Pre-cooling is done to remove the field heat of the harvested produce, which is detrimental to keeping quality of fruits and vegetables and it is done to retard ripening and senescence processes. Prompt pre-cooling conserves the weight and extends the storage life in tomato. PLW in storage can be reduced from 6 to 2.5% by employing pre-cooling treatments.
Methods of Pre-Cooling:
Several effective methods for rapid removal of heat from produce are in commercial use. The choice of method depends largely on the perishability and refrigeration equipment of the produce its adaptability to a specific method and the availability of facilities.
1. Hydro Cooling:
Cooling with cold water is rapid and effective method of pre-cooling used for cooling a wide range of fruits and vegetable in bulk before packing. Its use is limited for packed commodities because of the difficulty of achieving sufficient water flow through the containers. Flooding, spraying or immersion accomplishes hydro cooling. A properly designed flood system is more efficient than either the spray or immersion system because it combines a great volume of water with rapid movement of cooling medium over the product. When cooling completed, the product must be moved to a cold room or raid reworking will occur.
2. Air –cooling or Room Cooling:
The use of refrigerated air as pre-cooling medium is widely used for pre-cooling packed fruit, but the system is not widely used for vegetables. Pre-cooling with air can be accomplished in a conventional cold storage room, a special pre-cooling , a funnel cooler, or a forced air cooler. Cooling with air requires a longer time than cooling with water or vacuum.
3. Vacuum Cooling:
Leafy vegetables are commonly cooled by reducing atmospheric pressure in artificial hermetically sealed chambers. Reducing the atmosphere pressure also reduces the pressure or water vapour in the chamber and thus cooling is effected. The outstanding advantages of vacuum cooling are the sped and uniformity of cooling of adapted commodities. Leafy vegetables , particularly lettuce is difficult to cool with water or air, but they can be field packed and then cooled quickly and uniformity by vacuum. Commodities like tomatoes with epidermis which is resistant to water movement are not adopted to vacuum cooling.
4. Pre-cooling of Fruits:
Temperature protection of the fruit in the field immediately after harvesting is very important. This can easily be done by moving the fruits to the shade. It is essential to remove the field- heat of fruits in order to have extended shelf life.
Following Methods are Adopted to Pre-cooling the Fruits:
i) Air Cooling:
The air cooling is done by placing the fruits in the cold room. Fruits are placed in well ventilated containers in order to achieve some air exchange.
The hydro-cooling is an old and effective pre-cooling method for fruits. Fruits are dipped in cold water or spray the cold water on the fruits. Some chemicals are also mixed with water used in hydro-cooling to prevent the shade and diseases.
Hydro-cooling appears with a refrigerated CaCl2 solution (21 0C for 10 minutes) would prevent storage disorders. Pre-cooling has got effect on quality of fruits e.g. L-49 variety of guava. The reducing sugar percentage and ascorbic acid content was higher in pre-cooled fruits than without pre-cooled fruits.
iii) Vacuum Cooling:
It is costliest method of pre-cooling. It is rarely used in case of fruits. It is done by reducing the atmospheric pressure. It reduces the pressure of water vapour in chamber which result in evaporation of water from fruits which bring down the temperatures.
Pre-cooling treatment prior to low temperature storage can delay the development of chilling injury in grapes.
Pre-cooling of Vegetables:
When the vegetables are harvesting during hot weather, it is desirable to remove the filed heat of the harvested vegetables to retard the ripening and senescence. Cooling and storage losses can be reduced by pre-cooling when the vegetable harvested during hot period pre-cooling is of vital importance. Eg. Melons.
Stages of ripening a level of field heat and the vegetable also determines the need of processing. In case of Tomato above 26.7 0 C temperatures and ripening is to delayed there is no need for pre-cooling.
Pre- cooling can be done by Following Processes:
i) Placing the vegetables in a refrigerated truck with forced humidified air circulation.
ii) Placing ice in package.
iii) Placing ice in water and passing the vegetables through a spray of cold water.
iv) Vacuum cooling is also done.
v) Hydro-cooling is done by flooding spraying or immersing is a rapid and effective method of pre-cooling.
Water is an excellent material to transfer a heat from the produce to the cooling medium.
Highly perishable leafy vegetables and sealed crops are often cooled by reducing the atmospheric pressure. Leafy vegetables are especially difficult to cool with and then cooled rapidly and uniformity by vacuum cooling.
Asparagus cooled in water 0.5 to 1.0 0C for 9 to 12 minutes and then stored at 2 0C for 1-5 days followed by 2 days at 20 0C reduces water losses and quality reduction during storage.
Capsicum harvested at green when cooled in water at 7 to 10 0C for 10 minutes and then packed in poly bags with or without ventilation shows largest shelf-life than non-pre cooled fruits.
Hydro cooling increases the storage life of melons when stored in 21 % O2 + 0% CO2.