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

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monoglycerides, and other fat-soluble substances.

The retention of a-tocopherol is much greater than

that of g-tocopherol.

0010 The vitamin circulates in the lymph and blood

bound to lipoproteins, but, unlike some fat-soluble

substances such as vitamin A, no specific carrier pro-

tein for vitamin E has been discovered. There is a high

correlation between the total fat and the tocopherol

concentrations in blood serum. Thus, diseases associ-

ated with high serum lipids (hypothyroidism, dia-

betes, hypercholesterolaemia) produce high plasma

vitamin E levels, whereas those with low serum lipids

(abetalipoproteinemia, malnutrition, cystic fibrosis)

produce low vitamin E levels. (See Cystic Fibrosis;

Hyperlipidemia (Hyperlipidaemia); Malnutrition:

The Problem of Malnutrition; Retinol: Physiology.)

0011 Different tissues have widely differing vitamin E

contents, as shown in Table 2. The vitamin is most

concentrated in cellular fractions that are rich in

membrane lipids such as the mitochondria. The vita-

min E content of red blood cells is about 20% of that

in plasma, and there is a rapid exchange between

these two pools.

0012 Less than 1% of vitamin E is metabolized; how-

ever, there are reports of oxidation products of the

vitamin, such as the quinone and hydroquinone,

being found in tissues such as the liver and the lung.

Tocopherol is mainly excreted unchanged in feces.

Very little is excreted in the urine.

Biochemical Function

0013 Unlike most vitamins, there is no specific, enzymati-

cally controlled reaction for which vitamin E is

known to be a required cofactor. Instead, it is gener-

ally agreed that vitamin E acts as an antioxidant,

protecting tissue against the nonenzymatic process

of air oxidation of PUFA, as suggested by Olcott in

1941 and extended and elaborated by Dam in 1957

and by Tappel in 1961. (See Coenzymes.)

0014 Evidence for the antioxidant theory of the function

of vitamin E can be found in the following observa-

tions. The more PUFA in the diet, the more vitamin E

is required. Animals on vitamin-E-deficient diets and

subjected to oxidative stress conditions (such as high

ozone levels) develop indirect evidence of the pres-

ence of peroxidic products in their tissues. Animals

exposed to stressful oxidizing conditions are pro-

tected by increased intakes of vitamin E; examples

of oxidative stress include smog, cigarette smoke,

and some types of chemical carcinogens. Synthetic

antioxidants that are structurally unrelated to vita-

min E (e.g., ethoxyquin and diphenyl-p-phenylene

diamine) can be used to replace it in diets. Finally,

there is some evidence that other dietary antioxidants

(such as vitamin C and selenium) can partially ameli-

orate low intake levels of vitamin E. (See Ascorbic

Acid: Physiology; Selenium: Physiology.)

Vitamin E Deficiency

0015Overt deficiency of vitamin E in the absence of

nutritional deprivation is rare in humans, although

animals can be made vitamin-E-deficient by feeding

them synthetic diets that contain fats from which

vitamin E has been removed. A number of diseases

are known in various animals to result from a defi-

ciency of vitamin E; perhaps the most notable feature

of these deficiency states is the very great species

specificity of the manifestations of vitamin E defi-

ciency. Notable among the pathologies found are

muscular degeneration, encephalomalacia, and, as

discussed above, problems with gestation in some

animals (e.g., rats but not mice).

0016Vitamin E deficiency in humans is generally meas-

ured by the time to observe hemolysis of red blood

cells when the blood is exposed to hydrogen peroxide.

In this test, the ability of vitamin E to protect red

blood cell membrane lipids from undergoing de-

structive oxidation is measured. Normal levels of

vitamin E in adult human blood serum average

1.05 mg dl

1

, but the range is very wide.

0017The first documented cases of vitamin E deficiency

in humans were in patients with abetalipoprotein-

emia. These patients develop severe neurological

abnormalities during their first two decades of life.

Supplements of vitamin E (generally 100 mg per kg of

body weight per day) can prevent the disease if started

early enough in life. Children with cholestatic liver

disease also develop neurological abnormalities due

to inadequate delivery of vitamin E to the affected

tissues. Studies in hyperlipidemic rats demonstrate

that the transfer of vitamin E to tissues is impaired.

(See Liver: Nutritional Management of Liver and

Biliary Disorders.)

