Contents

Sr. No. Particulars Page No.

1 Introduction

1.1 Production

1.2 Chemical Constituent

1.3 Varieties

2. Post-Production Operations

2.1 Pre-harvest Operations

2.2 Harvesting

2.3 Curing

2.4 Cleaning

2.5 Sorting and grading

2.6 Packaging

2.7 Bulk Storage 2.7.1 Onion and its Physiology for Storage 2.7.2 Onion Storage Structure Requirements 2.7.3 Onion Storage Practices 2.7.4 Storage structures 2.7.4.1Pole Method 2.7.4.2Sika method2.7.4.3Split Bamboo storage structure2.7.4.4Nasik type storage structure2.7.4.5Forced air ventilated storage structure

2.8 Processing2.8.1 Dehydration of Onion2.8.2 Fermented products2.8.3 Oil2.8.4 Juice2.8.5 Other products

3. Overall Losses

4. Economic and Social Considerations

5. References

1. INTRODUCTION

Onion (Allium cepa L.) is a important vegetable crops in most parts of the world,

particularly the varieties that are grown for bulbs. It is a naturally packaged vegetable

consisting of fleshy, concentric scales enclosed in paper-like wrapping leaves,

connected at the base by a flattened stem disc.

Onion is an important crop in the tropics, which account for nearly 30% of total

global production. Export potential of onion is developing in several tropical regions

partly because if dried and packed properly, the bulbs can be transported for

considerable distances without deteriorating. Production of onion in India increased

during 2000-01 to 2005-06 (Fig 1). Major onion producing states are Karnataka,

Maharashtra, Orissa, Gujarat, Uttar Pradesh and Madhya Pradesh. Maharashtra is

leading onion growing state, accounting for about 1645 and 2469 million tonnes

production in 2004-05 and 2005-06 respectively. Gujarat accounts for about 1223 and

2128 million tonnes production in 2004-05 and 2005-06 respectively (Fig 2).

Onion plant and plantation

Onion is bulbous biennial or perennial herb that give off a distinctive and pungent

odour. Onions are grown on all type of soils such as sandy loam, silt loam and heavy

clay soils. The onion grows in mild climate without extremes of high or low temperatures

even though it can be grown under a wide range of climatic conditions. The

transplanting of seedlings is more common practiced for cultivation of onion. It is

generally transplanted in the month of August to September and harvested in the month

of October to December for kharif whereas rabi crop is transplanted in March to Apri

and harvested in May to July. In India, the yield of winter (rabbi) onion is around 25-30

tonnes per hectare and in rainy season (kharif) it is comparatively low. The onion plant

and bulb structure is shown in Fig. 3

Fig. 3 Onion plant and bulb structure

Chemical Constituent

Onions is consumed by all classes of people-poor and rich and hence assumes a

place of essential item. They are highly valued for their flavour and for their nutritional

value in supplying minor constituents such as minerals and trace elements (Table 1).

The bulbs are boiled and used in soups and stews, fried or eaten raw. They are also

preserved in the form of pickles. Onion leaves, especially the Spring onion, are also

used in salads and soup. There is a dearth of information on secondary and derived

onion products.

The proximate composition of selected Allium species is presented in Table 1.

The composition of onion varies according to variety and agronomic and environmental

conditions of growth. In general, varieties with high(up to 20%) dry matter are selected

for processing. Onions are a rich source of amino acids and γ-glutamyl peptide,

anthocyanins, flavonols, and phenolics. Nonstructural carbohydrates consisting of free

sugars, trisachharides, and frutcans, contribute the major portion of the dry weight of

onions. High-dry-matter onion cultivars have reduced glucose and fructose contents

and much higher fructan levels than varieties with low dry matter contents.

Onions are rich in sulphur-containing compounds. The enzyme alliinase

hydrolyzes the S-alk(en)yl cystein sulfoxides to produce pyruvate, ammonia, and many

volatile sulphur compounds associated with the flavour and odour of onion. This occurs

when cells are damaged or distrupted. Onions contain primarily the S-(l-propenyl),

propyl, and, to a lesser degree, methyl alliins. Thus the typical flavour of onion is due to

the presence of propyl- and l-propenyl- containing allicins and di- and trisulfides. More

than 80 compounds have been identified in freshly cut onions.

Table 1: Major nutrients and vitamins content in onion bulbs (Allium cepa var. cepa)

and leaves (Allium fistulosum) per 100g edible portion.

