Cooling

Normally chilling is the key to the cool chain transport and storage to reduce metabolism and extend shelf life. However, damage as a result of chilling injury can occur in some crops - usually those of tropical or sub-tropical origin - although some temperate crops, such as asparagus and potato, are susceptible at low but non-freezing temperatures (Saltveit, 2003).

Extensive microstructural changes may occur when ice crystallises. The rate of freezing becomes an important variable and affects the crystalline structure of ice and also affects the nucleation of crystals. Slow freezing generally causes extracellular growth as large crystals, while rapid cooling produces more uniform and smaller crystals (Aguilera and Stanley, 1990) with a higher associated quality as a result of lower levels of damage. This is a simplistic account since Ostwald ripening may occur during storage of the frozen material; this is where large crystals grow at the expense of smaller ones. Freezing disrupts tissues and cell walls by the formation of ice crystals; the temperature of freezing depends on the concentration of solutes in the tissue. As with heating, freezing disrupts the means of water transport across cell boundaries and turgor cannot be maintained.

This has consequences for texture loss and also for destructuring since the cells become deflated and the tissue becomes difficult to break open.

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