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Isolation and functional properties of protein fractions from rapeseed flour (Brassica campestris L. var. Echo) Kodagoda, Lakshman Punniyadasa

Abstract

A three stage method of extraction of proteins from rapeseed flour sequentially with water, 0.1 N hydrochloric acid, and 0.02 N sodium hydroxide has been developed. The first water extract contained mainly acidic and neutral proteins whereas the second hydrochloric acid extract contained basic proteins, which migrate to the cathode by pH 8.8 gel electrophoresis. This can partially be explained by higher arginine content. Ash content in the second hydrochloric acid extract, approximately 36%, was reduced to less than 10% by treating with 0.05 M oxalic acid. The water extract may be a good supplement to other vegetable products because of high lysine content. The approximate yields of proteins in the preparation of isolates were 11, 7, 41 and 61% respectively for the water, hydrochloric acid and sodium hydroxide extractions and for the single stage sodium hydroxide extraction. Proteins were lost in the whey during isoelectric precipitation, with a concomitent loss of a considerable amount of cystine and methionine. The emulsifying capacity of the water extract was better than that of the other extracts: 45 and 35 ml of corn oil per 10 0 mg of protein isolate respectively. The concentrates showed a similar trend, but the volume of oil was approximately 25% less for all preparations. In baking studies, the replacement of 5% of the wheat flour with rapeseed protein isolates prepared by isoelectric precipitation caused an approximate 10 to 15% decrease in loaf volume. The bread prepared with the concentrates decreased loaf volume by about 20%. The loaf volume was restored in most cases by adding 0.5% Atmul 124 (a mono- and diglyceride mixture manufactured by Atlas Chemical Industries) to the dough. With the isolates from the first water and second hydrochloric acid extracts an increase in loaf volume of 10 to 15% was observed over the control. With the isolates from the third sodium hydroxide and single stage sodium hydroxide extractions the loaf volume was restored without any increase. The bread prepared from the concentrates together with emulsifier showed a decrease of 12% in loaf volume except the third sodium hydroxide extract which had a 6% increase. Whip tests showed the best expansion of meringue when the protein isolate from the hydrochloric acid extraction was used at a level of 3% replacing egg white protein, an even better result than whipping the egg white alone. All other isolates showed a decrease in expansion. The concentrates except the water extract showed a slight decrease.

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