Effect of domestic processing on oligosaccharide content of some dry legume seeds

Raffinose family oligosaccharides (raffinose, stachyose and verbascose) present in legume seeds have been reported to cause flatulence. The object of this work was to study the effect of methods of domestic processing, including soaking in tap water and 0.5% sodium bicarbonate solution, cooking and autoclaving of unsoaked and soaked seeds, on oligosaccharide contents of four commonly consumed legumes in Egypt, namely: faba bean (Vicia faba L.), lentil (Lens culinaris), common bean (Phaseolus vulgaris) and cowpea (Vigna sinensis). The identification and quantitative determination of oligosaccharides using gas-liquid chromatography (GLC) indicated that the four raw legume seeds contained sucrose, raffinose, stachyose and verbascose. The raffinose family oligosaccharides account for 67.3, 63 2, 53.0 and 51.0% of the total soluble sugars in cowpea, faba bean, lentil and common bean, respectively. The major oligosaccharide was verbascose in faba bean and stachyose in the other three legumes. The soaking of legume seeds in tap water or sodium bicarbonate solution resulted in a decreased concentration of sucrose and the raffinose family of sugars in each case. The extent of losses was enhanced as the time of soaking was increased. The removal of oligosaccharides from legume seeds was higher in alkaline medium than in water. In addition, the loss of an individual sugar was not related to its solubility or molecular weight. Cooking of unsoaked seeds led to a large decrease in oligosaccharide content of all studied legumes compared with soaking for 12 h. The loss in the raffinose family oligosaccharides after cooking of soaked seeds was highest in common bean and lowest in faba bean. Autoclaving had a greater effect on removal of oligosaccharides than did cooking. In all cases, autoclaving of soaked seeds resulted in complete disappearance of raffinose.