PHYSICOCHEMICAL STABILITY OF SELECTED FROZEN AND DEHYDRATED FOODS: IN RELATION TO STATE/PHASE TRANSITIONS
SYAMALADEVI, ROOPESH MOHANDAS
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Water activity (aw) concept is traditionally used as a predictive means for microbial, chemical and physical changes in low and intermediate moisture. State of water and food solids, state and phase transitions are also relevant in explaining physicochemical stability of in non-equilibrium food systems as observed by many researchers. However, the relationship between the state and phase transitions and rate of physicochemical degradation reactions in food systems is still not apparent. The broad objective of this research is to evaluate the physicochemical stability of selected dehydrated and frozen food systems in relation to state and phase transitions. Water sorption isotherms were developed by isopiestic method and glass transition temperatures of raspberries were determined by differential scanning calorimetry to identify with the interactions between water and food solids. A state diagram of raspberry was developed, which included glass line; glass transition temperature versus solids content, freezing curve; initial freezing point versus solids content; end point of freezing Tm', corresponding solids content Xs', characteristic glass transition Tg' and corresponding solids contents Xs" of maximally-freeze-concentrated raspberry. Water activity varied significantly at equivalent water concentrations obtained using absorption or desorption however, the glass transition temperatures of raspberries were independent on the method of equilibration. Enthalpy relaxation experiments were conducted in freeze-dried raspberry powder at selected temperatures below its onset glass transition temperature (Tgi) to explore the molecular relaxation process as many physicochemical degradations continue to occur in the glassy state. A larger mean enthalpy relaxation time for raspberry powder at temperatures lower than the Tg suggests molecular level relaxations are much slower at temperatures smaller than Tg. Anhydrous glucose maltose and maltotriose were used to investigate the effect of molecular weight on enthalpy relaxation. Maltotriose may be a better encapsulant or food ingredient than glucose and maltose to reduce structural relaxation and protect bioactive compounds in raspberries during their glassy state storage of food systems as smaller relaxation enthalpies are observed at isothermal aging. Anthocyanin degradation in frozen and freeze-dried raspberries during storage was examined in relation to state/phase transition temperatures. Anthocyanin degradation was significantly greater in rubbery state of freeze-dried raspberries in comparison to the glassy state while state of the system did not influence anthocyanin degradation in frozen raspberries. The influence of state/phase transitions and temperature fluctuations on ice recrystallization during the frozen storage of salmon fillets was systematically investigated by observing the change in ice crystal size. Ice recrystallization was observed in the glassy state of frozen salmon, however, the rate was significantly smaller than that in the rubbery state due to the reduced mobility of unfrozen water. The influence of state/phase transition on physicochemical stability is largely dependent on the nature of food system and type of physicochemical degradation. The findings of this research are important to food industries because they can help optimize storage and distribution conditions and minimize quality loss in dehydrated and frozen foods.