Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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See also: Acids: Natural Acids and Acidulants; Chilled

Storage: Use of Modified-atmosphere Packaging; Chill

Foods: Effect of Modified-atmosphere Packaging on Food

Quality; Colorants (Colourants): Properties and

Determination of Natural Pigments; Controlled-

atmosphere Storage: Applications for Bulk Storage of

Foodstuffs; Fungicides; Ripening of Fruit; Sensory

Evaluation: Aroma; Storage Stability: Mechanisms of

Degradation; Parameters Affecting Storage Stability

Further Reading

Ballington JR, Kirkman WB, Ballinger WE and Maness EP

(1988) Anthocyanin, algycone, and aglycone-sugar con-

tent in the fruits of temperate North American species of

four sections in Vaccinium. Journal of the American

Society of Horticultural Science 113: 746–749.

Beaudry RM, Cameron AC, Shirazi A and Dostal DL

(1992) Modified atmosphere packaging of blueberry

fruit: effect of temperature on package oxygen and

carbon dioxide. Journal of the American Society of

Horticultural Science 117: 436–441.

Beaudry RM, Moggia CE, Retamales JB and Hancock JF

(1998) Quality of ‘Ivanhoe’ and ‘Bluecrop’ blueberry

fruit transported by air and sea from Chile to North

America. HortScience 33: 313–317.

Eck P (1988) Blueberry Science. New Brunswick: Rutgers

University Press.

Eck P (1990) The American Cranberry. New Brunswick:

Rutgers University Press.

Hancock JF and Draper AD (1989) Blueberry culture in

North America. HortScience 24: 551–556.

Hulme AC (ed.) (1970) The Biochemistry of Fruits and their

Products. London: Academic Press.

Kader AA, Kasmire RF, Mitchell FG et al. (1985) Post-

harvest Technology of Horticultural Crops. Extension

Bulletin 3311. University of California at Davis.

Kalt, W, Forney CF, Martin A and Prior RL (1999) Antioxi-

dant capacity, vitamin C, phenolics and anthocyanins

after fresh storage of small fruits. Journal of Agriculture

Food Chemistry 47: 4638–4644.

Luby JJ, Ballington JR, Draper AD, Pliszka K and Austin

ME (1990) Blueberries and cranberries (Vaccinium). In:

Moore JN and Ballington JR (eds) Genetic Resources

of Temperate Fruit and Nut Crops. Wageningen, The

Netherlands: International Society for Horticultural

Science.

Maas JL, Galletta GJ and Stoner GD (1991) Ellagic acid, an

anticarcinogen in fruits, especially in strawberries: a

review. Horticultural Science 26: 10–14.

Uphof JCT (1986) Dictionary of Economic Plants. Lehre,

Germany: J Cramer.

Usher G (1974) A Dictionary of Plants used by Man.

London: Constable.

Vander Kloet SP (1988) The Genus Vaccinium in North

America. Agriculture Canada Publication 1828.

Ottawa: Agriculture Canada.

Yarborough DE and Smagula JM (eds) (1996) Proceedings

of the sixth international symposium on Vaccinium

culture. Acta Horticulturae 446.

Factors Affecting Quality

W S Conway and M Faust, Beltsville Agricultural

Research Center, Beltsville, USA

C E Sams, University of Tennessee, Knoxville, USA

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001The primary goal of the fruit grower is to produce a

product of such quality that it is attractive to the

potential consumer in both appearance and taste. In

order for quality fruit to reach the consumer, it must

be properly grown in the orchard, harvested at the

time of proper maturity, and stored in such a manner

that the quality is maintained. Economic losses occur-

ring in postharvest channels are greater than often

realized, and the avoidable loss of quality ‘between

the farm gate and the consumer’ is the cause for real

concern. Fresh fruit increases several-fold in unit

value while moving from the field to the consumer,

because of the added cost of careful harvesting and

handling. Factors that affect fruit quality can be clas-

sified into three groups: (1) nutritional factors during

growth; (2) factors associated with the time of har-

vest; and (3) factors associated with storage. These

factors, as they relate to the quality of the major

temperate fruits, including apples, apricots, cherries,

peaches, pears, and plums, will be discussed.

Tree Nutrition

0002In order for the fruit tree to produce quality fruit, it

must be supplied with the proper mineral nutrients

and photosynthates. Orchard trees grow and produce

fruit in the same place for 15–50 years. By the time

these trees are mature enough to produce a crop, they

have developed extensive root and branch systems

and are able to acquire nutrients from the soil, inter-

cept light, and produce photosynthates. The peren-

nial nature of the tree and the necessity of producing

large crops of fruit regularly imposes demands for

nutrition not found in herbaceous plants or forest

trees. The tree has changing nutrient needs during

the year, particularly when it sets and raises its fruit.

These occasional high mineral nutrient needs may not

be met by uptake through the roots and must be

supplied by foliar application. Fruit nutrition must

often be concerned with the nutrition of not only

the whole tree but also individual organs. For

example, fruit should have a certain nutrient content

for maximum storability. This requires techniques

other than nutrition for maintaining the tree. First,

nutrients must be applied and the translocation and

2768 FRUITS OF TEMPERATE CLIMATES/Factors Affecting Quality

accumulation of these nutrients into the fruit must be

understood and horticultural techniques applied to

direct such nutrients into the fruit in quantities

to attain the goal. Mineral nutrients that affect fruit

quality include boron, calcium, magnesium, phos-

phorus, and potassium. A summary of known effects

of nutrients on fruit quality is given in Table 1.