0018The ratio of maternal and fetal blood tocopherol

levels is about 5, suggesting poor placental transport

of vitamin E. For this reason, premature infants have

tbl0002 Table 2 a-Tocopherol content of human tissues

Tissue mgg

1

Plasma 9.5

Red blood cell 2.3

Fat 150

Adrenal 132

Heart 20

Testis 40

Liver 13

Adapted from Machlin LJ (ed.) (1991) Handbook of Vitamins. New York:

Marcel Dekker.

5798 TOCOPHEROLS/Physiology

low vitamin E levels. Premature infants are often

given elevated oxygen therapy, since their lungs are

immature, and this places an added oxidative stress

on their systems. Since modern medicine allows

smaller premature infants to be saved than in the

past, there has been an increase in diseases in these

infants that may be ameliorated by vitamin E (such as

intraventricular hemorrhage). Premature infants are

now routinely given supplemental vitamin E.

PUFA Autoxidation and the Role of

Vitamin E

0019Polyunsaturated fatty acids have methylene groups

(—CH

— groups) located between two double

bonds, as shown in Figure 2. This type of chemical

functionality makes these compounds particularly

sensitive to the flameless oxidation in air that is called

’autoxidation.’ Figure 2 shows the steps involved in

LOO

LOO H + L

L H is a PUFA with a structure like linolenic acid:

+LH RH +L

LOO

+LH

2LOO

LOOL + O

LOO +

LOOH +

Tocopheryloxyl radical

COH

(CH

)

A series of stable products

Initiation

Radicals formed

in normal metabolism

or from certain types

of toxins

No antioxidant present:

Vitamin E present:

Propagation

Termination

Lipid

radical

Lipid peroxyl

radical

Lipid

radical

Lipid

hydroperoxide

fig0002 Figure 2 Chemical reactions involved in the autoxidation of a particular polyunsaturated fatty acid, linoleic acid. The autoxidation

sequence involves three different types of reactions: initiation, propagation, and termination. Initiation is the process in which radicals

are produced; propagation steps are those that occur over and over in the chain reaction and where products are formed; and

termination steps are those in which radicals are destroyed. Reproduced from Tocopherols: Physiology, Encyclopaedia of Food Sci-

ence, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

TOCOPHEROLS/Physiology 5799

this process. Free radicals are produced both in normal

metabolism and by the effects of toxins on tissue.

These primordial free radicals can cause the formation

of lipid-free radicals, as shown in the initiation se-

quence. The lipid radical, L

, then undergoes the

propagation sequence shown, leading to the formation

of lipid hydroperoxides, LOOH. These LOOH com-

pounds also are formed in enzyme-catalyzed reactions;

for example, the enzymes in the arachidonic acid cas-

cade serve to convert arachidonic acid (the acid with

20 carbon atoms and four double bonds) to several

types of hydroperoxides that have potent biological

properties, such as 15-hydroperoxyeicosatetraenoic

acid (15-HPETE). If vitamin E is not present, two of

the peroxyl radicals (LOO

) involved in the propaga-

tion sequence must find each other and collide in order

to convert radicals to stable, nonradical products. This

reaction is shown in the termination sequence. If vita-

min E is present, it reacts extremely rapidly with per-

oxyl radicals, converting them to nonradical products

and stopping the autoxidation chain reaction. One

tocopherol molecule can protect 100 or more PUFA

molecules from autoxidative damage.

See also: Antioxidants: Natural Antioxidants; Ascorbic

Acid: Physiology; Cancer: Diet in Cancer Prevention;

Coenzymes; Cystic Fibrosis; Dietary Requirements of

Adults; Fatty Acids: Properties; Hyperlipidemia

(Hyperlipidaemia); Liver: Nutritional Management of

Liver and Biliary Disorders; Malnutrition: The Problem of

Malnutrition; Phenolic Compounds; Retinol:

Physiology; Selenium: Physiology

Further Reading

Ciba Foundation (1983) Biology of Vitamin E. London:

Pitman Books.

Combs GF (1998) The Vitamins: Fundamental Aspects in

Nutrition and Health. London: Academic Press.

Diplock A, Machlin L, Packer L and Pryor W (eds) (1989)

Vitamin E: Biochemistry and Health Implications. New

York: New York Academy of Sciences.

Frei B (ed.) (1994) Natural Antioxidants in Human Health

and Disease. San Diego, CA: Academic Press.

