteristic curve scaling method (Kemp et al., 2001).
b 0.7679 (±0.2193)
C 0.0067 (±0.0001) In Figures 28.11 and 28.12, some important points,
b 0.4158 (±0.0129) A to E, are marked in the curves, illustrating the
transition between the different stages of the drying
process. The induction stage, that goes from A to B,
corresponds to the initial unsteady-state heating pe-
riod, and in most cases, it represents an insignificant

542 Part III: Commodity Processing

W (kg water/kg dry solids) 1.0

Figure 28.10. Influence of temperature 0.9
on desorption isotherms of pears using
Halsey model. 0.8 Halsey model
20 °C

0.7 50 °C
80 °C

0.6

0.5

0.4

0.3

0.2

0.1

0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
aw

Figure 28.11. Typical drying curve, showing the Figure 28.12. Typical drying rate curves, showing the
variation of moisture content with drying time (adapted rates of moisture removal as a function of time and
from Rizvi, 1986). moisture content (adapted from Rizvi, 1986).

portion of the total drying cycle. The unhindered dry- evaporation at the surface. The factors that influence
ing or constant rate period is represented by BC, the drying rate during this period are the surface area,
and during this stage, the drying surface is satu- the temperature and moisture gradients between the
rated with water as the rate of migration of wa- surface and the surrounding air, and the heat and mass
ter from the inside to the surface equals the rate of transfer coefficients. In food systems where mois-
ture transfer is controlled by capillary and gravity
forces, the constant rate period is significant and its
extension is determined by the food structure and

28 Pear Drying 543

by the mechanisms that govern the internal liquid can lead to rather good prediction of drying kinet-
movement. As drying proceeds, a critical moisture ics, although they do not have a physical meaning
content is reached, Wc, corresponding to point C, (Kiranoudis et al., 1997).
where the rate of migration of water from the inte-
rior to the surface is reduced to a degree that is no Drying rate curves were experimentally deter-
longer sufficient to saturate the entire surface, and mined for peeled pears in convective dryers at 30◦C,
this begins to dry. The critical moisture content usu- 40◦C, and 50◦C (Guine´ and Castro, 2002b). Every
ally increases with drying rate and thickness of the 24 h two samples from each dryer were collected
food. After point C, the surface temperature starts and their moisture contents (Mettler Toledo, HG53
to increase and the hindered drying or falling rate Moisture Analyser) were measured. The correspond-
period starts. This generally involves two different ing water activity was also analyzed (Rotronic Hy-
stages: the first falling rate period, CD, and the sec- grometer). All experiments were carried out until the
ond falling rate period, DE. In the former, the rate of moisture content of the pears reached about 20%,
moisture migration to the surface is lower than the since this corresponds to the optimum value for this
rate of evaporation, originating a continuously drier kind of dried fruit, considering its physical properties
surface until, finally, all the evaporation from the inte- and chemical composition (texture, consistency, elas-
rior of the food is completed (point D). In the second ticity, taste, and preservation capacity) (Guine´ and
falling rate period, the path for heat and mass transfer Castro, 2002b). From the batch drying curves (dry
becomes longer and more tortuous as the moisture basis water content vs drying time) obtained with
content diminishes. The limiting moisture content, two different fitting methodologies (Figs 28.13 and
known as the equilibrium moisture content, We, is 28.14), it is possible to verify that they follow the
reached when the vapor pressure of the food equals general pattern of a drying curve and that, for higher
the partial vapor pressure of the drying air, thus end- temperatures, the changes in the water content are
ing the drying process (Rizvi, 1986; Brennan et al., sharper, because the evaporation process is highly
1990). accelerated.

In typical industrial applications, the kinetic mod- The data were primarily fitted to cubic polynomi-
els developed are mostly empirical rather than being als, as suggested by Kemp et al. (2001)
mechanistic due to the complexity of the phenomena
involved. However, some empirical kinetic models W = y0 + at + bt2 + ct3, (28.10)
include parameters of phenomenological nature re-
lated to the most appropriate driving force, which where W is the dry basis moisture content and t the
drying time in hours. The fitting was done with the
software Sigma Plot, version 8.0 (SPSS, Inc.).

W (kg water/kg d.b.) 6

5 30 °C
40 °C
50 °C

4

3

2

1

Figure 28.13. Drying curves of

0 pears at 30◦C, 40◦C, and 50◦C:

0 50 100 150 200 250 300 350 experimental data and predicted by

Time (h) cubic fit.