0003 During the development of the tree, branches grow,

and leaves intercept more and more light and produce

more carbohydrates. At the same time, if the tree is

not pruned properly and the inside is shaded, these

shaded leaves are unproductive photosynthetically. In

apples, one fruit must be supported by 25–30 leaves

which are exposed to the sun to produce sufficient

carbohydrates to result in sweet, red fruit. Shade-

grown fruit are low in sugars and remain green.

Small fruit, such as cherries, require less support by

the leaves (1.5 leaves per fruit) than larger fruit. To

some extent, carbohydrates should be considered as

organic nutrients. If less than 75% of the light which

is measured above the canopy reaches the leaves, red

color will not develop. If the light is less than 50%,

sugar accumulation in the fruit is decreased and fruit

size will remain small. Similar to changing mineral

nutrient demands, the need for photosynthates also

changes. Photosynthates are especially needed soon

after bloom and again when fruit accumulates sugars

close to harvest. (See Carbohydrates: Classification

and Properties.)

Calcium

0004 Calcium is probably the most important mineral

element determining fruit quality. It is especially

important in apples and pears where it can prevent a

number of metabolic disorders. The effect of calcium

cannot be substituted by other factors. Calcium is

also important in other fruits because it has the gen-

eral effect of delaying ripening. Fruits high in calcium

can be transported better and maintain their quality

longer. The tissue calcium concentration at which

desirable effects are achieved is usually higher than

concentrations that the fruit normally accumulates.

Therefore, calcium accumulation into the fruit must

be forced by proper horticultural techniques. Calcium

nutrition is complicated by the fact that calcium is

needed most in the fruit. Consequently, calcium not

only needs to be taken up by the tree but also needs

to be directed to the fruit. (See Calcium: Properties

and Determination; Physiology.)

0005Increasing the calcium content of fruit is difficult to

achieve through normal fertilization. One method of

increasing calcium in the growing fruit is to apply it

to fruit trees as a spray. This spray must be applied to

the fruit itself because calcium applied to the leaves

will not translocate to the fruit or any other part of

the tree. Because of dilution during growth, calcium

concentration decreases depending upon the growth

rate of the fruit. This is an important factor in deter-

mining storage quality. Large fruits usually have very

poor storage quality. Small fruits, however, possess

high calcium concentrations and store well.

0006The direct application of calcium to the storage

organ would appear to be the most effective method

for increasing its calcium content. The calcium

content of apples has been increased by foliar sprays

and postharvest dipping in calcium chloride solu-

tions. Postharvest dipping with additives such as

food thickeners in the dip solutions further increases

calcium levels in apple fruit. Utilizing temperature

differentials, especially dipping warm apples into a

cold solution of calcium chloride, also results in add-

itional uptake of the solution. Even more successful

was vacuum or pressure infiltration of calcium chlor-

ide solutions into the fruit. In studies comparing treat-

ment of Delicious and Golden Delicious apples with

solutions of calcium chloride, the calcium concentra-

tion of fruit flesh was increased the most using pres-

sure infiltration, followed by vacuum infiltration,

with dipping being a distant third. In another study

comparing treatment of peaches with calcium chlor-

ide solutions, preharvest spray applications were less

successful than postharvest pressure infiltration.

tbl0001 Table 1 The importance of nutrient effects on fruit quality

Fruit Nutrients

Boron Calcium Magnesium Phosphorus Potassium Photosynthates

Apple þ þþþ  þþ þ þþþ

Apricot þ þ þ þþþ þþþ

Cherry þ

Peach þ þþ þþþ

Pear þþ þþ  þ þþ

Plum þþþ þþ

þImportance of deficiency levels; the effect of toxicity conditions.

Important during postbloom.

FRUITS OF TEMPERATE CLIMATES/Factors Affecting Quality 2769

0007 Increasing the calcium content of fruit tissue makes

the fruit more resistant to postharvest decay caused

by soft-rotting fungi. This may be attributable to

the fact that calcium ions bound to the pectins pres-

ent in the cell wall increase the number of cation

cross-bridges between pectic acids or between pectic

acids and other polysaccharides with acid groups, and

may make the cell wall less accessible to enzymes

produced by fungal pathogens that cause decay.

Fruit with sufficient or high calcium content maintain

quality in storage better than fruit which are low in

calcium. Calcium content of fruit flesh may be a

factor when considering whether to store fruit from

a particular orchard following harvest or sell it im-

mediately. Calcium analysis at harvest is often used

for making such decisions.

Potassium

0008 Potassium is needed in relatively large quantities.

When potassium levels are low, the fruit does not

color normally. It is likely that insufficient potassium

concentration decreases the photosynthesis of leaves,

which in turn lowers sugar concentrations, and this

affects the color. It is well established that cell exten-

sion is the consequence of the accumulation of potas-

sium in the cells. Maintenance of high turgor pressure

associated with potassium is needed for the enlarge-

ment of the fruit. This is especially important in

peaches and nectarines which need an extra water

supply during the last phase of fruit growth. Small

peaches are relatively poor in quality. (See Potassium:

Physiology.)

0009 The accumulation of organic acids is often the

consequence of unbalanced potassium transport.

When protein synthesis is impaired, potassium is in

excess, and acids are synthesized for charge balance.

Organic acids and potassium also accumulate in the

affected spots of the apple disorder Jonathan spot.

