Pmas Arid University Rawalpindi Pakistan

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PRE-COOLING OF
HORTICULTURAL CROPS
Pre-cooling: Purpose
• Removal of field heat
• Reduce energy required for cold storage
• Marketing flexibility
– Market at an optimum time (economy & quality)
– Market over a longer distance
Importance of pre-cooling
Pre-cooling is the first step of good
temperature management of fruits
and vegetables after harvest.
It is essential practice in any
successful cool chain management of
horticultural produce.
However, time and temperature are
the two most important features of pre-
cooling.
Speed of cooling depends upon:
Accessibility of produce to the refrigerating
medium,
Difference between the temperature of produce
and refrigerating medium,
Velocity of refrigerating medium, and
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Effect of pre-cooling
Pre-cooled -
Alive & Happy Not-Precooled-
Faster Sick and Die

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Pre-cooling temperature
Generally, horticultural produce
are cooled to their storage
temperature i.e.,
For example, grapes are cooled
to1-4°C, potato to 5–9°C, and
(Do not wash with water).
Mango, tomato & banana to be
cooled to > 10 °C.
All fruits and vegetables are mostly cooled
by room cooling and or mechanical
refrigeration.
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Pre-cooling temperature
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Advantages of Pre-cooling
Inhibition of the growth of decay
causing organisms,
Restriction of the enzyme activities,
Reduction of water loss from the harvested produce,
Reduction in rate of respiration and ethylene (C2H4)
liberation, and
Rapid wound healing.
Benefit of pre-cooling
• Prevent wilting
• Slow the decay rate
• Prevent quality loss due to softening
• Reduce ethylene production
• Minimize the impact of ethylene

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Principles of cooling
• Cooling temperature 0 – 14°C
• Reduce temperatures via different means
– Direct cooling - Indirect cooling
Heat sink
medium e.g.
cold water, ice
and mixture
Heat out
Heat sink
medium e.g.
cold air, cold
metal
Heat out
Selecting pre-cooling technique
• Nature of the produce
– Temperature requirement
– Susceptibility to wetting
• Package design
• Production capacity
• Economic factors
• Social factors
Pre-cooling techniques
• Air cooling
– Room cooling
– Forced air cooling
• Hydro cooling
• Ice cooling
– Top icing
– Liquid icing
– Individual package icing
• Vacuum cooling
• Evaporative cooling
Room cooling
• Insulated room equipped with refrigeration
unit
• Disadvantage
– Slow cooling rate
– Not suitable for produce in large containers

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Room cooling Forced-air cooling
• Fan assisted room-cooling
• The fan pulls cool air through packaged produce
and forces the hot air to leave the package
• Cooling rate depends on temperature and the
air flow rate
• 75 – 90% more efficient than room cooling

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Forced-air cooling
Anona Coconut Mango Pumpkin
Avocado Cucumber Melons Rhubarb
Banana Eggplant Okra Strawberry
Breadfruit Grape Orange Summer Squash
Brussels sprout Grapefruit Papaya Tangerine
Carambola Guava Passion fruit Tomato
Cassava Kiwi Pepper Pineapple
Cherimoya Kumquat Persimmon
Pomegranate Litchi Prickly pear
Hydro-cooling
• The flow of chilled water over produce
• Disadvantages
– Limited to produce that are not sensitive to wetting
– Not energy efficient (20-40% efficiency)
• Critical point
– Good water sanitization practice
– Proper packaging
Hydro-cooling

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Hydro-cooling: Packaging
• Wire-bound wooden crates
• Waxed fiberboard cartons
– Should not have solid top
• Mesh bags
• Bulk bin
Hydro-cooling: produce
Ice-cooling
• Ice continues to absorb heat as it melts
• Suitable for
– Produce with high respiration rate
– Dense product or palletized packages
• Relatively energy efficient
– 1 lb of ice cool 3 pounds of produce (85°F to 40°F)
• Maintain low temperature during
transportation
Top-icing
• Crushed ice is added over the top of the produce
• Produce
Broccoli Carrot Leek
Brussels sprouts Chinese cabbage Parsley
Cantaloupe Green onions Peas

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Individual package icing
• Add measured amount of crushed ice over
the produce
• Disadvantages
– Uneven cooling
– Labor intensive
– Limited to low volume product
• Some automated system is available using
ice dispenser & conveyor
Liquid icing
• Injecting slurry of ice & water into the
package through vents or hand holes
• Excellent cooling method for both large &
small operation
• Disadvantages
– Limited to produce that are not sensitive to wetting
– Warm, wet produce is prone to post harvest diseases
Liquid icing
Produce can be iced Produce can be damaged by
direct contact with ice
Artichokes Strawberries
Asparagus Blueberries
Broccoli Raspberries
Leafy green Tomatoes
Watermelon, cantaloupe Green beans
Carrot Cucumbers
Radishes Herbs
Spinach Garlic
Sweet corn Squash

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Ice-cooling: packaging
• Hold strength after wetting
• Example
– Wax fiberboard cartons:
• Minimal openings
• Insulation properties
– Baskets
– Wooden wire-bound crates
– Hampers
– Perforated plastic liners
Vacuum cooling
• Vacuum around the produce causes water to
evaporate rapidly thus reducing the temperature
• Vacuum is created by putting produce in the
metal container. Then, the air is evacuated
• Disadvantage: wilting (if overdone)
• Hydro vacuum cooling
– Spray water onto the produce before vacuum process
Vacuum cooling
• Produce
– High surface: volume ratio
– Produce difficult to cool with Forced-air or
Hydro-cooling
Brussels sprouts Chinese cabbage Snap beans
Carrot Leek Spinach
Cauliflower Lettuce Sweet corn
Celery Peas Swiss
chard

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Evaporative cooling
• Misting/wetting in the presence of dry air stream
(RH<65%) to cause evaporation
• Effective and inexpensive means of providing low
temperature & high RH conditions
• Good for warm season crop such as tomatoes,
pepper, cucumbers or eggplant
Comparison of cooling method
ALTERNATIVE COOLING
METHODS

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Solar assisted cooling chamber
• Temporary fruit storage at farm
• Hollow wall constructed from porous clay bricks
• The wall is kept moist
• Solar energy evaporated the water in the wall
reduce temperature
• Can achieve
– 4-5°C < ambient
– 85-90% RH inside the structure
High altitude storage
• Every 1000 m, the temperature decreases by
1.8°F (1°C)
• Storing produce at high altitude reduce energy
required for cooling
Night ventilation
• Requires
– Large temperature difference
between day and night
– Well insulated structure
– Close vents hole early in the
morning