Water activity and glass transition in dairy ingredients

Abstract

The glass transition temperatures (Tg) 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 Tg and ΔCp (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 Tg inflection values: the model gives an approximate inflection value of Tg corresponding to the measured one ±14°C. This lack of fit can be explained both by the accuracy of measurement of Tg and by not considering minor constituents of dairy powders (minerals and oligosaccharides) in calculation. From the values of Tg and ΔCp 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 (aw) 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 - Tg] and ΔCp, whose value is in agreement with the expected behavior of various dairy powders during drying and storage. © INRA, EDP Sciences, 2005.