Constituent Quantity

Major nutrients Bulbs Leaves

Water 87 90 91

Calories 48 36 30

Protein (g) 1.5 1.8 1.6

Fat (g) trace 0.5 0.3

Carbohydrate (g) 11 6 6

Fibre (g) 0.5 1.0 0.8

Calcium (mg) 30 40 55

Phosphorous (mg) - - 41

Iron (mg) 0.5 3.0 1.1

Vitamins

ß-carotene equiv. (µg) trace 328 630

Thiamine (mg) 0.04 0.05 0.06

Riboflavin (mg) 0.02 0.10 0.08

Niacin (mg) 0.3 0.5 0.5

Ascorbic acid (mg) 10 50 19

Source: (Platt, 1962; FAO, 1972)

2. Post-Production Operations

2.1 Pre-harvest Operations

The condition of onion leaves is a good indicator of the maturity and general

state of the bulb. Bulb onions which are to be stored should be allowed to mature fully

before harvest and this occurs when the leaves bend just above the top of the bulb and

fall over. As a practical guide, farmers should conduct sample counts on the number of

bulbs, which has fallen over in a field; and when the percentage of bulbs, which have

fallen over, reaches about 70-80% then the entire crop should be harvested. Harvesting

could commence earlier when 50-80% of the tops have gone over, before it is possible

to see split skins exposing onion flesh. Storage losses at optimum maturity are normally

lower than those harvested before the tops collapse. Bulbs generally mature within 100-

140 days from sowing, depending on the varieties and the weather. Spring onions

mature for harvesting after 35-45 days from sowing. Harvested crop should be allowed

to dry or cure and ripen in the sun for several days after lifting. Onions can yield up to

15-30 t/ha depending upon varieties under good growing and management conditions.

2.2 Harvesting

Onion is harvested depending upon the purpose for which the crop is planted.

Harvesting takes 45-90 days from field settling for green onions and 65-150 days for

bulb depending upon the variety. Bulbs are considered to be mature when the neck

tissues begin to soften and tops are about to abscise and decolourise. Development of

red pigment and the characteristic pungency of the variety are also important harvest

indices of onion.

Onions for use in the green stage are harvested as soon as they reach edible

size. The plants are pulled by hand, the roots are trimmed and the out side skin is

peeled off, leaving the leaves clean and lower portion above the bulb white. The onions

are then washed, sorted and tied in bunches. The size of bunch depends on local

market preference. If onions are to be transported to long distance then it is to be

packed in crates with crushed ice to avoid discolouration and wilting of leaves at high

temperature.

Onions for sale as dried bulbs or for storage should be harvested after tops have

started falling over. Best time to harvest rabi onion is one week after 50% tops have

fallen over. In kharif season since tops do not fall, soon after the colour of leaves

changes to slightly yellow and tops start drying, red pigmentation on bulbs develop and

also true shape and size develop, bulbs are harvested and kept in windrow for drying

the tops. Leaving the tops intact until complete drying increases dry matter, this might

be due to greater loss of water from the bulbs with intact foliage; or to movement for

materials from the tops to the bulbs. If tops are cut too close, the neck does not close

well and decay organisms have easy access to the bulb. Early harvest results in

sprouting of the bulbs and late harvest results in formation of secondary roots during

storage. In kharif season late harvesting results in doubles and bolting.

Harvested bulbs are placed in containers (basket, bins) or tied into bunches and

placed directly on the floor of a trailer for transport. These trailers can be pulled by an

animals or mechanical transport such as a tractor. Both packaging and transport

systems must be selected to ensure minimum handling damage to produce. Hard

surfaces should be cushioned with leaves, foam or other appropriate force decelerators.

2.3 Curing

Curing removes excess moisture from the outer layers of the bulb prior to

storage. The dried skin provides a surface barrier to water loss and microbial infection,

thereby preserving the main edible tissue in a fresh state. Curing also reduces

shrinkage during subsequent handling, reduces the occurrence of sprouting, and allows

the crop to ripen before fresh consumption or long-term storage (Opara and Geyer,

1999). The use of the word ‘curing’ for onion drying is rather inaccurate since no cell

regeneration or wound healing occurs as in other root crops such as yam and cassava.

Curing reduces bulb weight and since they are sold mostly on a weight basis, achieving

the desired level of dehydration is critical. Weight losses of 3-5% are normal under

natural curing conditions and up to 10 % with artificial curing.

In

traditional small-scale operations, onion curing is carried out in the field in a process

commonly called ‘windrowing’. It involves harvesting the mature bulbs and laying them

on their sides (in windrows) on the surface of the soil to dry for 1 or 2 weeks. In hot

tropical climates, the bulbs should be windrowed in such a way to reduce the exposed

surface to minimise damage due to direct exposure to the sun. In wet weather, the

bulbs can take longer time to dry and may develop higher levels of rots during storage.

The side of the bulb in contact with wet soil or moisture may also develop brown strains

or pixels, which reduce the appearance quality and value. Obviously, successful

windrowing is weather dependent and therefore cannot be relied upon for large scale

commercial onion production business. Bulbs harvested for storage require in total 14-

20 days of ripening or curing before being stored. Harvested onions may also be placed

in trays, which are then stacked at the side of the field to dry. In some tropical regions,

the bulbs are tied together in groups by plaiting the tops, which are then hung over

poles in sheds to dry naturally.

Harvested bulbs can also be taken straight from the field and cured artificially

either in a store, shed, barns, or in a purpose-built drier. This method is commonly used

when crops are stored in bulk but it can also be applied to bags, boxed or bins. Under

this method, bulbs are laid on racks and heated air is rapidly passed across the surface

of the bulbs night and day [O’Connor, 1979; Brice et al., 1997]. Curing may take 7-10

days and is considered complete when the necks of the bulbs have dried out and are

tight and the skins shriek when held in the hand. The control of humidity level in the

Fig. 3 Curing of onion

store is critical. Under very high humidity, curing is delayed and fungal infection can

increase. However, if relative humidity is too low (below 60%), excessive water loss and

splitting of the bulb outer skins can occur, resulting in storage losses and reduction of

bulb value. Placing onions on wire mesh in well ventilated conditions and using air at

about 30°C, 60-75% Rh and 150 m³ /h/m3 is generally recommended for artificial curing

of onions.

