ANALYSIS OF CYANIDE IN CASSAVA USING ACID-HYDROLYSIS OF CYANOGENIC GLUCOSIDES

Guided by the results of NMR experiments on the reactivity of linamarin in alkali and acid, an acid hydrolysis method was developed for cyanide analysis in cassava. Hydrolysis in 2.0 M H2SO4 at 100-degrees-C for 50 min of a cassava extract produced cyanohydrins which rapidly decomposed to cyanide ion in alkali. Excess pH 6 buffer was added, followed by chloramine-T and pyridine/barbituric acid (Konig reaction) to produce a purple solution which was measured spectrophotometrically at 583 nm. The colour intensity depended on pH and phosphate concentration, hence accurate results required similar solution conditions for KCN standards. The method gave reasonable agreement with results obtained by the use of linamarase, and adequate recoveries of added linamarin (70-95% dependent on conditions). Acid hydrolysis is cheaper than the enzymic method using linamarase, which is expensive. Also the pyridine/barbituric acid reagent used in the acid hydrolysis method is cheaper and more stable than the pyridine/pyrazolone normally used in the enzymic method. Six locally grown cultivars gave < 30 mg HCN kg-1 fresh tuber, and one cultivar (SM1-150) contained only 4 mg HCN kg-1 fresh weight. Analyses of the same cultivar grown more recently gave values of 13-27 mg HCN kg-1, showing the need for a study of the environmental factors influencing the cyanide content of cassava tubers.