Influence of cooling rate on glass transition temperature and starch retrogradation during low-temperature storage

An increasing amount of research evidence suggests a relationship between the glass transition temperature (Tg) and the optimum storage temperature for a frozen food. In addition, the Tg for a frozen food may be related to the cooling rate through the temperature range of phase transition for water. In order to optimize the storage stability of frozen foods, the conditions during the freezing process must be considered.; The objectives of the study were: (1) to gain a better understanding of factors influencing the state of water in a food as a result of a freezing process and storage at sub-freezing temperatures, (2) to determine the influence of cooling rate through the primary zone of freezing on retrogradation of gelatinized rice starch, (3) to investigate the relationship between starch retrogradation rate and glass transition temperature for rice starch gel, (4) to explore the physical changes in a rice starch gel during storage, resulting from different cooling rates and corresponding glass transition temperatures, and (5) to evaluate the applicability of the WLF equation for predicting the influence of storage temperature on the rate of starch retrogradation.; Sucrose solutions were selected as a model food system. The influences of water content, cooling rate, and annealing on Tg of sucrose solutions were evaluated. The results were compared with literature values obtained using similar methods in an effort to confirm the methodology used in this investigation. Rice starch with 65% w/w water content was heated to obtain starch gel as a second model food system. Differential scanning calorimetry (DSC) was used to measure the Tg and the enthalpy during retrogradation of the starch gel. Two cooling rates were used to investigate starch retrogradation and Tg of starch gel. After cooling to -50C, the starch gels were stored at -15, -10, -5, 0 and +5°C. The Tg and retrogradation of starch gels were measured after storage of 0, 1, 3, and 5 days.; Results indicated that the Tg was reduced at the higher cooling rate. Retrogradation enthalpy increased with storage time. In addition, retrogradation rate during storage was reduced at higher cooling rate. The retrogradation rates at storage temperatures above Tg were significantly higher than at storage temperatures below Tg. These results suggest that the retrogradation rate for gelatinized rice starch can be limited through the control of Tg, as established by the cooling rate. This approach can be used as a basis for optimizing frozen food storage stability based on both the rate of freezing and the temperature of storage.