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(1997) Effect of calcium and vitamin D supplementation

on bone density in men and women 65 years of age or

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Sørensen O (1987) Low serum levels of 25-hydroxyvita-

min D and 1,25-dihydroxyvitamin D in institutionalized

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Feldman D, Glorieux FH and Pike JW (eds) (1997) Vitamin

D, chapters 40–43. London: Academic Press.

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RIPENING OF FRUIT

A R N Teixeira and R M B Ferreira, Universidade

cnica de Lisboa, Tapada da Ajuda, Lisboa Codex,

Portugal

Introduction

0001 Most fleshy fruits attain physiological maturity at

an unripe state unsuitable for immediate consump-

tion. The ripening process that follows, either on the

tree or after picking, is the result of a number of

physiological and biochemical processes which are

revealed in a sequence of changes in color, texture,

aroma, and taste, leading eventually to a physio-

logical state at which the fruit is commercially

considered as edible. However, in the context of

sensorial changes the meaning of ripening is rather

ambiguous as it is based on a subjective appreciation

of a very intricate process that involves a number

of metabolic changes. Table 1 illustrates some of

the most important organoleptic and physiological

changes that occur during ripening and the biochem-

ical processes most likely to be responsible for them.

5006 RIPENING OF FRUIT

0002 The ripening process is mainly concerned with

changes in components formed during fruit growth

and development. So, many fruits are able to ripen

after being detached from the mother plant, provided

physiological maturity has been attained before

picking.

Respiratory Patterns – Climacteric and

Nonclimacteric Fruits

0003 Even after detachment from the plant, fruits continue

to display for some time the metabolic activities

characteristic of living organisms. As changes in re-

spiratory rates are good indicators of alterations in

general metabolism, fruit respiration is useful as a

reference to physiological state, which determines

the potential storage life of a fruit. The first system-

atic studies of fruit respiration were carried out by

Kidd and West in the 1920s with Bramley Seedling

apples. Fruits, picked at different stages of develop-

ment and stored at various temperatures, exhibited a

typical respiratory pattern characterized by a decline

in the respiration rate after picking which, sooner

or later, depending on the storage temperature, was

followed by a temporary rise in respiration (Figure 1).

The visible changes normally associated with ripening

(color, texture, and flavor) began at or shortly after

the maximum rate of respiration. The sudden change

in the respiration rate was recognized by Kidd and

West as a critical phase in the life of the fruit, denoting

the transition from maturity to senescence, and was

named ‘climacteric.’ The same authors showed later

that a similar increase in respiration also occurred in

apples attached to the tree. In this situation the rise in

respiration takes place at a slower rate but, eventu-

ally, reaches higher peaks than those attained by the

detached fruits. Subsequent work indicated that

many other ripening fruits exhibit a similar pattern

of respiration, most of them, mainly tropical and

subtropical fruits, displaying higher climacteric

peaks than apples (Figure 2a). On the other hand,

some fruits show a continuous decline in respiration

throughout maturation and ripening (Figure 2b)and

have been termed ‘nonclimacteric.’ A list of commer-

cially important climacteric and nonclimacteric fruits

is given in Table 2.

tbl0001 Table 1 Organoleptic and metabolic changes that occur during

ripening

Organoleptic and

physiological changes Biochemical changes

Texture Rise in water-soluble pectins and

decreasing degree of esterification

through pectolytic activity. Partial

cellulose breakdown

Color Degradation of chlorophyll and

unmasking of underlying pigments.

Synthesis of new pigments

(carotenoids or anthocyanins)

Aroma and taste Qualitative and quantitative changes in

carbohydrates and organic acids.

Synthesis of alcohols, esters, and other

volatile compounds

Rise in respiration Autocatalytic synthesis of ethylene.

