Thermal Transitions and Extrusion of Glycerol-Plasticized Whey Protein Mixtures

The effects of glycerol and moisture contents on the thermal transitions of whey protein isolate (WPI) powder-glycerol-water mixtures were studied. Mixtures with ratios of 100:0, 70:30, 60:40, and 50:50 WPI:glycerol on a dry basis (db) were preconditioned to 0.34 ± 0.01 (25.4 ± 0.4 °C) and 0.48 ± 0.02 (25.9 ± 2.2 °C) water activity. Differential scanning calorimetry (DSC) showed the existence of an endothermic peak starting at 148.3 ± 0.7 °C for 100% WPI preconditioned to a water activity of 0.34 ± 0.01. The onset temperature of this peak decreased with addition and increase of glycerol content, as well as with the increase in water activity from 0.34 ± 0.01 to 0.48 ± 0.02. An additional endothermic transition, important for extruding the mixtures into flexible sheets, occurred in mixtures containing 50% glycerol db, preconditioned to 0.48 ± 0.02 water activity. The onset temperature of the peak was 146 ± 2.0 °C. Whey protein-based sheets containing 45.8%, 48.8%, and 51.9% glycerol db were obtained using a Haake-Leistritz corotating twin-screw extruder. All samples were obtained at a screw speed of 250 rpm and a final barrel-temperature profile of 20, 20, 20, 80, 110, and 130 °C. Melt temperature at the time of sheet formation was 143 to 150 °C. Average thickness of the sheets was 1.31 ± 0.02 mm. Samples with 45.8% glycerol db had significantly higher tensile strength (TS) than samples with higher glycerol contents. Also, as glycerol concentration increased, sheet elastic modulus (EM) decreased significantly (P