Hayaishi O and Mino M (eds) (1987) Clinical and Nutri-

tional Aspects of Vitamin E. Amsterdam: Elsevier.

Lubin B and Machlin L (eds) (1982) Vitamin E: Biochem-

ical, Hematological and Clinical Aspects. New York:

New York Academy of Sciences.

Machlin L (ed.) (1980) Vitamin E: A Comprehensive

Treatise. New York: Marcel Dekker.

Machlin LJ (ed.) (1991) Handbook of Vitamins. New York:

Marcel Dekker.

Packer L and Fuchs J (eds) (1993) Vitamin E in Health and

Disease. New York: Marcel Decker.

Pryor WA (1984) Free radicals in autoxidation and aging.

In: Armstrong D, Sohal RS, Cutler RG and Slater TF

(eds) Free Radicals in Molecular Biology, Aging and

Disease, pp. 193–201. New York: Raven Press.

Pryor WA (2000) Vitamin E and heart disease: Biochemistry

to human trials. Free Radical Biology & Medicine 28(1):

141–164.

Sauberlich HE and Machlin LJ (1992) Beyond Deficiency:

New Views on the Function and Health Effects of

Vitamins. New York: New York Academy of Sciences.

Tocotrienols See Tocopherols: Properties and Determination; Physiology

TOMATOES

B McGlasson, University of Western Sydney,

Penrith South DC, New South Wales, Australia

Introduction

0001 Tomatoes are an integral part of human diet world-

wide and production in 1999 was estimated at 91.7 

t. The tomato is a member of the Solanaceae – the

potato family. Tomato fruit, often described as a

vegetable fruit, is consumed fresh and in several pro-

cessed forms, including dried, pure

e, paste, ketchup,

sauce, soup, and canned whole-peeled. It is a watery

fruit containing 5–7% dry matter. It contains rela-

tively low concentrations of vitamin C, provitamin

A and minerals, compared to other commercially

important fruit species. However, because it is con-

sumed in quite large quantities, it is a major source

of these nutrients, especially in western societies.

There is renewed interest in the dietary properties of

tomato because it is the principal source of lycopene.

5800 TOMATOES

Lycopene is an antioxidant that is thought to be a

factor in the prevention of several chronic diseases,

including cancer and coronary heart disease. There is

evidence that lycopene is absorbed more efficiently

from processed tomato products compared to fresh

fruit. Genetically, the tomato is probably the most

highly defined and best mapped of all domesticated

soft-fruited crop plants. An enormous range of com-

mercial cultivars is available. There are cultivars bred

specifically for the fresh market that produce fruit

ranging in diameter from 15 mm to more than

90 mm, and processing types especially suited to

machine harvesting. All modern cultivars incorporate

multiple resistances to many pathogenic diseases and

pests.

Genetic Origins

0002 It is thought that the original domestication of the

tomato took place in Mexico and probably resulted

from selection over many generations from the ances-

tral form now known as Lycopersicon esculentum

var. cerasiforme, the cherry tomato. The genetic

center of origin of the tomato is believed to be the

western coastal strip of South America which extends

about 80 km inland and to altitudes of 3000 m. This

coastal strip extends from Ecuador to Peru and Chile

(latitude 0–25



S). Geneticists, notably the late Pro-

fessor Charles Rick of the University of California,

Davis, USA, have collected, classified, and docu-

mented many wild species of Lycopersicon and

other closely related genera. Some of these wild

species have been sources of resistance against a

wide range of diseases and pests, which have been

incorporated into modern cultivars. The wild species

include ecotypes that are adapted to low and high

growth temperatures, arid and humid environments,

and saline soils. However, some of these species

cannot easily be crossed with the domesticated

tomato (Table 1).

Genetic Mutants

0003Many natural and some induced mutants have been

documented, and some of these have been shown to

have commercial application. The tomato is typically

diploid, comprising 12 pairs of chromosomes (2n ¼

24). Excellent gene linkage maps have been developed

by classical plant breeding using defined genetic

marker stocks and isozyme markers. Molecular geno-

mic maps have been derived from restriction length

polymorphism analysis. Extensive physiological and

biochemical research, especially on the ripening-

impaired mutants rin, nor and Nr, during the 1970s,

provided the basis and the stimulus for the consider-

able advances in knowledge of the molecular biology

of the genetic regulation of fruit ripening and cell wall

softening that have been achieved during the last 20

years.

