Se han revisado diversos metodos de determinacion de la actividad del agua. Los metodos pueden dividirse en aquellos que requieren el uso de instrumentos comunes de laboratorio y en aquellos que necesitan para su determinacion aparatos especiales. La principal ventaja de los que necesitan aparatos especiales es que la actividad del agua puede determinarse mas rapidamente

The water activity, pH and other compositional parameters such as the moisture, fat and NaCl contents of various food emulsions were determined: butters, margarine (W/0) and mayonnaises (O/W). Water activity was determined by a gravimetric method (PEC procedure) and an instrumental method (dew-point hygrometer). The mean a(w) values obtained for the samples assayed were 0.904 +/- 0.050 (butters), 0.914 +/- 0.028 (margarines) and 0.947 +/- 0.013 (mayonnaises). The a(w) of the butter and margarine samples could be predicted with an error less than 0.02 units from the NaCl content of their aqueous phase (m) by using the following simple linear regression equation: a(w) = 0.954 - 0.036m. This equation could not be used to predict the a(w) of the mayonnaises, which, however, were obtained by using the Chen equation for mixtures of solutes.

Melting processes of refrigerated fruits and vegetables (Jonathan apple, sour cherry, red currant, onion, cauliflower and cucumber) were studied using the DSC method. Prior to the tests apple and onion were stored without and the other products with refrigeration. The refrigerated products were melted in microwave space, then chopped and homogenized. Measuring was carried out with the juice obtained from the homogenized sample, and the sample containing the fibre was also measured. Besides the DSC examinations, the water content of the samples was also determined by drying in exsiccator at 105 C/degrees/ to constant weight. The DSC curve of the samples examined is a characteristic endotherm curve, with phase shift temperatures varying from product to product. An analysis of the temperature dependence of the phase shift temperature showed that at the beginning of intensive melting 7-23 o/o, while at the usual temperature of cold storage 1-10 o/o of the water content was in liquid state