2.4 Cleaning

Freedom from any impurity, which may materially alter the appearance or eating

quality, is essential. Soil and other foreign materials must be removed and badly

affected produce must be discarded. Cleaning is carried out by using air or by manually

removing unwanted materials on the bulb surface. Care should be taken to avoid

physical injury on the bulb during these operations.

2.5 Sorting and grading

Onion after curing and cleaning are graded

manually or mechanically using grader before they

go to storage or for marketing. The thick neck,

bolted, doubles, injured and decayed bulbs are

picked out. Sorting and grading is done after

storage also to fetch better price. The outer dry

scales usually rub off during the grading process,

giving the onions a better appearance for market. It has been experienced that if

storage is arranged after proper sorting and grading losses in storage are reduced.

Consumer preference for onion size varies from place to place. Hence graded onion

should be marketed according to consumer preference to fetch higher price.

For marketing of onion in domestic as well as export purpose the grading is most

essential. Big size bulbs are needed for Delhi market and medium size onions are

required for Calcutta, Pune and Lucknow. Banglore, Bhopal, Jabalpur and Hyderabad

requires medium to big size bulbs whereas, Bhubaneshwar, Guwahati and other

centres in North Eastern region prefers small sized onions.

Fig. 4 Onion grader

For export purpose NHRDF has given grading standards. For Saurashtra Big

onion light red onion are graded as Extra big (60 mm), Big (40 mm), medium (20 mm)

and small (<20 mm).

2.6 Packaging

Good packaging for onions must meet the following criteria: (a) strong enough to

retain the required weight of onions under the conditions of transport and storage, (b)

allow sufficient ventilation for the air around the bulbs to maintain relative humidity in the

required range, and (c) in many circumstances, provide a means of displaying legally

required and commercially necessary information (Brice et al., 1999).

There are many traditional methods for packaging onions for transportation

and/or storage purpose. These include 'string of onions', shelves and loose bulk. In

'string of onions' packing, the bulbs are tied together by means of their tops to produce

a bunch of bulbs is also a form of packaging. This is suitable for transporting small

quantity of crop, and during storage, the bunches are hung from the roof or from special

racks. Shelves for onion handling and storage are made from either wooden slats or

metal mesh on a wooden or metal frame, and are usually fixed in position with the bulbs

loaded and unloaded in the store. Ventilation (natural or forced) is usually achieved by

passing air over the shelves. To achieve adequate aeration of the bulbs, the depth of

bulbs on the shelves should be limited to 10 cm.

Onions are also stored loose bulk (instead of containers) by heaping the bulbs

directly on the floor or elevated platform. Because they are not restrained, the bulbs roll

during store loading to completely fill the storage space. Bulk storage permits maximum

utilisation of store space, and uniform aeration is easier to achieve than in stacks of

bags or other rigid packaging. However, where bulk storage is to be implemented, the

retaining walls must be strengthened when storing larger quantities of bulbs and

arrangements need to be made for re-bagging before subsequent marketing. It is also

difficult to inspect bulbs regularly under these storage conditions. Loose bulk handling of

onion is most suitable for large-scale operations where forced ventilation can be

provided during long-term storage.

Onions can be packaged and stored in a variety of containers such as boxes,

cartons, bags, bulk bins, pre-packs, plastic film bags, and stretch-wrapped trays.

Packages typically contain 25 kg and above, especially for transporting crop from field

to store and/or during storage. The same 25 kg bags or smaller bags may be used from

store to market place. The type of packaging to be used, depends on crop size, length

of storage and marketing requirements. A problem with packaging onions in boxes, net

bags and bulk bins is that if they are too large, and airflow pattern tends to be around

rather than through them. Under this condition, the respiration heat of the bulb results in

a warm, humid environment in the centre of the package, which can result in decay or

sprouting.

Onion Bags

Sacks and nets used for onion packaging fall into three groups: (i) general-

purpose jute sacks, as used for many agricultural commodities, (ii) open-weave sacks of

sisal-like fibre, (iii) open-mesh nets, normally of plastic materials and (iv) big bags, used

alternatively to crates, containing up to 1000 kg. Jute sacks are readily available, but

their disadvantages include: (i) generally too large - may contain 100 kg onions, hence

difficult to handle and an increased risk of mechanical damage; (ii) bulbs are not visible

through the fabric, and it is difficult to monitor condition during storage; (iii) there is

some resistance to airflow if they are used in an aerated store; (iv) difficult to label

effectively; and (v) recycled sacks may encourage spread of postharvest diseases.

Onions are packed in jute (hessian) bags for transporting to yard or brought as

loose. For safe handling 40 kg open mesh jute bags having 200-300g weight should be

used in domestic market. For export bigger size onions are packed in 5-25 kg open

mesh bags. In retail market onions are packed in plastic crates. Plastic netted bags;

attractive and better in strength are also used for packing onions.

