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The water spreading pressure was evaluated as a criterion of food stability in the relationship with the monolayer and minimum integral entropy of the water adsorbed on the surface of the freeze-dried Pleurotus ostreatus powder. The monolayer and the thermodynamic functions were calculated from the water adsorption isotherms obtained by a static method at 25, 35 and 45 °C, over a range of water activities from 0.11 to 0.79, and the surface of the powder was investigated using low-temperature nitrogen adsorption. The color parameters (L*, a*, b*), laccase activity and antioxidant capacity of the product were evaluated during storage at different water activities at 25 °C. The maximum color stability occurred in the range of water activity from 0.23 to 0.32, which coincided with the low tendency to contraction or spreading of the water adsorbed on the surface. These values correspond to the water activities of the monolayer and minimum integral entropy due to that the water film was in a condensed phase. The results also showed that the water activity affects the enzyme stability significantly, but slightly the antioxidant capacity of the mushroom during the storage. The study concludes that the analysis of the spreading pressure contributes to determining the more suitable conditions for storage of the dehydrated foods; in addition to that, it has the advantage of being calculated from an isotherm of water adsorption. [ABSTRACT FROM AUTHOR]

Fractal analysis, using water adsorption and an image analysis technique, was applied to investigate the effect of surface irregularities on the thermodynamic properties of matrix calcium-sucrose powder moisture sorption. The sorption isotherms were evaluated at 15, 25, and 35°C for calcium-sucrose powders obtained by a cryogenic process (CP) and spray drying (SD). Solubilization of the powders was observed after 20 days of storage at 25°C, and the powder CP had greater physical stability than SD. There was good agreement among the fractal dimension computed by scanning electron microscopy analysis and water adsorption isotherm. The surface fractal dimensions were between 2.19 and 2.44 for CP and were larger than those obtained for SD, which were between 2.11 to 2.26. The differential and integral enthalpies increased as the surface geometry became more irregular, also increased the interactions between the powder surface and water molecules. These results suggested that the increase in the matrix irregularities for a product can displace the minimum integral entropy toward high water activity, thereby improving the storage stability. [ABSTRACT FROM AUTHOR]