The effects of water and glass transition on the hydrolysis of sucrose by invertase in noncrystalline carbohydrate systems were investigated. Maltodextrin/sucrose (2:1) and maltodextrin/lactose/sucrose (1:1:1) were dissolved in distilled water. Invertase (10 mg/17.2 g) was added. Amorphous samples were produced by freeze-drying the solutions. Sorption isotherms were determined gravimetrically at 24 degrees C over the 0.113-0.763 aw, and over 0.239-0.764 aw, the glass transition, Tg was determined using differential scanning calorimetry (DSC). DSC and water sorption results suggested that samples remained noncrystalline. Sucrose inversion was analysed by monitoring glucose content during storage. Sucrose hydrolysis occurred at significant rates at 0.662 and 0.764 aw. The rate increase was not related to the apparent glass transition of the systems.

Relation between the thermodynamic parameters obtained from water sorption isotherms and the degree of reduction in the glass transition temperature (Tg), accompanied by water sorption, was quantitatively studied. Two well-known glassy food materials namely, wheat gluten and maltodextrin were used as samples. The difference between the chemical potential of water in a solution and that of pure water (), the difference between the chemical potential of solid in a solution and that of a pure solid (), and the change in the integral Gibbs free energy () were obtained by analyzing the water sorption isotherms using solution thermodynamics. The parameter correlated well with ΔTg (≡Tg − Tg₀; where Tg₀ is the glass transition temperature of dry material), which had been taken to be an index of plasticizing effect. This indicates that plasticizing effect of water on foods can be evaluated through the parameter .