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

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this disease, except to avoid olive varieties which are

susceptible to cold injury and to this organism, or by

quarantine. The root-knot, citrus and reniform nema-

todes can be suppressed by less irrigation frequency

and predacious nematode in olive.

0025 Peacock spot (Cycloconium oleaginum) and Verti-

cillium rot (Verticillium alboatrum) are the fungal

diseases of olive, occasionally causing heavy losses

in production and fruit quality. The problem is ser-

ious in humid areas, but can be controlled by various

fungicides such as Bordeaux mixture or lime sulfur.

Soil solarization of individual diseased trees, when

combined with chemical weed control, is effective

in controlling Verticillium wilt in olive trees. (See

Fungicides.)

Harvesting

0026 Traditionally, the olives are harvested by hand into a

receptacle in the autumn or early winter at the appro-

priate green-yellow stage. Olives for oil extraction are

collected by beating the branches of the tree with long

poles so that the fruit falls on to cloth sheets laid on the

ground. This is done after the fruit has turned black.

Beating of the trees tends to promote the spread of

olive-knot disease through injury to the fruit and tree.

In Greece and elsewhere, the scarcity of farm labor has

resulted in the introduction of plastic nets for collec-

tion of naturally falling fruit. To obtain better-quality

oil, the olives should not be left on the nets for more

than 15 days. Chemical sprays to induce fruit abscis-

sion are used to aid the mechanical harvesting of

olives. Mechanical shakers with stationary cloth,

canvas-covered, or mechanized collecting frames are

used to a limited extent, mostly for harvesting olives

for oil. A ‘beating-down’ machine for mechanical

picking of olives without damaging the trees has

been tested in Italy. The machine is particularly suit-

able for high-yielding trees with well-developed foli-

age, and for old trees, for which vibrating picking

machines are not recommended.

0027 For high-quality table olives, where uniformly ripe

fruit is a requisite, the fruit is handpicked using

ladders. The finest and most highly prized oil is

made from olives picked just before they begin to

soften and darken. The time of maturity may vary

within the cultivated area, among the trees within the

same grove, or within the fruits on the same tree.

0028 The measurement of Brix values allows for rapid,

inexpensive estimation of the oil content of olive

fruits at the harvest site and is useful in determining

optimum harvest time. The respiration rate is another

new ripening index for drupes and the harvesting of

olives is suggested to be conducted during the climac-

teric phase of the fruits.

Postharvest Handling and Storage of Fruit

0029Ideally, the extraction of oil should follow the harvest-

ing of the olives without delay. As this is impractical,

the olives must be stored for a period before process-

ing. During storage, several chemical and biochemical

changes may occur that lead to the deterioration of

the oil. The most serious damage is caused by enzymes

indigenous to the olive tissue and those produced by

the microflora on it. The pathogenic and saprophytic

fungi isolated from stored olives, except Saccharo-

myces spp., increase the acidity of the extracted oil,

and no aflatoxins are reported to be detected in

samples infected by Aspergillus flavus. As the olive

tissue respires, heat is produced, which, if not effect-

ively dissipated, accelerates the enzymatic actions.

Lipolysis, lipid oxidation, and other undesirable reac-

tions occur, leading to a lowering of the oil quality.

0030To minimize the effect of these activities, the olives

are stored in 25-cm-thick layers in cool buildings. The

use of perforated trays for spreading and stacking

saves space. The olives are better stored under water

in tanks containing mild preservatives, such as 3%

salt or 0.03% citric acid and 3% salt, or 2% metabi-

sulfite. Aerobic storage with a low concentration of

acetic acid gives stable color and texture even after 3

months; bulk storage can be done in 50% ethanol.

Aerobic storage and the maintenance of a CO

con-

centration of 10–15 mg per 100 ml throughout the

storage period eliminates shriveling. Fruit maturation

is delayed by refrigeration compared with storage at

room temperature and the firmness of the fruits

stored at room temperature decreases much more

rapidly. The sensory and chemical qualities of the oil

produced from fruits stored at 5



C can be maintained

for fruit storage durations up to 45 days, but with

storage at 8



C, these qualities are maintained for

storage durations up to 15 days. For fruits stored at

room temperature, oil quality deteriorates after 7

days of fruit storage. The processing period of oil

mills can be extended by freeze-storing of olives at

18



C for 90 days. This also helps to increase oil

extraction. (See Preservation of Food.)

Processing and Utilization

0031Oil is extracted traditionally by simple equipment

involving crushing, pressing, and separation of oil

from the liquor. It is now being replaced by modern

mechanical equipment of widely varying designs,

either as separate units or combined as a single-stage

process. The olive oil recovery processes have been

patented in many countries.