Increased protein synthesis and altered

metabolic control of respiratory

pathways

1.0

0.5

0.0

ml CO

−1

dry weight h

−1

1.0

0.5

0.0

ml O

−1

dry weight h

−1

0 50 100 150 200

Days in storage

fig0001 Figure 1 Climacteric rise in respiration of Rocha pears. Carbon dioxide evolution in fruits stored at 0



C (filled circles) and 1



(open circles) and oxygen absorption at 0



C (filled squares) and 1



C (open squares). From Teixeira AR, Carmona MA, Barreiro MJ,

Silva MJ and Cabral ML (1978) A counservac¸ a

o frigorifica de pera Rocha. Agronomia Lusitana 39: 57–84, with permission.

RIPENING OF FRUIT 5007

Climacteric Behavior and Ethylene Production

0004 It was recognized early in this century that ethylene

can cause physiological responses in various plant

tissues. The suggestion that fruit tissues themselves

release gaseous substances that may affect the

ripening behavior of nearby fruits was made by

Coussins, who reported, in 1910, that gases emanat-

ing from oranges caused premature ripening of

bananas. It was later showed that the active substance

released by ripening fruit was ethylene and that in

most climacteric fruits the rise in ethylene production

occurred at or prior to the beginning of the respiratory

upsurge. So the gas was considered responsible for the

initiation of the climacteric and the related ripening

processes and was named the ‘ripening hormone.’

0005The climacteric or nonclimacteric behavior of

fruits has been associated with the capacity of their

tissues to synthesize ethylene. In climacteric fruits the

respiratory increment is always accompanied, and

often preceded, by an increase in ethylene levels

within their tissues. This increase in endogenous

ethylene induces an autocatalytic process of ethylene

synthesis that, apparently, triggers off the respiratory

upsurge of climacteric fruits. On the other hand, in

nonclimacteric fruits the ethylene levels are always

low and usually tend to decrease slowly during

ripening at a rate approaching the rate of diminishing

respiration. Climacteric and nonclimacteric fruits

also differ in their responses to ethylene applied ex-

ogenously. The autocatalytic response of climacteric

fruits to applied ethylene leads to the onset of climac-

teric respiration which, once initiated, cannot be

stopped. Even if the exogenous gas is removed, the

endogenous levels of ethylene will continue to

increase once its catalytic synthesis starts. Thus, the

respiratory rate at the climacteric peak does not

depend upon the concentration of ethylene initially

present. The respiratory rate of nonclimacteric fruits

also responds to the application of exogenous ethyl-

ene, but does not exhibit an autocatalytic effect.

Thus, in nonclimacteric fruits, the magnitude of the

response varies proportionally to the concentration of

the gas applied and the process can be reversed; that

is, the respiration rate decreases to basal levels upon

(a)

200

150

100

Breadfruit

Cherimoya

ml O

or CO

−1

Mango

Tomato

Apple

01020

Time units

(b)

ml O

or CO

−1

0510

Time units

Strawberry

Grape

Pineapple

Cherry

Lemon

fig0002 Figure 2 Respiration patterns of some (a) climacteric and (b)

nonclimacteric fruits. From Biale JB and Young RE (1982) Respir-

ation and ripening in fruits – retrospect and prospect. In: Friend J

and Rhodes MJC (eds) Recent Advances in the Biochemistry of

Fruits and Vegetables, pp. 1–39. London: Academic Press, with

permission.

tbl0002Table 2 Fruits with climacteric and nonclimacteric patterns of

respiration

Climacteric Nonclimacteric

Apple Blueberry

Apricot Cacao

Avocado Cherry

Banana Cucumber

Mango Grape

Papaya Grapefruit

Peach Lemon

Pear Lychee

Plum Olive

Tomato Orange

Watermelon Pineapple

Strawberry

5008 RIPENING OF FRUIT

removal of ethylene, and can be reproduced by

renewed applications of the gas. Nevertheless, the

application of exogenous ethylene accelerates the

ripening in nonclimacteric fruits as it does in climac-

teric fruits when furnished in the preclimacteric

phase.

