Study On The Storage Stability Of Agaricus Bisporus Based On Water Activity And Glass Transition Theory

Based on the water activity (aw) and glass transition theory, this paper discussed the influence of the two theories on the storage stability of Agaricus bisporus. The results can provide theoretical basis and technical approach for quick frozen, freeze drying of Agaricus bisporus. The main research contents and conclusions are as follows:1. Moisture adsorption isotherms of Agaricus bisporus were determined at different temperatures (5,15,25and35℃) using a static gravimetric method. Nine models were used to fit experimental data. The net isosteric adsorption heat of Agaricus bisporus calculated by means of the Clausius-Clapeyron equation in the range of5-35℃and aw in0.11-0.93.The results showed that the adsorption isotherms of freeze dried Agaricus bisporus followed "J" shape. At selected aw, the EMC increased with the temperature reduced. Among the models tested, the Halsey model showed the best fit in the range of5-35℃and aw in0.11-0.93. The net isosteric adsorption heat of Agaricus bisporus decreased from12.12to0.30kJ/mol when the moisture content increased from8to50g/100g dry matter.2. Differential scanning calorimetry (DSC) was used to determine the Tg and freezing point (TF) of Agaricus bisporus. Gordon-Taylor equation and Clausius-Clapeyron equation were used to fit Tg and TF data. The state diagram of freeze-dried Agaricus bisporus was obtained by Gordon-Taylor equation and Clausius-Clapeyron equation and DSC data. According to the understanding of state diagram, the shelf life prediction model of the freeze-dried Agaricus bisporus was established by the Williams-Landel-Ferry (WLF) equation.The results showed that Tg decreased from-11.4to68.3℃as moisture content of Agaricus bisporus increased from0.029to0.298g/g (w.b.), And TF decreased from-1.2to-22.3℃as the solids content of Agaicus bisporus increased from0.299to0.574g/g (w.b.). The state diagram provided an estimate of maximally freeze-concentrated solutes at0.704g/g (w.b.) with the characteristics temperature of end point of freezing (Tm) being-31.9℃and glass transition (Tg) being-77.9℃. The state diagram developed for Agaricus bisporus in this work can be used in determining the stability during frozen storage and in dried conditions as well as in designing drying and freezing processes. With understanding of the state diagram, the best storing condition for Agaricus bisporus products could be proposed by WLF equation which can be expressed as follows:where Tg and T are glass transition temperature and storing temperature, respectively; τg and τ are time constants for crystallization at Tg and T, respectively; C1and C2are general constants with C1=20and C2=155; τg equals20years.3. The relationship between aw preservation theory and the glass transition theory and their effects on storage stability of freeze-dried Agaricus bisporus were discussed. The results assumed to provide a theory basis on freeze process, drying technology and the storage stability of dehydration products of Agaricus bisporus.The results showed that based on aw preservation theory, the predicted Tg will be lower than safe storage temperature. Likewise, based on the glass transition theory, the forecast will also lower than its safety aw values (namely corresponding aw at monolayer moisture content). Therefore, there is a considerable discrepancy in the temperature related stability criteria predicted by the concepts of water activity and glass transition.