Phosphorus

0010 Phosphorus is involved in the energy transfer mechan-

ism, including the generation of adenosine triphos-

phate (ATP) and the formation of sugar and alcohol

esters. In addition to these roles, phosphorus also has a

regulatory function in many enzymatic processes

where inorganic phosphorus controls the rate of reac-

tion. (See Phosphorus: Properties and Determination.)

0011 The uptake of phosphorus in fruit of the apple tree

closely follows the weight increase of the fruit, and

uptake continues until harvest. Phosphorus levels

in apples have been positively correlated with fruit

firmness and negatively with low-temperature break-

down; it is therefore important to insure high enough

levels of phosphorus in the tree to avoid these

disorders.

Magnesium

0012The fruit requires considerable amounts of magne-

sium. In the leaves of the apple tree the calcium

concentration on a dry-weight basis is about five

times that of magnesium, whereas in the fruit the

magnesium concentration is twice as high as that

of calcium. Starch usually accumulates in magne-

sium-deficient tissues (starch decomposition, sucrose

formation, and phloem loading require energy and

ATPase activity, which all depend on magnesium),

and thus photosynthates are not partitioned into the

fruit. Magnesium-deficient trees also usually produce

small fruits. The overall photosynthesis of the tree is

also severely affected by defoliation. Thus the small

fruit size of magnesium-deficient trees may be attrib-

utable to more than one factor affecting photo-

synthesis. (See Magnesium.)

0013Some disorders of fruit may be accentuated or their

development may be triggered by a high concentration

of magnesium. All of these effects are known to be

caused by either lower calcium concentration in the

fruit or factors which counteract the beneficial effect

of calcium. Bitter pit, the collapse of cell clusters usu-

ally after storage of the fruit, is generally prevented by

sufficient calcium and accentuated by high magnesium.

Boron

0014Boron-deficiency symptoms are often noticed in the

fruit before symptoms appear in the shoot. In apple,

the mildest effect of boron deficiency is a flattening of

the fruit. If the deficiency is slightly more severe,

internal cork, i.e., round or irregular brown regions

within the core area, is clearly visible upon cutting the

fruit. The dead cell masses become dry, hard, and

corky. In pears, similar brown areas are closer to the

surface and, if they develop early, the surface above

the spots is depressed. Plums show only malformed

fruits without browning. If boron deficiency develops

early, external cork develops on apple before the fruit

is half-grown. In the early stages the areas appear

water-soaked, after which they harden and crack.

The fruit appears dwarfedand misshapen. (See Boron.)

0015There is very little margin between sufficiency and

toxicity where boron is concerned. Boron toxicity

manifests itself in early maturation of fruit, prema-

ture fruit drop, shortened storage life, and senescence

breakdown in storage. Boron toxicity is often caused

by boron in the irrigation water. Irrigation water

should be free of boron.

Fruit Maturity

0016Fruits developing during the growing season usually

reach a point when they are mature. If they are

2770 FRUITS OF TEMPERATE CLIMATES/Factors Affecting Quality

harvested at this time, they can maintain high quality

in storage. If they are picked in an immature state,

and only a few days separates immature from mature

in the case of peaches or apricots, the quality of the

fruit remains low. If they are harvested when they are

overly mature, the storage quality is poor and break-

down may occur during storage. The transition from

the mature to the ripe state is accompanied by a

number of enzymatic processes. The fruit softens,

the green color (chlorophyl) is destroyed, yellow

under-color develops, acids are depleted to a large

extent, and starch is converted to sugars. The rates

of these processes depend upon the species, and the

determination of harvest maturity is more or less

critical in influencing the quality of the fruit. The

relative importance of maturity on fruit quality is

given in Table 2.(See Ripening of Fruit.)

0017 The time at which fruit mature is generally deter-

mined by genetics. In most cases it is well known that

fruit matures in a certain number of days after bloom

depending upon the variety (e.g., apples mature

80–200 days after bloom; for McIntosh it is 123

days, Delicious 150 days, and Fuji 200 days). Other

species follow similar patterns. The temperature

during the first 3 weeks after bloom influences the

time of maturity. If the temperature after bloom is

warm, maturation time is shorter; if the temperature

is cool, the time of maturation is longer.

Storage Conditions

0018 Once fruit has been harvested at the right stage of

maturity, it is important to maintain the quality of the

fruit during storage. Postharvest losses are economic-

ally greater than equivalent losses in the field prior to

harvest. Fruit with bruises, skin breaks, and other

external distortions should not be placed in storage.

Not only are these blemishes unsightly, but they

also provide avenues of entry for postharvest decay-

causing pathogens which will reduce fruit quality.

Mechanical damage can also increase the amount of

moisture loss during storage, again reducing the qual-

ity of fruit. Optimum maturity is a very important

factor to consider when deciding which fruit to put in

storage. Storage life of fruits can be reduced if they

are harvested in an overmature or undermature state.

(See Storage Stability: Mechanisms of Degradation;

Parameters Affecting Storage Stability.)

0019Refrigerated storage retards senescence caused by

ripening, softening, and textural and color changes.

Refrigeration reduces unwanted metabolic changes,

heat produced through respiration, moisture loss and

resultant shriveling, and slows decay caused by post-

harvest pathogens.