0006 The belief that ethylene is the only substance dir-

ectly responsible for the climacteric rise in respiration

and the ensuing ripening has recently been under

close reexamination. The existence of a strong link

between ethylene and respiration is well documented

in the literature, being observed in many other plant

tissues besides fruits. However, other volatile sub-

stances that are produced during ripening when ap-

plied exogenously may mimic the respiratory changes

caused by ethylene. On the other hand, the onset of

increased ethylene synthesis does not always precede

the respiratory upsurge in climacteric fruits. The lit-

erature reports a variety of fruits in which the increase

in ethylene production either coincides with (e.g.,

Cox’s Orange Pippin apples and Anjou pears) or

follows (e.g., avocado var. Fuerte and plum var.

Wickson) the rise in respiration. It has been suggested

that, in these fruits, ethylene is not the only factor

that determines the initiation of the climacteric and

of ripening. The minimum ethylene production rate

that triggers off the rise in respiration, although vary-

ing between species and cultivars, is always very low

(frequently less than 1 mlkg

1

, which is approxi-

mately equivalent to an internal concentration of

2 p.p.m.). So, the accurate determination of the rela-

tive timing of initiation of the rises in the rates of

ethylene synthesis and respiration is rather difficult,

particularly if, at this stage, other substances are

being formed whose effects add up to those of ethyl-

ene. Notwithstanding the evidence suggesting that the

stimulation of respiratory activity in fruits and other

plant tissues may be induced by a number of factors,

in the majority of climacteric fruits the autocatalytic

ethylene synthesis begins before the respiratory

upsurge and in most nonclimacteric plant tissues

respiratory activity is tightly coupled to ethylene con-

centrations. Apparently, ethylene stimulates metabol-

ism in general and may alter the control mechanism

of existing metabolic pathways and induce changes in

gene expression.

Climacteric Behavior and Ripening

0007 If the link between ethylene and climacteric respir-

ation is partially resolved, the role of the climacteric

respiratory rise in ripening remains obscure. It is now

evident that many of the physiological changes asso-

ciated with ripening (e.g., softening, color changes,

and ethylene synthesis) can be, with some manipula-

tion, separated from the respiratory climacteric.

Thus, some fruits can initiate softening and pigment

synthesis without an increase in respiration and the

application of certain plant hormones and of protein

synthesis inhibitors may prevent ripening changes

without obviating the climacteric rise in respiration.

Such observations do not support the opinion that the

increased rate of respiration supplies the extra adeno-

sine triphosphate (ATP) required as the energy source

for physiological transformations that occur during

ripening, such as synthesis of proteins, nucleic acids,

and pigments. In fact, the estimated energy demands

of ripening are much lower than those provided by

climacteric respiration; apparently, the energy gener-

ated by the basal respiratory rate is sufficient to drive

the ripening events.

0008The evidence does not uphold the existence of an

integrated relationship between the respiratory cli-

macteric and ripening, but rather suggests that the

rise in respiration during ripening may simply be a

facet of ethylene action. The suggestion that ethylene

may act by lowering the organizational resistance of

the cell, mainly by increasing membrane permeability,

has some experimental support. On the assumption

that fruit tissues, like other living tissues, react to any

injurious situation by changing their metabolism in

order to maintain a homeostatic condition and that

such changes are driven by respiration, it has been

suggested that the respiratory climacteric is largely a

homeostatic response, in an attempt to counterbal-

ance the injury imposed on mitochondria by the initi-

ation of senescence. That is, the degradative effects of

incipient cellular senescence, probably accelerated by

ethylene, impose a stress on mitochondria which

leads to an increased respiratory rate. Such a sugges-

tion is supported by the fact that mitochondrial struc-

ture and function, including coupling, are kept intact

throughout the ripening process.

Effects on Texture

0009The texture of a fruit depends on the turgor of the

cells as well as on the presence of supporting struc-

tures like cell walls and the cohesiveness of the

cells. The latter property is conferred by the middle

lamella, the intercellular structure that cements to-

gether the primary walls of contiguous cells and that

is composed mainly of pectic material. Structurally,

fruit cell walls are similar to primary cell walls of

other plant tissues whose main components are poly-

saccharides, namely cellulose, hemicelluloses, and

pectins. Essential for the structural integrity of cell

walls is the presence of calcium. Calcium ions

are particularly important in cell-to-cell adhesion,

the cementing effect being primarily attributed to

the formation of a calcium–pectate complex in the

RIPENING OF FRUIT 5009

middle lamella. The role of calcium in the mainten-

ance of cell wall structure is particularly important in

fruits in which the middle lamella gives a larger con-

tribution as a cell wall component than in other plant

tissues. The extensive softening that takes place

during the ripening of many fruits is mainly attributed

to changes occurring in cell walls, including the

middle lamellae. (See Cellulose; Hemicelluloses.)

