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·tencl J.: Water Activity of Skimmed Milk Powder in the Temperature Range of 20 -45 °C. Acta Vet. Brno 1999, 68: 209-215. Water activity data for adsorption and desorption of moisture from skimmed milk powder were investigated at temperatures in the range of 20 -45 o C and moisture content of the material tested from 3.2 to 20 % (wet basis). The experimental procedure used was a gravimetric dynamic method with continuous registration of sample weight changes. Four mathematical models of sorption isotherms (Chung-Pfost, Halsey, Henderson, and Oswin) were evaluated to determine the best fit for the experimental data. The modified Oswin equation was a good model for moisture adsorption and desorption of skimmed milk powder. Water sorption capacity decreased as temperature increased. The critical value of equilibrium moisture content of milk powder tested, corresponding to the water activity equal 0.6, was 11 % (wet basis) at the temperature of 20 o C. Repeated rehydration of the material brought an increase in the original equilibrium moisture content 3.2 % (wet basis) to 6.3 -8.1 % (wet basis) in relation to the temperature. It was also demonstrated that an increase in equilibrium moisture content was very small (about 4 % wet basis) in the range of water activity 0.1 to 0.9. Higher levels of water activity than 0.9 resulted in a marked increase of equilibrium moisture content and susceptibility to spoilage by microorganisms. The hysteresis effect between moisture adsorption and desorption was insignificant.

The study presents results of water sorption tests of whole milk powder in the temperature range of 5-35 °C and water activity (a w ) from 0.11 to 0.97. The experimental procedure used was the manometric static method. Four sorption models recommended in literature sources (ChungPfost, Halsey, Henderson, Oswin) were analysed and evaluated with the aim of a w prediction. The modified Oswin's equation was the best model for moisture adsorption and desorption of the whole milk powder. Critical values of the equilibrium moisture content (EMC) from the viewpoint of microorganism growth corresponding to the a w = 0.6 were calculated for the temperature range tested. The critical EMC was 7.82% and 8.51% wet basis (w.b.), for water adsorption and desorption, respectively, at the temperature of 20 ºC. Sorption capacity of samples tested decreased as temperature increased, and vice versa. The differences between the EMC values at a constant a w were small in the temperature range measured, and rehydration of the dried material resulted in hysteresis but this effect was non-significant. Sorption isotherm, moisture content, modelling, microbial growthWater activity (a w ) is a useful measure of water availability for the growth of various microorganisms (Roos 2002) and physicochemical stability of biological materials in general. Relationships between a w and growth of moulds, yeasts and bacteria were described by Beuchat (1981); the microbial proliferation starts at about a w = 0.6. Moisture adsorption (desorption) isotherms provide a graphical representation of the water adsorbed (desorbed) by a hygroscopic material at various near-ambient air humidity at a given temperature and pressure. Each point of the moisture sorption isotherm (MSI) corresponds to the equilibrium moisture content (EMC) of the wet material under nearambient air conditions. Under these circumstances, the heat and mass exchange between the material and surrounding atmosphere does not occur. At equilibrium, the a w is related to the equilibrium relative humidity (ERH) of the near-ambient air (Rao and Rizvi 1995). Moisture sorption isotherm of biological materials, especially of food, is usually described as a plot of an amount of water adsorbed/desorbed as a function of a w . Most of these materials follow a sigma-shaped curve corresponding to type II of the BET classification (Rao and Rizvi 1995). The course of the curve is the result of the additive effect of the Raoult's law, the capillary effect and surface-water interactions. There are two inflections, one around an a w value of 0.1-0.3, and the other at 0.7-0.9. These are the results of changes in the magnitude of separate physicochemical effects. There are numerous models for MSI and for predicting the relationship between EMC and a w at a constant temperature in literature data. These models are theoretical, namely BET and GAB and semi-empirical. For instance, Halsey, Henderson, Chung-Pfost, and Oswin, and typically, an empirical model is Peleg (Schuchmann et al. 1990). These models are ...

To reduce the adverse eff ects of hot air drying, ginger slices were pretreated prior to drying. The eff ects of 2 pretreatment methods (blanching versus dipping in 0.1% ascorbic acid solution) and drying temperatures (60, 70 and 80 °C) on the drying characteristics and the qualities of the dried ginger slices in terms of total phenolic content (TPC), antioxidant activities (DPPH and ABTS methods) and color were investigated. It was noted that the moisture content for both un-pretreated and pretreated samples decreased more rapidly when drying was performed at higher drying temperatures. However, the drying time of un-pretreated and pretreated samples was not signifi cantly diff erent. The TPC and antioxidant activities of the ginger slices increased markedly a er drying. The increase in these values was higher for pretreated samples. The dried ascorbic acid solution dipped sample exhibited the highest values of TPC and antioxidant activity. Moreover, the total color diff erence of the ascorbic acid solution dipped ginger was slightly lower than the total color diff erence of the un-pretreated ginger. However, the tested drying temperatures, ranging from 60 to 80 °C, did not aff ect the fi nal TPC, antioxidant activities, and color of the ginger samples.