73C-32 |
Enthalpic and entropic mechanisms related to moisture sorption on yogurt |
E. AZUARA1, C. I. Beristain1, and M. D. Ramos. (1) Instituto de Ciencias Básicas, Univ. Veracruzana, Apartado Postal 575, Xalapa, Veracruz, 91000, Mexico Water activity may significantly affect the cell viability or survival of lactic acid bacteria, which is an important quality index for yogurt powder as well as for fresh yogurt. Since sorption isotherms are generally affected by the processing method, it is important to determine and compare sorption isotherms of yogurt powder processed from different drying methods. Determination of the thermodynamic parameters provides a better insight into the sorption mechanism. The objective was to use a thermodynamic method to obtain information about the enthalpic and entropic mechanisms ocurring in the moisture sorption process and its relationship with the stability of the products. Moisture sorption isotherms of yogurt, concentrated yogurt, freeze dried yogurt and freeze dried concentrated yogurt, were selected and processed at 20, 35 and 50°C. Differential and integral thermodynamic parameters were determined using the method of Othmer. Enthalpy-entropy compensation was calculated by plotting enthalpy changes(DH) vs entropy changes(DS). Enthalpy-entropy compensation for yogurt, concentrated yogurt, freeze dried yogurt and freeze dried concentrated yogurt, showed two isokinetic temperatures. The first isokinetic temperature (TB1) was observed at low water actvities and it was controlled by changes in the entropy of water, whereas the second isokinetic temperature was considered to be enthalpy-controlled and it had the same value for all yogurt products (TB2=327.6± 0.9 K). The moisture zone controlled by entropy was larger for yogurt (0-9 g H2O/100 g dry solids) and freeze dried yogurt (0-7.7 g H2O/100 g dry solids) than for concentrated (0-4 g H2O/100 g dry solids) and freeze dried concentrated yogurt (0-3.6 g H2O/100 g dry solids). The best storage conditions for the yogurt products were found at the zone where entropic control finishes and enthalpic control begins. The enthalpy-entropy compensation provides a new tool to improve the stability of foods, modifying the zones controlled by enthalpy or entropy.
Session 73C, Food Engineering: Transport Processes and Kinetics
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