0010 In ripe fruit, cell walls and middle lamellae are

partly dissolved as a result of the action of a number

of enzymes that break down pectins and cellulose, of

which polygalacturonase, pectinesterase, and cellu-

lase are the most extensively studied and whose activ-

ities are often correlated with the rate at which

softening occurs during ripening. It is apparent from

such studies that the major changes occurring in cell

walls during ripening are associated with depolymer-

ization and solubilization of the pectic substances due

to the action of polygalacturonase and pectinesterase,

respectively. The solubilization of pectin during the

ripening of apples has been attributed to diminished

levels of calcium ions in the cell walls and there is

evidence that the softening of the apple fruit is associ-

ated with the transfer of divalent cations, particularly

calcium, from the cell wall into storage compartments

inside the cell. Postharvest calcium treatments that

are carried out to prevent some physiological dis-

orders, such as bitter-pit in apples, confer greater

firmness to the fruits during storage.

0011 The role of cellulase in tissue softening is not clear.

Though it seems likely that hydrolysis of cellulose

would weaken the cell wall structure, the experimen-

tal evidence is rather inconclusive as to its importance

as a primary cause of fruit softening.

Effects on Sweetness

0012 In fresh fruits the main compounds responsible for

sweetness are soluble carbohydrates, i.e., sugars. It is

apparent from the degree of sweetness of different

ripe fruits that their sugar contents vary widely.

Even within a particular species the sugar concentra-

tion in fruits may depend on the variety and on the

environmental conditions and agricultural practices

to which the mother plant has been subjected, prior to

harvesting. As most climacteric fruits are harvested

before ripening, considerable changes in sugar con-

tent may occur during storage. The average total

sugar contents of most ripe fruits lie in the range 5–

10% of fresh weight. Outside this range are, typically,

lime and lemon with lower sugar contents (0.5–

3.0%), and grape, cider apple and other fruits used

for alcoholic fermentation purposes with higher con-

tents (13–20%). (See Carbohydrates: Classification

and Properties.)

0013The main individual sugars responsible for sweet-

ness in fruits are the monosaccharides glucose and

fructose and the disaccharide sucrose. Further mono-

saccharides may also be present in fruits, but their

contribution to sweetness is negligible. The relative

proportions of glucose and fructose vary with species

but in many fruits glucose levels exceed those of

fructose. This does not necessarily mean a higher

contribution of glucose for sweetness since the

sweetening power of glucose is 2–3 times lower than

that of fructose. Two important exceptions are apples

and pears in which fructose may be present in concen-

trations up to three times those of glucose. In grapes,

berries, and oranges, both sugars are often present in

similar amounts. (See Fructose; Sucrose: Properties

and Determination.)

0014Sucrose is an important product of photosynthesis

and the main form in which carbon is translocated

from the leaves to other parts of the plant, including

fruits. So, it is not surprising that in most fruits su-

crose is the main respiratory substrate utilized for the

provision of energy and intermediary metabolites for

biosynthesis. The first step in the metabolism of su-

crose, catalyzed by b-fructofuranosidase (invertase),

is the hydrolysis of the disaccharide to glucose and

fructose. These facts explain the predominance of

glucose, fructose, and sucrose in most fruit tissues.

The sucrose concentration varies from fruit to fruit

but in most fruits the total hexose content exceeds

that of sucrose, and in some fruits (e.g., cherry, grape,

and tomato) sucrose is almost absent.