0020Controlled atmosphere (CA) is a process or system

which is used to supplement refrigeration in main-

taining fruit quality in storage. It involves a change in

storage atmosphere that replaces air with an atmos-

phere containing 1–2% oxygen, 1–5% carbon diox-

ide, and 93–98% nitrogen. In conjunction with

refrigeration, CA can extend the storage life of apples

and pears and result in quality of fruit beyond that of

refrigeration alone. Controlled-atmosphere storage

(CAS) cannot be applied to cherries, peaches, plums,

or apricots. (See Controlled-atmosphere Storage:

Effects on Fruit and Vegetables.)

Effect of Refrigeration and Controlled

Atmosphere

0021In order to obtain maximum results from cold stor-

age, it is necessary for the temperature in storage

rooms to be held fairly constant. The best tempera-

ture at which to store most apple cultivars is between

1



C and 0



C. If the temperature rises 1 or 2



above the upper limit of 0



C, or if apples are not

promptly cooled to the desired temperature, there is

a real danger of increased decay and ripening. This

danger increases as the period during which the

temperature is above the optimum is extended. At

1.5–2



C, 3 or 4 days would have little effect since

the temperature of the fruit will not rise as quickly as

that of the air, but 10 days at this temperature would

reduce the storage life of the fruit by about a week

and could result in greater losses owing to decay.

Conversely, if the temperature falls several degrees

below 1



C for apples, there is a chance that the

fruit will freeze.

tbl0002 Table 2 Relative effect of maturity at the time of harvest on fruit quality

Fruit Immature Overmature

Apple – – – Poor flavor, metabolic diseases – – – Storage breakdown

Apricot – Poor flavor, low sugars – Softening

Cherry – – – Low sugars – Softening

Peach – – – Poor quality – Softening

Pear – Poor flavor – – – Storage breakdown

Plum – – Poor flavor, high acids – Softening

– Relative importance of maturity on fruit quality.

FRUITS OF TEMPERATE CLIMATES/Factors Affecting Quality 2771

0022 It is important to maintain a uniform temperature

throughout the storage room. Temperature variation

within the room could result in a mixture of overripe

and prime fruit being removed from storage, as well

as increased losses owing to decay in some areas

of the room. A summary of storage requirements

and average storage durations for various temperate

climate fruits is given in Table 3.

Apples

0023 Apple cultivars such as Delicious and Golden Deli-

cious can be stored under proper cold-storage condi-

tions for 5–6 months. Early or summer-maturing

apples are more perishable then the later-maturing

cultivars, and go directly to market rather than

being stored for long periods. (See Apples.)

0024 Refrigeration, however, is desirable for storing

apples even for a short period. At 4



C, apples respire

and soften twice as fast as at 0



C, and three times as

fast at 15



Casat4



0025 For the majority of apple cultivars, the optimum

storage temperature is between 1



Cand0



Cin

combination with 90–95% relative humidity (RH).

Since the highest freezing point for apples is approxi-

mately 1.5



C, most apples can be stored at 1



or above. However, a 1



C storage temperature

requires well-controlled storage temperatures to

avoid freezing.

0026 As an example of the importance of proper storage

temperatures, for Delicious apples, the cultivar grown

in the greatest abundance in the USA, storage at

1



C will extend storage life approximately 25%

over fruit stored at 0



0027 While the more popular cultivars such as Delicious

and Golden Delicious apples can be stored at 1



to 0



C, some other cultivars, owing to a greater

susceptibility to low-temperature disorders, should

be stored at somewhat higher temperatures. Cultivars

such as Jonathan and McIntosh should be stored at



0028 CAS, as previously stated, uses refrigeration in a

gas-tight room, where the atmosphere within is con-

trolled so that this atmosphere is higher in carbon

dioxide and lower in oxygen than normal. Apples

stored in CA respire, ripen, and soften more slowly

and have fewer disorders than those in air storage.

This results in a longer storage life and longer shelf-

life after removal than fruit stored in air. These fruit,

however, must be harvested at the proper maturity

and be of the highest quality to realize the benefits

resulting from CAS. CA rooms in which apples are to

be stored usually contain an atmosphere of 1.5–3%

oxygen, 1–8% carbon dioxide, the remainder being

nitrogen. The exact combination of gases is cultivar-

dependent. The storage life in CA for McIntosh,

for example, may be nearly double that in regular

cold storage. The use of CA also allows the fruit

to be stored at slightly higher temperatures with

good results. Cultivars such as Jonathan and Yellow

Newton, which may develop low-temperature dis-

orders at 0



C in regular cold storage, may be stored

at 2



C and still retain excellent quality. In addition,

large quantities of Delicious and Golden Delicious,

the most popular apple cultivars, are stored in CA, at

0.5



Cto0



0029More recently, even better quality retention has

been realized with a lower oxygen level of 1% rather

than the 2–3% used for McIntosh, Delicious, and

Golden Delicious apples. Oxygen at this level,

however, must be maintained very precisely at 1%,

because when oxygen drops below 1%, injury to the

fruit may occur.

Apricots

0030Although apricots are seldom stored commercially

for long periods of time, they may keep well up to 3

weeks at 0.5



Cto0



C, with a RH of 90–95%

being desirable. Unfortunately, fruit picked when

they are mature enough to ship or store are at the

same maturity as those used for canning and lack the

taste of tree-ripened fruit. The biggest hazard of ship-

ping and handling apricots is decay, and since mature

fruit is more susceptible to decay than immature fruit,

it is not possible to allow the fruit to reach the stage of

maturity for fresh-market use. (See Apricots.)

