Fruit Vegetable Processing Postharvest Physiology I INTRODUCTION Fruits

Fruit & Vegetable Processing Postharvest Physiology 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. l Fruits and vegetables respire by taking up oxygen, and giving off carbon dioxide and generating heat; l they also transpire, i. e. , lose water in vapor form. l l 1

l 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. l Subsequent to harvest, the source of water, photosynthates and minerals are cut off, and they enter into a deterioration or perishable phase. 2

Several changes take place in cell-wall composition and structure that result in the softening of the fruits and vegetables. l In general, visual color gradually changes as chlorophyll is degraded and yellow pigment of the skin and flesh increases in content. l In fruits and vegetables respiration involves the enzymatic oxidation of sugars to carbon dioxide (C 02) and water, accompanied by release of energy. l However, other substances such as organic acids and proteins also enter the respiratory chain. l 3

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, . l Cellular water is lost because of respiration and transpiration, resulting in fruits and vegetables becoming soft, shriveled, and limp. l Anthocyanins that give the typical red, orange, blue, and other pigments of some fruits and vegetables may increase after harvest. l l 4

l l 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. 5

l l 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. 6

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. l Growth, development. prematuration, ripening, and senescence (Figure 1) are the most important phases in fruit and vegetable ontology. l The growth of fruits and vegetables begins with cell division and cell enlargement, which accounts for the final size. l 7

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Growth and maturation is referred to as “fruit development”. l Senescence is the period when anabolic and biochemical processes give way to catabolic processes—leading to aging and final death of the tissue. l Ripening generally begins during the later stages of maturation and is considered the beginning of senescence. l 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. l 9

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II. RESPIRATION l Respiration of fruits and vegetables is an index of physiological activity and potent storage life. l It is one of the basic processes of life and directly related to maturation, handling, transportation, and subsequently, storage life. l Respiration of fruits and vegetables involves the enzymatic oxidation of sugars to carbon dioxide, water, and release of energy (Figure 3). l 11

Other substrates such as organic acids, fats, and proteins also play an important role during the process of respirarion. l The energy produced by the oxidation of sugars is convened into the energy of adenosine triphosphate (ATP), as an energy carrier. l 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 > 6 C 02 + 6 H 20 + Energy l 12

As indicated above this respiration of fruits and vegetables involves the following aspects: l 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. l 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. l 13

l 2. Oxygen: the supply of 02, for normal respiration is generally adequate unless intentionally restricted as in the case of CA (Controlled Atmosphere) Storage. l 3. Carbon Dioxide: removal of respiratory CO 2 requires more attention than supply of 02 because CO 2 may be in excess even when supply of 02 is adequate. 14

A three to five percent reductions of 02 concentration would not have an adverse effect on a product, but a comparable increase in CO 2 could suffocate and ruin certain fruits and vegetables. l 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. l Increase in rate of respiration causes acceleration of substrate utilization. l 15

l l 5. Rate of Respiration: The rate of respiration determines the quantity of 02 that must he available per unit of time. The quantities of CO 2 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. 16

l 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. l The rate of respiration can he defined as the weight of CO 2 produced per unit fresh weight and time (mg CO 2/kg/h) (Table 1). l The rate of respiration may be expressed in ml CO 2/kg/h or the quantity of 02 taken up rather than CO 2 given out. 17

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l The classification of the rate of respiration is presented in Table 2. l 6. Initial Rate of Respiration: the rate of prevailing respiration or within a few hours varies depending upon crop and temperature) l 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. 21

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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 4 A). • 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. • 23

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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 Q 10. This indicates at each 10°C rise in temperature, the rate of reaction doubles. • However, Q 10 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. • 26

• • • 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. 27

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. • 28

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. • 29

• Ill. TEMPERATURE QUOTIENT OF RESPIRATION The temperature quotient (Q 10) 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. 30

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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 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. • 32

l 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. l Sweet as well as sour cherries develop scald when weather in is unusually warm and dry during the several weeks before harvest l Chemical reactions of respiration are controlled by temperature and ideally, one could expect a Q 10 of about 2. 5 for respiration. 33

l This means that for a 10°C rise in temperature, the respiration would double. l Rapidly growing young tissue respires faster than that which develops slowly. l 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. 34

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. l 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). l 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. l 35

l l 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. 36

Climacteric and nonclimacteric fruits and vegetables l A large number of fruits and ‘vegetables show a sudden and sharp rise in respiratory activity called the climacteric rise during the life cycle; l whereas others which do not show climacteric rise are called nonclimacteric fruits and vegetables. 37

l The time of harvest for climacteric fruits and vegetables is critical for their maximum storage life and quality. l Non-clirnacteric fruits and vegetables are allowed to ripen on plants or vines and the resulting maturity is regulated by storage. l Maturity tests such as color, brix, acidity, and others are employed to determine whether they can meet standard grades and can be legally sold. 38

The classification of edible fruits and vegetables according to their respiration pattern 39

