Removal of Cyanogens from Cassava Roots : Studies on Domestic Sun
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REMOVAL OF CYANOGENS JTROM CASSAVA ROOTS Studies on domestic sun-drying and solid-substrate fermentation in rural Africa Promotoren: Dr. ir. A.G. J. Voragen hoogleraar in de levensmiddelenchemie Dr. H. Rosling associate professor of internal medicine, at the Unit for International Child Health, Uppsala University Sweden A.J.A. Essers REMOVAL OF CYANOGENS F'ROM CASSAVA ROOTS Studies on domestic sun-drying and solid-substrate fermentation in rural Africa Proefschrift ter verkrijging van de graad van doctor in de landbouw- en milieuwetenschappen op gezag van de rector magnificus, dr. C.M. Karssen in het openbaar te verdedigen op vrijdag 2 juni 1995 des namiddags te half twee in de aula van de Landbouwuniversiteit te Wageningen CIP-DATA KONINKLIJKE BIBLIOTHEEK, DEN HAAG Essers, A.J.A. Removal of cyanogens from cassava roots: Studies on domestic sun-drying and solid-substrate fermentation in rural Africa / A.J.A. Essers. - [S.I. : s.n.1 Thesis Landbouwuniversiteit Wageningen. - With ref. - With summary in French, Portuguese and Dutch. ISBN 90-5485-378-6 Subject headings: cassava processing / cyanogenic glucosides / toxicity. Most of the research published in this thesis was financed by the Netherlands' Minister for Development Co-operation (project nr. RF/90/953). Cover photo: Basket with dried pieces of cassava root ABSTRACT Essers, A.J.A. (1995) Removal of cyanogens from cassava roots: studies on domestic sun-drying and solid-substrate fermentation in rural Africa. Ph. D. -thesis at Wageningen Agricultural University, The Netherlands, together with the Unit for International Child Health, Uppsala University, Sweden (131 p, English, French, Portuguese and Dutch summaries) 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 linarnarase 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 heap- fermentation, 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. 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. 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 Neurospora sitophila and Rhizopus oryzae were the most effective in cyanogen removal. Optimization of processing conditions, including the use of starter cultures, is recommended for ensuring safe products. Key words: cassava processing 1 cyanogenic glucosides I toxicity Preface The research presented in this thesis was initiated during my work as nutritionist in the Provincial Health Service of Nampula, Mozambique, from 1980 to 1985. During this multi task assignment, I became involved in studies of outbreaks of paralysis, associated with high consumption of poorly processed cassava. Nampula province is one of the most cassava dominated areas of Africa. During drought periods, roots of bitter cassava became the sole food crop due to food shortage. This led to the traditional cassava processing being shortened, which war linked to toxic eflects. The farming population tried to avoid the acute poisonings they experienced, by altering their processing techniques. It became my task to assist them in their experiments to turn bitter roots into a flour safe for consumption, in a short period of time. When rural Mozambique became too insecure, the investigations continued in other areas of Africa. The Department of Food Science of Wageningen Agricultural University provided an excellent environment for studying the mechanisms of cyanogen removal from cassava tissue. The Netherlands Ministry for Development Cooperation (DGIS) finded the research project 'DetoxiJfcation of cassava at household level in rural Afiica', from 1990 - 1994. Its aim was to provide knowledge for the improvement of the domestic processing of cassava. Part of the project was carried out in Uganda and Malawi, where it also aimed at increasing the national research capacity. The results have been given in a report which serves as a basis for extension activities. Field studies in Mozambique and Uganda, as well as laboratory studies in Wageningen are presented in this thesis. Together with colleagues from the Nutrition Department of the Ministry of Health, I recently had the opportunity to feed back results to the target population in Nampula province, Mozambique. Alternative processing methods were tested and adapted in the field with groups of apparently motivated female farmers. We formulated a message and trained local extensionists as part of a campaign to promote effective processing.' It is my hope that the results of these projects and this thesis may benefit the cassava farmers who are occasionally at risk of intoxication by the crop which forms the basis of their existence. I dedicate this thesis to the cassava cultivating peasant population in Africa. h he poem at page 1291130 was a result of the training course. Pan of a hugh market for heap-fermented cassava in Paida. Wcst-Nilc. Uganda (note the high stuffed sacks for export out of the county) CONTENTS Chapter Page Abstract Preface 1 General introduction 1 2 Insufficient processing of cassava induced acute intoxications and the paralytic disease konzo in a rural area of Mozambique. 7 3 Studies on the quantification of specific cyanogens in cassava products and introduction of a new chromogen. 2 1 4 Cyanogen removal from cassava roots during sun-drying. 39 5 The safety of dark, moulded cassava flour compared with white - a comparison of traditionally dried cassava pieces in north-east Mozambique. 6 Reducing cassava toxicity by heap-fermentation in Uganda. 7 Contribution of selected fungi to the reduction of cyanogen levels during solid-substrate fermentation of cassava. 8 Mechanisms of enhanced linamarin degradation during solid- substrate fermentation of cassava. 9 General discussion Summary Resume (French) Resumo (Portuguese) Samenvatting (Dutch) Acknowledgements Mother cassava (poem) Mam2 mandioca (poema) Curriculum Vitae CHAPTER 1 GENERAL INTRODUCTION The crop cassava Cassava is a tropical shrub of which the starchy storage roots, and in some areas also the leaves, are important as food. It was domesticated by Indians living in northeastern South America several thousands years ago. Cassava was brought by the Portuguese to West Africa during the last half of the sixteenth century. It was introduced to East Africa and Asia during the eighteenth century (Jones, 1959: 28- 32). Cassava continues to be an important crop in South America, it has become so in Asia, and in Africa it has become extremely important, with an estimated root production in 1986 was 30, 41, and 57 X 106 Mttannum, respectively. Cassava roots provide 30% of Africa's staple food (De Bruijn and Fresco, 1989). The crop's agronomic advantages, such as high productivity, ease and flexibility of cultivation, tolerance to drought, and its ability to grow well on relatively poor soils probably made its adoption in Africa so rapid and extensive. Drawbacks of cassava roots in terms of food are their bulkiness, their perishability, their low protein content and their potential toxicity. The roots contain about 65% water and they deteriorate within 4 days after harvest. This makes them difficult to transport and market. As the protein level is lower than that of other staple foods, cassava roots have to be supplemented with protein-rich foods to form a balanced diet. Cassava leaves contain a high amount of protein with a moderate net utilization. Considering this, the crop cassava is not low in proteins. The potential toxicity is related to the capacity of