0031Cooling apricots quickly to 4



C immediately after

harvest and then holding them at 0



C will retard

decay and ripening. If the storage temperature is

tbl0003 Table 3 Storage requirements and storage periods for fruits grown in temperate climates

Fruit Cold storage Controlled atmosphere

Temperature

(



C) Duration

Temperature

(



C) Duration

(months)

Apple  1 to 4 3–6 months 0.5 to 4.4 7–9

Apricot 0.5 to 0 1–3 weeks

Cherry 1 to 0 2–3 weeks

Peach 0.5 to 0 2–4 weeks

Pear 1.5 to 0 2–7 months 1.5 to 0 7–9

Depends on cultivar; range is given.

2772 FRUITS OF TEMPERATE CLIMATES/Factors Affecting Quality

kept at 4–7



C, the fruit will have a more ‘mealy’

texture after ripening then fruit kept at 0



Cherries

0032 Fresh sweet cherries have a handling limit from har-

vest to arrival at the market of about 2 weeks if

temperatures do not exceed 2



C. If sealed polythene

liners are used in containers, the cold-storage period

can be extended by about a week. Sweet cherries

covered with polythene and stored at 1



Cto0



for 2 weeks after harvest (90–95% RH is also desir-

able) will then have a week remaining for distribution

in marketing channels. If sweet cherries are held

longer than the 2–3 weeks indicated, there is a loss

of flavor and brightness. Moisture loss is a critical

factor detracting from the fresh appearance of both

the fruit and the stems. Cherries harvested in the field

should be covered with a moisture barrier at harvest

until packaged for shipment. Sweet cherries are ex-

tremely perishable and rapid deterioration occurs if

the fruit are not refrigerated. Sour cherries are usually

unsuitable for storage. They are harvested directly

into ice water for immediate cooling and may be

held at 0



C for several days to lengthen the process-

ing period. (See Cherries.)

Peaches

0033 Peaches are usually stored for only a short period and

usually only to escape an overabundance on the

market at a given period, or to extend their processing

life. Peaches harvested at the proper maturity can

usually be stored at 0.5



Cto0



C for 2–4 weeks.

Early-maturing cultivars have a shorter storage life,

while the later-maturing cultivars may be successfully

stored for 3–4 weeks. If peaches are stored for longer

than 3–4 weeks in cold storage, they often fail to

ripen when moved to higher temperatures. The flesh

of these fruit may become dry and mealy, or wet and

mushy, and may brown internally around the stone.

There is a deterioration of flavor, and a dull appear-

ance results. This malady is called internal break-

down. Storage of peaches at 2–5



C for 7–14 days

results in internal breakdown. (See Peaches and

Nectarines.)

0034 Rapid cooling of peaches to 4



C at harvest is ne-

cessary to retard ripening and decay. Brown rot, a

fungal disease of peaches, can cause large losses in

storage, and refrigeration is an effective method of

retarding the development of this disease.

Pears

0035 Pears are very sensitive to the temperature at which

they are stored. Most pears in the USA are grown in

the Pacific North-west, and are stored at 1



Cwith

aRHof90–95%. The recommended storage

temperature for pears is 1.5



C to 0.5



C with a

RH of 90–95%. As an example of the sensitivity of

pears to storage temperature, Anjou and Bartlett

(Williams) pears have approximately a 40% longer

storage life at 1



C than at 0



C. It is necessary to

maintain precise temperature control at these low

temperatures to prevent freezing. It is also necessary

to cool the fruit rapidly (remove field heat) to store

these fruit successfully for long periods. When pears

are being cooled, temperatures as low as 3.5



Cto

2



C can be used, but should then be raised to

1



C as the fruit temperatures approach 1



Since pears also lose moisture rapidly, an RH of

about 90% is desirable. (See Pears.)

0036Proper maturity at harvest is also conducive to

long storage life. If pears are harvested at full matur-

ity, they are less likely to be susceptible to physio-

logical disorders and will ripen properly after

storage. However, pear fruit gain much of their

weight close to harvest. If growers delay harvest by

a week, they may gain 1–2 (US) tons of fruit per acre

(2268–4535 kg ha

1

), but storage quality will be

poor.

0037The length of time for which pears can be safely

stored at 1



C varies with cultivar. Holding pears

beyond their normal storage period may result in fruit

which will not ripen properly. Anjou pears can usu-

ally be safely stored for 6–7 months, while pears of

the Bartlett (Williams) or Bosc cultivars can only be

stored for 3–4 months.

0038CA, as with apples but to a lesser extent with pears,

has been used successfully to extend the storage life of

pears and to maintain a greater capacity for ripening.

It is generally considered that 2–2.5% oxygen and

0.8–1.0% carbon dioxide is the optimum atmosphere

in which to store pears safely.

Plums

0039In general, plums, including fresh prunes, are not well

adapted to long cold-storage periods. Storage period,

however, differs according to cultivar. The damson-

type cultivars store better than the softer, oriental

cultivar fruit. Well-matured fruit of the firmer var-

ieties can be safely stored for 4–5 weeks at 0.5



Cto



C. Although ripening proceeds very slowly at these

temperatures, there is some loss in flavor. If stored

beyond 5 weeks, flesh browning and abnormal

flavors often result.

