1 Introduction Onion is a famous spice commodity grown all over the world and consumed in various forms. It has been in cultivation for more than 4000 years. The maximum diversity of Allium species is found in a belt from the Mediterranean basin to Iran and Afghanistan, i.e. Iran, north Iraq, Afghanistan, Soviet middle-Asia and west Pakistan,1 indicating the primary centre of origin. The earliest record comes from Egypt where onions appear as carvings on pyramid walls and in tombs from the third and fourth dynasties (2700 BC).2 It is thought that Romans took the onion north of the Alps. The onion was among the first cultivated plants taken to the Americas from Europe. Europeans took the species to East Asia during the last century.
The distinctive flavour of alliums have established plants as an essential part of the cuisine of the world. It is used as immature, mature bulbs as vegetable and spice as well as food for poultry and non-milking cattle. In India and China, onion is the basis of many dishes. Nearly every Indian recipe starts with the same procedure. Fry chopped or sliced onion slowly, add spices (frequently fresh garlic and ginger and dried spices like cumin, turmeric, chillies) and fry until the onion turns golden. The mixture (wet masala) may afterwards be pureed, simmered with tomatoes or yogurt or just added to boiled or fresh vegetables, meat, chicken and fish to form curry. The onion rings can be fried and used as fast food. Onion pakodas made out of sliced onion and chickpea flour is an important snack item in Indian hotels. Onion can be eaten raw or cooked; mild flavoured or coloured bulbs are often chosen for salad.
Alliums are also used widely in food processing. However, handling of bulk of onion and storage of bulbs for 56 months is cumbersome. Losses of fresh onion in storage have been reported to be about 25 to 30%.3 Value addition of raw onion in the form of processed products is the most practical solution. Whilst onion is traded in its raw form, the international market is increasingly focused on dehydrated products such as flakes, rings, granules, powder, etc., and processed onion as frozen or canned, or onions in vinegar and onions in brine.
Allium cepa is cultivated mainly as a biennial, but some types are treated as perennials. It is propagated by seeds, bulbs or sets (small bulbs). Very wide variation in shape, size
and colour is observed in this species. Intensive selection during domestication and natural hybridization might have created variability. There is a lot of controversy regarding the taxonomic position of Allium and related genera. In early classification onion was placed in the Liliaceae. Some British and American botanists included onion in Amaryllidaceae. In a more recent taxonomic treatment of monocotyledons, Allium and its close relatives are recognized as the distinct family Alliaceae, close to Amaryllidaceae. Robinowitch and Brewster1 suggested the following hierarchy:
1. Class: Monocotyledones
2. Super order: Liliiflorae
3. Order: Asparagales
4. Family: Alliaceae
5. Tribe: Allieae
6. Genus: Allium
Jones and Mann4 gave simple classification of genus Allium cepa for the use of horticulturists as under:
a. Common Onion Group (Allium cepa L. var. Cepa; Allium cepa L. ssp. Cepa, and ssp. Australe Trofim): Bulbs are large, normally single. Plants produce from seeds or from seed grown sets. The majority of cultivars grown for dry bulbs belong to this group. This is the most important group of trade grown all over the world. It includes hundreds of open pollinated varieties, land races and commercial F1 hybrids. Maximum diversity exists in southwest Asia comprising countries like India, Pakistan, the former Soviet Republics and also Mediterranean countries.
b. Aggregatum Group or Shallots (Allium ascalonicum auct. Non strand; Allium cepa L. ssp orientale kazak): Bulbs are smaller than common onions and several to many form an aggregated cluster. Reproduction is almost exclusively vegetative via daughter bulbs. Occasionally scapes are developed and in some types, seed production is possible. This group is of minor importance. Locally adapted cultivars are grown mainly in home gardens. Cultivars are more suited to humid tropical regions of Asia, West Africa and the Caribbean area.
c. Ever-Ready Onion (Allium cepa L. var. perutile stearn): Bulbs are narrow with shorter flower stalks and smaller umbel. Bulbs or leaves can be gathered at all times of the year. It is used mainly as salad onion. This group is again sub-divided into potato or multiplier onion and shallots.
(i) Potato or Multiplier Onion: The bulbs divide into between 3 and 20 bulb sets that are wider than they are long. They are covered by outer dry skins.
(ii) Shallot: Shallots form cultures of narrow, separate bulbs. The leaves and flowers are usually smaller than common onion. Shallots are suitable for high latitude and short season regions.
