INCORPORATING COLD CHAIN
TECHNICAL critical storage temperature . Prolonged storage periods necessitate freezing of most commodities in which state they can be held indefinitely . Temperatures in this case are maintained considerably below freezing ( -12 ° C to -25 ° C or lower ).
Meats and other non-living substances as a rule present few problems in their preparation for freezer storage , whereas fruits and vegetables often require considerable processing before they can be frozen in order to safeguard them against deterioration while in storage .
STORAGE CONDITIONS
With regards to storage conditions , we find that the main cause of deterioration of products such as meat , poultry , 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 – 90 % are usually applied in cold rooms .
When products are stored in vapourproof 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 .
MIXED STORAGE
With regards to units that are used for mixed storage we find that in small installations , separate facilities for each and every product are economically not justified , hence , a number of products are usually stored together . Storage conditions will then be a compromise , temperatures usually being above the optimum for some products to prevent damage to the more sensitive products stored in the same cold room . Sealing products in plastic or glass prevents evaporation and thus serves to maintain the quality of certain products .
Higher storage temperatures tend to shorten the storage-life of some of the products held in the mixed storage . When products are stored for short periods , however , this is not normally a serious problem . In larger installations , products are grouped for storage and only those requiring similar storage conditions are placed together .
Products which give off an odour should never be stored with other products . Dairy products are highly sensitive to this .
PREPARATION
Some consideration to the preparation of the food stuffs prior to storage is advisable . One of the most important factors that determine the maximum storage time of a product stored under refrigeration is the condition under which it enters the cold room .
Refrigeration only stops or slows down the natural process of deterioration and cannot restore an already deteriorated product to its original condition . Nor can refrigeration improve the quality of a product . For these reasons , only undamaged products of high quality are suitable for storage . Since deterioration of food products starts immediately after harvesting or slaughtering , it is important that products are cooled to the storage temperature as soon as possible . Products that need to be transported over long distances should be pre-cooled and shipped in refrigerated transport .
Other aspects of preparation include , chilling , blanching and the ageing of meat . Product chilling refers to the rapid cooling of a product from harvesting or slaughtering temperature to the storage temperature . Rapid chilling and high air velocities are desirable during this time so that the temperature and vapour pressure of the product are lowered as quickly as possible to avoid excessive moisture loss and shrinkage . High air velocities carry away evaporated moisture and prevent the recondensation of moisture on the surface of the product . High velocities also greatly increase the heat transfer rate . The humidity should be high when chilling to prevent excessive moisture loss .
Blanching is accomplished by immersing the product in boiling water or steam for a short period , the length of which depends on the nature of the vegetable . It ranges from one minute for hulled peas to 10 minutes for corn on the cob .
Refrigeration only stops or slows down the natural process of deterioration and cannot restore an already deteriorated product to its original condition . Nor can refrigeration improve
the quality of a product .
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