EFFECT OF FREEZING RATE ON THE THERMAL, MECHANICAL AND PHYSICAL AGING PROPERTIES OF THE GLASSY STATE IN FROZEN SUCROSE SOLUTIONS

The freezing of foods containing significant quantities of sugar and other carbohydrate-based systems involves the removal of water (plasticizer) as ice crystals leading to the complex formation of a metastable amorphous (unfrozen) phase. A study was undertaken using differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA) to characterize the influence of freezing rate on the formation and stability of the glassy state in freeze-concentrated sucrose solutions. Two thermal transitions were evident upon warming these solutions, denoted T-Tr1 and T-Tr2 in this study. Rapidly frozen systems exhibited concentration-dependent thermal behaviour (T-Tr1on and T-Tr2 values and level of devitrification) and reduced mechanical resistance to structural flow (TMA). The physical aging behaviour was freezing-rate-dependent (greater excess enthalpy for rapid freezing) and showed maximum excess enthalpy values in good agreement with T-Tr1 DSC measurements under the various treatments. Annealing at -35 degrees C for 60 min following slow freezing resulted in the greatest increase in T-Tr1, up to -40 degrees C (DSC), and an increase in specific volume below T-Tr2 suggesting that the T'(g) occurred in this temperature range. The freezing rate had a substantial influence on the resulting stability and formation of frozen systems which attest to the importance of kinetic variables in understanding food freezing.