Transcript ISO 9004: 2000 Quality Management Systems ‑ Guidelines for

Fruit & Vegetable Processing
Postharvest Physiology
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I. INTRODUCTION
Fruits and vegetables when harvested from
vines or plants are “living’’ structures,
continuing metabolic reactions and sustaining
physiological processes for a considerable time
during their postharvest period.
 Fruits and vegetables respire by taking up
oxygen, and giving off carbon dioxide and
generating heat;
 they also transpire, i.e., lose water in vapor
form.
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 The
respiration and transpiration losses
are made up by replenishing water,
photosynthates (sucrose and amino
acids), and minerals from the time-flow
of cell sap while fruits and vegetables are
attached to the plants or vines.
 Subsequent to harvest, the source of
water, photosynthates and minerals are
cut off, and they enter into a deterioration
or perishable phase.
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Several changes take place in cell-wall
composition and structure that result in the
softening of the fruits and vegetables.
 In general, visual color gradually changes as
chlorophyll is degraded and yellow pigment of
the skin and flesh increases in content.
 In fruits and vegetables respiration involves the
enzymatic oxidation of sugars to carbon dioxide
(C02) and water, accompanied by release of
energy.
 However, other substances such as organic acids
and proteins also enter the respiratory chain.
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Consequently, the loss of these reserves in fruits
and vegetables results in the production of
energy and the accompanying need for oxygen
(0,) and removal of CO,.
 Cellular water is lost because of respiration and
transpiration, resulting in fruits and vegetables
becoming soft, shriveled, and limp.
 Anthocyanins that give the typical red, orange,
blue, and other pigments of some fruits and
vegetables may increase after harvest.
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Apples, plums, pumpkins, and others enhance
color development in a packaging shed or in a
refrigerator.
 The skins of some fruits and vegetables develop
bloom or waxes after harvest that gives them an
attractive appearance which may aid in reducing
transpirational losses.
 Starchy fruits and vegetables undergo a
decrease in starch and increase in sugar and
acids after harvest.
 However, there may be changes in the kinds of
acids present.
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In certain cases as maturity advances, astringency
decreases caused by tannins or polyphenols.
 Volatiles and aroma components of many kinds of
fruits and vegetables are produced after harvest if
they are mature or ripe.
 However, when they are harvested rather immature
or at the ‘green’’ stage for distant shipment, they
do not yield typical aroma.
 For example, if Jordanian peaches are harvested
for shipment to Kuwait market they do not develop
as good aroma as when allowed to mature and
ripen on the tree.
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Ethylene is one of the volatiles synthesized in certain
fruits and vegetables at certain stages of maturity and
development; when it reaches a high enough
concentration, it triggers the ripening process and more
ethylene is produced and the process of ripening is
accelerated.
Growth, development. prematuration, maturation,
ripening, and senescence (Figure 1) are the most
important phases in fruit and vegetable ontology.
The growth of fruits and vegetables begins with cell
division and cell enlargement, which accounts for the
final size.
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Growth and maturation is referred to as “fruit
development”.
 Senescence is the period when anabolic and
biochemical processes give way to catabolic
processes—leading to aging and final death of the
tissue.
 Ripening generally begins during the later stages
of maturation and is considered the beginning of
senescence.
 The relative changes in weight, sugars,
chlorophyll, and acidity are common to most fruits
and vegetables (Figure 2) but other parameters
such as respiration, flavor, aroma, and carotenoids
can vary from commodity to commodity.
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II. RESPIRATION
 Respiration of fruits and vegetables is an index
of physiological activity and potent storage life.
 It is one of the basic processes of life and
directly related to maturation, handling,
transportation, and subsequently, storage life.
 Respiration of fruits and vegetables involves the
enzymatic oxidation of sugars to carbon dioxide,
water, and release of energy (Figure 3).
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Other substrates such as organic acids, fats, and
proteins also play an important role during the
process of respirarion.
 The energy produced by the oxidation of sugars
is convened into the energy of adenosine
triphosphate (ATP), as an energy carrier.
 The oxidation of sugars takes place in several
steps under control of specific enzymes. A simple
formula for respiration may be as follows:
Sugar + 602 > 6C02 + 6H20 + Energy
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As indicated above this respiration of fruits and
vegetables involves the following aspects:
 1. Substrate: the quantity of substrate
(predominantly sugars) in fruits and vegetables
available for respiration is a deciding factor for
their longevity at that temperate.
