Extraction of proteins from soybean residue (okara) and investigation of their physicochemical properties and their application as emulsifiers

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Soybean products and in particular soymilk have received significant attention by the public in recent years. Their increasing popularity is primarily due to their nutritional composition and potential health benefits as well as the fact that they are lactose-free and cholesterol-free which makes them ideal alternatives to cow milk especially by lactose intolerant people as well as people suffering from hypercholesterolemia. Okara is a protein-rich residue produced at significant amounts during soy milk production and thus constitutes an abundant and cheap raw material that can be used for the extraction of proteins with potential applications in the food industry as functional food ingredients. The aim of this research was to evaluate alkaline extraction and ultrasonication for the extraction of okara proteins, build our knowledge on the relationship between the extraction method, the protein structure and its functions, as well as investigate the application of the extracted proteins in a mayonnaise-like emulsion system. The results demonstrated that alkaline extraction in 0.1 M phosphate buffer at pH 12 and at 60 oC for 60 mins resulted in significantly higher protein extraction from okara (~ 36 % w/w) compared to pH 9,10 and 11 ranging from 4 – 14 % in these cases. The main protein components in the extracts were 11 S (glycinin) and 7 S (β-conglycinin) as observed with SDS-PAGE analysis; further separation and purification resulted in the protein content of 83-86 % protein compared to the starting okara material. The pH of the extraction had a considerable effect on the structure of the protein as demonstrated by the Fourier transform infrared spectroscopy. It also improved the physicochemical properties of the okara protein isolate (OPI) such as its protein solubility, zeta potential, size distribution and emulsion properties. An ultrasonication method further improved the extraction yield to higher than 100% (compared to the standard solvent method). Ultrasonication also improved the physicochemical properties without affecting the purity of the protein. The rheological data revealed that ultrasonication derived OPI had the best rheological properties compared to the conventional derived OPI and commercial protein isolate.