Glass transition and water effects on sucrose inversion by invertase in a lactose-sucrose system

Enzymatic changes are often detrimental to quality of low-moisture foods. I n the present study, effects of glass transition and water on sucrose inver sion ina lactose-sucrose food model were investigated. Amorphous samples we re produced by freeze-drying lactose-sucrose (2:1)-invertase (20 mg inverta se/49.4 g of carbohydrate) dissolved in distilled water. Sorption isotherms were determined gravimetrically at 24 degrees C. Sucrose hydrolysis was de termined by monitoring glucose content using a test kit and the amounts of fructose, glucose, and sucrose using HPLC. The glass transition temperature s, T-g, at various water contents were measured using differential scanning calorimetry (DSC). The BET and the GAB sorption models were fitted to expe rimental data up to a(w) 0.444 and 0.538, respectively. Water sorption and DSC;results suggested time-dependent crystallization of sugars at a(w) 0.44 4 and above. Significant sucrose hydrolysis occurred only above T-g, concom itantly with crystallization. Sucrose hydrolysis and crystallization were n ot likely in glassy materials.