Onion Processing

1. Dr. R. T. Patil
Director,
Central Institute of Post Harvest
Engineering and Technology, Ludhiana
Post Harvest Management and
Value Addition of Onion

2. Introduction
• The annual average world production is at
around 72 million metric tones.
• China is the leading producer and the other
major onion producing countries are India,
United States, United Arab Republic, the
Netherlands, Japan, Brazil, Bulgaria, Spain,
and Italy.
• In the international trade, these countries play
an important role in exporting the onions
contributing 74.6% of the total export (FAO
2008).

3. Characteristics
• Onion is a short term crop with less
pay-back period.
• Onion is highly perishable and
therefore high post harvest losses
occur during transportation and
storage.
• Onion are also grown in large
quantity in developed countries and
they have better quality and longer
storage life

4. Production & Utilisation
• Onion (Allium cepa) is an important export-
oriented crop of our country.
• Both immature and mature bulbs are used as
vegetable and condiment.
• The cultivation of onion is done at different
times throughout the year in different parts of
the country.
• The crop comes to the market three times in a
year Rainy Kharif 20%, Late little rainy Kharif
20% and Rabi 60% (April-May)

5. Nutritional and Medicinal
Importance
• Onions (Allium cepa L.) are grown as vegetable
as well as spice.
• They are highly valued for their flavour and for
their nutritional value in supplying minor
constituents such as minerals and trace
elements.
• It contains vitamin B and a trace of vitamin C
and also traces of iron and calcium.
• The outstanding characteristic of onion is its
pungency, which is due to a volatile oil known
as allyl-propyl disulphide.

6. Medicinal Effects
• Onion can lower blood lipids and prevent
hardening of arteries.
• It contains selenium that can induce the human
body to release large amounts of glutathione which
lowers the occurrence of cancer.
• Onion flavonoids have antidiabetic, antiaging, and
bacterial-inhibition effects.
• Presence of several sulphur compounds makes it
antiseptic. Onion paste is reported to be effective
for external applications in case of insect bites.
• Onion is said to possess stimulant, diuretic and
expectorant properties and hence useful in
flatulence and dysentery.
• High amounts of sulphur in onions make them
particularly effective in regenerating hair follicles
and stimulating hair regrowth

7. Pre-harvest Operations
• The post-harvest technology combined with pre-harvest
factors determine the shelf-life.
• Restriction on use of excessive and delayed nitrogen
application (not exceeding 100 kg/ha under normal
fertility condition and application of N in 2-3 splits within
60 days of transplanting), use of ammonium sulphate
instead of urea for top dressing, more use of organic
manures to fulfill nutritional requirement, use of
vermicompost, neem cake etc. have also showed
encouraging results.
• Similarly last irrigation given 10-15 days before
harvesting reduces microbial losses in stored onions
compared to withholding of irrigation before five days.

8. Pre-harvest Sprays
• Pre-harvest pesticides sprays of 0.02%
streptocyclin and 0.1% carbendazim ten days
before harvesting.
• Application of thiophanate methyl @ 0.1% +
streptocyclin or klorocin @ 0.2% + ekalux @
0.25% immediately after neck cut and combined
with field and shade curing is promising under
North Indian conditions
• Use of maleic hydrazide @ 2000-3000 ppm 75
days after planting for kharif onions and @ 2500
ppm combined with 0.1% carbendazim spray 10
days before harvesting in rabi onions gives
better post harvest storage.

9. Harvesting
• Stage of harvesting also determines the
shelf-life of onions
• Storage losses at optimum maturity are
normally lower than those harvested
before the tops collapse.
• Harvesting one week after 50% neckfall
and field curing by windrow method for 3-
5 days till foliage turn yellow is
recommended.

10. Curing
• Single most important post-harvest treatment required for the
long storage .
• Curing is a drying process intended to dry off the necks and
outer scale leaves of the bulbs to prevent the loss of moisture
and the attack by decay during storage.
• The essentials for curing are heat and good ventilation,
preferably with low humidity.
• Shade curing for 10 days is optimum to eliminate the chances
of microbial infection in storage.
• Curing of kharif onions in sun or solar dryer with foliage and
storage with dried foliage is better
• Curing in perforated crates with forced air circulation is
recommended for rabi onions.
• Bulbs cured artificially for 16-20 h were ready for market or
further storage compared to 8 – 10 days (240 h) in traditional
sun curing (Centre of AICRP on Post Harvest Technology).
• Artificially cured bulbs could be stored at ambient conditions
for 30 days with a maximum weight loss of about 10.5%

11. Grading
• 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.
• For local market the onions are graded
based on their size.
– Extra large onion (>6 cm dia.)
– Medium (4-6 cm dia.)
– Small (2-4 cm dia.)

12. Irradiation to Improve Shelf-
life
• Exposure of onion bulbs after harvesting
with 60-90 Gy inhibit their sprouting
regardless of crop season, environmental
condition and type of storage.
• However, to reduce the microbial and other
losses, combined use of irradiation with
improved storage and providing the
irradiation facilities at production level may
be considered.

13. Characteristics of Good
Onion
• be reasonably uniform in shape, size colour
and pungency of the variety /type
• be mature, solid in feel, reasonably firm with
tough clinging skins.
• be throughout cured and dried.
• be free from dust and other foreign material.
• be free from defective, diseased, decayed and
damaged bulbs caused by seed stems, tops,
moisture, dry sun scald burn, sprouting,
mechanical or other injuries and staining.
• be free from moulds, soft rot and insect attack.
• % of seed stem or bolted bulbs shall not
exceed 20% in Nasik kharif onions.