544 Part III: Commodity Processing

W (kg water/kg d.b.) 6
30 °C
Figure 28.14. Drying curves of pears
at 30◦C, 40◦C, and 50◦C: experimental 5 40 °C
50 °C
data and predicted by sigmoidal fit.
4

3

2

1

0
0 50 100 150 200 250 300 350
Time (h)

In addition, a sigmoidal function of the type The variation of the moisture ratio with the dry-
ing time is presented in Figure 28.15 for the three
W = y0 + 1+ a − t)/b] (28.11) temperatures studied, from which it is clear that the
exp [(x0 effect of increasing temperature on accelerating the
drying process.
was fitted to the experimental data with better results,
as can be confirmed from the comparison of Figures A first order kinetics of the form (Togrul and
28.13 and 28.14 (where it is evident that the cubic Pehlivan, 2003),
fitting introduces some degree of instability to the fi-
nal stage of the drying curve) and from the parameter MR = y0 + a exp(−bt), (28.12)
estimation and the corresponding correlation coeffi-
cients presented in Table 28.4. where t is expressed in hours, was fitted to the data,
using once again Sigma Plot. The results are summa-
Table 28.4. Results of the Fitting to the Batch rized in Table 28.5, the high values of the correlation
Drying Curves coefficient denoting the goodness of the fit.

Temperature MOISTURE DIFFUSIVITY

Parameters 30◦C 40◦C 50◦C Moisture diffusivity is an unquestionably relevant
transport property necessary for correct modeling and
Cubic fit 5.1131 5.9685 5.9353 understanding of food drying processes. However, its
y0 −0.0559 −0.1306 −0.4069 determination is quite complex, not only because of
a experimental difficulties, but also due to mathemati-
b 0.0002 0.0010 0.0098 cal problems in the solution of the diffusion equation.
c 0.0000 0.0000 −0.0001
R 0.9925 0.9912 As mentioned before, the drying of pears is crit-
0.9946 ically dependent on the temperature as well as on
the water and sugar concentrations inside the fruit.
Sigmoidal fit 0.2390 0.4199 0.3254 In fact, as the drying proceeds, some migration of
y0 −87.9603 27.6879 0.7858 the sugar occurs along with the water that flows from
x0 10.7263 the inside to the surface of the pears. This combined
a 25.2585 5.9720 −7.2140 with the fruit shrinking originates an increase in sugar
b −62.6247 −12.3753 0.9961 concentration close to the surface, offering an extra
R
0.9941 0.9985

28 Pear Drying 545

MR = (W−We)/(W0−We) 1.2 30 °C
1.0 40 °C
0.8 50 °C
0.6
0.4 Figure 28.15. Moisture ratio curves
0.2 50 100 150 200 250 300 350 for 30◦C, 40◦C, and 50◦C with
0.0

0

Time (h) exponential fit.

resistance to the water diffusion (Guine´ and Castro, where De is the effective diffusivity, W the dry basis
2002b). moisture content at time t, We the equilibrium mois-
ture content, W0 the initial moisture content, and Lx ,
Despite the complexity of the moisture transfer L y, and Lz represent the half size of the sample in
process and the significant change of the physical the three spatial dimensions. The evolution of the
characteristics of the fruits along the drying process,
it has been reported by many researchers that Fick’s pear dimensions along drying was studied by mea-
law seems to be reasonably good for predicting the
moisture distribution inside many food materials dur- suring the values of the pear radius and height as the
ing drying (Zogzas et al., 1994a; Bonazzi et al., 1997).
For the transient diffusion, assuming an uniform ini- moisture content diminished, and the values for Lx ,
tial distribution of the moisture content as well as a L y, and Lz were thus calculated from the following
uniform concentration at the surface for t > 0, the so- equations:
lution of Fick’s law can be approximated by (Konishi
et al., 2001) Lx = L y = 0.0129+0.0268[1−exp(−0.1281W )],

R2 = 0.923 (28.14)

W − We 8 3 ␲2 Lz = 0.0168 + 0.0099[1 − exp(−0.3285W )],
W0 − We ␲2 4
= exp − De t R2 = 0.945 (28.15)

× 1 + 1 + 1 , (28.13)

L 2 L 2 L 2 that were obtained by adjusting the experimental data
x y z with Sigma Plot 8.0 (SPSS, Inc.), and where Lx and
L y are both equal to the pear radius and Lz is half of
Table 28.5. Results of the Fitting to the the total height, expressed in meters [Lz = (h + r )/2
Moisture Ratio Curves (Fig. 28.1), and the fit of Eq. 28.15 was applied to the
experimental values of (h + r )/2].
Temperature
According to Equation 28.13, the values of De can
Parameters 30◦C 40◦C 50◦C be obtained from the slope of a semilog plot of the
moisture ratio [(W − We)/(W0 − We)] versus time,
y0 0.0177 0.0127 0.0287 knowing the evolution of pear dimensions.

a 0.9834 0.9901 1.0724 The dependence of the effective diffusivity on
temperature and moisture content is usually repre-
b 0.0383 0.0484 0.2111 sented by a modified Arrhenius relationship of the

R 0.9985 0.9989 0.9616