 The weight loss due to increased temperature and
respiration usually is more than to five percent
depending upon the structure of the fruits and
vegetables.
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2. Oxygen: the supply of 02, for normal
respiration is generally adequate unless
intentionally restricted as in the case of
CA (Controlled Atmosphere) Storage.
 3. Carbon Dioxide: removal of
respiratory CO2 requires more attention
than supply of 02 because CO2 may be in
excess even when supply of 02 is
adequate.
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A three to five percent reductions of 02
concentration would not have an adverse effect
on a product, but a comparable increase in CO2
could suffocate and ruin certain fruits and
vegetables.
 4. Energy: removal of heat from respiration is
vitally important; otherwise the life of fruits
and vegetables will be reduced to an increased
temperature around the commodity.
 Increase in rate of respiration causes
acceleration of substrate utilization.
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5.Rate of Respiration: The rate of respiration
determines the quantity of 02 that must he
available per unit of time.
 The quantities of CO2 removed at he same time.
 Increased rate of respiration will reduce the
storage life of product.
 Rate of respiration is a function of temperature
and available concentration of 02 around the fruits
and vegetables.
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In addition. some fruits such as potatoes will
have a lower rate of respiration than spinach
or lettuce due to inherent substrate available
for respiration and the anatomical variations
of the commodities.
 The rate of respiration can he defined as the
weight of CO2 produced per unit fresh
weight and time (mg CO2/kg/h) (Table 1).
 The rate of respiration may be expressed in
ml CO2/kg/h or the quantity of 02 taken up
rather than CO2 given out.
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The classification of the rate of respiration is
presented in Table 2.
 6. Initial Rate of Respiration: the rate of
prevailing respiration or within a few hours
varies depending upon crop and temperature)
 7. Average Rate of Respiration: It is
determined by measuring rates at a definite
time interval, summing the rates thus
determined, and dividing by the number of
intervals involved.
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8. Effects of Temperature and Days in Storage on
Rate of Respiration: the rate of respiration
generally increases as the temperature and the
storage duration of fruits and vegetables increases.
• However, at very high temperatures and at the very
long storage duration, the rate of respiration
decreases until the death of products. However, one
does not store fresh commodities at such high
temperatures (Figures 4 and 4A).
• 9. Effects of Commodity on Rate of Respiration:
the rate of respiration varies depending upon
commodity and variety, also the commodity will
vary with other varieties of the same commodity.
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10. Maturity of Fruits and Vegetables on
Respiration Rate: fruits and vegetables harvested at
early maturity for distant market respire faster than
those harvested at the firm-ripe maturity.
• 11. Van’t Hoff’s Law: this law indicates that the rate
of chemical reaction is controlled by temperature.
• He coined the term Q10.This indicates at each 10°C
rise in temperature, the rate of reaction doubles.
• However, Q10 for respiration may not always be
doubled; sometimes it may be more than doubled
depending upon the maturity and anatomical structure
of the fruits or vegetables.
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Postharvest physiology is influenced by
preharvest factors on the farm or in the orchard.
• Physiology of fruits and vegetables begins at the
time of blossoming or bud formation and is
affected by agricultural practices—fertilization,
variety, and irrigation—and by environmental
factors such as sunlight duration and quality,
temperature, humidity, etc.
• The genetics of fruits and vegetables determine
postharvest storage life. Those crops that are
most perishable such as lettuce, spinach,
strawberries and raspberries have a short growing
season life.
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In contrast, Winesap apples which
require 160 to 170 days to develop
have a longer storage life.
Summer cultivars of apples generally
have a shorter storage life because
they ripen earlier.
Likewise, early summer apples have a
higher respiration rate than fall apples;
they also have a greater number of
cells, more lenticels, and give off more
ethylene.
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•
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However, one should not conclude that
the differences in storage life of fruits and
vegetables can be explained simply by
length of growing season, respiration rate,
or amount of ethylene released.
It involves genetic factors which control
growth, development. postharvest
behavior, and physiological and
morphological variations.
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•
Ill. TEMPERATURE QUOTIENT OF RESPIRATION
The temperature quotient (Q10) is not
the same for all fruits and
vegetables, nor will it be the same for
another variety of the same fruit.
• The example is presented in Table 3.
• As a general rule, it can be said that an
apple or pear will ripen as much in a day
at 21 C as it will in a week at 0 C.
