Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 24, 2011
Publication Date: January 24, 2011
Citation: Qi, P.X., Onwulata, C.I. 2011. Physical properties, molecular structures and protein quality of texturized whey protein isolate: effect of extrusion moisture content. Journal of Dairy Science. 94:2231-2244. Interpretive Summary: Whey proteins, a nutrient-rich by-product of the cheese-making process, are not fully utilized in food products because their structure makes them incompatible with other ingredients in some food formulations. Using an extruder, a machine with two rotating screws inside a heated barrel, we changed the structure of whey proteins making them more useful as food ingredients. The results showed that the temperature and moisture inside the extruder changed the composition and structure of the whey proteins on the molecular level which in turn improved the functional and nutritional characteristics of the proteins. These results will lead to the development of new foods with enhanced nutritional quality.
Technical Abstract: Whey proteins, nutrient-rich by-products of the cheese manufacturing process, have many demonstrated functional and nutritional benefits. The environmental impact of disposing the large annual volumes (~25 billion pounds), can be mitigated by utilizing the whey proteins in foods. Successful extrusion texturization of whey protein isolate (WPI) has created an outlet for increased utilization of whey. This work explains the effects of extrusion texturization on the compositional and structural changes of WPI on the molecular level as a function of extrusion conditions: temperature (50 degrees, 75 degrees and 100 degrees C), and moisture content (30%, 40% and 50%). Changes in protein solubility (Bradford assay methods), protein profile (SDS-PAGE with or without the reducing agent, 2-mecaptoethanol), and quantitative compositional changes of beta-lactoglobulin and alpha-lactalbumin (HPLC) were determined. The results showed significant reduction in both the overall protein solubility, from 65% to 52% and beta-lactoglobulin content, 50% of total protein to nearly non-detectable levels, in the soluble fraction as a function of increasing temperature, suggesting the formation of intermolecular disulfide bonds attributable to the decrease in solubility in the extruded WPI products. Extrusion moisture appears to bear negligible impact on the solubility, composition, and structure of extruded WPI. Circular Dichriosm (CD) and intrinsic tryptophan (Trp) fluorescence spectroscopy showed structural changes of the soluble proteins; both the secondary structures of WPI and tertiary contacts changed considerably as temperature increased from 50 degrees C to 75 degrees C, but complete structural losses were observed for WPI samples extruded at 100 degrees C only.