Residual cyanide concentrations during the extraction of cassava starch

Abstract

SummaryCassava starch is traditionally extracted on a small scale in many tropical countries. The process consists of wet‐milling the washed roots, washing the starch from this milled pulp on vibrating trays or in mixing tanks, sedimenting the starch in wooden canals or concrete tanks and sun‐drying the product. This process was analysed during six production runs in two factories. The distribution of cyanide followed a similar pattern in both factories. Most of the cyanogenic glucosides (bound cyanide) in the roots are converted to free cyanide during the milling operation. Between 40 and 70% of the total cyanide appears in the water used to wash the starch from the disintegrated tissue, and between 5 and 10% appears in the fibrous residue used in animal feed. This residue also contains between 12 and 23% of the starch present in the cassava. The eluted starch is allowed to sediment for 1–3 days, after which it contains less than 4% of the cyanide present in the raw material. The dried product contains less than 1% of the quantity of cyanide present in the raw material; the residual concentration is 1–5 p.p.m. The factors involved in the removal of the cyanide during starch extraction are discussed, and their importance to more efficient large‐scale processes is indicated.