14. Graders for Onion
• The drudgery of human
labour involved for
grading of onion has
been reduced by hand
operated as well as
motorized onion
grader developed by
NRCOG, PUNE.
• The cost of grading with
machine is around Rs.
26/t as compared to Rs.
80/t in hand grading.
• The precision of grading
achieved by grader is
98% as against 50% in
hand grading.

15. Packaging
• For safe handling, 40 kg open mesh jute bags
having 200-300 g weight should be used in
domestic market.
• For export, common big onions are packed in
5-25 kg size open mesh jute bags. Bangalore
Rose and multiplier onions are packed for
export in 14-15 kg wooden baskets.
• Nylon net bags, when used for packing have
resulted in less storage loss because of good
ventilation.

16. Handling
• Bulbs intended for storage must be
free from cuts and handled with
extreme care.
• Onions should not be dropped on to
non-resilient surface from more than
6 feet height.
• If onions are to be stacked after
packing in store or trucks, the better
height is 2-2.5 metres.

17. Storage
• Onions should not be stored unless adequately dried either in
the field or by artificial means.
• It is necessary to dry the neck tissue and outer scales until they
rustle when handled otherwise the bulbs will rot in storage.
• Sprouting in onion is controlled by temperature. The
temperature between 10-25°C increases sprouting.
• Rooting is influenced by relative humidity (RH). More the
relative humidity more is rooting.
• Weight loss is more when temperature is above 35°C.
• Under ambient conditions the onions are stored at a
temperature of 30-35OC with RH of 65-70%.
• In cold storage, temperature is maintained at 0-2°C while the
RH is kept at 60-75%.
• Onions are stored in these sheds by spreading them on dry and
damp proof floor or racks. Periodical turning of bulbs or removal
of rotten, damaged and sprouted bulbs should be done.
• Well-ventilated improved storage structures with racks or tiers
having two or three layers of bulbs would be desirable for
proper storage.

18. Good Storage Practices
• Construction of storage godown should on raised platform
• Use of appropriate roofing material to prevent built up of high
temperature inside.
• Increased centre height and more slope is better for air circulation
and preventing humid microclimate inside godown.
• Bottom ventilation for free and faster air circulation to avoid
formation of hot and humid pockets between the onion layers.
• Avoid direct sunlight on onion bulbs to reduce sunscald, fading of
colour and quality deterioration.
• Restriction on width of each stack to 60-70 cm for cool humid
weather, 75-90 cm for mild and humid weather and 90-120 cm for
mild and dry weather conditions
• Restriction of stacking height to 100 cm for small and multiplier
onion and hot weather and 120 cm for mild weather and for big
onion to avoid pressure bruising.
• Cubicles should be made instead of continuous stack leaving
sufficient space for ventilation from all the sides.
• One cubic meter area of store accommodates about 750 kg
onions.

19. Mangalore tiled storage

20. Dehydration / Drying of
onions• The desirable traits for dehydration are white colored flesh, full globe
to tall global shape of bulbs with 5 to 6 cm in diameter, solids content
around 15%;
• After removing the tops, roots and outer skin onions are washed
thoroughly and then cut into slices 3-6 mm thick with a rotating slicer
or bread slicing machines.
• After cutting onion slices are carefully washed. Blanching is not
practiced, as the onion loses its flavor.
• The slices are dehydrated in a tray, cabinet, tunnel type or conveyor
belt type drier. The onions are dried when the ratio of prepared raw
material to drier product is about 9:1 (moisture content about 5%).
• Dehydrated onion has a storage life of about 12 months.
• Care is taken during dehydration to insure minimal powder production
because of its low value.
• Screening, grinding and packaging operations are carried out in
special dehumidified rooms (Below 30% RH ) because of hygroscopic
nature of the dried onions.
• A batch of 50Kg sliced onion takes about 8-9 hours drying time in tray
drier and 16-18 sunshine hours in greenhouse drying. The onions are
then milled into sliced, large chopped, chopped, ground, granulated
and powdered onions.

21. Value Added Products
• Onion salt is prepared by mixing 19 to 20% onion powder
with 78% free flowing pulverized refined table salt and 1to
2% anti caking agent which prevents water absorption,
caking, etc.
• Onion juice is also gaining popularity. It has fresh
characteristic flavour of onion because it is not subjected
to high temperature at which thiosulphates (primary
products associated with onion flavour) dissociate
resulting in cooked onion flavour.
• Natural dye-The pink colored peel in the onion is also an
excellent natural dye. It gives fast colour on cotton, silk
and woolen fabrics.
• Onion pickle-Peeling, Blanching for 5 minutes, Filling
blanched onions in a jar, Addition of salt, Standing for 24-
48 hours, Draining off water, Addition of vinegar and
spices and Storage

22. Poly house solar dryer for onion

23. CIPHET Process of Mechanical
Dewatering and Drying
• In drying main expense is the energy used to reduce moisture content
of onion.
• As a rule of thumb, for each 1% reduction in feed moisture, the dryer
energy input can be reduced by up to 4% (Bureau of Energy
Efficiency, India 2004).
• Process-The onions were washed, peeled and mechanically
dewatered prior to drying with centrifugal juicer.
• Effect of dewatering on energy consumed and quality parameters like
pyruvic acid content, ascorbic acid content, sugar content, rehydration
ratio, non enzymatic browning and color change were evaluated.
• Partial mechanical dewatering resulted in as high as 60.98 % less
consumption in energy than conventional method besides getting high
value products i.e. onion juice and onion powder.
• The optimum combinations are 60% dewatering and 60-70 C
temperatures.
• Onion juice was concentrated to 75% solids for increasing its shelf life.
• Storage studies of onion powder showed that there has been no
significant loss in quality even after 2 months storage at room
temperature.

24. Website: www.ciphet.in
Email-ciphet@sify.com
Ph. 0161-2308669