See also: Apples; Apricots; Boron; Calcium: Properties

and Determination; Physiology; Carbohydrates:

Classification and Properties; Controlled-atmosphere

Storage: Effects on Fruit and Vegetables; Magnesium;

Peaches and Nectarines; Pears; Phosphorus:

FRUITS OF TEMPERATE CLIMATES/Factors Affecting Quality 2773

Properties and Determination; Potassium: Physiology;

Ripening of Fruit; Storage Stability: Mechanisms of

Degradation; Parameters Affecting Storage Stability

Further Reading

Conway WS (1989) Altering nutritional factors after

harvest to enhance resistance to postharvest disease.

Phytopathology 79: 1384–1387.

Faust M (1989) Physiology of Temperate Zone Fruit Trees.

New York: John Wiley.

Hardenburg RE, Watada AE and Wang CY (1986) The

Commercial Storage of Fruits, Vegetables, and Florist

and Nursery Stocks. US Department of Agriculture,

Agriculture Handbook 66. Washington, DC: US

Government Printing Office.

Improvement and Maintenance

of Fruit Germplasm

R M Brennan and S Millam, Scottish Crop Research

Institute, Invergowrie, Dundee, UK

Background

0001 Fruit crops have been cultivated for centuries, both

commercially and in amateur gardens, and many

species covering a wide range of trees, shrubs, and

nonwoody perennials have been the long-term

subjects of germplasm improvement programs. For

example, different varieties of apples (Malus spp.)

were recorded by the ancient Greeks, and selection

and breeding of apple have continued since then.

In contrast, the cultivated strawberry Fragaria 

ananassa, an octoploid interspecific hybrid, was de-

veloped in the eighteenth century, and the highbush

blueberry Vaccinium corymbosum was only domesti-

cated in the twentieth century.

0002 Fruit crop species have certain common character-

istics: they are long-lived, usually highly heterozy-

gous, and clonally propagated from an elite mother

plant. They are also invariably economically minor

crops compared with large-scale arable species such

as cereals or potatoes, as reflected in the relative

research and development resources available. How-

ever, fruit is increasingly recognized as an important

component of a healthy diet, and consumption of

temperate fruit and fruit products, especially juices

and dairy derivatives, has risen sharply over the past

decade.

Fruit Germplasm Improvement

Breeding Objectives

0003Breeding objectives in fruit have traditionally con-

centrated on improving yield or reducing the

influence of abiotic factors such as low temperatures

on yield. However, resistance to pests and diseases

is increasingly important, particularly as chemical

controls are reduced on environmental and toxico-

logical grounds. There is also a rising demand for

fruit of a defined quality in terms of biochemical

characters (content of flavonoids, vitamins, soluble

solids, etc.) as well as physical characters (fruit

size and shape, skin strength, juice yield). The

move towards healthier diets worldwide, but espe-

cially in western Europe and the USA, has impac-

ted strongly on breeding and selection priorities,

and screening of germplasm, including potential

donor species, for nutritional attributes such as anti-

oxidant activity has been reported in several fruit

crop species.

0004Other breeding objectives such as machine harvest-

ability, storage potential, environmental adaptability,

and precocity of cropping remain important for most

breeding programs.

0005Financial support for the great majority of fruit

breeding programs in western Europe has in the last

15 years moved from the public to the private and

commercial sectors. The products from these pro-

grammes are more exactly aligned with the sponsors’

specific requirements, especially regarding fruit

quality, although much of the underpinning genetics

research remains in the public domain.

Breeding Methodology

0006The breeding of new fruit cultivars, as with most

clonally propagated crops, begins with the selection

of parental genotypes with complementary character-

istics suitable for the achievement of the breeding

objectives. After hybridization between the parental

types, promising phenotypes are selected from the

segregating F

population, multiplied vegetatively

through further screening cycles and eventually re-

leased as cultivars. The timescale over which this

process operates is substantial: in small fruits, the

time from initial hybridization to commercial release

is in the order of 15–20 years, and this may be even

longer for top fruit. It is therefore vital that (1) the

most appropriate hybridizations are made in the first

instance, (2) the progenies produced are sufficiently

large to allow enough variation for selection of

the desired phenotypes, and (3) the most efficient

and timely selection criteria are used to select out

appropriate phenotypes. The latter process can be

2774 FRUITS OF TEMPERATE CLIMATES/Improvement and Maintenance of Fruit Germplasm

significantly assisted by the use of molecular-marker

technologies (see below).

0007 The majority of fruit crop species, particularly tree

fruit, produce little or no fruit during the first 2–5

years at least, and these early crops cannot in any case

be used as an accurate guide to future cropping

worth. It is usual to use this juvenile period for the

assessment of vegetative characters such as plant

habit and pest/disease resistance. It is clearly import-

ant that desirable genotypes be selected as early as

possible to maximize the efficiency of the breeding

program. The increased emphasis on development of

molecular markers will potentially give major ad-

vances in terms of early selection of desired segregants

from a seedling population. Also, there are sometimes

morphological markers that can be used, for

example, in Rubus where progenies can be screened

out at the cotyledon stage for the presence of glandu-

lar hairs linked to the later development of undesir-

able spiny plants.

0008 Once the plants are producing flowers, the

evaluation of fruit quality and cropping potential

can begin. Promotion of earlier flowering by training

to a single stem, glasshouse culture, and high-density

planting has been achieved in apple, while grafting

of seedling scions on to mature trees has been

used extensively to hasten fruiting in both apple

and grape.