2.7 Bulk Storage

2.7.1 Onion and its Physiology for Storage

Every agricultural commodity is required to be stored properly to prolong the availability

with minimum qualitative and quantitative losses. Onion is not an exception. The onion

bulb is a natural food store for the plant, but it is a living system undergoing a process of

development towards sprouting, and is subject to decay by various disease causing

organisms. The objective of storage technology is to maintain the bulbs for as long as

possible in an unchanged sound condition with longer shelf life, and allow them to

transport and market after removal from store without much losses.

It is necessary to have the knowledge of the physiology of dormancy and epidemiology

of storage disease while thinking of long term storage. Systems to provide long dormant

condition and suitable condition which is unfavorable for disease development can be

engineered using the physical principles of temperature and humidity control. Also in

this process economic and technological constraints will have to be looked into. For this,

two basic strategies i.e. high temperature dormancy of onion bulbs and maintaining

storage temperature at around 300 C need to be exploited.

The physiological and pathological processes that proceed within a store of onion bulbs

interact with the physical process of heat and water vapour exchange so as to mutually

influence the environment within the store. Main factors which influence onion storage

and bring change in the bulbs are summarized as under in sequence:

With time, sprouting and internal root development proceed.

Sprouting and internal root development change bulb shape, tension of skins and

crack the skins.

This increases the conductivity of skins to water vapour and ultimately rate of

water loss from the bulbs.

Increase in sprouting increases respiration.

Increase in respiration increases outputs of heat, CO2 and water loss from the

bulb.

Diseases are developed in store when there are favorable conditions and bulbs

thus get deteriorated.

Bulb deterioration due to diseases will also increase respiratory outputs. The

onion skins has vital role in the physical and physiological processes in the

storage, as it is the main barrier to water loss and to CO2 exchange. 65-70%

relative humidity is desirable to maintain the skin fairly flexible and elastic. At

lower RH, the skin become very brittle and gets easily cracked notably when skin

moisture content falls below 20%.

Ventilation is needed to maintain humidity between 65-70% and lack of this often

adversely affects the quality and quantity by increase in water loss and

respiration.

Ventilation is also needed to dissipate heat produced by bulbs.

With time, requirement of ventilation for the above will also increase.

Design of store should, therefore, match the requirements.

High humidity with high temperature favors spread of pathogens within the store.

It is necessary to counter the above changes by proper monitoring of internal

environment. Heat and water vapour must be removed or introduced as necessary

either by using heating or refrigeration or ventilation or a combination of all the

mechanisms depending upon the economics. However, under Indian conditions in onion

growing states designs to exploit natural ventilation is most economical.

2.7.2 Onion Storage Structure Requirements

For effective long storage of onion the parameters essential to be looked after are the

bulb size, choice of cultivars, cultivation practices, time of harvest, field curing, removal

of tops, drying, grading, packing, storage conditions (optimum storage range of relative

humidity 65% to 70% with the temperature ranging between 250 C to 300 C).

Salient Features of Improved Storage Structures are:

1. Construction of structure on a raised platform to prevent moisture and dampness

due to direct contact of bulbs with the soil.

2. Use of Mangalore tile type roof or other suitable materials to prevent built up of

high inside temperature.

3. Increased centre height and more slope for better air circulation and preventing

humid micro climate inside godown.

4. Providing bottom and side ventilations for free and faster air circulation and to

avoid formation of hot and humid pockets between the onion layers.

5. Avoid direct sunlight or rain water falling on onion bulbs to reduce sun scald,

fading of colour and quality deterioration.

6. Maintenance of stacking height to avoid pressure bruising.

7. Periodical disinfection of structures and premises to check rottage.

8. Cost effectiveness of structures is based on utilization of locally available

material for the construction.

For onion storage, technology may be either with natural ventilation or with forced

ventilation. Although cold storage systems are used in certain countries for onion, this is

normally not adopted in India due to poor economics and lack of cold chain facilities

required to maintain the quality in the high ambient temperature prevalent in our

country. Onion storage in ventilation condition is quite satisfactory when the

temperature is maintained between 25oC to 30oC with a relative humidity range of 65%

to 70%. This environment reduces the storage losses, which are in the form of

physiological loss in weight, rotting and sprouting. The onion storage structure should

be oriented in the North - South direction i.e., length facing the East-West direction. The

storage of onion will be on raised perforated platform of 0.60 m height with bottom and

side ventilations. The ground clearance may be 60 cm with side opening of upto 80%.

Height of storage under ventilation storage should be in the range of 90 cm to 150 cm.