0032The oil obtained from sound fruit by pressing with-

out further treatment is called virgin olive oil. After

4264 OLIVES

the first pressing, the residual pulp is still rich in oil

and is usually recrushed and repressed with or with-

out the addition of warm water. Oil obtained from

the second pressing tends to have a more intense color

and higher acid content, as well as a weaker aroma.

This latter oil, together with inferior virgin oil, is

further subjected to refining treatments such as neu-

tralization, deodorization, bleaching, and winteriza-

tion; this helps in removing acid, color, and odor. The

oil so obtained is called refined olive oil, which is

largely used for mixing with the first extraction to

produce edible grades. Refining procedures decon-

taminate oil containing fenthion residues. The

remaining press cake still retains about 4–5% oil,

but more than 50% of the total oil in the cake can

be obtained in a single step by direct extraction in a

continuous centrifugation system, after adding about

I%Na

by weight of cake. It is then necessary to

dry and further deoil the cake for better stability

towards autoxidation. The dried cake is extracted

with hexane and the resulting ‘sulfur oil’ is repeatedly

rectified. The extraction with hexane results in an oil

with higher acidity and moisture content, but a lower

percentage of impurities.

0033 Olive oil has been classified by the United Nations

Conference on Olive Oil, held in Geneva in 1961, into

four groups according to the method of preparation

and acid content:

0034 Virgin olive oil. Oil extracted by pressing, free of

admixtures, called ‘extra’ when the oleic acid does

not exceed 1 g per 100 g, and ‘fine’ if the acid

content does not exceed 1.5 g per 100 g and the

flavor is perfect; ‘ordinary’ olive oil may contain

acid up to 3.0 g per 100 g and have a slight off-

flavor; if the oil has a definite off-flavor it is classi-

fied as ‘lampante.’

0035 Refined olive oil. This oil may be called ‘pure’ when

it is refined from virgin oil, and ‘second-quality’

when it is refined from solvent-extracted oil.

0036 Blended olive oil. Blended oil can be called ‘pure’

when it consists of a blend of virgin and refined

oil, and ‘blended’ when it contains a blend of

virgin and second-quality refined oil.

0037 Industrial oils. These oils are obtained by extrac-

tion of olive residue with solvents.

Storage and Packing of Oil

0038 Olive oil undergoes deterioration during storage. It is

important to store the oil extracted from olives of

different varieties separately. The storage drums and

tanks should be of inert material, impermeable to oil,

and lined with epoxy resins, enameled tiles, or glass;

and the oil should be stored at a constant temperature

of about 15



0039Olive oil is packed in several types of retail contain-

ers including bottles made of glass, polyvinyl chloride

(PVC) or polyethylene, tin-plated cans, and in tetra-

briks. The containers should provide protection of the

oil from light, and leave a minimum volume of head

space. Packing of oil under vacuum and inert gases is

also recommended.

Table Olives

0040Many different methods of converting olives into

olive products exist in different olive-growing coun-

tries. The traditional method of preparing table olives

by removing their bitterness through alkaline treat-

ment, which grew up in the environs of Seville, Spain,

is rapidly being replaced by technically advanced

methods. Production has been increased and costs

have been cut, and the industry has remained highly

competitive. Sevillian green olives are the predomin-

ant type used (70%), followed by oxidation-darkened

olives (25%), and untreated black olives (5%). The

chief innovations in the last 25 years have been mass

fermentation, development of conservation in an aer-

obic medium, development of automatic stoning and

stuffing machines, developments of pastes, fixing of

pack conditions and treatments, and general mechan-

ization of all handling, packing, and storage of the

end product.

0041The major commercial methods are as follows.

Green Fermented Olives (Spanish Method)

0042The fruits are picked when they are still firm and light

green in color. The principal varieties processed in

Spain are the Sevillano and Manzanillo. The fruit is

covered in large containers with 1.8% lye solution

and kept at 28



C for 4–8 h. The treated olives are

frequently washed with water for 24–36 h.

0043The washed fruits are then covered with brine in

fermentation tanks. This changes the color of the

olives from deep green to the typical ‘olive-green’

shade. The product, on attaining equilibrium, con-

tains 6–8% NaCl. Spices may now be added to the

fermented olives according to consumer preference.

Closed containers help prevent the development of

surface yeasts and molds.