Interspecific Hybridization

0009 For many fruit crop species, the use of interspecific

hybridization is increasing, in order to utilize natur-

ally occurring sources of pest and disease resistance,

fruit-quality components, etc. within the available

germ plasm (Figure 1 shows a typical hybrid) Some

examples are:

0010blackcurrant (Ribes nigrum): Ce gene for resistance

to Cecidophyopsis ribis introgressed from goose-

berry R. grossularia;

0011highbush blueberry (Vaccinium corymbosum): re-

duced chilling and greater soil pH adaptability

from V. darrowi, dwarfing habit from V. angusti-

folium;

0012apple (Malus domestica) – improved winter

hardiness from M. asiatica and M. pumifolia.

The ease with which hybrids can be made across

species barriers varies considerably, and the range of

techniques for achieving this is described below. Many

of the domesticated fruits that are presently in exist-

ence are originally hybrids between species, e.g., apple

(Malus domestica), where several species have

contributed to present-day cultivars, strawberry

(Fragaria ananassa), which is a hybrid between the

North American species F. chiloensis and F. virginiana,

and highbush blueberry (Vaccinium corymbosum),

which is a tetraploid hybrid formed from several dip-

loid races of Vaccinium from the southern USA. In

many genera, notably Fragaria and Vaccinium, there

are ploidy barriers to interspecific hybridization, but

nevertheless there is considerable interest in breeders

for overcoming these problems, for example, in the

creation of synthetic octoploids in Fragaria.

0013For top fruit species (Malus spp., Pyrus spp., Pru-

nus spp.), there is also considerable effort devoted to

rootstock breeding. The earliest progenies of these

fruits were grown from seed, and the best selections

were then propagated on seedling rootstocks of

fig0001 Figure 1 (see color plate 67) Interspecific hybridization in Rubus breeding (R. phoenicolasius is used as the donor of resistance to

raspberry beetle (Butyrus tomentosus)), showing the wild accession (left), a commercial red raspberry (right), and the resultant F

hybrid.

FRUITS OF TEMPERATE CLIMATES/Improvement and Maintenance of Fruit Germplasm 2775

the same or closely related species. Now, the variable

seedling rootstocks have been supplanted by the

selection and breeding of uniform, often dwarf, root-

stocks, and this type of program is of key importance

to the future development of these fruits.

Genetic Resources and Genebanks

0014 The elimination in the wild of potentially valuable

species material will inevitably have serious repercus-

sions for fruit breeders unless there is adequate

conservation of representative genotypes. Most tem-

perate fruits have centers of origin in the northern

hemisphere, and it was suggested by Vavilov in the

1930s that wild species show the most genetic vari-

ation around or near their center of origin. These

centers of diversity, both primary and secondary, are

crucially important in the conservation of genetic

resources, and the collection of relevant germplasm

is seen as a priority by many breeders and researchers.

In fruit germplasm collecting expeditions, it is now

recognized as important for the wild populations to

be both sampled for specific characters relevant

to existing needs within the breeding program but

also randomly sampled, for less predictable needs in

the future.

0015 In addition, there are several more recently domes-

ticated fruits where accessions from the wild have

formed the basis of commercial production; for

example, some of the first blackcurrant (Ribes

nigrum L.) cultivars released in Scandinavia were

superior selections from the wild, and present day

cranberry (Vaccinium macrocarpon) production in

the USA relies heavily on cultivars selected from the

wild – a total of 132 wild selections compared to only

seven from controlled hybridization.

0016 Germplasm collections must contain adequate

representation of all important taxa and, where ap-

propriate, specific ecotypes and accessions that may

possess unique and important characteristics that can

be utilized in long-term plant improvement programs.

The ability to access new sources of important char-

acters, and to generate sufficient variation in progen-

ies to select out improved seedlings of commercial

value, is key to the future of fruit breeding.

0017 Plant genetic resources can be maintained in situ,

whereby whole environments are conserved and the

plants therein preserved through the protection of

their habitat. For the maintenance of entire popula-

tions, this method of conservation may be appro-

priate. However, for accessibility, it is invariably

preferable to have the germplasm maintained in an

ex situ form.

0018 Ex situ genebanks, where diverse genotypes are

collected and maintained, usually at research centers,

in field or laboratory conditions, can provide both a

core collection of representative accessions and an

active collection that can be easily utilized by the

breeder. To be of significant value, material within

the collection must be fully phenotyped and charac-

terized, and increasingly, molecular data on each

accession are collated.

0019For clonally propagated material like fruit germ-

plasm, it is best to keep the material as an active field-

based collection, or clonal repository, although this

has associated problems in terms of cost (land use,

culture, labor), pests, and pathogens (causing damage

and potential loss of accessions and hence erosion of

diversity) and manageability (only relatively low

numbers of accessions, particularly of large plants

such as fruit trees, can be maintained compared to

seed banks).

0020The use of seed banks is of limited use in clonally

propagated crops such as fruits, but it can be import-

ant for the storage of wild populations, since often the

preferred method of collection in the wild is to obtain

fruits of the desired accessions to avoid removing the

entire plant. Also, many breeding programs maintain

seed of controlled hybridizations, in most cases to

allow further populations from interesting crosses

to be planted out.

0021In vitro collections are predominantly used for the

long-term storage of base collections of representative

genotypes. The plant material can be cryopreserved in

liquid nitrogen at 196



C, by which all the normal

plant processes are suspended. Systems for success-

fully cryopreserving a range of plant parts (apical

meristems, cell cultures, shoot sections) of several

fruit crop species have been developed, but cryopre-

servation is best considered as a complementary

method for situations where other conservation strat-

egies are not appropriate.

