Post-harvest precooling of fruit and vegetables


Pre-cooling is the key component in the preservation of quality for perishable fresh produce in post-harvest systems. It is likely the most important of all the operations used in the maintenance of desirable, fresh, and saleable produce. Precooling is defined as the removal of field heat from freshly harvested produce in order to slow down metabolism and reduce deterioration prior to transport or storage.

Proper pre-cooling preserves product quality by:

  •  inhibiting the growth of decay-producing microorganisms
  •  restricting enzymatic and respiratory activity
  •  inhibiting water loss
  •  reducing ethylene production

The importance of precooling

  1. Importance of lag time between harvest and cooling

Field heat can cause rapid deterioration of some horticultural crops and therefore it is desirable to remove this heat as quickly as possible after harvesting. When it comes to produce quality, every minute counts and that precooling is among the most cost-effective and efficient quality preservation methods available to commercial crop produces. For example, strawberries experience increasing deterioration losses as delays between harvesting and cooling exceed 1 hr.

 

  1. Influence of precooling on the respiration rate

The rate of deterioration after harvest is closely related to the respiration rate of the harvested product, therefore the reduction of respiration rate is essential to preserving market quality. Since the rate of respiration is influenced by temperature, precooling to remove the field heat before storage will reduce the respiration rate, and hence deterioration will decline accordingly. For example reduction in the temperature of 9.50 C in grapes halved the rate of respiration and doubled their keeping quality.

 

  1. Influence on metabolism

The increase in the rate of deterioration is related to the metabolic processes of the crop. Within the plants' temperature range, the rate of deterioration increases logarithmically with increasing temperature. Metabolic rates double for each 100 C rise in temperature. it can be seen that the quicker the temperature is reduced the fewer losses that can occur. Hence, precooling is essential in order to reduce metabolic changes such as enzyme activity and to slow the maturation of perishable produce.

 

  1. Effects of rapid cooing on ethylene

The reduction in temperature has the added advantage of reducing the production and sensitivity of the produce to ethylene that accelerates ripening and senescence. Therefore, the faster and more promptly the field heat and hence temperature is reduced after harvest, the quicker these deteriorative processes are retarded and hence the more of the initial quality can be maintained.

 

Methods for Precooling Produce

 There are seven principal methods of pre-cooling fresh produce:

1) Room cooling

Room cooling: a simple pre-cooling practice that includes placing the crops in a refrigerated room or container. Although the practice has relatively low energy requirements, it’s very slow and therefore recommended for crops that decay slowly.

2) Forced-air cooling

Forced air cooling removes field heat by using fans that circulate cool air throughout the storage chamber at high speed. This method is effective for pre-cooling of already packed berries and stone fruits.

3) Hydro-cooling 

Hydro-cooling: one of the most common and effective pre-cooling methods. The method includes spraying or immersing the crops in cold water. One more advantage of hydro-cooling is that it also cleans the crop. However, various pathogens can be spread by water. Therefore, hydro-cooling requires adequate water quality and sanitation management. For this purpose, some farmers use chemicals to prevent the occurrence of bacterial or fungal diseases.

4) Ice cooling 

Ice cooling: a method that includes applying crushed or granulated ice into a storage container or a box. As the ice melts, the cool water reduces the heat of the crops. The method is mainly used during crop transport.

 5) Vacuum cooling

Vacuum cooling: a method in which air is drawn out of the chamber. That way, pressure allows evaporation of crop moisture. Since a high amount of heat is required for water evaporation, this method is the fastest way of pre-cooling. This method is highly effective for leafy vegetables. In addition to vacuum cooling, farmers can also add water spraying in order to speed up the process of reducing thermal energy in the crops.

6) Cryogenic cooling

In cryogenic cooling, the produce is cooled by conveying it through a tunnel in which the liquid nitrogen or solid CO2 evaporates. However, at the above temperatures, the produce will freeze and thus be ruined as a fresh market product

7) Evaporative cooling

Evaporative cooling is an inexpensive and effective method of lowering produce temperature. It is most effective in areas where humidity is low. Dry air is drawn through moist padding or a fine mist of water, then through vented containers of produce. As water changes from liquid to vapor, it absorbs heat from the air, thereby lowering the produce temperature. The incoming air should be less than 65 percent relative humidity for effective evaporative cooling. It will only reduce temperature.

Considerable loss in quality and shelf life can occur as a result of holding harvested produce in the field before pre-cooling

Once the crops are harvested, farmers have a big responsibility to do their best in order to protect the final yield and preserve crop quality. Pre-cooling is a simple but effective practice that reduces fruit respiration rate, and therefore, it's most definitely worth a try.

 

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