Class transition and texture in a typical Spanish confectionery product: Xixona turron

The water sorption isotherm at 25C and glass transition temperature were determined in a typical Spanish confectionery product (Xixona turron), as well as in the fat-free product, The samples were equilibrated at water activity levels from 0.113 to 0.755. The GAB and BET models were fitted to sorption data and the monolayer moisture contents were 2.8 and 6.0 (g of water/100 g of dried sample) for turron and fat-free turron, respectively. The sorption data indicate a lipid interaction with hydrophilic constituents. The analysis of the glass transition temperature (Tg) of the product as a function of its moisture content is used to determine the critical water content (CWC) at which the product becomes rubbery, at a determined temperature. From sorption isotherm at the same temperature (25C) we can also determine the critical water activity (CWA). The plasticizing effect of water in Xixona turron can be modeled by the Gordon and Taylor equation. The fat also has a plasticizing effect, which is more obvious at low water content. A Texture Profile Analysis (TPA) was performed on samples with different moisture contents. Only samples with a moisture content lower than CWC showed fracturability, due to its glassy state, In the rubbery state hardness, gumminess and chewiness decrease when water content increases. Elasticity and cohesiveness increase in line with water content, more strongly for those moisture levels which are higher than BET monolayer value.