A reason to be bitter : Cassava classification from the farmers' perspective

Sammanfattning: Ethnographers report from both South America and Africa that cassava farmers classify cultivars as belonging to either of two groups, "bitter" or "sweet", and that farmers prefer to grow "bitter" cultivars as the staple crop. It remains contentious if the two groups and whether bitterness in taste reflects the content of cyanogenic glucosides in cassava roots. This is of importance since dietary cyanogen exposure may have adverse health effects. These studies aimed at elucidating the social and biological role of cyanogenesis in a cassava dominated farming and food system in Malawi, where small-scale farmers utilise the soaking method for processing roots from "bitter" cultivars. In 1994 and 1995 a qualitative interview survey in 13 communities in Nkhata-Bay district, in Malawi showed that social factors were the main reasons for farmers preferring "bitter" cassava. The need to process the roots dictates rigorous planning and bitterness confers protection from theft and vermin. In 1996, determination of urinary linamarin (the cyanogenic glucoside in cassava) and thiocyanate (the main cyanide metabolite) as biomarkers of dietary cyanogen exposure, was conducted on 176 women farmers. The low urinary levels found indicated that dietary cyanogen exposure was negligible inspite of frequent consumption of food made from soaked roots from "bitter" cassava cultivars. In 1996 two roots from 246 plants of the 10 most grown cultivars, about 24 plants from each cultivar were harvested. Farmers' scoring of bitter taste of the root tip was a good predictor of cyanogenic glucoside levels (r = 0.65). Mean cyanogenic glucoside level in 132 roots from "cool" cultivars was 29 mg HCN eq kg-1 fresh weight (Cl 25-33, range 1 - 123), and in 360 roots from "bitter" cultivars 153 mg (Cl 143 - 163, range 22 - 661). Farmers' distinction of "bitter" and "cool" cultivars predicts glucoside levels (r = 0.56). Taste scoring for bitterness by the taste panel was strongly correlated (r = 0.87) with cyanogenic glucoside levels suggesting that cyanogenic glucosides convey the bitter taste in cassava. DNA fingerprinting using SSR-markers showed that farmers had a high ability to distingush plants with specific genotypes as belonging to named cultivars. Their distinction of "bitter" and "cool" cultivars with high and low levels of cyanogenic glucosides appears to have influenced the genetic pool of cassava in this area since the genotypes of the two groups separated into two clusters in principle component analysis. Single women's food security was particularly compromised if they did not opt to grow the "bitter" cassava cultivars. Processing bitter roots was not a problem per se, rather the need for mechanised mills was perceived as the rate-limiting factor. Farmers' ethnoclassification and experience enables them to have more benefits than disadvanatges from cyanogenesis. These findings have implications for cassava breeding and extension programmes.