Comparison of chemical and microbiological methods in the estimation of methionine in cowpea (Vigna unguiculata) seeds

Abstract

1. Meals were prepared from the seeds of fifteen varieties of cowpea (Vigna unguiculata), one of lima bean (Phaseolus lunatus) and one of yam bean (Sphenostylis stenocarpa), and their methionine content was determined by six different methods.2. Total methionine content was determined by two chemical methods (ion-exchange chromatography and a colorimetric procedure) and by two microbiological methods. The ‘available’ methionine content was determined by microbiological assay with Streptococcus zymogenes.3. All the different methods for total methionine determination gave similar results, with much the same high extent of precision.4. The values for ‘available’ methionine content were similar to or marginally higher than the corresponding microbiological assay values for total methionine content. There was no indication that the methionine in any of the test samples was not completely available.

Methionine and cystine contents of bean (Phaseolus) seeds

“…None of the beans which were analyzed in the present study had such high levels of cystine. Evans et al (1976) analyzed cowpeas for methionine by ion-exchange, colorimetric, and microbiological (Streptococcus zymogenes) techniques. Their values of 1.18% by ion-exchange, 1.08% by colorimetric, and 1.22% by microbiological procedures compare with 1.15% methionine in cowpea protein reported in the present work.…”