Respiratory patterns vary from growth and development and also from fruit to fruit and vegetable to vegetable; l most leafy vegetables are of non-climacteric nature (Figure 5). l 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. l It takes place within the cell at the site of the various enzymes that participate in the process of respiration. l 40

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l l l Respiration and ripening can be retarded by reducing the amount of O 2. Ethylene, if O 2 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. ’ 42

IV. ETHYLENE PRODUCTION AND EFFECTS The Chinese knew in ancient times that pears could be ripened by exposing them to the smoke of incense burned in closed rooms. l 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. l Ethylene is the active degreening agent in stove gas and a concentration of 4 ppm would degreen lemons in 6 to 8 d. l 43

l After this discovery, ethylene became generally used for degreening citrus fruits, bananas, honey dew melons and tomatoes. l Ripe bananas give off ethylene that ripens green bananas during shipping. l 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. l Production of ethylene depends upon fruits and vegetables (Table 4). 44

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A classification of fruits and vegetables according to their ethylene production rates is presented in Table 5. l However, there is no consistent relationship between ethylene production capacity of the produce and its perishability. l Ethylene gas inhibited the sprouting of potatoes. l Small quantities of ethylene is produced by practically all plant parts and tissues, fruits, vegetables, flowers, leaves, roots, tubers, seeds, and fungi. l 46

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l 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, l applied ethylene increases the respiration of non-climacteric fruits and vegetables, l the magnitude of the increase being dependent on the concentration of ethylene (Figure 6). 48

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There is a relationship between the physiological age and the response of cantaloupes and tomatoes to continuous treatment with ethylene. l Because of ethylene’s marked accelerative effects on ripening of both climacteric and nonclimacteric fruits, it is considered to be a plant growth hormone. l The effect of ethylene as a ripening stimulant can be inhibited by CO 2 concentrations in or around the fruits and decreased O 2. l 50

l l l 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. 51

l l l 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. 52

l It is believed that an inhibitor of ripening is present in the leaves of the trees. l Pears are another fruit that must be picked before they are tree ripe in order to develop a good eating quality. l Some cultivars of pears must be exposed to colds storage temperatures before they ripen normally. l Recent research has shown that low temperatures bring about synthesis of ethylene in pears. 53

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. l 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. l Picking fruit at the peak of respiration offers the best storage quality. l 54

l l 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. 55

The chemical 2 -chloroethyl phosphoric acid (Etheral, CEPA, Ethephon) is one of these. l 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. l Although ethylene is a useful chemical in the control of growth and ripening responses, it has some harmful effects. l 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. l 56

VI. TRANSPIRATIONAL LOSS l Water is lost from fruits and vegetables as they grow on a tree or a vine; l they may decrease in volume during the warm and dry part of the day, but regain their moisture at night. l With an increase in the relative humidity of the storage atmosphere, there is a decrease in transpiration. l After harvesting, the process of transpiration continues but there is no way to replenish it. 57

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. l In most fruits and vegetables with 5 to 10 % loss in moisture content, the product are visibly shriveled as a result of cellular plasmolysis. l The pedicels of cherries and calyx of strawberries turn brown and dry and the berries become dull and loose luster. l Hence, quick cooling is necessary to preserve fresh appearance. l 58

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. l 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. l 59

l l l 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 semipermeable 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. 60

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A. MINIMIZING TRANSPIRATIONAL LOSS There is only one way to reduce shriveling and drying of fruits and vegetables in storage rooms and that is by increasing RH. l 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. l Hydrocooling fluid should contain a fungicide to prevent microbial growth. l 62

B. RELATIVE HUMIDITY AND TEMPERATURE l 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; l thus water vapor is readily transferred from the humid interior of the leafy vegetables of fruits to the relatively dry air. l 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. 63

The amount of moisture the air can hold before it becomes saturated rises with temperature increase. l More water is required to saturate air at 15. 6°C than at 4. 4°C. l 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. l 64

Further, water has a greater tendency to evaporate as its temperature rises. Hence, RH is always expressed with temperature. l As the temperature increases, the quality of the produce decreases (Figure 7); l likewise the quality also decreases as fruits and vegetables experience a postharvest field delay as seen in Figure. l 65

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C. AIR MOVEMENT High velocity air causes rapid evaporation by continuously removing water that is saturated. l 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. l 68

D. ATMOSPHERIC PRESSURE Water evaporates more rapidly in lower atmospheric pressure than in higher. For every 10% decrease in pressure water loss will increase 10%. l 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. l Therefore, during air transportation of fresh fruits and vegetables, appropriate pressurization should be maintained especially over 5, 000 ft altitude. l 69

VII. CHILLING INJURIES l Chilling injury is a disorder induced by low nonfreezing temperatures which occurs in certain susceptible plants or produce (Table 6). l Usually this damage can occur in tropical fruits and vegetables when stored al low refrigerated temperatures (Table 7). 70

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l Chilling injury affects sweet potatoes, bananas, and most tropical and subtropical fruits. l Chilling injury induces decay and can be avoided by storing at higher temperatures. l Vegetables such as potatoes and sweet potatoes should l Susceptibility of various vegetables to chilling injury are presented in Table 8 73

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