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Thus it is apparent that refrigeration is an
effective means of extending the commercial life of
fresh produce.
• Fruit growers and shippers have learned by practical
experience that the growing season has a powerful
influence on the storage age of fruits.
• For example, it has been recognized that t is
hazardous to store or to ship Bartlett pears grown in
cool coastal areas to distant places, because they often
develop core breakdown—a physiological disorder
that makes pears too soft and mushy.
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Some cultivars of apples grown in cool
climates cannot tolerate storage temperature of
0 C and must be stored at higher temperatures
such as (2.2—4.4 C) In order to avoid low
temperature breakdown.
 Sweet as well as sour cherries develop scald
when weather in is unusually warm and dry
during the several weeks before harvest
 Chemical reactions of respiration are
controlled by temperature and ideally, one
could expect a Q10 of about 2.5 for respiration.
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This means that for a 10°C rise in temperature,
the respiration would double.
 Rapidly growing young tissue respires faster
than that which develops slowly.
 The rate of respiration of asparagus is one of
the highest rates of all fruits and vegetables
because of the rapidly growing shoots of the
plant.
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Fruits and vegetables vary in respiration rates and
there are differences between cultivars and their
maturities—so it is not to be expected that the
respiration rate will be a fixed value at any given
temperature.
 It tends to be more constant at temperatures of (0 to
4.4°C) than at higher temperatures of (21.1 to
26.7°C).
 At the temperature range (0— 4.4°C), where fruits
and vegetables are held the longest-time, their heat of
respiration is a factor to be included in calculating the
refrigeration requirements for refrigeration storage
and transportation.
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In the case of fruits and vegetables, after harvest, fast
cooling is generally desirable especially for
perishable soft fruits such as berries and leafy
vegetables.
 This not only reduces metabolic activity of fruits and
vegetables, but also controls fruit decay.
 Fungi and other microorganisms increase rates of
respiration as do bruises and mechanical injuries;
 the most serious consequences of holding fruits and
vegetables at high temperature is the hastening of
ripening, and shortening of storage and marketing
life.
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Climacteric and nonclimacteric fruits and vegetables
A large number of fruits and ‘vegetables
show a sudden and sharp rise in
respiratory activity called the climacteric
rise during the life cycle;
 whereas others which do not show
climacteric rise are called nonclimacteric
fruits and vegetables.
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The time of harvest for climacteric fruits and
vegetables is critical for their maximum
storage life and quality.
 Non-clirnacteric fruits and vegetables are
allowed to ripen on plants or vines and the
resulting maturity is regulated by storage.
 Maturity tests such as color, brix, acidity, and
others are employed to determine whether they
can meet standard grades and can be legally
sold.
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The classification of edible fruits and vegetables
according to their respiration pattern
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Respiratory patterns vary from growth and
development and also from fruit to fruit and vegetable
to vegetable;
 most leafy vegetables are of non-climacteric nature
(Figure 5).
 Respiration is not merely a catabolic process, but it
provides energy to synthesize enzymes, cell
membrane constituents, and other material necessary
for life of the cell.
 It takes place within the cell at the site of the various
enzymes that participate in the process of respiration.
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Respiration and ripening can be retarded by reducing
the amount of O2.
Ethylene, if O2 is present, will increase the respiration
rates and other metabolic processes as well.
The ethylene may come from the fruit or the
vegetable itself or be added to the atmosphere.
In a fruit or a vegetable that has climacteric rise in
respiration, ethylene treatment initiates the rise
earlier, but the rates reach no higher levels.
The climacteric in respiration of certain fruits
generally occur at the onset of processes involved in
ripening.
The peak of respiration does not always coincide with
peak of ripening.’
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IV. ETHYLENE PRODUCTION AND EFFECTS
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The Chinese knew in ancient times that pears
could be ripened by exposing them to the smoke
of incense burned in closed rooms.
 Many years ago in Florida and California, oranges
were colored or more correctly “degreened’’ by
exposure to fumes from kerosene stoves or
exhaust from a gasoline engine in a special
coloring room.
 Ethylene is the active degreening agent in stove
gas and a concentration of 4 ppm would degreen
lemons in 6 to 8 d.
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After this discovery, ethylene became
generally used for degreening citrus fruits,
bananas, honey dew melons and tomatoes.
 Ripe bananas give off ethylene that ripens
green bananas during shipping.