For a 25 MT storage, the size of onion storage area will be 4.5m X 6.0m. The width of

storage may be reduced depending upon the availability of local construction material

and ambient condition. The length of storage structure may be increased to suit the

requirements of the individual farmers. The minimum overhang of 1.5 m on the

windward side and 0.5 m on all other sides should be provided to protect the produce

from sunlight and rain. At leeward side, the opening below the platform should be

closed to direct the air upward for better ventilation. Where storms/ cyclones are

expected, leeward side should not be closed when the windward side is open. During

storm there should be a provision to close the windward side. Emphasis should be laid

for better area utilization efficiency. The overall dimensions of a 25 MT structure may be

6.5 m X 7.0 m. The dimensions can be adjusted depending upon the capacity and site

conditions. The roof of the structures may be either Mangalore tile type or ACC sheets

for a single tire arrangement or RCC for two tier systems. In case of Mangalore tiles,

proper fixation should be done at the ends to prevent damage by air. If cheaper

materials are available which can prevent heat built up at the top of the structure, they

can also be used. The foundations should only support the pillars to bear the load of the

structure and wind. Continuous half brick thick wall may be provided on the leeward

side below the storage platform to serve as a wind barrier. MS angles may be used for

the truss and pillars. Half split bamboo sticks supported by MS angle frames may be

used for storage of onion. Side walls can also be of chain link (GI wire) type. It has been

observed that such structures can be constructed with an investment cost between Rs.

1500 to Rs. 2000 per MT. Therefore, adequate care is to be taken for economizing the

structures.

2.7.3 Onion Storage Practices

Onions are stored either loose or in bags. The beneficiaries are advised to sort the

onions prior to storage and thereafter atleast once in thirty days to take out the rotten/

infected onions in order to avoid further spread of diseases/ losses. Generally, a loss of

about 20-30 % is there during a storage season in the form of weight loss of onions

which can be controlled with proper care. However, the other types of losses can be

controlled to a greater extent if the structure is designed to facilitate maximum natural

ventilation through the stored onion and sorting is done at regular intervals.

General Requirements

The objectives of onion storage are to extend the period of availability of crop,

maintain optimum bulb quality and minimise losses from physical, physiological, and

pathological agents. Bulbs selected for storage should be firm and the neck dry and

thin. Discard thick-necked bulbs because they are most likely to have high moisture

content than optimum for storage, and therefore would have short storage life. Skin

colour should be typical of the cultivar. Microbial infections such as Aspergillus niger

occur during production of onions but these will only develop on the bulbs during

storage where the storage environment is conducive for their growth. Prior to storage,

crop must be cleaned and graded, and all damaged or diseased bulbs removed. Careful

harvest and pre-storage treatments with minimal mechanical loads are important to

achieve a long storage period. Both store room temperature, relative humidity, and

atmospheric composition affect the length of storage that can be achieved. Several

technology options are available for bulk storage of onions, including low-temperate

storage, high-temperature storage, ‘direct harvest’ storage and the use of controlled

atmosphere (CA) stores. The recommended storage conditions under these systems

are summarised below.

Advantages

In bulk storage, it is easier to control ventilation, temperature and humidity using

automatic control devices. This is because of the uniformity within in bulk storage, which

does not exist in the wooden bin system. The loading and unloading operation may also

be simplified by the use of conveyors for direct loading and unloading from trucks. The

bulk storage system is also simpler to clean than the cleaning of a stack of wooden

bins.

Disadvantages

Handling onions in bulk requires much more labour and reinforced storage structure

leading to high initial cost. The mixing of onions in the bulk storage structure also makes

it difficult to prevent the spread of disease by isolation. When handling onions in bulk,

packers for export will find it harder to identify good growers because of product mixing.

This makes it more difficult to award growers according to their ability. Storing in mass

means that onions are handled individually, leading to a greater chance of causing

injury to the bulb. This also means that structures to reinforce the walls will be needed

to support the onion load and false floors installed to enable ventilation from under the

bulk. A bulk storage unit once empty of bulbs, has little value in any other way.

Storage at Low Temperature

For successful low temperature storage, good ventilation and a low level humidity

in the range of 70-75% is essential. To maintain good quality crop, the period of storage

varies but may be up to 200 days. For maximum storage period and minimum losses

bulbs should be fully mature at harvest, and dried until the ‘neck’ of the bulb is tight. For

large-scale commercial storage, onions can be stored under refrigeration and the most

commonly recommended conditions are 0°C with 70-75% rh. Regular ventilation and

monitoring of both temperature and relative humidity in the store are necessary to avoid

significant fluctuations in environmental conditions. During the first few days of storage

the fans should provide an adequate airflow, to remove water in the outer skins and to

dry bruises. High air speed is needed for a period of up to 1 week, until the skin of the

upper onion layers in the bulk rustles. Excessive humidity in-store will lead to the

development of roots and promote rotting while higher temperatures will result in

Table 2: Recommended refrigerated storage conditions for onion bulb

Temperature (°C)

Relative humidity(%)

Length of storage

-3-0 70-75 6 months

-3 85-90 5-7 months

-2 75-85 300 days

(-2) - (-0.6) 75-80 6 months&

-1-0 70-80 6-8 months

-0.6 78-81 6-7 months

0 75-85 6 months

0 65-75 -

0 70-75 20-24 weeks*

0 70-75 -

0 65-70 1-2 months#

0 65-70 6-8 months†

0 - 230 days

0 70-75 or 90-95 up to 120 days

0 80-85 30-35 weeks§

1-2 80-85 30-35 weeks¥

1 87 -

1.1 70-75 16-20 weeks‡

4 - 170 days

8 - 120 days

12 - about 90 days

20 - 25 days

*= With 16.3% loss (red onion); #= Bermuda cultivar; †= Globe cultivar; ‡= With 14.2% loss (red onion); &= Superba cultivar; §= Optimum storage conditions, 7% maximum water loss before becoming unsaleable; ¥= Probable practical storage conditions, 7-10 days shelf-life (approx.) at 20°C after storage, 7% maximum water loss before becoming unsaleable. Compiled from (Thompson, 1996; Thompson, 1982).

sprouting and promote development of pathological disorders such as Botrytis rots

(Thompson, 1982) Bulbs freeze below -3°C and a range of storage temperatures and

relative humidities have been recommended for safe storage of onions (Table 2).