2 Chemical structure and influences on flavour
The importance of onion lies in the flavour that it imparts to various other dishes. A
common onion contains the following nutrient components:5
Moisture 88.692.8% Protein 0.91.6% Fat Trace0.2%
Carbohydrates 5.29.0% Ash 0.6%
Energy 2338 cal 100g 1
Elements mg 100g 1 fresh weight
Vitamin D 0.3 mg Riboflavin 0.05 mg Nicotinic acid 0.2 m Vitamin C 10.0 mg Folic acid 16.0 g Biotin 0.9 g Pantothenic acid 0.14 mg
Among various free amino acids which vary greatly, glutanic acid and orginine are abundant in onion. The concentration of these amino acids is higher at the centre of the bulb and decreases towards the outer scales.
Onion is characterized by its distinctive flavour and pungency which is due to sulphur containing compounds available in the scales of bulbs. The sulphur is in the form of various non-protein amino acids which include the precursors of volatile flavour compounds. These precursors are odourless, non-volatile amino acids of general name S- alk(en)yl cysteine sulphoxides.6 The precursors occur naturally in four types:
1. CH3SCH2CH(NH2) COOH O
2. CH3CH2CH2SCH2CH(NH2) COOH O
3. CH3CH = CHSCH2CH(NH2) COOH O
4. CH2CHCH2SCH2CH(NH2) COOH
Intact onion cells have no odour, but when cells are disrupted the enzyme alliinase is released. This enzyme hydrolyses the S-alk(en)yl cysteine sulphoxides to produce pyruvate, ammonia and many volatile sulphur compounds associated with flavour and odour.7 The enzyme is confined to cell vacuole, whereas the flavour precursors are confined to cytoplasm. The enzyme has access to precursors only when cells are disrupted. Boiled onion does not produce flavour as the enzyme is destroyed before having access to the flavour precursors. The tear-producing character of onion on cutting is known as the lacrimatory factor. Lacrimator is formed enzymatically during the hydrolysis of S-propenyl cysteine sulphoxide.8 Thus alliums with S-propenyl cysteine sulphoxide have a tear-producing effect and alliums with S-allyl cysteine sulphoxide resemble the taste of garlic.
Besides the bulb, the flavour precursors are also available in leaf blades, base plate, and roots of onion. However, they are absent in seeds.9 The taste of onion differs from variety to variety and within varieties grown under different conditions and different growth stages. Flavour precursors are synthesized in leaf blades and transported to the scales of bulb where they are stored.10 Younger blades are more productive than older ones. Precursor content increases during bulbing and then gradually decreases towards maturity. During storage of bulbs, the flavouring compounds increase; although the bulb is in a resting stage, it is still metabolically active. The increase in sulphur content is maximum until the sprouting of bulbs. The level of sulphur suddenly drops after sprouting. The reduction of flavour at the end of storage period may be due to metabolism and translocation of the flavour precursors themselves for nutrients for developing roots.11
Kopsell et al.12 observed increase in trans-(+)-S-(1-propenyl)-1-cysteine sulphoxide in seven cultivars under study. There was decrease in S-methyl-L-cysteine sulphoxide.
As well as genotype and stage of growth, environmental factors exercise great effect on flavour strength. The bulbing response in onion is a function of temperature and photoperiod. Plants grown at higher temperature under optimum photoperiod bulb more rapidly than those grown at lower temperatures. A threefold increase in volatile sulphur at higher temperature was observed by Platenius.13 The varieties grown in India and Pakistan produce bulbs with higher pungency as they mature during temperature range between 2535sC. Similarly high amount of watering produce bulbs with less flavour.13
Soil fertility status and high amount of sulphur containing fertilizers alter the flavour strength. More pungent bulbs were produced by Platenius on peat than sandy soil.13
Addition of sulphur in major nutrients was found to increase pungency in onion bulbs.14
A global review of area and production of onion shows that it is grown in 126 countries over an area of 2.3 million hectares producing 40.0 million tons of dry onion. Sixty-two per cent of the worlds production is from Asiatic countries. Among them, China (19.3%) and India (17.8%) are the giants by area as well as by total production. The worlds onion productivity is 17.16 tons/hectare.15 About 90% of onion is consumed within the countries of production. Total import of dry onion in the world is worth $118 million, while export is $910.5 million.16
Onion cultivation and processing is at its most advanced in countries such as the United States. Systematic breeding programmes for the development of high TSS white onion varieties launched by various research organizations and seed companies in the United States has been the backbone of the processing industry. Development of suitable
varieties in short-day onion grown in Asiatic countries holds good promise with the available input. India exports dehydrated onion (4124 tons) and processed onion (9095 tons) worth Rs.178.11 million and 191.85 million, respectively.17
3.1 Dehydrated onion Onion is dehydrated in the form of flakes, rings, kibbles and powder. A large part of dehydrated onion production is used as seasoning in production of catsup, chilli sauce and meat casseroles as well as cold cuts, sauces, soup, mayonnaise, salad dressing, sweet pickles, dog food, potato chips, crackers and other snack items. Food service outlets also use dehydrated onion because of its convenience in storage, preparation and use. Processing units require white onion varieties with important attributes:
white onion with globose shape, free from greening
high total soluble solids (>20%)
high degree of pungency
high insoluble solids
low reducing to non-reducing sugars ratio to avoid caramelization
good storage quality
Important cultivars for dehydration include white Creole, Southport White, Dehydrator No. 8, Dehydrator No. 14, VH-12, etc. Indian varieties are of short-day type and do not possess total soluble solids more than 15%. However, due to high pungency they make good quality dehydrated flakes and granules. The coloured varieties like dark red, red and yellow are also used for dehydration purposes but the quality of dehydrated produce is inferior to white varieties.