 Similarly emanations of ripe pears or apples
ripen other fruits because of ripe pears and
apples giving off ethylene which accelerates
the ripening of other unripened fruits.
 Production of ethylene depends upon fruits and
vegetables (Table 4).
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A classification of fruits and vegetables according to
their ethylene production rates is presented in Table 5.
 However, there is no consistent relationship between
ethylene production capacity of the produce and its
perishability.
 Ethylene gas inhibited the sprouting of potatoes.
 Small quantities of ethylene is produced by
practically all plant parts and tissues, fruits,
vegetables, flowers, leaves, roots, tubers, seeds, and
fungi.
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Very low concentrations of ethylene are
required to produce biochemical and
physiological responses in climacteric
fruits and vegetables, such as acceleration
of the ripening process, and in contrast,
 applied ethylene increases the respiration
of non-climacteric fruits and vegetables,
 the magnitude of the increase being
dependent on the concentration of ethylene
(Figure 6).
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There is a relationship between the physiological
age and the response of cantaloupes and
tomatoes to continuous treatment with ethylene.
 Because of ethylene’s marked accelerative
effects on ripening of both climacteric and nonclimacteric fruits, it is considered to be a plant
growth hormone.
 The effect of ethylene as a ripening stimulant
can be inhibited by CO2 concentrations in or
around the fruits and decreased O2.
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These conditions prevail in controlled atmosphere
storage.
Effects of ethylene on fruits and vegetables held at 0
to 4.4°C is not possible to detect;
nor is it detectable at higher temperatures of about
35°C.
Wholesalers and retailers should know that fleshy
fruits and vegetables give off large quantities of
ethylene at increased storage temperature.
fleshy fruits and vegetables should not be store and
shipped with susceptible commodities such as green
and leafy vegetables, carrots, and lettuce.
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Also, fruits and vegetables should be stored at low
temperatures to increase their shelf life;
otherwise their quality and storage life reduces.
Apples, pears, carrots, etc. should not be stored in
the same room or in a transportation container.
Ethylene is synthesized within the cell
enzymetically from methionine.
The sites of reaction within the cell are
mitochondria.
The avocado will not ripen on the tree but will
ripen and show a climacteric rise in respiration
after picking.
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It is believed that an inhibitor of
ripening is present in the leaves of the
trees.
 Pears are another fruit that must be
picked before they are tree ripe in order
to develop a good eating quality.
 Some cultivars of pears must be
exposed to colds storage temperatures
before they ripen normally.
 Recent research has shown that low
temperatures bring about synthesis of
ethylene in pears.
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It is now believed that accumulation of ethylene
in fruits and vegetables precedes the rise in
respiration, triggering the climacteric rise of
unripe fruits earlier.
 Climacteric rise in respiration is an indication of
the onset of senescence, indicating that the
fruits should be harvested before it starts this
rise in respiration.
 Picking fruit at the peak of respiration offers the
best storage quality.
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The effects of various growth regulators on fruits and
vegetables ripening are attributed to inducing
ethylene production.
 e.g., the stimulation of the ripening of figs by the
application of 2,4,5-T.
 Chemicals that are used to bring about abscission of
fruits and vegetables important in fruit thinning and
mechanical harvesting, have been shown to cause
ethylene production.
 Ethylene-releasing chemicals are in commercial use
in agriculture to bring about desired changes in plants
or plant products.
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The chemical 2-chloroethyl phosphoric acid
(Etheral, CEPA, Ethephon) is one of these.
 This chemical breaks down, releasing ethylene within
the plant tissue and modifying plant flowering,
vegetative growth, dormancy, abscission, fruit
maturation and ripening, disease, and freeze
resistance.
 Although ethylene is a useful chemical in the control
of growth and ripening responses, it has some
harmful effects.
 It can cause premature ripening in fruits, defoliation
in plants, lethal damage to nursery stock, petal fall,
failure in bud opening in flowers, russeting lettuce,
and bitterness in carrots.
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VI. TRANSPIRATIONAL LOSS
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Water is lost from fruits and vegetables
as they grow on a tree or a vine;
 they may decrease in volume during the
warm and dry part of the day, but regain
their moisture at night.
 With an increase in the relative humidity
of the storage atmosphere, there is a
decrease in transpiration.
 After harvesting, the process of
transpiration continues but there is no
way to replenish it.