0015A significant fraction of the sugar translocated to

the growing fruit is metabolized and used in various

biosynthetic processes, another fraction being stored

after conversion into starch, the most common

reserve carbohydrate in plants. Starch breakdown is

one of the most important biochemical changes that

occur during the ripening of many fruits. Some fruits

may contain, when mature but unripe, large amounts

of starch, the levels of which decrease dramatically

during ripening. For instance, bananas may contain

up to 20% starch at the mature green stage and less

than 1% when fully ripened. Large reductions in

starch content also occur during the ripening of

other fruits, notably mango, apple, and pear. The

increased levels of sucrose, glucose, and fructose pre-

sent in such fruits when ripened result primarily from

the enzymatic hydrolysis of starch. However, some

fruits do not accumulate starch during their develop-

ment, attaining their mature unripe state with little or

no starch at all (e.g., melon, pineapple, plum, and

grape). In these fruits, carbohydrates are stored in

the form of soluble sugars, their sweetness depending,

largely, on the transport of sucrose from other parts

of the plant, leaves being usually the main source.

5010 RIPENING OF FRUIT

Consequently, the optimum sugar content of these

fruits cannot be reached if they are harvested before

ripening. (See Starch: Structure, Properties, and De-

termination.)

Effects on Aroma and Taste

0016 Aroma and taste are considered to be the two major

components of flavor, a rather subjective attribute to

which all the other senses contribute in an integrated

manner. Most of the changes in aroma and taste,

together with changes in color and texture, that

are responsible for the commercial acceptability of

fruits take place during the ripening process. (See

Flavor (Flavour) Compounds: Structures and Charac-

teristics.)

0017 In spite of the important advances in flavor chem-

istry made in the last decades, our understanding

concerning the metabolism of the compounds in-

volved and the way they interact in the perception

of aroma and taste is very incomplete. Aroma, being

the sensation associated with smelling, requires the

presence of volatile, relatively hydrophobic com-

pounds. On the other hand, substances responsible

for taste and other nonspecific saporous sensations

are water-soluble and most nonvolatile, usually being

present at higher concentrations than those respon-

sible for aromas.

0018 Despite the relatively low lipid contents of most

fruits, lipid metabolism plays an important role in a

number of ontogenic events, including ripening and

senescence. Many of the most important aliphatic

fruit volatiles are formed through an oxidative deg-

radation of unsaturated fatty acids such as linoleic

and linolenic acids. For instance, the action of lip-

oxygenase and a lyase on linolenic acid leads to the

formation of 2-trans-hexenal or 2-trans-6-cis-nona-

dienal (depending on the site-specificity of the

enzymatic attack), these being important aroma con-

stituents of tomatoes and cucumbers, respectively.

The aldehydes and ketones generated by lipoxygenase

action can be converted further, by the action of

dehydrogenases, to the corresponding alcohols,

which usually have stronger aromas than the parent

carbonyl compounds. For example, the dehydrogen-

ation of 2-trans-6-cis-nonadienol gives 2-trans-6-cis-

nonadienal, which is important in the aroma of

cucumbers and melons.

0019 The class of compounds that most frequently

makes the major contribution to the pleasant fruity

aromas developed during ripening is the esters of

carboxylic acids. These are generated by esterifica-

tion of the alcohols formed by reduction of the alde-

hydes derived either from oxidation of long-chain

fatty acids or from amino acids. The mechanism of

ester formation has not been satisfactorily explained

but it is thought that esterification of the alcohols

occurs with acyl-coenzyme A derivatives of fatty

acids acting as acyl donors. While all the esters pre-

sent may contribute to the characteristic aroma, cer-

tain individual esters have been associated with the

aroma of specific fruits (Table 3). Some exceptions

are known. For example, the aroma of peaches is

attributed to the presence of lactones and that of

raspberries to the ketone 1-(p-hydroxyphenyl)-3-

butanone.

0020Volatile terpenoids are largely responsible for the

characteristic aroma of citrus fruits. The monoter-

penes citral and limonene exhibit distinct aromas of

lemons and limes, respectively. Sesquiterpenes are

also characteristic aroma compounds, such as b-

sinensal in oranges and nootkatone in grapefruits.

0021Of the four basic taste sensations (sweet, sour,

bitter, and salt), the dominant changes that occur

during fruit ripening involve an increase in sweetness

and a decrease in sourness (mainly due to acidity).