Ventilation must be carefully applied inside the store to achieve the required

temperature and humidity levels without inducing condensation of water on the surface.

Onion Storage at High-temperature

Onions can be stored at high temperatures of over 25°C at a range of relative

humidities (75-85%) which is necessary for minimising water loss. It has been

mentioned in the literature that storage at temperatures of 25-30°C is shown to reduce

sprouting and root growth compared to low-temperature storage (10-20°C). However,

weight loss, desiccation of bulbs, and rots occurred at high temperatures, making the

system uneconomic for long periods of storage that is required for successful onion

marketing (Thompson et al., 1972; Stow, 1975). In tropical climates, high-temperature

storage of onions can be achieved under ambient storage condition. Under these

conditions, ventilation must be carefully applied inside the store to achieve the required

temperature and humidity levels.

Controlled Atmospheric (CA) Storage of Onions

CA is used in combination with cold storage to extend the storage life of onions.

Recommended air composition and temperature regimes are summarised in Table 3.

Commercial CA storage of onion bulbs is limited partly because of variable success and

inconsistent effects on bulb quality. However, high carbon dioxide (0-5%) and low

oxygen (1-3%) levels in combination with low temperature storage has been shown to

reduce sprouting and root growth (SeaLand, 1991; Hardenburg et al., 1990). The

combination of CA storage (5% CO2, 3% O2) and refrigerated storage (1°C) also

resulted in 99% of the onion bulbs considered marketable after 7 months storage;

however, 9% weight loss occurred (Smittle, 1989).

Onion response to CA storage varies among cultivars. Therefore, experiments

should therefore be conducted under local conditions to determine the appropriate level

of gas composition suitable for safe storage of local cultivars. CA storage generally

increases the pungency of characteristic cultivars.

No commercial benefit has been identified for varieties with long storage potential.

Onions are damaged by < 1% O2 and 10% CO2.

Table 3: Recommended controlled atmosphere composition for storage of onion bulbs

Carbon dioxide (%) Oxygen (%)

Temperature(°C)

0 1-2 -

0 1-2 0-5

0-5 1-2 0-5

0-5 0-1 0-5

5 3 1

5 3 1

5 3 -

5 5 4-5

10 3 4-5

Source: (Thompson, 1996)

2.7.4 Storage structures

Onion bulbs, throughout the country, are stored by conventional methods. There

are different types of storage structures used in different parts of the country. They are

fabricated from the locally available material like bamboo. These structures are made

directly on the ground or are constructed on a raised platform made of bamboo or other

similar material.

Fig. 6 Pole method

Fig. 7 Sika method

In Paniapat district of Haryana and Jalalbad in Uttar Pradesh, structures are made of

bamboo/Sarkanda nets and thatched roof with ‘sirki’ which is covered on top with jute

cloth. The size of 3.0m x 1.2m x 1.2m has the capacity to store 40 quintals. The

bottom net is fixed at about 15-20 cm height from the ground level to have aeration.

In Nasik and Pune areas, the structures have side walls made of bamboo or locally-

avaialable wood spaced 1-2cm apart. Roof is made of either thatch, asbestos sheet or

tiles/tin sheet. There is, however, no bottom ventilation but flooring is done with the

help of soil, stone particles and sand and it is raised from ground level. Before loading

onion, sticks are spread on the floor. The size of the structure varies from area-to-area.

Normally width is kept as 1.5m and height is about 1.5m the length may vary from 13.5

to 30m. Its capacity is 200-450 quintals.

In Bihar and Gujarat, these structures have 3-4 tiers and the floor are made of bamboo

pieces spaced 1-2cm apart. The depth of loading is generally 30-60cm. Ventilation in

structure is provided by raised platform and windows on the side walls. Cold stores are

rarely used for storage of onion in India.

2.7.4.1 Pole Method: Cured bulbs are tied into

bunches with leaves. Bunches are tied around the

pole as shown in Fig. 6. Bunches are arranged in

such a way that bulbs can get maximum aeration.

Inspection during storage is possible hence, this

method of storage helps to discard rotted bulbs so

as to prevent further disease to other bulbs.

2.7.4.2 Sika method: Cured bulbs are tied

into bunches with leaves. Bunches are hanged on

poles. Hanging bulbs get maximum aeration. Similar

to pole method inspection during storage is possible

and discarding of rotted bulbs is easy in this method

also.

