Glass Transition and Glycerol Effects on Sucrose Inversion in Pullulan-Sucrose Systems

Glass transition and glycerol effects on sucrose inversion were investigated in pullulan-sucrose systems at various aw using DSC, sorption isotherms, and determination of sucrose hydrolysis by spectrophotometry. Water and glycerol plasticized the systems. DSC thermograms indicated phase separation in the systems depending on water content. Glycerol increased sucrose hydrolysis at 0.538 aw. INTRODUCTION Quality and shelf life of low-moisture, amorphous foods are often affected by enzymatic changes during storage. Changes in the physical state and structure often result from glass transition 1, . The glass transition occurs over a temperature range and it may be accompanied by an increase in diffusion-controlled physical or chemical processes. In the glassy state, the food systems have a high stability and viscosity, but at temperatures above the glass transition, various time-dependent and diffusion-controlled changes can be observed , supporting an assumption of increased rates of chemical and enzymatic reactions. Food materials are significantly plasticized by water. At increasing water contents, the materials also have higher water activities (aw) and several reactions in low-moisture food systems have been shown to exhibit higher rates above specific water contents and aw. Other low molecular weight compounds, e.g., polyols, are also reported to plasticize food materials. For example, glycerol is a non-toxic and colorless liquid with a sweet taste and syrup-like consistency that can be mixed with water. It is used as a softener or plasticizer to improve flexibility, smoothness and pliability of cellophane film and casings used as sausage skins as well as in starch-based edible films and coatings. Our previous studies47 suggested that invertase activity appear at intermediate water activities, and in lactose-sucrose models the rate of sucrose inversion increases concomitantly with lactose crystallisation. Pullulan is a water-soluble extracellular polysaccharide. Due to its non-toxicity, nonirritating properties and resistance to oxygen permeability, it is used for producing films, binders, adhesive thickeners, viscosity improvers, and coating agents. Thus, pullulan has a number of potential uses in the pharmaceutical and food industries, and in biotechnology. However, investigations on physical properties of pullulan-based products are still scarce. The objective of the present study was to investigate the effects of glycerol and water on sucrose inversion in pullulan-sucrose blends in order to elucidate pullulansucrose-glycerol interactions and to examine the relationship of the glass transition and invertase activity. EXPERIMENTAL The pullulan-sucrose-invertase-glycerol (PSIG) system was prepared as follows: Glycerol (5g) was mixed to 500ml of distilled water. Sucrose (33.3g) was dissolved into the solution and pullulan (66.6g) was added under a soft heating and stirring until dissolution was completed. The solution was cooled at 5°C. Invertase (58.2mg) dissolved in 5ml of distilled water beforehand was added to the mixture and quickly mixed. Aliquots of the solution (5ml) were prepared in 20ml glass vials and pre-frozen at –20°C for 2h. The samples were then stored in a freezer at –80°C for 24h and freeze-dried for 4 days at a pressure < 0.1 mbar. The freeze-dried samples were stored for 5 days in vacuum desiccators over P2O5 for further dehydration. The same procedure was followed in preparation of pullulan-sucrose (PS) and pullulan–sucroseinvertase (PSI) systems without invertase and glycerol and glycerol addition, respectively. PSI and PSIG were used for water sorption isotherm, differential scanning calorimetry (DSC) and kinetic studies while the PS was used for physical characterization of the system using water sorption isotherm and DSC studies. 0 5 10 15 20 25 30 35