CYANOGEN REDUCTION DURING LACTIC FERMENTATION OF CASSAVA

Lactic fermentation is amongst the most common processing techniques used for cassava in Africa. There are two types of fermentation: that of grated roots and of soaked roots. Knowledge of the mechanisms of cyanogen reduction during lactic fermentation is important so that effecient processing techniques can be identified and promoted and less efficient techniques can be improved or restricted to low cyanogen varieties of cassava. An understanding of the mechanisms involved also allows the effects of changes in processing technique to be predicted.

Although fermentation of both grated and soaked cassava roots involves lactic acid bacteria and a reduction in pH value, the mechanisms of cyanogen reduction are different. By comparing changes in cyanogens during natural fermentation with those occurring in the absence of microbial growth, it is possible to determine the relative roles of microbial and plant enzymes in the processes.

Micro-organisms play little or no role in cyanogen reduction during the fermentation of grated cassava. Although a high proportion of the micro-organisms present have the ability to hydrolyze linamarin, 95% of the initial linamarin content is hydrolyzed within 3 hours of grating. This demonstrates that grating is the key step in cyanogenic glucoside hydrolysis bringing endogenous linamarase into contact with linamarin.

In soaked roots, microbial growth is essential for efficient cyanogen reduction. Although there is some reduction in cyanogens in the absence of microbial growth, efficient cyanogen reduction only occurs when microbial growth takes place and the roots soften (or ret). The mechanisms of cyanogen reduction are more complex than for grated roots, but leaching of cyanogens from the softened roots plays a significant role. On the third day of fermentation, it was demonstrated that after root softening, approximately one third of the initial linamarin of the roots was present in the soaking water.

The implications of these mechanisms of cyanogen reduction and the effects of subsequent processing stages are discussed.