ex-pansion when the unfrozen inner part of the product undergoes phase transition. When the stress generated in the interior of the product exceeds the resistance of the frozen material at the surface, freeze-cracking takes place. With high-pressure-shift freezing (HPSF), this problem may be avoided since the initial formation of ice is instantaneous and homogenous throughout the volume of the product, thus eliminating internal stresses.
High pressures can be applied during freezing of foods in dif- ferent ways: Phase transition can occur either under constant pressure (pressure-assisted freezing to obtain ice I or other types of ice: ice II, ice III, and so on) or due to a pressure change (pressure-shift freezing) as Knorr and others (1998) pointed out. Two types of high-pressure-shift freezing can be distinguished: one in which expansion occurs gradually (always near the equi- librium curve), and the other in which expansion to atmospher- ic pressure occurs suddenly, thus achieving considerable super- cooling at atmospheric pressure. Fuchigami and others (1997a,
1997b) conducted pressure-assisted freezing experiments to ob- tain several ices different from ice I, and high-pressure-shift freezing experiments with gradual expansion (lasting 1 min from 200 MPa to atmospheric pressure) in carrots. They found that the less harmful freezing methods, for vegetable structure, were pressure-assisted freezing inducing formation of ice III, with smaller specific volume than liquid water, and gradual high-pressure shift freezing. However, these authors did not use high-pressure-shift freezing with abrupt expansions.
The objective of the present work was to analyze the effect of
freezing on the microstructure of 2 whole fruits of large size— peach and mango —by comparing traditional methods and high-pressure-shift freezing with abrupt expansions.