Microorganisms and fermentation of cassava in relation to the cyanide levels and product quality.

A study was carried out in Mwanza, Kilimanjaro and Coast regions, Tanzania, to assess the influence of micro-organisms involved in traditional cassava fermentation. The effectiveness of selected local cassava processing methods in reducing the cyanide levels and influences on other properties in the finished products were compared. Cassava fermenting micro-organisms were isolated and their linamarase enzymatic activity studied. Different products known as “Udaga,” “Makopa” and “Kivunde” are produced in Mwanza, Kilimanjaro and Coast regions respectively. “Udaga” processing in Mwanza involves covering a heap of peeled and chopped fresh cassava tubers with leaves and left to ferment for three to four days. This process forms a black product due to growth of moulds. “Makopa”, a yellowish golden colour product is obtained after sun drying of the peeled and chopped cassava tubers. “Kivunde” is produced through soaked fermentation of peeled or unpeeled tubers to give a bright white product. Lower residual cyanide levels were found in “Kivunde” than in “Makopa” and “Udaga”. “Kivunde” processing reduced the cyanide content from 556.9 to 8.3 mg CN- eq/kg for the bitter cassava and from 215.2 to 8.2 mg CN-eq/kg for the sweet cassava. In “Makopa” cyanide levels was reduced from 446.8 mg CN-eq/kg to 20.0 mg CN-eq/kg dry weight for the bitter cassava and from 121.9 mg CN-eq/kg to 13.8 mg CN-eq/kg for the sweet cassava. In the case of “Udaga”, cyanide reduction was from 313.2 to 6.4 mg CN- eq/kg for bitter cassava and from 130.5 to 11.5 mg CN-eq/kg for the sweet cassava. Moulds, yeast, and bacteria were isolated from the fermenting cassava products. During the fermentation process, succession of micro-organisms was observed, with the bacteria being the first to appear, followed by yeast and lactic add bacteria and finally by moulds. A sharp pH drop was observed to slow down the rate of cyanogenic glucoside degradation due to the suppression of the amylolytic micro-organisms and change of equilibrium in the linamarin hydrolysis reaction equation Characterisation of 164 microbial isolates showed four bacteria strains (AMU-3F, AMU-IB, AMU-31 and AMU-OE) all of the genus Lactobacillus plantunim and a yeast Rhodotorula SPP (AMU 1A) which could affect fermentation of cassava following the “Kivunde” process. The API-ZYME test showed the above five isolates to have a linamarase activity ranging from 3 to 5 nanomoles/ml/min of both a and (J-glucosidase, and the same range for Valine arylamides activity. Other isolates with the ability of reducing cyanide levels to below 100 mg CN eq/kg also produced butyric add which contributed to off aroma of the fermented products. Nutritionally, there was no significant different between the nutrient content of “Makopa”, “Udaga” and “Kivunde”. The four Lactobacillus plantunim strains could also ferment amygdalin, a glucosidic cyanogen from bitter almond and raffinose which is considered to be an antinutritional factor. Comparison between spontaneous, back-slopping and use of the above strains as starter cultures under “Kivunde” processing method showed the cultures to be more efficient in cassava detoxification It is recommended that cassava from most of the studied areas in Tanzania should not be consumed raw before processing, and Kivunde process leads to a safer product than “Makopa” and “Udaga”. Due to the effect of pH on cyanogenic glucoside degradation, it is recommended that research be done to incorporate in the starter culture some useful micro-organisms with sufficient amylolytic activity. For easy handling of the culture organisms, it is also recommended to study the ways of producing the inoculum in a powder or pellet form as is for the case of baker’s yeast.