Physicochemical Changes in Frozen Products: Glass transition and rheological behavior in frozen starch sucrose hydrocolloids systems

Abstract

Food systems based on gelatinized starch undergo syneresis and texture changes related to amylose and amylopectin retrogradation; these recrystallization processes can affect the acceptability of food products. Many diffusion-controlled deteriorative phenomena have been related to the glass transition temperature. The glass transition temperature of the unfreezable matrix (T´g), determines the stability of frozen systems. Maintaining the storage temperature below T´g the diffusional deteriorative processes are minimized because the frozen matrix is in the glassy state. T´g onset values range between 4.5 and 5.5ºC for corn starch pastes with or without hydrocolloids. The presence of sucrose and different hydrocolloids (1% xanthan gum, guar gum or sodium alginate) in gelatinized starch systems lead to T´g values below 22oC ( determined by DSC).Viscoelastic behavior of the frozen pastes was tested by oscillatory rheometry. An increase of the dynamic moduli G* after slow freezing and during storage at 18°C was related to sponge formation due to amylose retrogradation in starch-sugar systems. The addition of hydrocolloids prevented the sponge formation, however certain textural damage was evidenced. Hydrocolloids addition was beneficial from the technological point of view by decreasing textural damage during frozen storage at temperatures above T´g. The effect of the gums could not be attributed to modifications in T´g but to the lower molecular mobility induced in the rubbery state by increasing viscosity. The storage of food at the usual commercial temperatures (-18oC, above T´g) affects the quality of sugar systems without gums due to amylose and amylopectin retrogradation. However when hydrocolloids are included in the formulations, the usual storage conditions allows maintaining acceptable textural attributes.