Phase Separation of Ice Crystals in Starch-Based Systems During Freezing and Effects on Moisture Content and Starch Glass Transition

摘要

To better understand the effects of freezing in starch-based food, this work focused on characterizing the decrease in moisture content of the starch fraction during freez-ing, for various total moisture contents (31.0%-245.6% dry-weight basis [dwb]) and under different freezing temperatures (-50° to –20°C), by using differential scanning calorimetry to measure the proportions of freezable and nonfreezable water. At low total moisture content (31.0%-71.6% dwb), most of the water was bound to the starch fraction, whereas, at high total moisture content (103.4%-245.6% dwb), the starch fraction became saturated with a constant moisture content in the range 33.9%-42.5% dwb, whereas the amount of freezable water increased from 60.9% up to 211.7% dwb. The proportion of nonfreezable water also decreased with freezing temperature, from 49.5% dwb at –20°C to 41.4% dwb at –50°C, in the case of a 71.6% dwb total moisture-content system. At low temperatures, the freezing equilibrium was reached more quickly, from 4 min at –50°C to 30 min at –30°C. At –20°C, the equilibrium was not reached after 1 h. By reporting the decrease in moisture content of the starch fraction on a starch state diagram, it became apparent that although all samples even-tually reached the glassy state, samples frozen at –20°C remained in the rubbery state during most of the freezing process. These results suggest that real starch-based prod-ucts frozen at –20°C may experience significant retrogradation before reaching the more stable glassy state.