Pre Cooling of Horticultural crops

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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 goodtemperature management of fruitsand vegetables after harvest.

It is essential practice in anysuccessful cool chain management ofhorticultural produce.

However, time and temperature arethe two most important features of pre-cooling.

Speed of cooling depends upon:

Accessibility of produce to the refrigeratingmedium,

Difference between the temperature of produceand 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 produceare cooled to their storagetemperature i.e.,

For example, grapes are cooledto1-4°C, potato to 5–9°C, and(Do not wash with water).

Mango, tomato & banana to becooled to > 10 °C.

All fruits and vegetables are mostly cooledby room cooling and or mechanicalrefrigeration.

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Pre-cooling temperature

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Advantages of Pre-cooling

Inhibition of the growth of decaycausing 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 icingProduce 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 beansCarrot Leek SpinachCauliflower Lettuce Sweet cornCelery 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