Comparisons of the use of desiccators with or without vacuum for water sorption and glass transition studies

Water sorption properties of food solids are often determined gravimetrically using samples stored over various salt solutions. In such studies desiccators with or without vacuum can be used. However, the apparent equilibrium water contents may differ depending on the technique used. The objectives of the present study were to investigate the resultant isotherms when desiccators with or without vacuum were used for freeze-dried wheat dough, gluten, denatured gluten, native, and gelatinized starch and to determine the glass transition temperatures for gluten, denatured gluten, and gelatinized starch using Differential Scanning Calorimetry (DSC). Using vacuum desiccators, all five materials were dehydrated over P₂O₅ until a constant weight was achieved. Using desiccators without vacuum, gluten and native starch were dried in vacuum for 3 days at 30°C. Triplicate samples were rehumidified in desiccators with or without vacuum over salt solutions ranging from a_w 0.113 to 0.843 at 24-25°C. The BET and GAB sorption isotherm models were fitted to two water sorption data. Equilibrium water contents were achieved after 2-3 days of storage using vacuum desiccators, while equilibrium time was 2-3 weeks using desiccators without vacuum over the whole range of relative humidity (RH) for all materials. The GAB sorption isotherm for all materials showed clearly that water contents were higher at the high water activities (> 0.6) and lower at the low water activities (< 0.4) using vacuum desiccators, because of a probable difference in humidity between the external atmosphere and inside the desiccators with or without vacuum.