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The moisture content of most fruits and vegetables is
high and weight loss during transportation and
storage can be a serious economic factor, especially if
fruits are sold by weight.
 In most fruits and vegetables with 5 to 10 % loss in
moisture content, the product are visibly shriveled as
a result of cellular plasmolysis.
 The pedicels of cherries and calyx of strawberries
turn brown and dry and the berries become dull and
loose luster.
 Hence, quick cooling is necessary to preserve fresh
appearance.
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The weight loss of fruits and vegetables in
storage depends upon size, maturity,
composition and structure, air surrounding
them, storage temperature, relative humidity,
velocity of air in the storage, thickness of
cuticles, size and number of stomata and
lenticels, and other factors.
 A practical way to minimize this effect is to
cool the fruit quickly using hydrocooling
containing antifungal chemicals which will
both cool the fruits and control the adhering
fungal growth.
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Similarly, lettuce and other leafy vegetables are cooled
by sprinkling cold water on them followed by vacuum
treatment.
Preventive loss of water from fruits and vegetables can
be attempted both by reducing respiration as well as
transpiration.
Fruits and vegetables should be precooled before storage
at lower temperatures.
Sometimes, it is essential to package the produce in
semi-permeable polyethylene or mylar bags.
When dry fruits and vegetables, such as nuts or dried
fruits, are stored in polyethylene containers, the problem
is to maintain desirable low RH (about 60%) and to
avoid fungal growth.
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A. MINIMIZING TRANSPIRATIONAL LOSS
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There is only one way to reduce shriveling and
drying of fruits and vegetables in storage rooms
and that is by increasing RH.
 Vegetables as well as fruits can be protected
from a lower RH by using various types of
permeable polyethylene bags or films or by
providing moisture in the form of ice or
hydrocooling.
 Hydrocooling fluid should contain a fungicide to
prevent microbial growth.
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B. RELATIVE HUMIDITY AND TEMPERATURE
 Water loss is rapid at low relative humidity (RH)
and slower at higher RH because the air in the
room contains less water vapor than it can hold at
the temperature of the room;
 thus water vapor is readily transferred from the
humid interior of the leafy vegetables of fruits to
the relatively dry air.
 In contrast, if the RH in the room is 100% (water
saturated atmosphere), the air in the room and
fruits or vegetables are balanced in respect to
moisture content, the gradient between the two is
low, and moisture loss is nil.
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The amount of moisture the air can hold before
it becomes saturated rises with temperature
increase.
 More water is required to saturate air at 15.6°C
than at 4.4°C.
 Accordingly, at 15.6°C and 90% RHI, the air is
drier than in a room at 4.4°C and 90% RH
resulting in rapid dehydration of the produce.
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Further, water has a greater tendency to
evaporate as its temperature rises. Hence, RH is
always expressed with temperature.
 As the temperature increases, the quality of the
produce decreases (Figure 7);
 likewise the quality also decreases as fruits and
vegetables experience a postharvest field delay
as seen in Figure.
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C. AIR MOVEMENT
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High velocity air causes rapid evaporation by
continuously removing water that is saturated.
 Air movement should be sufficient enough to
effectively remove respiratory heat from the
produce after it has cooled to the temperature
of the room, trailer, or rail car.
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D. ATMOSPHERIC PRESSURE
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Water evaporates more rapidly in lower atmospheric pressure
than in higher. For every 10% decrease in pressure water loss
will increase 10%.
Thus the rate of water loss in an airplane will be about 20%
more than a sea level due to the difference in pressure alone.
Should the lower air pressure be coupled with lower RH and
relatively higher temperature, a significant amount of water
can be lost from produce during air transit.
Therefore, during air transportation of fresh fruits and
vegetables, appropriate pressurization should be maintained
especially over 5,000 ft altitude.
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VII. CHILLING INJURIES
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Chilling injury is a disorder induced
by low nonfreezing temperatures
which occurs in certain susceptible
plants or produce (Table 6).
 Usually this damage can occur in
tropical fruits and vegetables when
stored al low refrigerated
temperatures (Table 7).
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Chilling injury affects sweet potatoes,
bananas, and most tropical and
subtropical fruits.
 Chilling injury induces decay and can
be avoided by storing at higher
temperatures.
 Vegetables such as potatoes and sweet
potatoes should
 Susceptibility of various vegetables
to chilling injury are presented in
Table 8
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