Glass Transition Changes During Osmotic Dehydration

摘要

The changes in Ataulfo mangoes (Mangifera índica L.) osmotically dehydrated were evaluated through Modulated Temperature Differential Scanning Calorimetry (MTDSC) in order to determine their thermodynamic behavior due to water loss (WL) and solid gain (SG) during the process. These values were related with the effective diffusion coefficients for water (D_(ew)) and solids (D_(es)) into the fruit. The samples were analyzed under frozen conditions. With the derivative of heat flow signal was possible distinguish among overlapping curves that cannot be seen easily in the heat capacity (Cp) signal, even though in the last one the molecular motion changes are best represented. This manner, the glass transition temperature (Tg) corresponding to sucrose in concentrated solution around the solid matrix, was found between -46 and -53° C; also the value referred as Tg' for freeze sucrose solutions as cited by other authors. A direct relationship between these temperatures and the non freezing water content (Wg) was found. Mango Tg variation was in increasing with the WL decrement, as expected, and the melting enthalpy increase exponentially (-144 to -40 J/g) with the soluble solids increment into the fruit (18 to 43° Bx). As moisture content decrease by dehydration, the food matrix became more viscous and the water and solids transfer are delayed; these changes were reflected in the decrease of effective diffusion coefficients, D_(ew) and D_(es) (1.096x10~(-08) -3.679x10~(-08) m~2/s), while Tg in mango samples increase (-31 to -24°C). All these results show the useful with the MTDSC analysis to follow the osmotic dehydration inside the product.