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 gravimetrical ly using samples stored over various salt solutions. In such studies desicc ators 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 desic cators with or without vacuum were used for freeze-dried wheat dough, glute n, denatured gluten, native, and gelatinized starch and to determine the gl ass transition temperatures for gluten., denatured gluten, and gelatinized starch using Differential Scanning Calorimetry (DSC). Using vacuum desiccat ors, all five materials were dehydrated over P2O5 until a constant weight w as achieved. Using desiccators without vacuum, gluten and native starch wer e dried in vacuum for 3 days at 30 degreesC. Triplicate samples were rehumi dified in desiccators with or without vacuum over salt solutions ranging fr om a(w) 0.113 to 0.843 at 24-25 degreesC. 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 equ ilibrium time was 2-3 weeks using desiccators without vacuum over the whole range of relative humidity (RH) for all materials. The GAB sorption isothe rm 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 bet ween the external atmosphere and inside the desiccators with or without vac uum.