The shelf life of most foods is enhanced by storage at low temperatures. Figure 1.2 shows the shelf life of several food products as a function of temperature1. The life of all products shown increases as the storage temperature drops, but for certain food products, such as bananas and certain kinds of apples, the optimum storage temperature is higher than the freezing temperature. For the products shown in Figure 1.2 and for most meats, fish, fruit, and vegetables, the storage life can be prolonged by maintaining the product at low temperatures. Many foods are stored in the unfrozen state and rarely are frozen, for example, bananas, apples, tomatoes, lettuce, cabbage, potatoes, and onions. The optimum storage temperatures of several products, from the perspective of both preservation and economy, are shown in Table 1.1. Some fruits can be stored at temperatures slightly below 0°C (32°F) without freezing, because the water in the fruit is in solution with sugars and other substances providing a lower freezing point than 0°C (32°F). Certain meat and poultry products maintain their quality better when stored superchilled at temperatures about -1°C (30°F) at which temperatures they remain unfrozen.
Immediately following harvest, the fruits or vegetables are often warm. While the products would cool eventually if stored in a refrigerated room, the rate of such cooling might be too slow. Sometimes a precooling process3 (distinguished from room cooling) is applied. Forced air cooling, application of ice, hydrocooling where the product is inundated with chilled water, and vacuum cooling are examples of methods of post-harvest cooling. Vacuum precooling is one of the methods used wherein a load of product, such as lettuce, is placed into a chamber and a vacuum pulled until a small amount of liquid water on the leaves evaporates. The process provides rapid internal cooling of the lettuce head. Sometimes refrigeration plants are loaded on truck trailers (Figure 1.3) and moved to growing areas during the harvest season.