Abstract

Various measures are taken to prevent or inhibit the crystallization of sugar in these products. However, there is still a mechanistic type of approach needed to understand the crystallization of glucose in a complex composition as the exact cause is not clearly defined and the control measures are not yet fully effective. Several factors such as the concentration of sugars, ratio of glucose to fructose and amount of organic acids may all contribute to the crystallization of glucose. The state diagram is a critical tool to understand this process, which implies to carry out experiments to determine the change of the glass transition temperature (Tg) as a function of raisin water activity. Three raisins varieties (Flame, Thompson, Golden) were thin-sliced and placed in desiccators which contained saturated salt solutions under vacuum. Nine different saturated salt solutions were chosen to generate various relative humidities (RH) (range of 0.1 – 0.90). The samples were equilibrated at 25 oC for 2-5 weeks depending on RH. Water activity was measured in Aw Lab Meter and thermal transition was estimated by Differential Scanning Calorimetry (DSC1, Mettler Toledo). Thermal procedure was cooling from 25 to -70°C at 10 °C/min, isothermal at -70 °C for 10 min and heating from -70 °C to 160 °C at 10 °C /min. The state-diagram and the corresponding isothermal sorption were constructed. Tg varied from -20 to -60°C at a water activity of 0.25 and 0.7, respectively, and non-significant differences were found among all three varieties (p<0.05). The water activity of raisins was 0.51, 0.56, and 0.54 for Flame, Thompson and Golden, respectively, corresponding to a Tg of -37, -42, and -41°C, respectively.