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

摘要

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..