Introduction
HE MOST COMMON PROBLEMS IN FROZEN FRUITS AND VEGETA
bles are chemical reactions that cause changes in flavor, col- or, texture, and nutritional value, and physical damage that causes loss of turgor and texture changes. The relationship be- tween the final quality of the product and the freezing rate has been widely studied (Rahman and others 1971; Nguyen and Phan 1973; Martí and Aguilera 1991; Fuchigami and others
1994). From the physical point of view, slow freezing rates pro- duce few ice crystals of large size that alter the structure of the product and result in important quality losses. Fast freezing rates produce a large number of small ice crystals that cause less damage. The freezing rate of a food (IIR 1972) is conditioned by intrinsic factors of the product such as thermophysical proper- ties, size, shape, and by extrinsic factors such as the temperature of the freezing medium, the freezing system employed, and so forth. In fruits or vegetables of regular geometry and small vol- ume (peas, bilberries, and so on), it is easy to establish appro- priate conditions for almost homogenous freezing throughout the volume using traditional freezing methods. Nevertheless, freezing large-size foods involves problems due to thermal gra- dients (Otero and others 1998). Although at the surface of the food the freezing rate may be high enough to produce small ice crystals, inside the product local freezing rates decrease consid- erably. causing the usual quality losses. Cryogenic freezing with liquid nitrogen achieves high freezing rates even at the center of the product. This system has the disadvantage of freeze-cracking, which causes critical and irreversible damage. According to Hung and Kim (1996), freeze-cracking occurs be- cause the rapidly frozen outer zone opposes further volume