0044The fermented olives, before marketing, are graded

according to shape, size, and color. Sometimes the

olives are pitted and stuffed with pimentos, onions,

almonds, anchovies, or other products. The fer-

mented brine is filtered (or replaced by fresh brine)

for covering the olives in bottles or cans prior to

pasteurization.

0045Olives at different stages of ripeness suffer major

losses during the pitting process; green unripe olives

are excessively firm,and the cell wall of changing-color

OLIVES 4265

olives is too disorganized to give good industrial

results. The treatment with high concentration of lye

and its 100% penetration into the flesh, irrespective

of fruit size, results in significant textural decline and

changes in the composition of polysaccharides that

are most important to the cell wall structure.

0046 Among the major spoilage organisms, zapatera – a

butyric acid producer – is important.

Canned Ripe Olives (American Style)

0047 The principal commercial varieties packed are the

Ascolano, Manzanillo, Mission, and Sevillano.

Straw-yellow to cherry-red olives are graded

according to color and size to insure uniform lye

penetration. The fruits are placed in 5.8% brine for

6 weeks or longer in large wooden or concrete tanks,

and the brine concentration is gradually raised to

10%. The fruit is then transferred into shallow vats

and treated 4–8 times with gradually decreasing

strength of lye (3.0–0.5% NaOH). Each treatment is

followed by exposure to air for 1–5 days, either with

frequent turning in the presence of air or by bubbling

air through the immersed fruit. The olives are then

leached by frequent changes of water for 5–7 days

until all traces of NaOH are removed. (See Canning:

Principles.)

0048 The washed olives are pasteurized, cured in 2–3%

NaCl for 2–6 days, graded, and packed into enameled

cans covered with 2.5–3.5% NaCl solution, sealed

and sterilized at 116



C for 1 h. Pitting and stuffing

of olives are often practiced.

Black (Naturally Ripe) Olives (Greek Method)

0049 The Greek black olive industry uses naturally ripe,

fully matured and dark-purple fruits of mainly Cala-

mata, Conservolea, andMagaritici varieties. The fruits

are covered with brine, and the concentration is grad-

ually increased to prevent shriveling. The covering

brine concentration is kept at 10% NaCl during winter

and increased to 15–16% during summer to prevent

spoilage. The bitterness is lost within 3–6 months, and

the lactic acid formed during fermentation does not

exceed 0.5%. The dark-purple anthocyanins of the

olives turn light red during fermentation.

0050 The olives, while being prepared for marketing, are

exposed to air until the dark color is regained. The

fruits are carefully sorted and graded according to

international standards, and repacked in fresh brine

containing about 8% NaCl and 0.5–0.75% acetic

acid.

0051 Antifungal treatment trials of ‘Greek-style’ black

olives indicate that sorbic acid 0.075% is the most

efficient preservative, followed by 0.075% benzoic

acid and 0.032% calcium proprionate. In the olives

inoculated with Aspergillus parasiticus, the preserva-

tives totally inhibit aflatoxin formation, while the

toxin can be detected at low concentrations in all

controls.

0052The spoilage organisms encountered are zapatera,

galazoma, and film-forming pectolytic organisms

which disintegrate the ‘meat’ of the fruit. This last

problem can be prevented by means of a thin layer of

paraffin oil on the surface of the brine.

Utilization of Byproducts

Olive Cake or Meal

0053The press residue may be used as a fuel,manure, animal

feed, soil conditioner, fiber (cellulose and lignin) for

food use, or in single-cell protein production. Sludge

obtained from olive-processing plants enriched with

nitrogen fertilizer can be applied to olive orchards

growing on alkaline alluvial soils and as manure for

maize. The spent form of olive oil cake can be incorp-

orated into the diets of rabbits at 30% level. Mixtures

of organic acids of low molecular weight and fatty

acids can be recovered from the waste gases produced

during drying of cake. Pleurotus eryngii can be grown

on a medium containing olive husks.

Olive Stones

0054The olive stones find use in molded products, plastics,

and furfural manufacture. Activated carbon produced

from olive stones has a high adsorption capacity

against lead.

Olive Vegetation Water

0055Tocopherols, flavor compounds, antioxidants, and

anthocyanins can be recovered from olive vegetation

water. Olive oil deodorization distillate contains squa-

lene in a concentration range of 10–30%. Squalene in

high purity with 90% yield can be recovered by super-

critical carbon dioxide extraction. The application of

100–200 m

waste waters from olive oil-processing

plants per hectare, 45 days after sowing, markedly

increased barley and sunflower yields and was

reported to have no adverse effects on soil microor-

ganisms. (See Antioxidants: Natural Antioxidants;

Flavor (Flavour) Compounds: Structures and Charac-

teristics; Tocopherols: Properties and Determination.)

