Folate contents of legumes determined by optimized enzyme treatment and stable isotope dilution assays

Stable isotope dilution assays (SIDAs) were adopted to quantify folates in legumes. Deconjugation of polyglutamic to monoglutamic vitamers was achieved by a combined treatment with rat plasma and chicken pancreas conjugase. Additionally, conjugase inhibitors were effectively removed by strong anion exchange prior to deconjugation. Determination of various dried legume seeds by the optimized SIDA technique revealed total folate contents between 10 μg/100 g (green peas; Pisum sativum) and 318 μg/100 g (soybean; Glycine max). Total folate concentrations of canned legumes were analyzed in a concentration range between 4 μg/100 g (green beans; Phaseolus vulgaris) and up to 69 μg/100 g (black-eyed peas, cow peas; Vigna unguiculata). Deep frozen products of green peas and green beans were found to contain relatively high folate concentrations up to 146 μg/100 g (green peas). A comparison to literature data, analyzed by means of microbiological assays, indicated significant lower SIDA values for some dried legume seeds. The folate pattern measured by SIDA revealed 5-methyltetrahydrofolate as the predominant vitamer in frozen peas, dried lentils, dried black-eyed peas and mung beans, whereas in fresh beans and soybeans tetrahydrofolate was most abundant and in peanuts 5-formyltetrahydrofolate was found to be the most important folate vitamer.