State Transitions and Enzymatic Activity in Low Moisture Carbohydrate Food Systems

Effects of thermal transitions (glass transition, crystallization), water, and glycerol on invertase activity in freeze-dried carbohydrate systems were investigated. Efficacy of κ-carrageenan to delay sugar crystallization in freeze-dried carbohydrate systems was also studied. Water sorption properties and glass transition temperatures, Tg, were determined for freeze-dried carbohydrate-sucrose systems embedded with invertase, and stored under various relative humidity (RH) conditions, at 24°C. Invertase activity was determined by measuring glucose, fructose, and sucrose contents using HPLC or glucose content using an enzymatic method as a function of storage time. The extent of crystallization in lactose-sucrose-invertase (LSI) and lactose-sucrose-invertase-carrageenan (LSI-carrageenan) was determined by powder X-ray diffraction and reflected on changes in water sorption behavior. DSC thermograms of samples stored at various RH conditions were obtained by scanning samples at a heating rate of 5°C/min from at least 35°C below the glass transition temperature ranges. Relationships between RH, water content, Tg, and rates of sucrose hydrolysis were analyzed. Crystallization in lactose-sucrose-invertase systems was observed at RH as low as 44.4% and it became substantial above RH 53.8% concomitantly with sucrose hydrolysis. In non-crystallizing systems, sucrose hydrolysis was effective at RH 66.2 % and above, but was not directly related to Tg. In systems composed of maltodextrin matrix, crystallization was not manifested in changes of sorption properties. Maltodextrin could be used in preparation of food materials to delay crystallization. Water sorption study and X-ray diffraction results demonstrated that κcarrageenan was effective in delaying crystallization. This result is of great importance for industrial processes where crystallization needs to be controlled. Glycerol enhanced the rate of sucrose hydrolysis. Plasticizers, such as glycerol, can affect the enzyme activity even at low RH. Water is the most important plasticizer but also a key factor controlling the activity of the enzyme and possibly plasticization of the enzyme itself.