Nutritional and Health Aspects

0056The use of olive oil, whether in medicine or as

source of nutrition, has its roots in ancient history.

During the last few decades, renewed interest has

been generated in the nutritional and health aspects

of the olive. The oil is used largely for culinary

4266 OLIVES

purposes, in wool combing, the manufacture of toilet

preparations, and in the pharmaceutical industry.

005 7 Besides its physiological advantage in being rapidly

and completely digested, the oil also has clinical

importance e.g., antiulcer activity, effectiveness in

combating gallbladder disease, and in lowering

plasma cholesterol level. Omega-6-polyunsaturated

fattyacidinoliveoilmayexacerbateadjuvant-induced

arthritis. Olive pollen extract can induce an asthmatic

response. (See Fatty Acids: Dietary Importance.)

Seealso: Acids:NaturalAcidsandAcidulants;

Antioxidants:NaturalAntioxidants; Canning:Principles;

Carbohydrates:ClassificationandProperties;

Colorants (Colourants):PropertiesandDeterminationof

NaturalPigments; Fats:Classification; Fatty Acids:

Properties;DietaryImportance; Flavor (Flavour)

Compounds:StructuresandCharacteristics;

Fungicides; Phenolic Compounds; Preservation of

Food; Ripening of Fruit; Tocopherols:Propertiesand

Determination

Further Reading

Alves Mdo CFGP and Pinheiro Alves Mdo CFG (1991)

Rationalization of national olive oil production

capacity: a critical review, from harvesting to storage of

olive oil. Revista de Ciencias Agrarias 14(4): 59–71.

Codex Alimentarius WHO/FAO (1970) Recommended

international standard for olive oil, virgin and refined

olive residue oil. CAS/RS-33.

Damania AB (1995) Olive, the plant of peace reigns

throughout Mediterranean. Diversity 11(1–2): 131–132.

di Giovacchino L and Di Giovacchino L (1997) From olive

harvesting to origin olive oil production. OCL Oleagi-

neux, Corps Gras, Lipides 4(5): 359–361.

FAO (1998) Production Year Book, vol. 52. Rome: Food

and Agriculture Organization. (www.fao.org)

Fernandez Diez MJ (1971) The Olive. In: Hulme AC (ed.)

Biochemistry of Fruits and Their Products. New York:

Academic Press.

Katsoyannos P (1992) The Olive Pests and their Control in

the Near East. FAO Plant Production and Protection

Paper no. 115, ix þ 178 pp. Rome: FAO.

Kiritsakis A and Markakis P (1987) Olive oil: a review.

Advances in Food Research 31: 453–482.

Mangold HK and Fedeli E (1997) Olives, olive oils and

the Mediterranean diet. Rivista Italiana delle Sostanze

Grasse 74(8): 349–352.

Santos Antunes AF (1980) Technology of olive oil produc-

tion. Boletim Instituto de Azeite e Productos Oleagino-

sis 8(1): 23107.

ONIONS AND RELATED CROPS

S Khokar, University of Leeds, Leeds, UK

G R Fenwick, Institute of Food Research,

Norwich, UK

Introduction

0001 The genus, Allium (family Alliaceae), is large and

geographically diverse, comprising over 500 species,

most of which have little commercial importance.

Alliums are distributed across warm temperate and

temperate regions and into the subArctic belt. The

major centers of genetic diversity are Afghanistan,

Iran, and Pakistan with the Mediterranean basin

being regarded as a secondary center. In addition to

the onion (A. cepa L.), cultivated vegetable Alliums

include garlic (A. sativum L.), leek (A. ampeloprasum

L.), Japanese bunching onion (A. fistulosum L.), rak-

kyo (A. chinense G. Don), Chinese chives (A. tuber-

osum Rottl. ex Spr.), chives (A. schoenoprasum L.),

and shallot (A. cepa L.).

0002The onion is by far the most important of the culti-

vated Alliums. Production is mainly in the Northern

Hemisphere, with relatively little in unfavorable trop-

ical regions (where shallots are grown). Cultivation

experience, cultural attitudes, social beliefs, and cu-

linary practices have resulted in specific Alliums being

prized in particular regions; for example, leek and

shallots in Western and Northern Europe; kurrat in

Egypt and the Eastern Mediterranean; rakkyo, Japan-

ese bunching onion; and garlic in China and Japan.

