Food Crops Postharvest Deterioration

Harvest is a major event for any food that is derived from plants. As a plant part is severed from the plant it loses its source of supply of nutrients and its repository for metabolic waste products. Until the detached plant part undergoes conventional food processing, it continues to live, respire, transpire, and senesce, ultimately leading to death. Postharvest deterioration continues until the item is either processed or consumed. Handling techniques have been developed to slow the physiological processes to provide a product that is satisfactory to the consumer.

Food crops are categorized as agronomic or horticultural. Agronomic (field) crops are primarily the grains and oilseeds that are harvested in a dry to semidry state and tend to be relatively stable to handling and storage as long as they are protected from moisture and insects. Horticultural (garden) crops, which comprise fruits, vegetables, and nuts, tend to be much more perishable, requiring sophisticated handling systems to transport them from field to consumer.

Estimates vary widely on how much of a crop is actually consumed (1,2). Processing techniques such as canning, freezing, and drying are designed to minimize these losses and extend the length of the season they are available for consumption. Postharvest handling techniques that manipulate the storage environment extend the life of the product while keeping it in a fresh state. Minimal processes such as cutting, slicing, and dicing increase the appeal and convenience of the item frequently at the expense of greater perishability (3). Losses of edible product begin in the field during harvesting and loading; continue during transport to the processing plant, packinghouse, or market; during storage at any point in the distribution scheme; and during food preparation or even by the consumer at the point of consumption. Losses may be complete resulting from the discarding of part (removal of outer leaves) or all (discarding a rotten fruit) of a given item. Frequently losses are more subtle and less tangible such as loss of acceptability, nutritional quality, or economic value. An understanding of the scope of losses incurred for a particular fruit or vegetable requires an understanding of the complexity of the handling and distribution system for that item (4,5).

Key concepts provide insight into the perishability of most fresh horticultural crops. Physiological deterioration of a fresh item begins at harvest and continues until processing or consumption. Respiration is the metabolic breakdown of food constituents to release the necessary energy to sustain the healthy tissue. Transpiration is the release of moisture from the surface of the fruit or vegetable. Senescence is genetically programmed deterioration that leads to cell and tissue death (4). Quality refers to the properties of a particular fruit or vegetable that make it unique and influence its purchase and consumption by the consumer. Shelf life is the time period a product can be maintained at an acceptable level of quality (5).

Although most harvested products are at their peak of quality at harvest, climacteric fruits continue to ripen after detachment from the plant. These fruits (eg, apples, bananas, pears, and tomatoes) will develop color, flavor, and textural attributes during postharvest storage. In many cases climacteric fruits do not develop full flavor off the plant, but the perishability of the fully ripe fruits precludes distribution.

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