Traditionally onion dehydration is performed by sun drying in India, on a domestic scale. Various types of solar driers have also been designed for dehydration purposes.18
However, controlled drying under optimum temperature and time gives good quality product. Cabinet drying at 5560sC for 1015 hours gives a better quality dehydrated product than sun drying and drying in solar huts.19, 20 Commercial dehydration is achieved by forced hot air with the total process divided into three stages: drying at 75, 65 and 5560sC, the conditions of dehydration becoming milder as the moisture content falls.5 Van Arsdel et al. 21 have given a schematic representation of the process for onion dehydration as shown in Fig. 1.
The process is completed by placing the dehydrated onion pieces in bins where the final moisture content (~4%) is achieved via the circulation of warm air currents. The approximate composition (100 g 1) of dehydrated onion should be as shown in Table
3.2 Onion oils and other onion products
There are a number of other onion products.
Onion oils: Concentrated oils extracted from onion can be used to impart the flavour of onion to processed food without the difficulties of handling a large bulk of fresh bulbs.6 Onion oil is obtained by the distillation of minced onions which have been allowed to stand for a number of hours before distillation. The oil is of dark amber
coloured liquid. The yield of oil varies from 0.002 to 0.03%. One gram of oil is equivalent to 4.4 kg fresh onions or 500 g onion powder.22 Use of onion oil is very safe from a microbiological contamination point of view, but there is the problem of flavour being lost. Onion oil is also used in non-alcoholic beverages, ice creams, confectionery, baked goods, condiments, meats and pickles.5
Onion juice: Onion juice with low flavour component is another processed product. Massarated pulp of onion is flash heated (140160sC) and then cooled at 40sC. The product is evaporated to 7275% solids to facilitate preservation. During processing aromatic components may be removed so that the product has a low flavour profile.22
Onion salt: Onion salt is prepared by mixing 1920% onion powder with 78% free flowing pulverized refined table salt and 12% anti-caking agent which prevents water absorption, and caking, etc.23
Ash 3.2 g Ca 363 mg Fe 3 mg Mg 122 mg P 340 mg K 943 mg Na 54 mg Zn 2 mg Vitamins (ascorbic acid) 15 mg
Onion pickles: Pickled onions are eaten in large quantities in many European countries. Onion pickles are prepared out of two types, namely (i) brown or dark red onion 2845 mm diameter and white or silver skin (pearl or cocktail) onions between
10 and 28 mm in diameter.5 These onions are produced by planting with high plant density. The onions are first peeled and allowed to ferment in 10% brine solution for
2496 hours. During fermentation sugars from the bulbs are converted to lactic acid and a small amount of acetic acid and alcohol. The fermentation is controlled by adding small quantities of lactic acid. The pickled bulbs are bottled in vinegar, possibly darkened with caramel and pasteurized at 80sC.6
Vinegar from onion: A new type of vinegar can be produced from onions that have been rejected for other conventional purposes because of low quality.24 Horiuchi et al.24 tested several types of onion as raw material for vinegar production. Vinegar was produced successfully from juice of red onion cv. Kurenai by batch culture using yeast and Acetobacter aceti. The vinegar produced from onion had a higher potassium content, while sodium was lower than in conventional vinegars. The total amino acid and organic acid contents of onion vinegar was much higher than in other kinds of vinegars. The commercial feasibility of this new type of product needs to be assessed. In a country like India a colossal quantity of red onions go to waste during the lean season and these can best be utilized for value addition.
4 Functional properties
Besides use as a condiment and spice for flavouring and enriching various cuisines, onion has been known for its high medicinal properties for thousands of years. Chinese, Indians and Egyptians have known about its various medicinal properties since antiquity and these have been well documented. Charak Samhita, an ancient Indian medical treatise describes many curative uses of onion. Augusti25 listed various traditional uses of onion, including:
It acts as stimulant, diuretic and expectorant and mixed with vinegar, it is useful in the case of sore throat.
