CHEMICALLY DETERMINED TOTAL AND AVAILABLE METHIONINE IN BEANS (PHASEOLUS-VULGARIS L) AND ISOLATED PROTEIN-FRACTIONS

The total and non-oxidized methionine contents in Phaseolus vulgaris L . whole beans and in isolated protein fractions from seven bean variet ies were studied. The total methionine contents determined by classica l ion-exchange chromatography (IEC) after performic acid oxidation and HCl hydrolysis, ranged in bean meals from 1.30 to 1.53% with an avera ge of 1.42%. Total methionine in bean protein extracts, which accounte d for both water- and salt-soluble proteins was similar (1.45%); water -soluble albumins showed an average methionine content of 1.20%. The n on-oxidized (considered available) methionine contents in whole beans measured by reaction with BrCN and gas chromatography of methylthio-cy anate formed, were unexpectedly higher than total methionine. These re sults might indicate that interfering compounds, probably S-methylcyst eine or its dipeptide with glutamic acid, present in variable amounts in leguminous seeds, over-estimated the potentially available methioni ne in whole beans. The protein extracts and isolated albumin fractions were devoid of these interfering substances and, as expected, signifi cantly lower available methionine contents were observed. The protein extracts showed values of methionine equivalents, determined as methyl thiocyanate, between 0.70 and 0.90% in protein, 37% lower than total m ethionine determined by IEC, corresponding to an available methionine content of 63%. The albumins showed methylthiocyanate values varying b etween 0.65 and 0.85% in protein, on average 32% lower than values obt ained by IEC, corresponding to 68% available methionine.