0004Since about 1980, the tomato has become a favor-

ite model system for gene technologists. The tomato is

an excellent experimental subject because it is easy to

tbl0001 Table 1 The species of Lycopersic on

Subgroup Characteristics

esculentum complex Hybridize readily

esculentum Normal tomato

esculentum var. cerasiforme (cherry tomato) Adapted to wet, tropical conditions

cheesemanii Yellow to orange fruit, diameter 6–9 mm

Source of jointless gene, aid to machine harvesting

hirsutum and its forms Green fruit with purplish stripes, diameter 1.5–2.5 cm Source of cold tolerance, and

resistance to several insect pests, root knot nematodes, some fungi and bacteria

parviflorum and chmielewski Yellow-green fruit, diameter 1–1.4 cm

chmielewski is a source of high solids

pennellii Small, green fruit

Initially classified in the genus

Solanum

Resistant to drought and some sucking insects

pimpinellifolium (currant tomato) Resistant to Fusarium and bacterial speck

peruvianum complex Do not hybridize readily with esculentum complex

chilense Green fruit, diameter 1–2 cm Source of the Tm2

gene

Resistance to the tobacco mosaic virus

peruvianum Green fruit, diameter 1–3 cm Very diverse species which includes coastal and

mountain races

Potentially valuable source of genes for resistance to many fungal and viral diseases

and insect pests

Adapted from Taylor IB (1986) Biosystematics of the tomato. In: Atherton JG and Rudich J (eds) The Tomato Crop, pp. 1–34. London: Chapman and Hall.

TOMATOES 5801

transform and regenerate in tissue culture. The plants

can be grown easily under protected growing condi-

tions year-round and fruit mature within about 3

months from sowing. Genetic transformation and

gene technology have been used to reduce the amounts

of cell wall-softening enzymes and ethylene production

to slow ripening, and to introduce genes for resistance

to glyphosate herbicide. (See Pesticides and Herbi-

cides: Types, Uses, and Determination of Herbicides.)

Fruit Composition

0005 Botanically, the tomato fruit is classified as a berry. The

fruit can be divided into skin, pericarp, and locular

contents. Jelly-like parenchyma cells which fill the

locular cavities surround the relatively large number

of hard, small seeds borne per fruit. Small-fruited cul-

tivars, such as those grown for the fresh trade in cool

climates or for machine harvesting for processing,

generally contain few locules, whereas large-fruited

cultivars tend to be multilocular. The locular juices

contain higher concentrations of organic acids than

the outer locular walls but the reverse is true for the

reducing sugars. Blended tissue has a pH of 4.0–4.7.

0006 Table 2 shows data for a fresh-market cultivar

representative of modern determinate types grown

outdoors in countries such as the USA and Australia.

There is considerable variability in the concentration

of nutrients, which are affected by genetic differences

and especially by irradiance, growth temperatures,

irrigation, and salinity. The fresh tomato fruit

contains a comparatively low concentration of dry

matter. The edible portion of the fruit comprises

99% of the fresh weight. The inedible portion

includes the seeds and the skin which are removed

during the production of the pure

e, paste, and soup.

0007 Because of the relatively large amounts of fresh and

processed products consumed, the tomato is an im-

portant source of vitamin C, b-carotene, and minerals

such as potassium in the human diet. Standard

commercial cultivars contain predominantly the re-

ducing sugars glucose and fructose. Although sucrose

is the main metabolite translocated in the tomato

plant, acid invertase, present in the vacuoles of the

fruit cells, insures that all but trace amounts of su-

crose are hydrolyzed. However, some species other

than L. esculentum are known to accumulate a sig-

nificant proportion of sucrose. L. chmielewski,

which lacks vacuolar acid invertase, has been crossed

with L. esculentum to produce high-sugar lines.

However, breeding of commercial cultivars with

high soluble solids has proven difficult because

sugar metabolism appears to be controlled by several

quantitative traits. Green tomatoes store a small

quantity of starch, which reaches a maximum when

the fruit are about half-grown. Starch is hydrolyzed

rapidly once ripening begins.

0008Detailed information is available on the concen-

trations of (1) organic acids, in addition to citric and

malic, that are involved in the tricarboxylic acid cycle

of aerobic respiration, and (2) the enzymes which

catalyze this cycle and related metabolic pathways.

