MAKING SAFE FLOUR FROM BITTER CASSAVA BY INDIGENOUS SOLID SUBSTRATE FERMENTATION

In some African communities, a stage of solid substrate fermentation is included in the sun-drying process of transforming cassava roots into flour. The dynamics of cyanogen removal, the role of the microflora involved, and cyanogen and mycotoxin contamination of the final products were studied in Uganda. Dominant mycelial growth was from the fungi Neurospora sitophila, Geotrichum candidum and Rhizopus oryzae. No mutagenicity, cytotoxicity or aflatoxins could be detected in the flours. The fermentation process was significantly more effective in reducing cyanogen levels than sun-drying alone, but did not always result in innocuous levels of cyanogens.

Laboratory experiments showed that non-inoculated incubation of disinfected root pieces reduced cyanogenic glucoside levels, and separate inoculation with fungi and a Bacillus sp. isolated from on-farm fermented cassava caused significant additional reduction. Several of the micro-organisms involved were capable of displaying linamarase activity. However, enhanced glucoside breakdown in root pieces was associated with root softening but not with the micro-organisms' linamarase capacity. Addition of a cell wall-degrading enzyme preparation under aseptic conditions caused softening of root pieces and a significant reduction of cyanogenic glucosides, but adding linamarase did not. It is proposed that the main contribution of micro-organisms to linamarin level reduction in solid substrate fermenting cassava is their cell wall-degrading capacity which facilitates the contact between endogenous linamarase and the cyanogenic glucosides.

Inclusion of a stage of solid substrate fermentation during processing into flour appeared instrumental in reducing the potential toxicity of cassava roots and was more effective than sun-drying alone; there is room, however, for optimization to ensure safer products.