Essential oil from onion contains a heart stimulant, increases pulse volume and frequency of systolic pressure and coronary flow.
Onion consumption lowers blood sugar, lipids and cholesterol.
Fresh onion juice has antibacterial properties due to allicin, disulphide and cysteine compounds and their interactions.
Antiplatelet aggregation effect in human and animal blood has been reported due to regular consumption of onion.26, 27
The antioxidant activity of onion (Allium cepa) and onion scales has been studied in lipid oxidation models2833 and in radical scavenging assays.34, 35 Both yellow and red onion were poor antioxidants towards oxidation on methyl linoleate33 contradictory to high antioxidant activity towards oxidation of LDL.35 Onion had also a poor antioxidant score in the ORAC activity test while garlic (Allium sativum L) expressed a score four times higher.34 Yin and Cheng36 reported that the presence of garlic bulb, garlic greens, Chinese leek, scallion, onion bulb, and shallot bulb significantly delayed lipid oxidation of phosphatidylcholine liposomes. While allicin 4 is responsible for the antioxidant activity of garlic bulb,37 compounds other than allicin are involved in determining the antioxidant effect of other Allium members. According to Velioglu et al.38 anthocyanin- rich vegatables including red onion scales generally showed very strong activities towards oxidation of -carotene linoleic acid model systems. Similarly, green onion tops were reported twice as active as green onions with quercetin 5 included in the antioxidant substances.28, 29
5 Quality issues
5.1 Dry onions
For export from India the following specifications have been defined by the Agricultural
Processed Products Export Development Authority (APEDA).39
A. General big onions:
1. 46 cm bulb diameter, light red to dark red colour, globular, pungent onions are suitable for Asian markets and Arab countries.
2. 34 cm bulb diameter, globular, pungent and light red colour onions are suitable for Bangladesh market.
3. 78 cm bulb diameter, globular and oval round shape, yellow or brown colour
onions are suitable for European and Japanese markets.
B. Small onions:
23 cm bulb diameter, dark red to violet red, globular shape onions are suitable for
Malaysia and Singapore markets.
C. Multiplier onions:
2.53.5 cm bulb diameter, dark red colour, bigger size bulblets onions are suitable for Malaysia, Singapore, and Sri Lanka markets.
As per international quality standards40 dry onions should be intact, sound, clean, sufficiently dry for intended use, free from abnormal external moisture, free from off odours and the stem must be twisted or clean cut. Shape and colour should be typical to the variety. The size should be uniform with minimum variation in the group. The size of group can be of 1020 mm, 1525 mm, 2040 mm, 4070 mm and 70 mm plus. They should be free from abnormal swelling, doubles, sprouting and saprophytic fungus. The bulbs should be packed in sacks of jute or nylon nets with appropriate capacity varying from 2540 kg. Consumer prepacks of 12 kg capacity, such as nets, plastic film bags or stretch-wrapped trays can be used.
Sliced 90 400
Large chopped 90 300
Minced 150 180
Granulated 150 140
Agglomerated coarse 150 140
Agglomerated fine 150 140
5.2 Dehydrated products The American Dehydrated Onion and Garlic Association have standardized quality and grade specifications for dehydrated onion products.41 Based on particle size the products are classified as shown in Table 2.
In all products moisture should be 5.0%. The products should be free from black or dark brown pieces, seed stems, sediment or sediment attached to onion, extraneous vegetable matter (such as tops, rootlets, and other harmless vegetable matter), outer roots, metallic particles, hair, etc. The material should be packed in moisture barrier material like multi-walled polythene bags, fibre drums and stored under cool and dry conditions. Exposure to high temperature and light reduces the colour quality of dehydrated products. Freshly harvested as well as stored bulbs are used for dehydration. During harvesting, handling and storage, the bulbs carry a heavy load of harmful bacteria, fungi yeast and mould. There is every possibility of passing these microbes to the final product. The
count of various microbes should be at tolerable levels as follows: Aerobic plate count < 500.000/g
Yeast and mould < 5000/g Coliforms < 200/g Salmonella Absent/25g E. Coli Absent/g
5.3 Other onion products Onion is processed in the form of pickled onion, as onion in brine and onion in acetic acid. For processing, bright white onions with globose shape, fully cured, free from rots, mould, fungus, external damage, sprouting and greening are used. Bulbs of 1625, 2545 and 4570 mm diameter grade are used for processing. Smaller grade fetches better
Salinity as NaCl 16% 0.5% 5% 0.5% Acidity as acetic acid 0.3% 0.5% 4% 0.2% CaCl2 0.5% 0.5%
prices. The chemical composition of onions in brine and acetic acid should be as shown in Table 3.42 The processed onions are packed in food grade HMHDPE barrels of
220240 litre capacity.
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