The principal phenolic compound found in tomato

fruit is chlorogenic acid. Alkaloid glycosides are

common throughout the Solanaceae. These com-

pounds taste bitter. a-Tomatine is commonly found

in the tomato. Quite low concentrations are found in

unripe fruit and these decrease by about half during

ripening. In contrast to solanine in potato, a-tomatine

appears to pose a low risk of toxicity to people.

0009Detailed information on the free amino acid

composition of fresh fruit is available. Glutamic

acid is the major component. Glutamic, aspartic,

and g-aminobutyric acids, plus glutamine, comprise

tbl0002Table 2 Nutrient composition of tomato

Nutrient Content (per100 g edible portion

)

Energy

(kJ) 56

Gross constituents (g)

Water 94.7

Protein

1.0

Fat 0.1

Dietary fiber 1.6

Carbohydrates (g)

Glucose 0.9

Fructose 1.0

Sucrose 0

Starch 0

Organic acids (g)

Citric 0.43

Malic 0.08

Oxalic 0

Other 0

Vitamins (mg)

Vitamin C 18

Thiamin 0.04

Riboflavin 0.02

Nicotinic acid 0.7

b-Carotene (equivalent)

0.34

Minerals (mg)

Potassium 200

Sodium 6

Calcium 8

Magnesium 10

Iron 0.3

Zinc 0.2

Edible portion is 99% of the fruit.

Energy was calculated as g((protein  17) þ (fat  37) þ

(monosaccharides  16) þ (disaccharides  16.8) þ (starch  17.6) þ

(organic acids  10)).

Protein was calculated as %N  6.25.

b-Carotene was calculated as mg b-carotene þ 0.5  mg (b-carotene þ

cryptoxanthin).

Adapted from Wills RBH (1987) Composition of Australian fresh fruit and

vegetables. Food Technology in Australia 39: 523–526 with permission.

5802 TOMATOES

about 80% of the total free amino-nitrogen-contain-

ing compounds in fruit. These compounds contribute

to tomato taste.

Pigments

0010 Traditionally, tomatoes are thought to be red, but

pink, orange, yellow, white and even black strains

occur naturally. The typical red color is caused by

Lycopene, which is usually the predominant pigment.

Lycopene is a carotenoid pigment, but it is not con-

verted to vitamin A when consumed by people. Lyco-

pene is more concentrated in the outer fruit wall

(pericarp) than in the internal locular tissue. The

distribution of b-carotene follows a reverse pattern.

0011 During ripening, the chloroplasts are transformed

into chromoplasts. Chlorophyll disappears com-

pletely within about 4 days at 20



C, and lycopene

reaches a maximum by about 6 days.

0012 There are relatively small increases in the carotene

pigments during ripening. Lycopene synthesis is opti-

mum at 13–22



C. Lycopene concentrations range

from 2 to 7 mg per 100 g edible portion. At higher

temperatures, synthesis is inhibited and is completely

stopped above 30



C. The fruit color is then yellow.

There are many mutants that affect pigment synthe-

sis, including some which retain chlorophyll when the

fruit is ripe, e.g., Evergreen and Greenflesh. The crim-

son gene (og

) increases lycopene concentrations at

the expense of b-carotene, and the high-pigment gene

(hp) increases total carotenoids, thus enhancing the

color of ripe fruit. (See Carotenoids: Occurrence,

Properties, and Determination.)

Production Systems

0013 Domesticated cultivars display two kinds of growth

habit – indeterminate and determinate. Indeterminate

or vine types typically continue to grow, flower, and

set fruit as long as the plants remain healthy, and

growing conditions are suitable. These types require

support. They may be trained on stakes or trellises, or

suspended on twine from overhead wires. In general,

a considerable amount of labor is required to prune

and train the plants. Indeterminate cultivars for the

fresh market are used in commercial glass or plastic

houses, and yields of 250 t ha

1

have been reported in

a full growing season in the UK. A high proportion of

fresh market tomatoes produced in glass or plastic

houses are grown hydroponically. Plants are either

grown in a solid medium with the nutrients being

allowed to run to waste or in a recirculating aqueous

medium. This system enables the mineral compos-

ition and concentration of salts in the medium to be

precisely controlled.

