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 ...

Brunensis, 2013, LXI, No. 6, pp. 1769-1777 Water sorption tests of Roselle (Hibiscus sabdariff a L.) carried out under laboratory conditions are presented together with mathematical analyses of the moisture sorption isotherms (MSI's). Moisture equilibrium data for adsorption and desorption of water from Roselle powder were investigated at near ambient air temperatures in the range of 5 and 35 °C and water activity (Aw) ranging from 0.11 to 0.97. The manometric method has been used for water sorption tests. Models for MSI's are exponential equations. Coeffi cients of determination are 0.998 and 0.996 (for adsorption and desorption at 5 °C, respectively), 0.998 and 0.999 (for adsorption and desorption at 20 °C, respectively), and 0.998 and 0.999 (for adsorption and desorption at 35 °C, respectively). The equilibrium moisture content (EMC) of Roselle samples increased with an increase of Aw at a constant temperature both for adsorption and desorption. Adsorption curve equates to desorption curve at higher temperatures of tests carried out. Critical values of EMC of samples tested corresponding to the Aw equal to 0.6 were between 13.401% moisture content wet basis (MC w.b.) and 15.934% MC (w.b.) for moisture adsorption and desorption, respectively. These values are useful for storing conditions optimisation from point of view microorganisms grow and structural changes analyses. Crystal structure changes were observed during adsorption and desorption in the microscope, too. It was found out glass transition in dependence on the water content of samples tested.water activity, equilibrium moisture content, hysteresis, prediction, grow of microorganisms, glass transition Roselle (Hibiscus sabdariff a L.) belongs to the family Malvaceae. It originated in India and it is widely distributed in tropical and subtropical regions as a potential new food crop of considerable economic potential. Roselle is a short-day annual erect shrub and can grow to a height of 1-3 m, depending on the species. Its calyces have been suggested for the production of so drinks, Roselle tea, jam, juices, and natural food colourants (Chen et al., 2005;Chang et al., 2012). The extracts of calyces and leaves of Roselle show a signifi cant antihyperlipidemic activity (Ochani and D'Mello, 2009).Roselle calyx (Fig. 1) is usually harvested at high MC (w.b.), 85%. Therefore, drying is an important postharvest treatment prior to reduce the MC and to increase the shelf life. Moreover, the main purpose of drying the products is to minimize packaging requirements, to prolong the shelf life and to reduce shipping weights (Vengaiah and Pandey, 2007). Tab. I shows the chemical composition of Roselle calyces (Babalola et al., 2001).

Received: August 30, 2013VLÁŠEK VÁCLAV, LANGOVÁ JITKA, ŠTENCL JIŘÍ: Eff ect of modifi ed atmosphere packaging on stability of three kinds of bread. Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis, 2013, LXI, No. 6, pp. 1881-1887 The study evaluates eff ect of ordinary and modifi ed atmosphere N 2 and CO 2 on bread packaging and changes of selected bread parameters, moisture content (MC) and water activity (Aw), in real storage time. MC has a signifi cant infl uence on physical properties of bread and Aw characterizes its microbiological stability. Both parameters are functionally dependent. Manometric method was used for measurement of Aw and gravimetric method was used for measurement of MC. Samples of three kinds of bread, gluten-free, Šumava bread, and Bavorský bread, packaged into barrier wrappers, were stored at the room temperature of 20 °C. MC and Aw were measured in real storage time in intervals of three days for all kinds of bread, always in crust and crumb separately. Individual measurements were fi nished when a mould were discovered in samples.

Moisture sorption isotherms (MSI’s) of skimmed milk powder in the temperature range of 5–20 °C were determined using manometric method. MSI’s, which show the water content versus water activity (Aw) at a constant temperature, are used to describe relationships between water content and equilibrium state relative vapour pressure (RVP). The equilibrium moisture content (EMC) of skimmed milk powder samples is growing with an increase of Aw at a constant temperature both for water adsorption and desorption. Isotherms were found to be type II of Brunauer-Emmett-Teller classification. It is the type most common for foods. The shape of created isotherms was sigmoid. Structural modifications of crystals were observed during adsorption in the microscope, too. Critical value of EMC of tested samples corresponding to the Aw equal to 0.6 for adsorption was 6.50% MC (w.b.) at temperature 5 °C, 9.15% MC (w.b.) at temperature 10 °C, and 7.71% MC (w.b.) at temperature 20 °C. These values determine optimal conditions for storage from the point of view microorganisms grow, Aw<0.6.