Water activity and glass transition in dairy ingredients

Abstract

The glass transition temperatures (T g ) of various dairy powders were determined by differential scanning calorimetry among the sorptions isotherm, and were also calculated using the extended Couchman-Karasz equation, taking into account the dry values of T g and Cp (heat capacity changes during glass transition) and the contents in each amorphous component category. The results obtained showed a statistically significant correspondence between measured and calculated T g inflection values: the model gives an approximate inflection value of T g corresponding to the measured one 14 C. This lack of fit can be explained both by the accuracy of measurement of T g and by not considering minor constituents of dairy powders (minerals and oligosaccharides) in calculation. From the values of T g and Cp measured and/or calculated in this study, it is possible to anticipate the behavior of a dairy powder under given temperature (T) and water activity (a w ) conditions during drying and storage with regard to stickiness and caking. We propose the calculation of a stickiness and caking sensitivity index according to the values of [T -T g ] and Cp, whose value is in agreement with the expected behavior of various dairy powders during drying and storage.