0014Most of the tomatoes commercially grown out-

doors have a determinate habit. This habit is con-

trolled by sp, the self-pruning gene. The plants have

a compact bush habit and set fruit over a brief period

of about 1 month. Determinate fresh-market toma-

toes may be grown as a ground crop in dry climates or

may be staked or trellised in regions with relatively

high rainfall. Production on stakes or trellises facili-

tates spraying to control foliage diseases and pests.

Little or no pruning or training is required. Recently

some outdoor growers have replaced determinate cul-

tivars with indeterminate or ’gourmet’ cultivars. This

has been made possible by the advent of improved

cultivars incorporating the rin gene. Fruit of these

cultivars remain firm when ripe and can be allowed

to ripen to an advanced color stage (vine-ripe) before

they are harvested. Gourmet tomatoes are expected

to have better flavor because they are harvested when

ripening is well underway and they may have higher

sugar levels compared to determinate cultivars

because they have higher leaf–fruit ratios. Tomatoes

for processing are universally grown as a ground crop

on raised beds. Irrigation by trickle is well suited to

outdoor tomato production. Water and nutrients can

be applied efficiently to the root zone according to the

demands of the crop.

0015Most fresh-market tomatoes are harvested by

hand. Fruit at early color stages is harvested at

frequent intervals from trellised plants. Some harvest

machines have been developed but they have not been

widely adopted because of the risk of excessive fruit

damage and limited cost savings. Mechanical harvest

aids have been developed and are more widely used.

These machines transport pickers along the rows and

convey the harvested fruit into bulk bins carried on

the machine. On-ground plantings are harvested up

to four times, and a high proportion of the fruit is

harvested green. Green fruit is usually held in

controlled-temperature ripening rooms with the con-

tinuous addition of ethylene to reduce the time to

the commencement of ripening.

0016In countries where labor costs are high, processing

tomatoes are harvested by machine when most of the

fruit is fully ripe. These machines cut the bushes at

ground level, elevate the bushes into a shaker section

to separate the fruit from the bushes, and then present

the fruit either to an automatic color sorter, or to

people riding on the machine who remove clods of

soil and unripe or overripe fruit by hand. The sorted

fruit is accumulated in large bins, traveling with the

harvest machines, and then carried by road to the

factory.

0017The development of the cultivars and the machine

harvesters was a revolutionary step pioneered by the

late GC Hanna of the University of California, Davis,

TOMATOES 5803

USA, in the 1950s. Cultivars with the following char-

acteristics are required: plants are compact; fruit is set

and ripened over a confined period; fruit stores well

on the bush when ripe; fruit detaches easily and with-

stands the physical rigours of machine harvesting.

Processors pay growers on the basis of total solids

per tonne of fruit. The incorporation of the j

jointless

gene in modern cultivars enables detachment of the

fruit free from calyces and pedicels.

Fruit Growth and Ripening

0018 Tomatoes take 6–7 weeks from flowering, depending

on temperature, to reach full size. Cell division con-

tinues for about 2 weeks after flowering, but the bulk

of the increase in fruit size is the result of cell expan-

sion. In normal cultivars, the first appearance of red

or pink color at the blossom end of the fruit signals

the completion of growth and the beginning of

ripening. Laboratory studies with fruit harvested at

a mature green stage have shown that ripening

actually begins about 2 days before the external

color change. An early indication of ripening is a

small increase in ethylene production, which can be

measured with a sensitive gas chromatograph.

0019 The tomato is a climacteric fruit in which ripening

is accompanied by an increase in both respiration and

ethylene production (Figure 1). A natural increase in

endogenous ethylene production initiates ripening

and regulates or integrates many of the biochemical

events in ripening. The disappearance of starch, the

destruction of chlorophyll, and the synthesis of

lycopene, aroma, and polygalacturonase, a cell wall-

hydrolyzing enzyme, are highly integrated with the

changes in respiration and ethylene production.

These events either do not occur or are diminished

and protracted in the ripening-impaired mutants rin,

nor, Nr, and alcobaca. Fruit produced by F

hybrids

of mutant lines and normal cultivars has intermediate

traits. The fruit ripens but may not develop accept-

able color and flavor. The severity of these effects

depends on the mutant gene and the genotypic back-

ground of the parent lines. However, many cultivars

incorporating rin are being grown for the fresh

market. The benefits include a slower rate of ripening

and softening, so that the fruit has a longer shelf-life.

Incorporation of ripening-impaired mutant genes in

cultivars for processing appears to have no benefits.

