Rapid cooling of horticultural produce
A guide to system selection
©Crown Copyright 1982 - 1993. Reproduced by Garlicworld under licence from the Controller of Her Majesty's Stationery Office.
The quality of most vegetables deteriorates rapidly after harvest. However, the rate of deterioration depends very much on the nature of the commodity and conditions under which it is harvested. Lettuce, for example, exposed to hot dry conditions, wilts within a few hours of harvest but bulb onions show no ill effects at all.Deterioration of Vegetables
Successful marketing depends on reducing this deterioration to a minimum so that the quality of the product reaching the consumer closely resembles that of the freshly harvested crops. Cool-chain handling is an aid to better quality but the chain must remain intact: produce should be displayed on refrigerated shelves for maximum shelf life.
Deterioration falls into three categories:
These biochemical changes lead to yellowing of the leaves, browning of cut surfaces and the development of unwanted flavours and odours. Like all living organisms, vegetables respire and this, through a series of complex chemical reactions, produces heat. The amount of heat produced varies from one vegetable to another. Sprouting broccoli, spinach, peas and sweetcorn have particularly high respiration rates and release large amounts of heat. As a result they are highly perishable and maintenance of quality is best achieved by very rapid cooling. However, bulb onions and main-crop potatoes have much lower respiration rates, and need not be cooled as quickly.
- wilting due to moisture loss;
- rotting due to fungal and bacterial attack;
- colour, flavour and texture changes due to biochemical activity.
Respiration rate of all vegetables increases 2½ times for every 10°C rise in temperature. So cooling crops from 20°C to just 10°C will extend market life. Most crops sell by eye appeal and the 'length of shelf life' is based on this factor. However, for crops such as broad beans, peas and sweetcorn, loss of sugars is equally important. At high temperatures the sugars are converted into starch and loss of 'sweetness' occurs. Thus rapid cooling not only affects the visual quality but also taste. Similarly, asparagus becomes more fibrous with time.