Differences in the physical state and thermal behavior of spray-dried and freeze-dried lactose and lactose/protein mixtures

The physical state and thermal behavior of dried food ingredients are important in the control of processing and storage stability of such materials. The physical structures of spray-dried and freeze-dried anhydrous and crystalline lactose, lactose/whey protein isolate (WPI), lactose/Na-caseinate and lactose/gelatin mixtures were observed by scanning electron microscopy (SEM). Glass transition, Tg, and instant crystallization temperatures, Tcr, were determined using differential scanning calorimetry (DSC). Particles in spray-dried amorphous lactose were spherical, and in lactose/protein mixtures it was also spherical with some dents. Freeze-dried lactose and lactose/protein mixtures resembled pieces of broken glass. Crystals formed from spray-dried lactose were tomahawk-like but those formed from freeze-dried lactose had needle-like or rod-like structures. Tg and Tcr of freeze-dried lactose and lactose/protein mixtures were slightly higher than those of spray-dried lactose and lactose/protein mixtures at corresponding water contents. But Tcr of lactose/Na-caseinate and lactose/gelatin mixtures were lower in freeze-dried than in spray-dried materials. Time-dependent lactose crystallization was observed at RVP 44.1% and above in both dehydrated materials, except in freeze-dried lactose/Na-caseinate and lactose/gelatin. These results indicated that freeze-dried and spray-dried materials have different physical and thermal behavior suggesting that different microstructures and product properties are obtained with different drying methods. Industrial relevance Lactose is often applied as a mixture with other sugars and proteins in the food industry. Hence, understanding the physical state and thermal behavior of different dehydrated ingredients has a great importance in the development of proper processing and self-life control procedures for such ingredients and products. This manuscript provides some information about storage stability of lactose in the presence of proteins under various moisture conditions. Data on water sorption and glass transition can be used to predict changes during processing and storage of spray-dried and freeze-dried lactose and lactose/protein mixtures. The crystallization data allow prediction of crystallization behavior, as a physical state-dependent phenomenon, and therefore, stability of lactose and lactose containing food products.