Storage Temperatures

0020 Tomatoes are chilling-sensitive. Storage of green fruit

at less than 10



C for more than a few days inhibits

ripening. Severely chilled fruit will not ripen when

returned to normal ripening temperatures. In less

severely chilled fruit, ripening may be retarded but

the fruit is much more susceptible to pathogenic

decay. The recommended storage and transport

temperature for tomatoes is 12



C. Carefully handled

fruit, harvested at an early color stage, may be stored

for up to 3 weeks. Green and partly colored fruit

should be ripened at 13–22



C to the desired stage.

Ripe fruit may be stored satisfactorily for about 4

days at about 5



C. (See Chilled Storage: Principles.)

Sensory Quality

0021Many countries have quality standards for fresh-

market tomatoes. The criteria include size, color,

and freedom from blemishes, decay, and insect pests.

There are no dramatic changes in gross components,

such as sugars and acidity, that enable the establish-

ment of objective quality standards. The pressure of

competition, on growers in countries such as the USA

and Australia, to reduce production costs led to the

adoption of large-scale production of determinate

cultivars which soften slowly and have a long shelf-

life (Figures 2–5). Fruit of the first introductions of

these new cultivars sometimes retained a tough

woody texture, even when fully colored. Even if to-

matoes have optimum levels of sugars and acids, a

tough texture greatly reduces their acceptability to

consumers. The universal criticism of fresh tomatoes

is tough texture and lack of flavor. This has led to an

increased production of cherry and gourmet cultivars

that are harvested at advanced color stages and

generally sold at a premium price compared to

standard cultivars. (See Sensory Evaluation: Practical

Considerations.)

03 6 912

Starch

Ethylene

PG1

PG2

Lycopene

Softening

Chlorophyll

Respiration

First color

(breaker)

Day of ripening

fig0001Figure 1 Changes in metabolism and composition during

ripening. PG, endopolygalacturonase. Reproduced from Toma-

toes, Encyclopaedia of Food Science, Food Technology and

Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993,

Academic Press.

5804 TOMATOES

0022 Flavor of tomatoes is a product of the interaction of

taste and aroma. Research has shown that taste is the

more important factor. Taste principally involves an

interaction between total sugars and titratable acid-

ity. Most modern cultivars appear to have adequate

acidity, but sugar levels are frequently too low. Other

taste enhancers include the amino-nitrogen com-

pounds, especially glutamate, and other unidentified

compounds which contribute to ’tang.’ Measurement

of total soluble solids by refractometer gives a rough

indication of the level of taste components because

sugars comprise more than half of the soluble solids.

However, refractometric values are often not well

correlated with taste. Incorporating titratable acidity

(mmol per 100 ml of juice) in a multiple regression

equation improves the correlation, but it remains

fig0002 Figure 2 (see color plate 134) Harvest aid for fresh market tomatoes. Fruit are sorted and accumulated on a hopper on the machine.

The fruit are periodically transferred to half-tonne bins for transport to the packing house.

fig0003 Figure 3 (see color plate 135) Mechanical harvesting of processing tomatoes. Plants are cut at ground level, and elevated on to a

shaker which separates the fruit from the plants.

TOMATOES 5805

unsatisfactory. Other components that contribute to

taste remain to be identified. Improved analytical

procedures allow direct measurements of the major

taste components but are too expensive for routine

use.

0023 Despite these limitations, practical observations

show that raising total soluble solids from the

common level of 4.0–4.5% to over 6.0% gives a

highly significant improvement in flavor scores. Cul-

tivars with the genetic potential to accumulate high

sugar concentrations and, consequently, higher total

soluble solids are essential, but it is necessary to opti-

mize growing conditions to enable full expression of

this genetic potential. Research in several countries

has examined the feasibility of raising the electrical

conductivity of the irrigation solution, especially

fig0004 Figure 4 In Australia fresh-market tomatoes are mostly volume-filled into 10-kg cartons for marketing.

fig0005 Figure 5 (see color plate 136) Tray packs of glasshouse tomatoes, harvested once ripe.

5806 TOMATOES

during the fruit growth phase, either by the addition

of salts, such as potassium chloride, in outdoor pro-

duction, or by the addition of sodium chloride to

hydroponic systems in glass or plastic houses. Ultim-

ately the best way to increase sugar levels will be by

plant breeding but this goal remains elusive because

the regulation of sugar concentrations involves

several quantitative genetic traits.