However, changes in bitterness (due to terpenoids)

are important in citrus fruits, as are changes in astrin-

gency (a taste sensation due to the association of

phenolic substances, including tannins, with proteins

in the saliva) that occur during the ripening of

bananas, grapes, and other fruits. The changes in

the concentration of acids during fruit development

and ripening differ with the type of fruit. The optimal

acidity is rather high for citrus fruits, somewhat less

for pome fruits, and even less for tomatoes. Most fruit

tissues accumulate excess acid in the preripening

stage of development but, when ripe, each fruit has

a range of acid-to-sugar ratio corresponding to opti-

mum taste. In most fruits, citric and malic acids are

the major contributors to the desired degree of acid-

ity. (See Acids: Properties and Determination; Phen-

olic Compounds.)

0022In postharvest fruits, as in other plant living tissues,

organic acids are in a constant state of flux. They

may arise or be used up through the operation of

tbl0003Table 3 Volatile esters associated with the aroma of individual

fruits

Ester Fruit

Benzyl benzoate Cranberry

Ethyl butanoate Orange

Ethyl 2-methylbutanoate Melon

Ethyl 3-methylbutanoate Blueberry

Isopentyl acetate Banana

Methyl anthranilate Concord grapes

Ethyl 2-methylbutanoate Apple

Methyl (3-methylthio)propanoate Pineapple

Methyl and ethyl derivatives of

trans-2-cis-4-decadienoate

Bartlett pear

RIPENING OF FRUIT 5011

glycolysis, the citric acid cycle and, possibly, the

glyoxylate cycle. Much of the loss of organic acids

observed during ripening is attributed to their oxida-

tion in respiratory metabolism as suggested by the

increased respiratory quotient (RQ, the ratio of

carbon dioxide evolution to oxygen absorption)

during the climacteric. The RQ is approximately 1.0

when sugars are the respiratory substrates, increasing

to about 1.3 when malate or citrate is the substrate,

and further increasing to 1.6 when tartrate is respired.

The rise in RQ during the climacteric is often accom-

panied by an increased activity of certain decarbox-

ylation enzymes, such as malic enzyme and pyruvate

decarboxylase. There is evidence that malic enzyme is

synthesized de novo during the climacteric of pome

fruits.

Changes in Color

0023 During the ripening of almost all fruits there occurs a

disruption in the organization of chloroplasts and

their reorganization into chromoplasts. As the photo-

synthetic apparatus is dismantled and the chlorophyll

degraded, the color of existing carotenoids is

unmasked. This phenomenon may be responsible for

the yellow-to-red colors of the ripe fruits, as in

bananas where yellowing is due to the unmasking of

carotenoids as a consequence of chlorophyll degrad-

ation. Although carotenoids are normally synthesized

in green tissues, in most fruits that contain carote-

noids ripening is accompanied by increased biosyn-

thesis. During postharvest storage such synthesis may

be affected by the surrounding atmosphere (decreased

oxygen and increased carbon dioxide or nitrogen

concentrations inhibit pigment development in the

endocarp of some cultivars of oranges and in toma-

toes), by light (it appears that light may stimulate its

synthesis but it is not required for its induction; in

detached green tomatoes it has been shown that light

increases carotenoid synthesis, and red light is more

effective than white or green light; far red light has an

inhibitory effect which suggests a phytochrome-

mediated response) and temperature (its effects vary

from fruit to fruit but in most cases the optimal

temperature for carotenoid synthesis is relatively

low; it is around 24



C for lycopene formation in

tomatoes, the pigment not being formed above



C but in other fruits in which lycopene is the

main carotenoid, such as watermelons and red grape-

fruits, its synthesis is not affected by temperature).

(See Colorants (Colourants), Properties and Deter-

mination of Natural Pigments.)