The dramatic increase in global travel, interest in

exotic and ethnic foods, and associated culinary prac-

tices, allied to improvements in agronomic practices

and varietal- and postharvest characteristics have

combined to encourage demand for fresh and pro-

cessed vegetable Alliums. This has been intensified

by prevailing enthusiasms for both ‘healthy’ eating

and naturally occurring health remedies and medi-

cines.

0003Onions and other cultivated Alliums have long

been prized for their culinary properties, flavor, and

taste and for their use as natural medicines. More

ONIONS AND RELATED CROPS 4267

recently, their nutritional and health-enhancing prop-

erties have attracted much scientific investigation

and consumer interest, thereby encouraging primary

production and stimulating postharvest activities and

opportunities. This brief article addresses the com-

position, flavor, postharvest handling, and processing

of Alliums, emphasizing their significance for human

health and well-being. The focus is mainly, but not

exclusively, on onion and garlic.

History

0004 The onion has been cultivated for over 4000 years;

the earliest records are found in Ancient Egypt, in-

cluding carvings dated to the third and fourth dynas-

ties (2700 bc). In addition to being eaten, onions had

significance in funeral rituals and embalming. During

their Exodus (1500 bc), the Israelites recalled the

foods of Egypt, including onion, leek, and garlic;

mention was made of onions in Indian manuscripts

dating from the sixth century bc. Locally grown

onions were among the fermented vegetables given

to the builders of the Great Wall of China (third

century bc). Greek and Roman authors, notably

Theophrastus and Columella, described the botany

and cultivation of the onion, whereas others, such as

Hippocrates and Dioscorides, focused on its medi-

cinal properties. These culminated in the writings

of Pliny, which addressed the cultivation, uses, and

history of onions, garlic, and leek, and, together with

the work of the military physician Galen, these had a

great influence on the practice and writings of herbal-

ists in medieval times and later. More extensive infor-

mation has been published in the reviews of Fenwick

and Hanley.

Production

0005 Onion production in Asia accounts for more than

60% of the global production; together, India and

China account for more than a third; in comparison,

European production is less than 10%. Asian produc-

tion of garlic exceeds 80% of the global volume, with

China alone producing almost two-thirds. The world

production of dry onions and garlic in 1999 is

summarized in Table 1.

Composition

0006 Onions and garlic are found in kitchens all over the

world, being used as cooked vegetables, in salads, and

as a flavoring in meat, poultry, and vegetarian dishes.

Onions additionally possess excellent processing

characteristics and may be canned, pickled, frozen,

and fried. Dehydrated onion and garlic powders and

flavor extracts also serve to enhance the value of these

Alliums in the home and in the food industry. The

proximate composition of selected Alliums is listed in

Table 2. The cultivar, agronomic and environmental

conditions during growth, and postharvest conditions

all affect the composition. Protein, fat, and fiber

contents are low in comparison with other vegetables,

and starch is absent. Carbohydrates include sucrose,

glucose, fructose, and fructose polymers. Green onion

tops have also been found to be rich in b-carotene and

to contain significant amounts of vitamin C. Alliums

are rich in amino acids and g-glutamyl peptides

(including precursors of the characteristic pungent

flavoring compounds), as well as biologically active

secondary metabolites such as anthocyanins, flavo-

nols, and phenolics.

Flavor

0007Alliums are rich in sulfur-containing compounds that

are broken down by endogenous enzymes to yield a

number of more volatile, less stable chemical com-

pounds. These are considered to be responsible for

much of the biological activity found in Allium

extracts and oils, as well as determining the particular

flavor characteristics of individual members of the

family. (See Flavor (Flavour) Compounds: Production

Methods.)

Flavor Biochemistry

0008Intact alliums have no pungency, since the volatile

products are only released following the interaction

of the enzyme, alliinase, with the S-alk(en)ylcysteine

sulfoxide (alliin, I) which occurs when tissue is dam-

aged or disrupted (Figure 1). The initial products of

this enzymic hydrolysis are ammonia, pyruvate, and

tbl0001Table 1 Production of dry bulb onions and garlic ( 10

tonnes)

Country Onion Garlic

World 41 527 9 042

Asia 25 511 7 585

Africa 2 780 178

North and Central America 3 370 301

South America 2 796 286

Europe 3 454 266

People’s Republic of China 11 290 5 990

India 4 429 451

USA 2 995 224

Turkey 2 300 106

Iran 1 210

Japan 1 240

Republic of Korea 936 484

Brazil 921 63

Spain 985 170

From FAO Production Data (1999).

