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, T-g, and instant crystallization temperatures, T-cr, 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. T-g and T,, 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 T-cr 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. (c) 2005 Elsevier Ltd. All rights reserved.