Crystallization and x-ray diffraction of crystals formed in water-plasticized amorphous spray-dried and freeze-dried lactose/protein mixtures

Effects of proteins (whey protein isolate [WPI], Na-caseinate, and gelatin), drying method, storage relative vapor pressure (RVP), and time on lactose crystallization and crystals formed were investigated using x-ray diffraction (XRD). Crystallization was observed from increasing peak intensities of XRD patterns. Lactose in lactose/protein (5:1, 3:1) mixtures crystallized in samples stored at RVP of 44.1% and above in both spray-dried and freeze-dried materials, except in freeze-dried lactose/Na-caseinate and lactose/gelatin mixtures, which showed lactose crystallization at 54.5% RVP and above. The rate of crystallization increased with increasing RVP and storage time. The rate of crystallization in spray-dried materials was higher than in freeze-dried materials, and the crystallization rate decreased with increasing protein content. Lactose crystallized mainly as alpha-lactose monohydrate in spray-dried lactose/WPI and lactose/gelatin mixtures. Crystals formed in freeze-dried lactose/WPI and lactose/gelatin mixtures were anhydrous beta-lactose and alpha-lactose monohydrate crystals. Lactose crystallized as alpha-lactose monohydrate in both spray-dried and freeze-dried lactose/Na-caseinate mixtures. Trace amounts of anhydrous beta-lactose were present in spray-dried lactose/WPI (5:1) and lactose/gelatin (5:1) mixtures. Peak intensities of XRD patterns for anhydrous beta-lactose decreased with increasing protein content and storage time. The crystallization data were successfully modeled using Avrami equation at an RVP of 65.6% and above. These data are important in understanding and predicting storage stability of lactose- and protein-containing food and pharmaceutical materials.