0024 The color of many ripe fruits, including pomes,

several types of berries, grapes, oranges, cherries,

peaches, plums, bananas, and figs, is due to the

production of anthocyanins. These are conspicuous

water-soluble plant pigments of phenolic character

that accumulate in the vacuoles and are responsible

for the pink, red, violet, and blue colors of fruits,

flowers, and vegetables. The colours of anthocyanins

are strongly affected by pH and can be intensified

and stabilized by intermolecular associations with a

number of metal ions such as those of aluminum,

iron, manganese, and copper. The anthocyanins are

typically located in the epidermal layers of the fruits

but, like carotenoids, may also be present in the flesh.

Anthocyanin synthesis during ripening is strongly

stimulated by light. Apparently, this stimulatory

effect has at least two mechanisms. By increasing the

photosynthetic rate, light increases the availability of

the carbohydrate needed to supply the energy and

building blocks for anthocyanin biosyntheis. The

second photoreaction involves the activation of

phytochrome, a photoreceptor pigment that mediates

several physiological processes in plant cells, includ-

ing the synthesis of anthocyanins. Preharvest applica-

tions of some growth regulators, such as alar

(N-dimethylaminosuccinamic acid), can induce

earlier formation of anthocyanins in fruits.

Postharvest Storage of Fruits

0025After picking, the ripening of fruits can be controlled

by temperature adjustment or by changing the

concentration of gases (usually carbon dioxide and

oxygen) in the surrounding atmosphere. These two

major methods of fruit storage are usually referred to

as ‘air refrigeration’ and ‘controlled-atmosphere stor-

age,’ respectively, the latter being normally combined

with some degree of refrigeration. Although effective

in retarding ripening, both methods of environmental

control impose stresses on fruits that may lead to

different types of physiological disorders.

Low-temperature Storage

0026Lowering the storage temperature is one of the com-

monest methods for extending the shelf-life of fruits.

With the exception of tropical and subtropical fruits,

the lowest temperature that, in theory, still permits

normal metabolism is near the freezing point of a

fruit, which in most cases lies between 0 and 2



The reduction of storage temperature decreases the

rate of respiration and of metabolism in general by its

direct effect on the rate of the enzyme-catalyzed reac-

tions. In climacteric fruits, low-temperature storage

also delays the onset of ripening by retarding the

autocatalytic production of ethylene and so extending

the preclimacteric phase of ripening. Lowering the

temperature also reduces the degree of response to

exogenous ethylene.

5012 RIPENING OF FRUIT

Controlled-atmosphere Storage

0027 A decreased rate of respiration and of other metabolic

reactions, including ethylene synthesis, can also be

achieved by altering the composition of the atmos-

phere in which the fruits are stored, usually by

increasing the carbon dioxide and decreasing the

oxygen concentrations. In controlled-atmosphere

storage the proportion of the gases is carefully con-

trolled, usually within + 1% of the desired value.

When the control of the gas concentrations is less

accurate, the practice is named modified-atmosphere

storage. For example, in a confined atmosphere the

carbon dioxide-to-oxygen concentration ratio may be

increased simply as a consequence of the normal re-

spiratory process or by sublimation of solid carbon

dioxide (dry ice). Another type of controlled-atmos-

phere storage is hypobaric storage in which the fruits

are kept in partial vacuum (0.1–0.3 atm). The de-

creased partial pressure of oxygen and the reduction

of internal ethylene levels due to the increased diffu-

sibility of the gases formed within the tissues retard

the ripening of the fruits. (See Controlled-atmosphere

Storage: Applications for Bulk Storage of Foodstuffs.)

0028 The efficacy of controlled-atmosphere techniques

in extending the storage life has been well demon-

strated for a number of fruits. Nevertheless, their

commercial use has been somewhat limited. Con-

trolled-atmosphere techniques are expensive and

some of the more profitable fruits either do not re-

spond favorably to atmosphere regulation or have

such a rapid market turnover that the need for an

improved storage procedure is curtailed. Neverthe-

less, conventional controlled-atmosphere storage is

currently used for apples and pears and the practice

of modified-atmosphere consumer-sealed packs is

becoming more popular due to the discovery of new

packaging materials having a selective gas permeabil-

ity that allows the retention of adequate gas concen-

trations when equilibrium is reached. Hypobaric

storage is successfully employed in the storage of cut

flowers but, to our knowledge, it has not been utilized

for the commercial storage of fruits. (See Chilled

Storage: Use of Modified-atmosphere Packaging.)

