Mpoko Bokanga

The biosynthesis of cyanogenic glucosides in roots of cassava

Abstract Linamarin is the main cyanogenic glucoside of cassava. De novo synthesis of linamarin in cassava roots was demonstrated in vivo by feeding [ 14 C]valine to excited segments of phelloderm. In vitro , a microsomal enzyme system isolated from cassava roots was shown to catalyse the conversion of valine to acetone cyanohydrin, the aglucone of linamarin. An antibody raised against cytochrome P450 TYR , the enzyme which catalyses the initial step in the biosynthesis of the cyanogenic glucoside dhurrin in sorghum, cross-reacts with a major polypeptide of similar molecular mass in cassava microsomes. Cyanogenic glucosides are known to accumulate in cassava roots, but hitherto de novo synthesis has only been demonstrated in the leaves, suggesting translocation of cyanogenic glucosides from leaves to roots. Our results show that at least part of the cyanogenic glucosides are synthesized in the roots. The data demonstrate that acyanogenic cassava roots cannot be obtained solely by blocking the transport of cyanogenic glucosides to the roots from other parts of the cassava plant.

Ecological variation of intake of cassava food and dietary cyanide load in Nigerian communities

AIM To study the ecological variation of intake of cassava foods and dietary cyanide load. DESIGN Ecological study design. SETTING Five communities in south-western Nigeria where tropical ataxic neuropathy (TAN) was described as endemic (area A), 11 communities in south-western Nigeria where TAN was described as absent (area B), and five communities in northern Nigeria (area C). SUBJECTS Subjects were randomly sampled from selected communities. Intake of cassava foods was estimated from dietary history and dietary cyanide load was estimated from urine thiocyanate concentrations. Residual cyanogens in cassava food samples from the community markets were determined. RESULTS In total, 1272 subjects from 21 communities - 238 from area A, 659 from area B and 375 from area C - were selected. Intake of cassava food per person per week was 17 meals in area A, 10 meals in area B, and one meal in area C. Geometrical mean urine thiocyanate concentrations were 73 micromol l(-1), 51 micromol l(-1) and 17 micromol l(-1) in areas A, B and C, respectively. Mean residual cyanogen content in cassava food samples was 16 mg HCN eq kg(-1) (confidence interval (CI) 13-18) in area A, and 13 mg HCN eq kg(-1) in area B (CI 11-14). CONCLUSION This study shows that the intake of cassava foods and dietary cyanide load is high in several communities in south-western Nigeria, predominantly in communities where TAN has been reported. Dietary cyanide load in these communities appears to be determined by the combination of frequency of intake and cyanogen content of cassava foods. Measures to improve the effectiveness of removal of cyanogen from cassava roots during processing are needed in the affected communities.

Moisture stress during growth affects the breadmaking and gelatinisation properties of cassava (Manihot esculenta crantz) flour

Breadmaking quality (judged by the regularity of the crumb) of cassava (in cassava flour/defatted soy flour/glyceryl monostearate recipes (80/20/3, w/w), Cereal Chem 68 323–327 1991) is related to crop age and the season of planting and harvesting. When taking the data sets of the dry season (J Sci Food Agric 66, 193–202 1994) and rainy season (ibid 68 167–174 1995) together, the following additional interpretation results: breadmaking quality of cassava is positively related to flour from roots with unrestricted growth, and therefore influenced by rainfall distribution, time of planting and harvesting. Variability in baking performance of cassava flour samples is related to the length and severity of the dry season, the growth behaviour of the genotype and the crop age. The highest flour breadmaking quality is obtained after 6 months of unrestricted growth of cassava irrespective of the planting season.