0022The use of in vitro storage of plant accessions over

short- to medium terms in conditions where growth is

reduced, usually at c.4



C, is another potentially

important method for the maintenance of valuable

germplasm. By reducing the temperature at which

in vitro cultures are grown, often with the presence of

osmotic or growth regulatory inhibitors in the culture

medium, subculturing intervals can be extended from

a few weeks to periods of a year or more. However,

the effects on stability have not been quantified for

many species.

0023Overall, in vitro techniques are potentially useful

for the maintenance of fruit germplasm, but consider-

ably more research is required on long-term stability

and the risks of mutations before they can be rou-

tinely used. The latter is now fairly easily monitored

using molecular techniques, although the risk of

mutations varies with species also. The use of

2776 FRUITS OF TEMPERATE CLIMATES/Improvement and Maintenance of Fruit Germplasm

in vitro genebanking has similar requirements in

terms of replication and plant health as conventional

genebanks maintained in the field or as seed.

Future Breeding Requirements and Prospects

0024 Fruit production in some parts of the world, notably

Asia, is presently in a period of significant increases,

particularly for apples, pears, and strawberries. How-

ever, fruit breeding is not showing a similar increase,

either in number of breeding programs or available

resources, and in general, there is a trend towards

fewer major cultivars worldwide. A highly restricted

range of cultivars are grown in a range of locations;

for example, cv. Cox apples can be obtained from the

UK, continental Europe, and New Zealand, and

Elsanta strawberry accounts for the majority of pro-

duction throughout Europe. This leads inevitably to a

loss of local cultivars and a contraction of the genetic

base for the crop in general. The breeders’ ability to

produce new and improved cultivars, responding to

developing market requirements, is thus impaired.

0025 The increasing interest worldwide in reduced input

production systems, often incorporating aspects of in-

tegrated pest management, together with a need to

drive down production costs, has led to an increasing

emphasis on the development of pest- and disease-

resistant cultivars of fruit. Also, processors and

marketing groups are demanding specific fruit-quality

traits in the available fruit, and the achievement of

these objectives through plant breeding represents a

major challenge to breeders into the future.

Biotechnological Approaches to Fruit

Crop Improvement

0026 Conventional crop-breeding techniques have enabled

major advances to be made in the yield and quality of

temperature fruit crops this century. However, in

recent years, the impact of a range of techniques

based on plant tissue culture, recombinant DNA

technology, or a combination of both has become

realized. This has facilitated the more precise and

targeted genetic enhancement of a number of temper-

ate fruit crop species to date and offers the potential

for future wider application.

Plant Tissue Culture

0027 The concept of totipotency (first postulated by

Schwann and Schleiden in 1838) is unique to plant

cells. The capacity for individual cells to regenerate

into intact plants underpins all plant tissue culture

manipulations. Fruit crops, historically, were among

the first species to be investigated in cell culture, with

the culture of grapevines in vitro (albeit in a relatively

unsophisticated form) reported as early as 1944.

Most temperate fruit species have now been estab-

lished in tissue culture, and these systems are of

significant economic and scientific importance. Prob-

lems associated with the establishment of, particu-

larly woody, fruit germplasm in vitro include the

long reproductive cycle of the plants, which restricts

the availability of the juvenile tissue necessary for the

establishment of a tissue culture system to a short

time period. However, the advent of increased know-

ledge in this area in recent years has resulted in the

range of species amenable for such technology being

widened.

0028Micropropagation Propagation of plants under

sterile conditions, in optimized environmental condi-

tions, at a high density is known as micropropaga-

tion. The principal advantage is the provision of

clean, clonal material in high volumes at greatly

accelerated rates of proliferation compared to con-

ventional propagation. Simple and cost-effective

methods for the micropropagation of such species as

strawberry, grape, kiwifruit, apple, raspberry, and

blackcurrant are in widespread use for the routine

commercial production of plants, regardless of

climatic or seasonal constraints.

0029Associated advantages of the use of sterile micro-

propagation techniques include the potential for

cleaning up and rejuvenating diseased or damaged

plant material. This has increasing importance, for

the provision of clean starting material for applica-

tion in breeding programs or as high-quality stock for

initiating conventional propagation. Meristem cul-

ture, in which the apical 12–20 cells are cultured

in vitro, sometimes with associated heat treatment,

is a valuable method for the elimination of viruses

from fruit germplasm and has been widely used in the

production of virus-free lines of apple, strawberry,

and other fruits. A range of other applications can

be facilitated by the use of micropropagation. These

include the conservation of unique or valuable mater-

ial in vitro, the relatively easy storage of material for

programmed cropping systems, and enabling the

transport of material in a cost-effective form.

0030Regeneration from tissue explants Micropropaga-

tion is based on the use of preexisting buds, e.g.,

shoot tips or axillaries. However, methods exist for

the de novo initiation of shoots from explants such as

leaf discs of apples and strawberries, stem sections of

Ribes, nonclonal material such as immature embryos

and cotyledons of Prunus spp. (peach, cherry, and

apricot), and immature achenes of Fragaria. Often,

a callus phase involving de-differentiated cells is in-

volved prior to the regeneration of shoots, generally

mediated by the application of exogenous growth

FRUITS OF TEMPERATE CLIMATES/Improvement and Maintenance of Fruit Germplasm 2777