ISHS Acta Horticulturae 368: International Symposium on Postharvest Treatment of Horticultural Crops
REDUCING THE WATER LOSS OF HORTICULTURAL AND ARABLE PRODUCTS DURING LONG TERM STORAGE |
| Author: | A. Scheer |
| Abstract:
Weight loss of harvested arable and horticultural products consists for a great part of moisture loss due to the evaporation of water from the product. This depends greatly on the vapour pressure difference (VPD) between the air in the product and the surrounding air. The smaller this VPD the less the water loss. This vapour pressure difference is determined by:
During the cooling down period it is hardly possible to reduce the weight loss with the common direct expansion cooling system. Only the application of a well designed pump circulation cooling system gives a better result. The best result in reducing water loss can be achieved by a high cooling rate in combination with a 'wet' cooling system and a forced air cooling method. A well designed wet cooling system creates the highest relative humidity, resulting in a small VPD. During the time of storage and mainly during the long term storage of pears, apples and root vegetables etc. it is very possible to get less water loss than usual with common cooling systems. It is well known that cooling always results in drying out of the fresh product. The control system and the quality of the refrigeration plant have a very great impact on the water loss during the cooling time. In this lecture attention will be focused on the effects of evaporator control on the water loss. From a computer calculation of the heat- and waterbalance in a cold store it was found that on/off switching of the evaporators with short periods of operation and periods of non-operation could perform better than a frequently used method of restricting water loss by means of a cooling capacity control. Reduced cooling capacity always means a longer cooling operation time and a greater effect on water loss. This theory about the effect of water loss by controlling the evaporators has been verified in two small cold storage rooms with 3.5 ton Golden Delicious and Elstar during a 180 day storage experiment. An extra reduced absolute weight loss of 1 % was achieved based on weight measurements of product samples from the cold stores. Reducing the water loss by an extra 30 % is possible by a simple on/off control cooling system equipped with a good quality air cooler. This means in practice enlarging the adjustment of the air temperature difference between the on/off period. The variations in air temperature and the lowest acceptable product temperature | |
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