Loss of residual cyanogens in a cassava food during short-term storage

Residual cyanogens in gari, the most popular cassava food in West Africa, is implicated in the causation of tropical ataxic neuropathy. Gari is eaten by soaking its granules in cold water or by adding boiling water to make a food called eba. This study was conducted to determine whether loss of the residual cyanogens in gari during short-term storage and when gari is made into eba will reduce dietary cyanide load in consumers. Fifteen samples of gari, nine roasted from cassava mash fermented for at least 4 days (type A) and six roasted from cassava mash fermented for only 1 day (type B), were stored for 4 weeks. Free cyanide, linamarin, and cyanohydrin in gari and in eba made from the gari were determined at weekly intervals for 4 weeks. Free cyanide was absent in all samples of gari. Mean cyanohydrin dropped from 8.4 mg HCN Eq/kg dry weight to 4.6 mg HCN Eq/kg dry weight in type A gari, while it dropped from 3.0 mg HCN Eq/kg dry weight to 1.3 mg HCN Eq/kg dry weight in type B gari. Mean linamarin dropped from 6.6 mg HCN Eq/kg dry weight to 2.8 mg HCN Eq/kg dry weight in type A gari, while it dropped from 1.7 mg HCN Eq/kg dry weight to 0.4 mg HCN Eq/kg dry weight in type B gari. Loss of linamarin and cyanohydrin was significant at P < 0.001 for type A gari and at P < 0.002 for type B gari when the weekly levels were compared with initial values. When gari was made into eba, 36% of cyanohydrin and 47% of linamarin were lost from type A gari, while 38% of cyanohydrin and 5% of linamarin were lost from type B gari. Loss of linamarin and cyanohydrin when gari was made into eba was significant for both types of gari at P < 0.001. This study shows that the loss of cyanohydrin and linamarin in gari during short-term storage and when gari is made to eba will reduce dietary cyanide load in consumers.

High cassava production and low dietary cyanide exposure in mid-west Nigeria

OBJECTIVE To investigate if high cassava production levels indicate high consumption and high dietary cyanide exposure in three villages situated within the area of Nigeria with higher cassava production than predicted by a geographic model for cassava production in Africa. DESIGN Exploratory assessment of: cassava production and processing by qualitative research methods and quantification of residual cyanogens in products; cassava consumption by food frequency and weighed food records and dietary cyanide exposure by urinary thiocyanate and linamarin. SETTING Rural communities of Afuze, Ebue and Ofabo in mid-west Nigeria. SUBJECTS 110 subjects from 42 households in three villages for food frequency interviews; 118 subjects in nine Ofabo households for weighed food records. RESULTS Cassava cultivation was reported to have increased in the preceding 20 years. It was consumed daily by 37 (88%) households, but its mean contribution to daily energy intake was only 13% The range of residual cyanogens in cassava foods was 0 to 62 mg HCN equivalent/kg dry weight (dw). Ten samples (19%) had levels above the 10 mg HCN equivalent/kg dw FAO/WHO safety limit. Mean urinary thiocyanate and linamarin were 51 and 20 micromol/L, indicating low cyanogen intake and dietary cyanide exposure. CONCLUSION High cassava production levels did not result in high consumption and high dietary cyanide exposure levels, therefore cassava production levels cannot be used to predict consumption or cyanide exposure levels in the study area. A large part of the production is explained by intensive sales.

Milling reduces the goitrogenic potential of cassava

Cyanide exposure from consumption of insufficiently processed cassava has been implicated in aggravating iodine deficiency disorders (IDD). The cyanide metabolite, thiocyanate (SCN) may interfere with iodine (I) uptake of the thyroid gland. A study on 217 women in an IDD endemic area in western Tanzania showed that 98% consumed cassava daily. Total and visible goitre rates were 72.8% and 13.3%, respectively. Median urinary iodine was 3.6 micrograms/dl indicating moderate iodine deficiency. Processing methods which remove cyanogens from cassava roots have changed with time. Urinary thiocyanate (mean; 128 mumol/l) was moderately increased, but women who frequently milled cassava had significantly lower urinary thiocyanate levels. This indicates that mechanical milling could reduce the goitrogenic potential of cassava and we conclude that IDD in the studied area is mainly due to iodine deficiency and sustainable iodine supplementation should be given highest priority.