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Physical chemistry of freezing: Impact of solutes - carbohydrates |
T. P. LABUZA, Dept. of Food Science & Nutrition, Univ. of Minnesota, 1354 Eckles Ave., Saint Paul, MN 55108 This presentation will review the basic physical chemistry of the freezing process with emphasis on the thermodynamic (freezing point depression) and dynamic (glass transition as a function of temperature) interactions. Of key here is the fact that carbohydrates can be used to lower freezing point effectively, can allow a frozen food to be texturally soft below the freezing point and are used in nature to give cryo-protection to multicellular organisms. Firstly the basis of freezing point depression will be reviewed from the standpoint of Raoult's Law. A comparison will be made between the theoretical depression and the actual depression for the common sugars added to foods, i.e. sucrose, fructose and glucose as well as to other carbohydrate ingredients such as starches, gums and trehalose. Use of the equilibrium diagram will be demonstrated to show the effect of rapid cooling (non-equilibrium) and the consequent move towards the eutectic temperature. Give these lines the next part will review the determination of the Tg curves for sugars as a function of moisture. Emphasis will be placed on the limitations of the DSC methods used as well as the large variability in published data. A question will be brought as to the validity of the extrapolation to the 0% sugar level (100% water) value of -135 C. A comparison of the same ingredients will be made from the literature data available. Give this the state diagram for the sugars will be constructed and used to illustrate the examine the practicality of the premise that frozen foods need to be stored below the glass transition curve.
Session 9, Function of carbohydrates in frozen and refrigerated foods
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