Pole and sika storage structures are useful for

farmers holding small quantity of onion. It is difficult

Fig. 8 Split Bamboo storage structure

Fig. 9 Traditional single tier storage structure

to handle onion during and after storage for commercial level. Tying of onion requires

humid atmosphere which is available during late night. Labour requirement to tie the

onion is labour intensive. Similarly, at the time of marketing trimming of leaves is labour

intensive. That’s why commercially this method of onion storage is difficult.

2.7.4.3 Split Bamboo storage structure: 100 to 800 kg onion can be stored in

structure made from bamboo split. These structures (Fig. 8) are placed on raised

wooden or steel platforms with provision of good aeration. Two concentric bamboo

structures are made with different diameter. Central hollow space on a platform is

found to provide good aeration, having minimum losses like spoilage, sprouting and

peeling. Onions are stored in annular space of bamboo structures. This structure is

suitable to store.

2.7.4.4 Traditional single tier storage structure: In Gujarat onion storage

structures have single to double tiers. The tops are made of tiles or corrugated cement

roofs. Farmers are also constructing chimney type storage structure having top with

RCC ceiling. In single tier structure windward sides are made from bamboo splits as

Fig. 11 Chimney type storage structure

shown in Fig. 9. The loading height is 1.5 to 2 m at all the places. There is enough

ventilation through windows in the walls, and also raised flooring at some height. The

losses in these structures vary from 30-50%. As per direct discussion with farmers of

Rajkot, Jamnagar and Junagadh district; onion stored for five months (15th May-15th Oct)

in chimney type storage structure have least losses.

2.7.4.5 Turbine air ventilated storage

structure:Where electricity is not available, wind-powered turbines can help keep

storerooms cool by pulling air up through the building. Vents at the floor level are

especially useful for cooling via night air ventilation.

Fig. 10 Traditional two tier storage structure

Fig. 12 Turbine air ventilated storage structure

The turbine illustrated below can be constructed of sheet metal that is twisted to catch

the wind, and attached to a central pole that acts as the axis of rotation. Warm air in the

storage room rises, causing the turbine to rotate, expelling the air and initiating an

upward flow of warm air. The turbine should be placed on the peak of the roof of a

storage structure.

Turbine air ventilated storage structure has a turbine air ventilator at top of the structure

as shown in Fig. 12. No energy is required to operate the ventilator to remove heat from

the storage structure. The utilization of turbine air ventilator may add some ventilation to

stored onion. An experiment was conducted at Junagadh Agricultural University, during

the month June to October 2007, it was found that after one month of storage of onion,

weight loss in turbine air ventilated storage structure found to be higher (7.56%) than

traditional storage structure (4.06%) subsequently, there was more cumulative weight

loss in traditional storage structure at the end of second, third and fourth month of

storage. However in forced air ventilated storage structure the cumulative weight loss

was less (22.87%) as compared to traditional storage structure and turbine air ventilated

storage structure after four month store.

Spoilt onion in traditional storage structure was higher than turbine air ventilated

storage structure through out the experiment. Here also, the minimum spoilage loss was

found in forced air ventilated storage structure.

Fig. 14 Nasik type storage structure

Fig. 13 Forced air ventilated storage structure

2.7.4.5 Forced air ventilated storage structure: Perforated concentric

structures can be used with natural

ventilated or forced air ventilated using

blower at bottom. Forced air ventilated

system can be utilized in traditional

single tier storage structure using

blower and pipe with hole. In

perforated concentric structures

losses can be reduced from 10% to

5-6% whereas in traditional storage structure losses can be reduced from 24% to 10%

in about 3 months using forced air ventilation.

2.7.4.4 Nasik type storage structure: The Nasik type storage structure (Fig. 14)

is an improvement over the local thatched type of bamboo structure. The capacity of

onion to be stored is 1 tonne. The onions can be stored for five months.

Fig. 25 Comparison of different losses and balance in different storage structures

Fig. 26 Dehydrated onion slices

2.8 Processing

Onion bulbs are generally chopped into desired sizes and shapes using a knife. Many

commercial devices are also available for chopping onions. In some food preparations,

the onions are blended with other ingredients to produce the desired flavour.  

Onions used for processing should have high pungency, since the dehydrated product

is primarily used as a flavouring agent and some of the pungency is lost during the

dehydration process. White bulbs are preferred to either yellow or red varieties. The

pigment quercetin in yellow onion is a bitter principle with inferior flavour. For economy

in field harvesting and plant preparation, large bulbs are desired for processing. The

bulbs should be able to hold up in common storage for 2-3 months with a minimum of

rot, shrinkage, or sprouting.

2.8.1 Dehydration of Onion

An experiment conducted at Junagadh Agricultural University recommended

that the dehydration of Talaja local white cultivars of onion should be carried at 76 °C

drying temperature and 27 m/min velocity of air, keeping 3 mm thickness of slice to get

the good quality of dehydrated flakes with minimum drying time i.e., 58 min and the

bacterial counts of the final product falling in the range of acceptable limit of

international standard.

