Effects of polyols on the stability of whey proteins in intermediate-moisture food model systems.

The objective of this study was to investigate the influence of polyols on the stability of whey proteins in an intermediate-moisture food model system and to elucidate the effect of polyols on the hardening of whey protein-based bars during storage. Four major polyols, glycerol, propylene glycol, maltitol, and sorbitol, were evaluated in model systems, which contained whey protein isolate, polyols, and water. The results showed that glycerol was the most effective polyol in lowering water activity and provided the soft texture of intermediate-moisture foods, followed by sorbitol and maltitol. These three polyols stabilized the native structure of whey proteins, provided a desired texture, and slowed the hardening of the model systems. Propylene glycol should not be used in whey protein-based high-protein intermediate-moisture foods because it caused changes in protein conformation and stability as observed by differential scanning calorimeter and Fourier transform infrared spectroscopy and resulted in aggregation of whey proteins and hardening of the bar texture during storage, causing loss in product quality.

Effects of moisture-induced whey protein aggregation on protein conformation, the state of water molecules, and the microstructure and texture of high-protein-containing matrix.

Moisture-induced protein aggregation through intermolecular interactions such as disulfide bonding can occur in a high-protein-containing food matrix during nonthermal processing and storage. The present study investigated the effect of moisture-induced whey protein aggregation on the structure and texture of such high-protein-containing matrices using a protein/buffer model system. Whey proteins in the protein/buffer model systems formed insoluble aggregates during 3 months' storage at temperatures varying from 4 to 45 degrees C, resulting in changes in microstructure and texture. The level of aggregation that began to cause significant texture change was an inverse function of storage temperature. The protein conformation and the state of water molecules in the model system also changed during storage, as measured by differential scanning calorimetry and Fourier transform infrared spectroscopy. During storage, the model system that had an initially smooth structure formed aggregated particles (100-200 nm) as measured by scanning electron microscopy, which lead to an aggregation network in the high-protein-containing matrix and caused a harder texture.

Moisture-induced aggregation of whey proteins in a protein/buffer model system.

Moisture-induced protein aggregation in a dry or intermediate-moisture food matrix can contribute to the loss of product acceptability. The present study evaluated the molecular mechanisms and controlling factors for moisture-induced whey protein aggregation in a premixed protein/buffer model system. Insoluble aggregates rapidly formed during the first 3 days of storage at 35 degrees C with a slower rate afterward. Evaluation of the insoluble aggregates by solubility tests in solutions containing SDS/urea/guanidine HCl/dithiothreitol and gel electrophoresis showed that the formation of intermolecular disulfide bonds was the main mechanism for protein aggregation, and all major whey proteins were involved in the formation of insoluble aggregates. Effects of various factors on aggregation were also investigated, including moisture content, medium pH, and the addition of NaCl. The dependence of aggregation on moisture content was bell-shaped, and the maximal extent of aggregation was achieved at a moisture content of around 70-80% on a dry weight basis.