COMPARISON OF METHODS OF ANALYSIS OF CYANOGENS IN CASSAVA

The analysis of cyanogens in cassava normally involves three steps: (i) extraction from cassava, (ii) hydrolysis to cyanide, and (iii) analysis of cyanide. There are some methods (not considered here) that replace (ii) and (iii) with direct analysis of linamarin. Extraction is normally carried out in dilute acid. Enzymatic hydrolysis using linamarase is a simple method, but the enzyme is expensive and often difficult to obtain in developing countries. An alternative which avoids the use of linamarase is hydrolysis in 2 M H₂SO₄ at 100°C, which is not quite as simple but gives good results.

For analysis of cyanide an electrochemical method using a cyanide electrode has been evaluated. The method gives reasonable results but is less sensitive and selective than the colorimetric methods based on the König reaction, and requires a blank determination for each analysis because of electrode interferences. Three colorimetric methods have been evaluated with regard to the dependence of the absorbance on the pH of the final colored solution. There is usually a small region of pH over which the absorbance is independent of pH, but some previous methods have worked in regions in which the absorbance is sensitive to small changes of pH. Acetate and phosphate buffers gave different absorbance behavior for the unstable pyridine/pyrazolone/bispyrazolone reagent and for the pyridine/barbituric acid reagent. Smelly and toxic pyridine was eliminated by use of isonicotinic acid/barbituric acid, which also gave an absorbance plateau at pH 5.5 – 6.5. A simple protocol is given, which will be useful in developing countries for analysis of cyanogens in cassava, using acid hydrolysis followed by isonicotinic acid/barbituric acid.