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 varieties were studied. The total methionine contents determined by classical ion-exchange chromatography (IEC) after performic acid oxidation and HCl hydrolysis, ranged in bean meals from 1.30 to 1.53% with an average of 1.42%. Total methionine in bean protein extracts, which accounted for both water- and salt-soluble proteins was similar (1.45%); water-soluble albumins showed an average methionine content of 1.20%. The non-oxidized (considered available) methionine contents in whole beans measured by reaction with BrCN and gas chromatography of methylthiocyanate formed, were unexpectedly higher than total methionine. These results might indicate that interfering compounds, probably S-methylcysteine or its dipeptide with glutamic acid, present in variable amounts in leguminous seeds, over-estimated the potentially available methionine in whole beans. The protein extracts and isolated albumin fractions were devoid of these interfering substances and, as expected, significantly lower available methionine contents were observed. The protein extracts showed values of methionine equivalents, determined as methylthiocyanate, between 0.70 and 0.90% in protein, 37% lower than total methionine determined by IEC, corresponding to an available methionine content of 63%. The albumins showed methylthiocyanate values varying between 0.65 and 0.85% in protein, on average 32% lower than values obtained by IEC, corresponding to 68% available methionine.