Precooling is of significant importance in maintaining the quality and the shelf-life of fruits and vegetables. Precooling is usually the first step in postharvest operations of fruit – and is done as soon as possible after harvest. Ideally, precooling is considered the key element in modern marketing chain of fruits and vegetables and is accomplished before the produce is cold stored or loaded into refrigerated trucks for shipment.
Fruits that are harvested at relatively high temperatures contain large amount of field heat. This high field heat leads to moisture loss due to continuing physiological processes of the harvested fruit resulting in loss of quality. The field heat, if not properly removed after harvest, causes water loss, wilting and shriveling leading to an undesirable damage in the appearance and quality of produce. Various precooling techniques such as hydro-cooling, forced air cooling, room cooling, icing, cryogenic cooling and vacuum cooling are now being used in the horticultural industry.
Hydrocooling (HC): Hydrocooling is a simple low-cost method that quickly lowers the temperature of the product. It utilizes chilled or cold water for lowering the temperature before further packaging of the product. Various types of hydrocooler are available, some of which are: conventional (flood) type, immersion type and batch type. In the batch system, chilled water is sprayed over the product for a certain period, depending on the initial product temperature and season. Water has high thermal conductivity and uniform contact between cold water and the surface of a product causes temperatures to drop quickly.
Forced Air Cooling (FAC): Forced air or pressure cooling is a modified type of room cooling and is accomplished by exposing produce packages to high air pressure on one side than other. There are different types of forced air-cooling arrangements such as air circulation at high velocity in refrigerated rooms, forcing air through the voids in bulk products as it moves through a cooling tunnel on continuous conveyors and forced air flow by the pressure differential technique through packed produce. Forced Air Precooling (FAC) helps reducing the rate of respiration of horticultural products and process of senescence and hence, adopted in a wide range of commodities. 
Room Cooling (RC): Room cooling is an effective method to precool the produce that is sensitive to free moisture or surface moisture and the produce that does not deteriorate rapidly. Room Cooling is simply placing the produce to be cooled in a refrigerated room in order to cool the product to the desired temperature. It is very slow method of precooling and therefore recommended for crops that decay slowly.
Vacuum Cooling (VC): Vacuum cooling uses a negative pressure (vacuum pump) to allow evaporation to occur. The moisture within the product evaporates at this low pressure, which cools the product. The main drawback of vacuum cooling is that it dehydrates the product to some degree.
Ice Cooling (IC): Ice cooling (or ice injection cooling) is the method of mixing ice and water into a slurry that is “injected” into the product packaging and cools by direct contact.
Important tips for precooling of produce
- Do not load pre-cooling facility beyond its optimum capacity.
- When stacking produce, allow adequate air-circulation to ensure all vegetables or fruits can be evenly cooled.
- Use proper vented boxes and baskets for forced-air cooling, and waxed cartons or Styrofoam boxes for hydro- cooling.
- Transfer produce out from the pre-cooling facility immediately after pre-cooling, to avoid overcooling or dehydration of the vegetables.
- Use potable water for hydrocoolers to minimize any food safety concerns.
- Separate ethylene-sensitive vegetables from ethylene producing ones.
Kashika Mahajan is an M.Tech. in Processing & Food Engineering from Punjab Agricultural University, Ludhiana. Her area of research interests are precooling, storage and postharvest handling of fruits and vegetables. Currently she is working as Asstt. Manager in Global AgriSystem Private Ltd Gurugram.
Dr Kuldeep Singh Bhullar is serving as Fruit Scientist in School of Organic Farming, Punjab Agricultural University, Ludhiana. He has 14 years of experience in nutrient management, production, breeding and postharvest management of fruit crops.
Dr. BVC Mahajan is specialized in postharvest horticulture and has more than 30 years of
experience in research on packaging, storage and ripening of fruits and vegetables. He has
developed several protocols for export and distant marketing of fruits and vegetables and
published 130 research papers in peer reviewed Journals. Currently, he is serving as the Director of Punjab Horticultural Postharvest Technology Centre at Punjab Agricultural University, Ludhiana.
