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Preparation, decolorization and functional properties of the protein isolates extracted from rapeseed meal Keshavarz, Elaheh

Abstract

Rapeseed meal was extracted successively with water, hydrochloric acid at pH 2 and sodium hydroxide at pH 10 according to the three stage method of Kodagoda et al. (40); with some modifications. The prior extraction of meal with organic solvents was discontinued to decrease the loss of protein during preparation of protein isolates. The oxalic acid treatment used by Kodagoda et al. for lowering the high ash content in the acid-extracted protein fraction (IA₁) was not essential and was eliminated from the procedure. Decolorization of the protein isolates was attempted using hydrogen peroxide as an oxidizing agent and sulfur dioxide as a reducing agent. At the concentrations of 0.02, 0.2 and 2%, hydrogen peroxide significantly improved the colour of the acid-extracted and the base-extracted protein isolates. The effect of hydrogen peroxide on the amino acid composition of the protein isolates with and without added cysteine was studied under the different conditions of pH and temperature. The hydrogen peroxide treatment decreased the methionine and cystine contents of the protein isolates except that the added cysteine protected cystine in the protein, whereas, the sulfur dioxide treatment was effective for colour improvement without damaging the amino acid composition of the protein isolates. The water-extracted protein isolate (IW) was passed through an activated charcoal column to eliminate glucosinolates according to the method of Woyewoda et al. (76). Amino acid analysis showed a slight decrease in phenylalanine and tyrosine of the protein isolate. Some functional properties of the protein isolates were studied. The base-extracted protein isolate (IB) revealed the best emulsification capacity and stability, whereas the second acid-extracted protein isolate (IA₂; the first and second protein isolates were prepared from the acid extract by precipitating at pH 3.8 and 7.0 respectively) showed the greatest foamability. However, the hydrogen peroxide treatment slightly deteriorated the emulsification properties of protein isolate IB and improved the whipping ability of protein isolate IA₂. It is assumed that conformational changes in protein molecules caused by the hydrogen peroxide treatment affected the functional properties of these protein isolates. The total phospholipid content of the protein isolates was determined and a significant correlation with the emulsification capacity of the protein isolates was observed.

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