Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2011
Publication Date: 3/23/2011
Citation: Qi, P.X., Onwulata, C.I. 2011. Physical properties, molecular structures and protein quality of texturized whey protein isolate: effect of extrusion temperature. Journal of Agricultural and Food Chemistry. 59:4668-4675.
Interpretive Summary: Whey proteins, nutrient-rich by-products of the cheese-making process, have not been fully utilized, leaving manufacturers with the challenge of disposing about 9 billion pounds of whey annually, without impacting the environment. Using extrusion technology, a powerful food processing operation, we changed the structure of whey proteins making them more useful as food ingredients in many commercial food products such as protein enriched snacks. This work explains how temperature during the extrusion changes the composition, structure and protein quality of whey protein isolate on the molecular level. Our results provide the basic understanding on how the extruder changes the molecular properties of whey protein isolate, and shows that the functional and nutritional characteristics are changed as well, enhancing the nutritional quality.
Technical Abstract: Extrusion is a powerful food processing operation, which utilizes high temperature and high shear force to produce a product with unique physical and chemical characteristics. Texturization of whey protein isolate (WPI) through extrusion for the production of protein fortified snack foods has provided a new and viable outlet to alleviate the increasing environmental impact caused by cheese production. However, the relationship between controlling processing conditions and creating products with the right texture and stability, i.e. the formation of micro- and macrostructures in concentrated protein systems, is not fully understood. In this work, we investigate the effect of temperature; one of the prime conditions during high moisture (50%) extrusion on the physical, chemical properties and protein quality of extruded WPI. Bradford assay methods were used to assess protein solubility of texturized whey protein isolate (WPI). SDS-PAGE techniques in the presence and/or absence of reducing agent, 2-mercaptoethanol were used to study changes in individual protein profile and composition in WPI as a function of changing extrusion temperature from 5 deg C to 100 deg C. HPLC was used to analyze quantitatively compositional changes of beta-lactoglobulin and alpha-lactalbumin, two of the most abundant whey proteins in bovine milk. The results showed significant reduction in both the overall protein solubility and beta- lactoglobulin content as a function of increasing temperature, suggesting that the formation of intermolecular disulfide bonds may be attributable to the increasing amount of aggregates seen in the extruded WPI products. Circular Dichriosm (CD) and intrinsic Trp fluorescence spectroscopic techniques were applied to study the structural changes of the soluble proteins. It was shown that both the secondary structures of WPI and tertiary contacts underwent considerable changes as temperature increased from 50 deg C to 75 deg C, but complete structural losses were only observed for WPI samples extruded at above 75 deg C. Available lysine was analyzed chemically using ninhydrin assay method. Lysine retention was negatively influenced by increased temperature during extrusion cooking compared to treatment by heating only, other extrusion factors such as shear force etc are also important in determining the protein quality of the final WPI products. In addition to the lysine loss, there was also a decrease in free SH-containing cysteine residues as revealed by DTNB (5,5'-dithiobis(2-nitrobenzioc acid)) reactions, caused by combined multiple extrusion conditions.