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fish , fruit , vegetables , cheese and eggs , is the loss of surface moisture by evaporation into the surrounding air . This is really important as the effects can destroy a product , sometimes called ‘ freezer burn ’. It causes shrivelling and wilting in fruit and vegetables and the product loses mass and vitamins . It causes discolouration and shrinkage in meats and cheeses . Eggs lose moisture through their shells .
Loss of moisture , or dehydration , will occur whenever the vapour pressure of the product is greater than the vapour pressure of the surrounding air . The vapour pressure of the product depends on its temperature – with the higher the temperature , the higher the vapour pressure . The vapour pressure of the air depends on its dew point temperature , which is a combination of dry bulb temperature and humidity . The lower the humidity at a particular dry bulb temperature , the lower the vapour pressure .
An air relative humidity of 100 % could represent the ideal conditions for preventing evaporation losses . However , product decay is also at a maximum at these conditions . Thus , controlled air movement and a humidity between 80 and 90 % are usually applied in cold rooms .
When products are stored in vapour-proof containers , the humidity and the air velocities are not critical . Hydroscopic products such as dried fruits absorb moisture and require storage at low relative humidities . The rate of evaporation depends on the vapour pressure difference , the amount of exposed product surface areas and the air velocity over the product .
The following tables give storage conditions for perishables . There are complete storage condition charts available , the following being examples to give some guidance . You will see that the relative humidity and maximum storage times are given , not just temperature .
STORAGE CONDITIONS FOR PERISHABLE PRODUCTS
Supplied by Grant Laidlaw
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RACA Journal I April 2024 www . refrigerationandaircon . co . za