Glass transition and storage stability for explosion puffing dried jujube powder

Abstract: Since the 1980s, water activity has been commonly used to evaluate the food storage stability, and food storage will be more stable in or below the moisture content of monolayer molecular layer. Water activity is related to the composition, temperature and physical state of the compounds, and the physical state of food compounds is also related to the stability. But recently, some scholars had found the limitations of using water activity to assess the food storage stability, so the glass transition theory was proposed. Glass transition is a well-known change in the state of amorphous materials, and the characteristic temperature is the glass transition temperature. When the temperature is lower than the glass transition temperature, the system is in glass state, energy is low, viscosity is high, and the molecular chain is segmented into "frozen" state, so the system is relatively stable. On the contrary, when the system temperature is higher than the glass transition temperature, the system is in rubbery state, which leads to the viscosity reduction, the free volume increasing and a variety of changes in motion by diffusion-controlled reactions acceleration, so the system is unstable. Therefore, food in glass state is often considered to be stable, and the glass transition temperature has become an important predictor of food quality. With the deep understanding of water activity and glass transition theory, more and more scholars had combined these 2 theories together to predict the stability of food storage. In order to investigate the suitable storage condition of explosion puffing dried jujube (Zizyphus jujube cv. Huizao) powder, moisture sorption isotherm and glass transition temperature were determined at 25°C by the gravimetric method and differential scanning calorimetry, respectively. Three water sorption models and Gordon-Taylor equation were used to fit the moisture sorption isotherms and the glass transition temperature data, respectively. The state diagram of jujube powder was developed using the fitted moisture sorption isotherm and glass transition temperature curve to determine the suitable storage condition. Results showed that the equilibrium moisture content of explosion puffing dried jujube powder increased with the increasing of water activity, and the equilibrium moisture content (dry basis) was increased from 0.06 to 0.76 g/g as water activity increased from 0.113 to 0.902. The sorption behavior displayed "J" type isotherms, which may be due to the high sugar content of jujube powder. The best model to describe the moisture sorption isotherm characteristics of jujube powder was GAB model (R2=0.9968). The fitting curve indicated great fitting effect. The glass transition temperature was decreased with the increasing of moisture content, and the glass transition temperature was decreased from 29.90 to -35.02℃ as moisture content (wet basis) of jujube powder was increased from 0.064 to 0.175 g/g. The Gordon-Taylor equation was used to fit the glass transition temperature nonlinearly, and the correlation coefficient (R2) was 0.9912, which revealed good fitness. When dry basis moisture content was at or below 0.1223 g/g, and the temperature was at or below -0.062℃, the storage condition was suitable for jujube powder.