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

The freezing of foods containing significant quantities of sugar and o ther carbohydrate-based systems involves the removal of water (plastic izer) 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 c haracterize the influence of freezing rate on the formation and stabil ity 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 conce ntration-dependent thermal behaviour (T-Tr1on and T-Tr2 values and lev el 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 t he various treatments. Annealing at -35 degrees C for 60 min following slow freezing resulted in the greatest increase in T-Tr1, up to -40 d egrees C (DSC), and an increase in specific volume below T-Tr2 suggest ing that the T'(g) occurred in this temperature range. The freezing ra te had a substantial influence on the resulting stability and formatio n of frozen systems which attest to the importance of kinetic variable s in understanding food freezing.