4268 ONIONS AND RELATED CROPS

an alk(en)ylthiosulphinate (allicin, II). The latter,

which possesses odor characteristics typical of the

freshly cut tissue, can undergo further nonenzymic

reactions to yield a variety of compounds, including

thiosulfinate [III] and di- and trisulfides [IV]. These

compounds have slightly differing flavors and odors,

and may impart a cooked note to steam-distilled

onion or leek oils.

0009 Not all alliums contain the same substituted cyst-

eine sulfoxides; onion contains primarily the S-(1-pro-

penyl), S-propyl, and, to a lesser extent, S-methyl

derivatives (Ia–c, respectively). Thus, the typical flavor

of onion is due to the presence of propyl- and 1-

propenylthiosulphinates and corresponding di- and

tri-sulfides. In contrast, garlic contains predominantly

S-(2-propenyl)cysteine sulfoxide (S-allylcysteine sulf-

oxide, Id) and, and none of the 1-propenyl isomer.

Its flavor is consequently due to the presence of

2-propenylthiosulphinate and related di- and tri-

sulfides. Hive and leek contain relatively more

S-methylcysteine sulfoxide, which yields methylthiol-

sulfinate (possessing a ‘cabbagy’ note), dimethyldisul-

fide, and related compounds.

0010 Since it is possible for both symmetrical (II, R ¼ R

)

and mixed (II, R 6¼ R

) thiolsulfinates to be formed

during the enzymic breakdown of alliins, and since

these can yield all of the breakdown products men-

tioned above and which can subsequently enter into

secondary chemical reactions, the composition of the

flavor volatiles of freshly chopped onion, leek, and

garlic can become very complex. Almost 100 com-

pounds have been identified in freshly cut onion and

garlic, and processing – especially that involving ther-

mal treatment – causes additional chemical reactions

to occur. These give rise to thiphenes, polysulfides

and the products of amino acid–sugar (Maillard)

reactions.

Lachrymatory Factor

0011Alone amongst the common alliums, the onion con-

tains large amounts of S-(1-propenyl) cysteine sulfox-

ide (Ia). This compound is unique because the action

of alliinase does not give rise to allicin, but rather to a

mixture of unstable isomers of thiopropanal-S-oxide

(V), which comprise the lachrymatory principles of

the onion. The mode of formation of these com-

pounds and their properties suggests that the prob-

lems of peeling onions may be reduced if carried out

under running water, or if the onion is first chilled.

Estimation of Flavor Strength

0012The development of simple and reliable methods for

the assessment and comparison of strength of flavor

in onions and garlic, which may be required to iden-

tify cultivars with advantageous processing character-

istics, has been the subject of much investigation.

Since the enzymic cleavage of flavor precursors by

alliinase generates a range of breakdown products,

these have been used to measure pungency. The

lachrymatory principle has been measured colori-

metrically and spectrophotometrically, but these

determinations are necessarily confined to onions. A

robust method for pyruvate employs the color reac-

tion with 2,4-dinitrophenylhydrazine; however, this

cannot be employed for dehydrated products due to

the presence of interfering by-products formed during

dehydration and storage. Gas-chromatographic

methods, with initial flavor concentration, have also

been used to measure the headspace volatiles. The

tbl0002 Table 2 Composition of cultivated alliums (fresh-weight basis)

Water

(g)

Energy

(kcal)

Protein

(g)

Fat

(g)

Carbohydrates

(g)

Sodium

(mg)

Fiber

(g)

Carotene

(mg)

Sodium

(mg)

Iodine

(mg)

Folate

(mg)

Onion, fried 65.7 164 2.3 11.2 14.1 10.0 3.2 40 4 6 38

Onion, raw 89.0 36 1.2 0.2 7.9 5.6 1.5 10 3 3 17

Garlic 64.3 98 7.9 0.6 16.3 1.6 4.1

04 3 5

Leeks 92.2 21 1.2 1.2 2.6 2.0 2.4 575 6 40

Chives 91.0 23 2.8 0.6 1.7 1.7 1.9

2300 5 1.6 45

Rakkyo 86.0 50 2.2 0.3 12.0

Nonstarch polysacchrides.

From Bender AE and Bender DA (1999) Food Tables & Labelling. Oxford: Oxford University Press.

R S CH

CHCO

(I)

(II)

R S S R

(III)

R S S R

(V)

(IV)

(S)

n = 2, 3

(a) R or R =

(b) R or R =

(d) R or R =

fig0001 Figure 1 Sulfur-containing flavor volatiles from alliums.