0024 The volatile compounds that contribute to aroma

have received much attention. Green tomato fruit

produces low levels of volatile compounds, but more

than 400 volatile compounds have been detected in

ripe fruit. While a considerable number of these com-

pounds contribute to tomato aroma, it is probable

that only about 15 compounds have an influence on

flavor quality. They include alcohols, aldehydes,

carbonyls, and sulfur compounds. A few compounds

may produce off-flavors but most evince a positive

response. Tomatoes that have high levels of soluble

solids tend to produce larger amounts of aroma.

0025 The flavor of tomatoes is changed considerably by

processing. An essential step in processing is denatur-

ation of enzymes and sterilization by heating to about

100



C. Although the major taste components remain,

volatile aroma compounds associated with fresh fruit

are lost. Additional flavor compounds are generated

nonenzymatically during processing. A large propor-

tion of processing tomatoes are concentrated in the

form of paste which contains a minimum of 24%

total solids. Paste can be stored for long periods

before conversion to sauce, ketchup, and soup.

Cell-Wall Softening

0026 Cell walls comprise a-cellulose, pectin, hemicellulose,

and some protein. During ripening, the fruit softens,

and softening is accompanied by changes in the

pectin and hemicelluloses. Cellulose remains intact.

Several hydrolases that attack cell walls are present

in tomato fruit. Endopolygalacturonase (PG) has

received particular attention and it is produced de

novo during ripening (Figure 1). However, the extent

of softening is not closely correlated with the amount

of enzyme produced, although cultivars with firm

fruit generally accumulate less enzyme than do culti-

vars with softer fruit. It appears that there are several

factors that limit the action of PG in vivo. These

factors may include the extent of methyl esterification

of polygalacturonate, the amount of calcium bound

to pectin, the distribution of PG in the cell walls, and

the activity of other hydrolytic enzymes that act on

cell wall components.

0027 There are three closely related isozyme forms: PG1,

PG2A and PG2B. PG1 comprises one subunit of

PG2A or PG2B, plus a nonenzymatically active

ancillary subunit. PG1 is the first form produced

during ripening and may be the active form of the

enzyme within the fruit. It has been suggested that the

ancillary subunit plays a key role in binding the active

PG2A or PG2B to its substrate. Normal cultivars

continue to accumulate relatively large amounts of

the small PG forms for several days after the fruit

become fully ripe. Fruit of rin and nor produce little

or no PG and soften very slowly. The amount of PG

present in ripe fruit is especially important to proces-

sors. An important factor determining the yield of

products such as ketchup, which is manufactured

per tonne of raw fruit, is the viscosity (consistency)

of the product, and the extent of depolymerization

of pectin has a major influence on viscosity. When

tomato tissue is comminuted, the vacuolar acids

(principally citric acid) chelate calcium bound to

pectin. The pH of the blended tissue is optimum for

PG activity, and the protective calcium is removed

from the pectins, which are rapidly solubilized unless

the enzyme is denatured. A key step in paste manu-

facture is the ’hotbreak.’ The tomato tissue is heated

as rapidly as possible to 100



C because PG remains

highly active up to about 80



C. Tomatoes trans-

formed with DNA for PG in an antisense configur-

ation have greatly reduced PG activity and yield

products with higher viscosity.

Future Developments

0028Tomato breeders, in cooperation with plant physiolo-

gists, biochemists, and gene technologists, will

continue to develop the commercial tomato. The

tomato is an excellent model system for research on

the genetic regulation of the biochemical events asso-

ciated with ripening. Large catalogued international

collections of wild tomato species are being main-

tained and researched. There is also enormous poten-

tial to improve resistance to pests and diseases, yield

of solids for processing, and sensory quality. If further

medical research confirms a role for lycopene in

reducing chronic diseases in people, increased con-

sumption of fresh and processed tomatoes selected

for high levels of lycopene can be expected. A large

amount of germplasm remains to be elucidated in

species such as L. peruvianum, in which there are

significant compatibility barriers with L. esculentum.

Molecular biology provides the tools for overcoming

these barriers.

See also: Carotenoids: Occurrence, Properties, and

Determination; Chilled Storage: Principles; Flavor

(Flavour) Compounds: Structures and Characteristics;

Fruits of Temperate Climates: Commercial and Dietary

Importance; Ripening of Fruit

TOMATOES 5807