Dehydrocyanation of cultivars of lima bean (Phaseolus lunatus L) and its effect on tryptophan content

Three cultivars (red, black and white) of lima bean were studied to determine the best medium in which to cook them in order to reduce their hydrocyanic acid content to tolerable levels. The study also investigated the effects of processing media on the trytophan content of lima beans. Samples of each cultivar were parboiled in each of three media, 0.94 M HCl, 0.47 M H2SO4 and distilled water, for various times. Portions of each cultivar were also parboiled in the three media after enzyme incubation. Boiled samples were dried, pulverised and analysed for cyanide and tryptophan contents. There were 96.82, 99.78 and 99.62% reductions in cyanide content of the red cultivar after boiling in water, dilute HCl and dilute H2SO4 respectively for 30 min. The corresponding values for the white cultivar were 96.39, 99.59 and 94.77% respectively. Dehydrocyanation was more effective in enzyme-incubated samples when they were parboiled in dilute mineral acids. The residual cyanide content in enzyme-incubated samples was below tolerable limits after 30 min of treatment. However, the longer the parboiling time, the higher was the trytophan loss. Therefore parboiling for 30 min in dilute HCl will produce the desired result of insignificant residual cyanide content and minimal tryptophan loss in all samples.