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2006 IFT Poster Presentation (PDF)
Glycerol content effect on the tensile properties of whey protein sheets formed by twin-screw extrusion
V. M. HERNANDEZ1, T. H. McHugh2, J. de J. Berrios2, D. Olson2, J. Pan2, and J. M. Krochta3. (1) Dept. of Biological & Agricultural Engineering, Univ. of California, Davis, 1 Shields Ave., 126 Cruess Hall, Davis, CA 95616, phone: 530-753-9830, fax: 530-752-4759, vmhernandez@ucdavis.edu, (2)USDA-ARS-Western Regional Research Center, Albany, CA 94710, (3) Dept. of Food Science & Technology, Dept. of Biological & Agricultural Engineering, Univ. of California, Davis, CA 95616
The use of whey protein-based edible films as barrier materials provides the opportunity to reduce packaging waste while controlling oxygen, aroma and lipid migration that affect food quality and shelf-life. Continuous extrusion of whey protein films through a die constitutes a more efficient method with commercial potential compared to solvent-casting and compression molding. Mechanical properties play an important role in the manufacturing and handling of stand-alone films that could be used as food wraps or pouches for dry foods. It was hypothesized that extruding whey protein isolate powder with varying glycerol contents can result in the formation of transparent, flexible sheets with different mechanical properties.
The objective of this research was to study the effect of glycerol plasticizer on the tensile properties of extruded whey protein sheets.
A Haake-Leistritz co-rotating twin-screw extruder with six independent heating and cooling zones and a length-to-diameter ratio of 30:1 was used to extrude whey protein sheets containing 45.77, 48.79 and 51.86% glycerol (d.b.). Samples were collected at a screw speed of 250 rpm and a barrel temperature profile of 20-20-20-80-110-130°C. Tensile properties in the machine direction were determined using an Instron Universal Testing Machine and a crosshead speed of 50 mm/min.
Melt temperature for all three types of sheets was 143-150°C, as recorded by a thermocouple imbedded in the slit die at the time of sheet formation. The average thickness of the sheets was 1.31±0.02mm. Sheets containing 45.77% glycerol (d.b.) had a significantly higher tensile strength than sheets with higher glycerol contents. Furthermore, as glycerol concentration increased, elastic modulus decreased significantly. No significant differences were found in percent elongation at break among the samples.
Extrusion of whey protein sheets constitutes the first step towards extrusion of thinner edible films, which can work together with conventional packaging to improve food quality, while reducing solid waste.