In indystry for dehydration of onion, initial operations such as cleaning, removing tail,

cap and loose skins are carried out by using traditional manual method. Skin layer is

removed by using abrasive centrifugal mechanical peeler. The peeled onion bulbs are

then transfer to a agitating water tank where the agitations of water creates washing

effect and which helps to remove other foreign material attached to the bulbs. After

washing the bulbs are supplied to the slicer. The clearence in the cutting blades of the

slicer is maintained such that it makes slices of required sizes; generally 3 mm in

thickness. These slices are fed to conveyor in uniform bed thickness. The conveyor

pass through dehydrator. The dehydrator used for dehydration is of zigzag type while

the air flows cross to the material flow. The hot air enters in the dehydrator from the

bottom portion and passes through the conveyor. The air is heated through steam

radiators. The onion slices are dried in conveyor from 85-90% moisture content to 15-

18% moisture contnent in 4 to 5 hrs. The dehydrated slices obtained from dehydrator is

again dehydrated in bin dryer to reduce moisture content upto 5-7%. Continuous

stirring is required in the bin dryer. The material is then transfer to dry and cool section

because it contain heat and if the temperature of material is not reduce quickly it starts

to absorb the moisture from the atmosphere and the moisture content of the produce is

increased, so to avoide these the product is quickly transfer to the cooling section. The

temperature in the cooling section is maintained 15-20oC. Other operations like grading

of different type of product, removal of damage product pieces, removal of metal and

hair like pieces, packaging and storage are carried out in the cooling section. For

grading of the products the rotating sieve machine is used, while for removing the

metal piece the magnetic separator and metal separating machine are used, also to

remove hair like material the cations generating machine is used. The burnt and

damage pieces of product are removed manually. The final packaging is carried out in

the cooling section, according the packaging size required by the buyers. The product

is packed in four products such as Kibbled, Powder, Chopped and Flakes.

Washing in agitating water

Drying(Continuous, counter flow type dehydrator)

Feeding Conveyor

Slicing(3 mm thickness)

Mechanical peeling (Removal of single layer of sking by using

centrifugal abrasive peeler)

Hand peeling (Manual removal of Cap, Tail and loose skin)

Sorting

Raw material

Bin drying

Grading

Cleaning

Packaging

Storage

Fig. 27 Onion dehydration flow chart

Removal of top Removal of skins

Washing Onion slicer Conveyor for dehydration

Dehydrated onion flakes Bin dryer Sieve grader

Magnetic separator Onion kibbles / flakes

Onion skin Onion kapcha Onion powder

Fig. 28 Various unit operations in onion dehydration industry

2.8.2 Fermented products

Fermented preparations are made from crushed or ground onion leaves and tops.

These prdoucts are used to flavour food at times when onions are not available. Onions

scales may be sun-dried for the same purpose.

2.8.3 Oil

Onion oil is obtained by distillation of minced onions; the yield of oil ranges from 0.002

to 0.03%, depending upon the raw material and processing conditions. It has a flavour

strength 500 times that of the dehydrated product.

2.8.4 Juice

Onion juice contains both flavour and aromatic compounds, their precursors and

sugars. They may be blended with volatile oils to resotre a rounder flavour profile. The

juice, which is visous and dark brown in colour, can be mixed with a support (such as

propylene glycol, lecithin, or glucose) to produce an oleoresin having a flavour intensity

10 times that of dehydrated onion powder.

2.8.5 Other products

Canned and bottled onions are used in catering industris. Onion rings are common

products in fast food industries.

Onion pickled is popular in many countries. Onion is steeped in 10% brine for 24-96

hours, with lactic acid being added to control fermentation. They are washed, covered

in vinegar, and pasteurized; bisulfite may be added to maintan the colour. The

undesirable yellow spots caused by flavonoids reduce the quality of bulbs for pickling,

hence flavonoid-free varieties shuoud be used.

3. OVERALL LOSSES

Estimated loss of total onion crop in developing countries is high and can reach

20-95% (Anon, 1978). Post-harvest losses of 16-35% have been reported (Steppe,

1976). Exact data on the nature and extent of these losses at each step in the

postharvest chain is not readily available in the literature However, losses of over 9%

has been reported for Spring onion between wholesale and retail (Amuttirantana and

Passornsiri, 1992).

4. Economic and Social Considerations

Although onions are important food sources in comparison with the other major

root and tuber crops (cassava, yams, potatoes and edible aroids), they are part and

parcel of human diet in both developed and developing countries. Due to their ability to

grown in both tropical and temperate regions, the growing and handling of onions has

received considerable attention in agricultural research and development. Despite the

advancements in mechanical harvesting, curing and storage of onions, losses can be

high for many small-holders and medium-scale farmers when lack access to improved

techniques due to their limited financial resource. Improvements in hand harvesting and

curing techniques can enhance the efficiency of small-scale farmers, particularly those

who grow most of their crop to sale. These improvements must be targeted to improve

to the welfare of rural farmers, particularly women and children who carry out most of

the operations in subsistence farms.

5. References

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Anon. 1978. Report of the Steering Committee on Postharvest Food Losses in Developing countries. National Research Council, National Science Foundation, Washington DC.

Anon. 2005. Tropical, Subtropical Fruits and Flowers Cultivation. By NIIR Board, Published by National Institute Of Industrial Re.

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