Glass transition, water plasticization, and lactose crystallization in skim milk powder

Effects of water content, storage time, and glass transition (physical state) on lactose crystallization and the resultant crystal forms in skim milk powder were investigated. Samples of freeze-dried skim milk containing amorphous lactose were stored at various relative humidities at room temperature (24 degrees C). Crystallization was observed from time-dependent loss of sorbed water during storage and from increasing intensities and peak areas of X-ray diffraction patterns. Lactose was found to crystallize largely as an anhydrous mixture of alpha- and beta-lactose in a molar ratio of 5:3. At high relative humidities, traces of other crystal forms were also found. The rate of crystallization, up to a maximum extent, which depended on crystallization conditions, increased as the relative humidity during storage increased. The rate of crystallization could be successfully modeled using the Avrami equation when relative humidity was greater than or equal to 66.2%. The relationship between the extent of crystallization and relative humidity was parabolic; the maximum crystallization occurred at a relative humidity of 70%. The corresponding water content and temperature difference between storage temperature and glass transition temperature were predicted to be 17% (wt/wt) and 61 degrees C, respectively. Glass transition seems to control lactose crystallization in skim milk powder, and the data obtained can be used in predicting lactose crystallization in milk products containing lactose.