Removal of cyanogens from cassava roots : studies on domestic sun-drying and solid-substrate fermentation in rural Africa

A.J.A. Essers

Research output: Thesisinternal PhD, WU

Abstract

<p>Cassava is an important staple crop, but its potential toxicity has led to some health problems in Africa. The potential toxicity comes from endogenous cyanogenic glucosides, mainly linamarin, which may degrade by linamarase to cyanohydrins and subsequently to hydrocyanic acid (HCN). A study into a small outbreak of paralysis and poisoning in a cassava-dominated rural area of Mozambique revealed that the walking disability was konzo, a recently identified disease, and suggested that insufficient processing of the bitter cassava roots was a factor in its causation. The usual processing stages to turn roots into flour, sun-drying and heapfermentation, were studied in Uganda and The Netherlands. For evaluation of initial and resulting levels of the cyanogenic compounds, an analytical assay was tested and improved. Mechanisms of cyanogen removal from cassava by sun- drying and heap-fermentation were elucidated, to allow for its optimization.<p>Sun-drying removed cyanogens insufficiently from roots with high initial levels. Dynamics of cyanogen levels are described. Continuing drying below moisture levels of 15% did not diminish linamarin levels further, but it was useful for further removal of the cyanohydrins formed. The dehydration rate influenced linamarin breakdown negatively. Reducing the size of the pieces to speed up drying, as done during the konzo outbreak, therefore resulted in higher residual linamarin levels. Linamarin breakdown can be enhanced by reducing the initial dehydration rate. Microbial contamination may need to be controlled to prevent the formation of microbial toxins.<p>In Uganda and Mozambique certain communities promote fungal growth by heaping and covering the peeled roots. Their aim is to improve the palatability and reduce the toxicity. Cyanogen removal by this solid-substrate fermentation appeared more effective than by sun-drying alone, but several samples of this flour from rural households still had undesirably high levels of cyanogens. Screening of 30 flour samples for mycotoxins was negative, but the formation of mycotoxins cannot be excluded. The humid incubation of cassava extends the time of physiological cell-wall degradation, which allows for linamarase-linamarin interaction. The microflora had an additional positive effect on cyanogen removal by enhancing the cell-wall degradation. The linamarase activity shown by several microorganisms was of lesser importance. The food grade fungi <em>Neurospora sitophila</em> and <em>Rhizopus oryzae</em> were the most effective in cyanogen removal. Optimization of processing conditions, including the use of starter cultures, is recommended for ensuring safe products.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Voragen, A.G.J., Promotor, External person
  • Rosling, H., Promotor, External person
Award date2 Jun 1995
Place of PublicationS.l.
Publisher
Print ISBNs9789054853787
Publication statusPublished - 1995

Keywords

  • storage
  • manihot esculenta
  • cassava
  • africa
  • derivatives
  • cyanogens

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