Effect of Soaking Solution Concentration on Resistant Starch and Oligosaccharide Content of Adzuki (V. angularis), Fava (V. Faba), Lima (P. lunatus) and Mung Bean (V. radiata L.)
Lorraine L. Niba
Legume starches are typically more resistant to digestion than various other starches, rendering them low glycemic index foods, and potential substrates for beneficial colonic fermentation. The presence of oligosaccharides however reduces acceptability of various legumes. This study aimed to quantify resistant starch levels in some commonly consumed legumes - adzuki, fava, lima and mung bean - and assess the effect of varying soaking solution concentration on resistant starch and oligosaccharides. Legume samples were treated for twelve hours with either 1 % or 5 % citric acid or sodium bicarbonate solutions. A control sample was soaked in distilled water. These were autoclaved at 120 oC and freeze dried. Samples were analyzed for resistant starch by the procedure of McCleary and Monaghan 2002, and for oligosaccharides using a Megazyme? analysis kit. Adzuki bean had the lowest resistant starch content in the distilled water controls (2.94 g /100 g dry weight), while fava bean had the highest resistant starch content (5.15 g /100 g dry weight). Treatment with 5 % bicarbonate solution, decreased resistant starch in all legumes, while soaking in 5 % citric acid solution resulted in increases in resistant starch. Soaking in 5 % citric acid or sodium bicarbonate resulted in decrease in total starch, except for lima bean. There were minimal changes in oligosaccharide levels. Concentration of pre-process soaking solution therefore has considerable effect on resistant starch formation and retention, and a less noticeable effect on oligosaccharide levels.