Comparative effect of boiling and solid substrate fermentation using the tempeh fungus (Rhizopus oligosporus) on the flatulence potential of African yambean (Sphenostylis stenocarpa L.) seeds

The tropical African yambean (AYB, Sphenostylis stenocarpa L.) is a protein-rich underutilized African legume. The presence of the flatulence- and diarrhoea-causing raffinose family oligosaccharides (RFO: raffinose, stachyose and verbascose) or α-galactosides has limited the food use of African yambean seeds. To reduce this limitation, non-traditional processing methods are required. Seeds of three varieties were (i) examined for the flatulence- and diarrhoea-causing RFO and (ii) fermented with Rhizopus oligosporus for tempeh production. The traditional tempeh production process involved dehulling, soaking in water for 24 h, boiling in water for 30 min, inoculation and fermentation. In addition, the traditional tempeh procedure was modified by using 1% citric acid solution instead of water for soaking and cooking. Comparisons with traditionally cooked beans, which involved boiling in water for 4 h, were made. Boiling seeds for 4 h resulted in 8–30% reduction of total α-galactosides in the three varieties, while the traditional tempeh procedure resulted in an almost complete loss (98%) of the same (P < 0.05). The modified procedure resulted in a bacteria-free tempeh but α-galactoside reduction was 22–39%. Both tempeh production processes were clearly more effective than was traditional cooking in reducing the flatulence potential of the AYB seeds.