ONIONS AND RELATED CROPS 4269

amount and composition of the volatile fraction is

largely dependent upon the conditions (degree of

tissue disruption, temperature, time), so that all of

these variables must be kept constant if reliable com-

parisons are to be conducted. Alternative approaches

involve measurement of alliinase or g-glutamyltrans-

peptidase activity. It is possible to monitor the flavor

precursors, including g-glutamyl peptides and alliins;

however, such methods provide an indication of

potential flavor, since the presence of the appropriate

enzymes is necessary to release the volatile com-

pounds.

Factors Determining Flavor

0013 Amongst the many factors that have been found to

have an effect on flavor are genotype, physiological

age and condition, storage, and agronomic and

environmental conditions. Thus, the range of flavor

intensity in onions may vary 10-fold, with the most

pungent being selected for processing. Sprouting of

onions during storage leads to an increase in flavor

as a result of a rise in transpeptidase levels and the

conversion of g-glutamyl peptides to alliins. Alliums

grown under dry or arid conditions are smaller and

visually less attractive than those grown in an abun-

dance of water, but generally possess stronger flavors.

The application of sulfur to the soil during growth

significantly increases flavor, pungency and (in

onions) lachrymatory character. The available sulfur

is used preferentially for plant growth; only when

these growing requirements are met is the secondary

metabolic pathway, leading to alliin and allicin,

enhanced.

Nutritional and Health Aspects

0014 Garlic bulb and onions, two members of the Allium

family, have been widely reported to possess a signifi-

cant antioxidant activity which has been ascribed to

allicin, a known scavenger of peroxyl radicals. Recent

studies have indicated that, in addition to this scaven-

ger, other compounds could also be involved. The

onion contains a number of quercitin, isorhamnetin,

and kaempferol conjugates and, indeed, is one of the

major sources of flavonols in European diet. A rela-

tively small range in content of quercitin conjugates

for red (110–295 mg kg

1

) and yellow (119–286

mg kg

1

) cultivars have been reported. Flavo-

noids have been shown to offer protection against

degenerative conditions such as cardiovascular dis-

ease, cancer, and aging.

0015 Many alliums, including noncultivated wild types,

have been extensively used for their therapeutic and

medicinal properties. Most traditional medical prac-

tices include references to onion, garlic, and other

alliums in the form of extracts, decoctions, concoc-

tions, and poultices. Onions and garlic oils have

proved effective in the treatment of a number of

conditions, including intestinal worms, stomach

ulcers, eye disorders, gastrointestinal disturbances,

hypertension, and malarial fevers. Beneficial antitu-

mor, hypoglycemic, hypolipemic, antiatherosclerotic,

antiplatelet aggregation effects have also been re-

ported. The active principles in many cases have

been shown to be the sulfur-containing alliin

derivatives.

0016A number of ‘odor-free’ garlic products, including

capsules, extracts, and tablets, are commercially

available. Since the biologically active principles also

possess pungency, some care is needed to ensure that

the treatment employed to remove odor has not at the

same time reduced or removed the medicinally active

compounds. (See Garlic.)

Postharvest Storage

0017Postharvest losses of onions may range from 10 to

30%, with even higher losses being reported from

tropical regions. A number of factors have been

shown to contribute to these figures; as a result, culti-

vars have been selected for improved storage charac-

teristics, methods for sprout control developed, and

storage practices and technologies optimized. Post-

harvest losses may be minimized if damage and bruis-

ing are reduced during harvest, storage, and packing;

adequate ventilation of storage areas is vital if mois-

ture accumulation on tissue surfaces, and subsequent

disease proliferation, is to be avoided.

Dormancy, Sprouting, and Storage

0018Bulb dormancy is controlled by a complex interaction

of growth inhibitors and promoters. Natural dor-

mancy commences with harvest maturity and con-

tinues for 4–9 weeks, during which period, the bulbs

will neither sprout nor continue to grow due to

the influence of inhibitors translocated from the

green leaves. However, these inhibitors are gradually

destroyed with time. Once dormancy has passed,

roots may emerge and leaf shoots appear; sprouting

is optimal at 10–15



C. Onions may be stored without

sprouting if maintained at 0–5



C and 65–70%

relative humidity (RH). Sprouting has also been in-

hibited by controlled-atmosphere storage atmosphere

storage at reduced oxygen levels (for example, 5–10%

,3%O



C), g-irradiation or preharvest foliar

treatment with maleic anhydride (MH). Onions

treated with MH and held between 2 and 0



C and

65–70% RH can be stored for 6–7 months without

any deterioration of quality.