Physiological Disorders

0029 Different fruit species or even different cultivars of

the same species may exhibit different tolerances to

low temperatures and to modified atmospheres. Ex-

posure of a particular fruit either to a temperature or

to an atmosphere with a carbon dioxide-to-oxygen

concentration ratio outside the recommended range

usually causes injuries that lead to a decrease in both

quality and storage life. The extent of such injuries

depends strongly on the duration of exposure.

0030The mechanism by which physiological disorders

are induced is not well understood. Chilling injuries

are generally attributed to a deregulation in metabol-

ism leading to the underproduction of some essential

metabolites and the overproduction of substances

whose accumulation has toxic effects. For example,

accumulation of ethanol, acetaldehyde, and oxalo-

acetic acid has been associated with low-temperature

breakdown of certain fruits, such as apples. Visible

effects of cold injury include tissue necrosis, surface

pitting, internal browning, and failure to ripen. Some

fruits (e.g., pears) normally exhibit a low susceptibil-

ity to cold injury, but for most fruits there is a critical

temperature below which the fruits may be damaged,

particularly if submitted to prolonged exposure. As

recovery may often take place after short exposures to

potentially damaging temperatures, it is believed that

a minimum time of continuous exposure is required

for injury to occur. In fact, interrupted exposures

are often effective in diminishing damage. The lower

temperature limit tolerated by fruits varies with their

origin. Many fruits from temperate regions (e.g.,

apples) may tolerate temperatures in the range 0–



C, whilst the banana becomes susceptible at

temperatures below 12



C. Subtropical fruits, such

as pineapples, avocados, and citrus fruits, show inter-

mediary limits, often below 8



0031Identical metabolic imbalances are induced by

extremes in atmospheric composition. Toxic levels

of ethanol, acetaldehyde, and succinic acid have

been shown to accumulate in certain fruits prior to

injury symptoms. Some fruits, such as citrus and

pome fruits, are rather susceptible to high carbon

dioxide levels and the use of low carbon dioxide

concentrations (1–5%) is recommended. For more

tolerant fruits, such as cherries, grapes, plums, and

strawberries, higher concentrations (15–30%) are

usually advised. During storage, a minimum concen-

tration of oxygen is required to support respiration.

Below this level, anaerobic metabolism takes over

and ethanol is produced. Although many fruits

appear to tolerate oxygen concentrations below 5%,

the exact limit for normal ripening is variable,

depending on carbon dioxide concentrations, tem-

perature, and duration of exposure.

Ionizing Radiation

0032Although this remains an isolated rather than an

extensive method of food preservation, the applica-

tion of ionizing radiation is effective in delaying the

ripening of fruits. Depending on the fruit, doses of

0.3–3.0 kGy can prevent, temporarily or perman-

ently, the onset of fruit ripening. However, the

minimum dose required to achieve this effect often

RIPENING OF FRUIT 5013

exceeds the maximum tolerable dose. Even when

successful in prolonging storage life, fruit irradiation

is frequently hampered by a rapid softening of the

tissues, possibly due to a direct or indirect activation

of pectic enzymes. Actually, the mechanisms respon-

sible for the observed effects of ionizing radiations on

fruits are far from clear, but this is not surprising if

one considers how numerous are the doubts over

the biochemical and physiological control of fruit

ripening even under normal conditions. (See Irradi-

ation of Foods: Applications.)

See also: Acids: Properties and Determination;

Carbohydrates: Classification and Properties;,

Cellulose; Chilled Storage: Use of Modified-atmosphere

Packaging; Colorants (Colourants): Properties and

Determination of Natural Pigments; Controlled-

atmosphere Storage: Applications for Bulk Storage of

Foodstuffs; Flavor (Flavour) Compounds: Structures

and Characteristics; Fructose; Irradiation of Foods:

Applications; Phenolic Compounds; Starch: Structure,

Properties, and Determination; Sucrose: Properties and

Determination