4270 ONIONS AND RELATED CROPS

0019 Garlic bulbs intended for consumption are held at

ambient temperatures and can remain in good condi-

tion for several months. However, for extended stor-

age of up to 8 months, bulbs should be stored at 2to



C and 60% RH. Sprouting most readily occurs

between 5 and 10



C, and inhibition may be achieved,

as described for onions. Unlike onions and garlic,

leeks are in active growth when harvested; they can

have a postharvest life of up to 8 weeks at 0



C and

95% RH. Controlled atmosphere treatment (for

example, 10% CO

þ1% O

) can extend the storage

period further. Leeks are stored upright to avoid

curvature of the pseudostems.

Storage Diseases

0020 Postharvest diseases of onions, which will reduce the

quality and, thereby, the value of the crop, are largely

determined by the growing and storage conditions.

The major disease of stored onions in temperate

regions is neck rot, caused by Botrytis allii. This may

be controlled by seed and set treatment with benomyl/

thiram and effective postharvest drying. Other soil-

borne diseases (notably, white rot, pink rot, basal rot

and smudge) are best controlled by combinations of

field treatment and good husbandry.

Processing

Dehydration

0021 Onions were included in the first dehydration experi-

ments carried out in the late eighteenth century. A

major program of onion and garlic dehydration was

conducted in California in the 1920s, and dehydrated

onion was already extensively used in food processing

by the end of the next decade. It was the onset of

World War II that provided the necessary impetus for

the development of commercial food dehydration

techniques. Onions for dehydration are generally

white or yellow and have a high solids content (18–

20% and above) and strong pungency. After prelim-

inary cleaning, peeling, and slicing/chopping, onions

are dehydrated at 75–60



C, the temperature being

reduced as the moisture content decreases. The final

moisture content of 4% is achieved through warm

air circulation. Discoloration may be minimized

by effective curing and conditioning whilst the appli-

cation of fluidized-bed techniques may overcome

problems of scorching and agglomeration. In general,

8–10 kg of raw onions produce 1 kg of dehydrated

product. Garlic may be dehydrated under similar

conditions, with 5 kg of fresh bulbs producing 1 kg

of dried product. Experiences of hot-air drying of

chives have revealed problems such as an undesirable

flavor, texture, and color. Flash-freezing and drying

have proved effective in overcoming these problems.

Dehydrated leek products are available for use in

soups and other products, and dry green onions

have been used to replace more expensive shallots

and chives. Onion and garlic powders are used in

the manufacture of onion and garlic salt. Both dehy-

drated onion and garlic are prone to discoloration, a

problem that can be minimized by optimizing the

drying conditions or employing approved additives.

(See Drying: Theory of Air-drying; Physical and

Structural Changes.)

Onion and Garlic Oils

0022Onion oil is obtained by the distillation of minced

onions. The oil, obtained in a 0.002–0.03% yield,

depending upon the source and processing condi-

tions, has a flavor enhancement 500 times that of

the dehydrated product. Garlic oil comprises 0.1–

0.25% of the fresh weight of the clove, 1 g being

equivalent to 200 g of dried garlic. The intense pun-

gency of these products makes them difficult to use

directly, and they are usually diluted in vegetable oil,

or encapsulated.

Onion and Garlic Juices

0023Onion and garlic juices contain both flavor and

aroma-active compounds, their precursors and sugars.

They may be blended with volatile oils to restore a

‘rounder’ flavor profile. The juices, viscous and dark

brown in color, can be mixed with a support (such as

propylene glycol, lecithin or glucose) to provide an

oleoresin with a flavor intensity 10 times greater than

that of the dehydrated powder.

Others

0024Canned and bottled onions are used by the catering

industry, especially in North America. Onion rings,

coated in batter, are widely used in fast food outlets;

these are supplied frozen, usually being extruded

rather than machine-cut. Pickled onions, popular in

the UK, are prepared from both ‘brown’ and pearl/

cocktail onions (28–45 mm and 10–28 mm, respect-

ively). Typically, brown onions are steeped in brine

(10%) for 1–4 days, and lactic acid is added to

control fermentation. They are then washed, covered

in vinegar, and pasteurized; bisulfite may be added to

maintain the color. Silverskin onions tend not to be

pasteurized since this adversely affects the texture and

flavor. Fermentation of Alliums has been described

for thousands of years; onions, garlic, and leek are

included in a variety of fermented products such as

kimchi, ragi, sauces, and pickles. Many varieties of

salamis and sausages are also strongly flavored with

garlic before drying.

ONIONS AND RELATED CROPS 4271

See also: Drying: Theory of Air-drying; Physical and

Structural Changes; Chemical Changes