Detoxifying cassava

Strategies that minimize one risk…may augment another risk… Peasant farmers are perfectly conversant with such linkages. The neglect of peasant agriculture by both donors and governments is among the deeper causes of current crises, along with the increasing inequality that deprives them of secure tenure to land and other resources, reducing benefits they can expect to receive from stewardship of these resources.

That comes towards the end of a lengthy, information-dense and closely argued paper on cassava’s domestication, evolution and chemical ecology by Doyle MacKey and others. ((McKey, D., Cavagnaro, T., Cliff, J., & Gleadow, R. (2010). Chemical ecology in coupled human and natural systems: people, manioc, multitrophic interactions and global change Chemoecology, 20 (2), 109-133 DOI: 10.1007/s00049-010-0047-1)) Coincidentally — or perhaps not — McKey also has a paper out just now on the evolutionary ecology of vegetatively propagated crops in general. ((McKey, D., Elias, M., Pujol, B., & Duputié, A. (2010). The evolutionary ecology of clonally propagated domesticated plants New Phytologist, 186 (2), 318-332 DOI: 10.1111/j.1469-8137.2010.03210.x)) But that’s something for a future post, perhaps. Back to cassava.

Well, as I say, there’s a lot of stuff in the paper, but let me focus on a just a couple of things here. Some cassava varieties are high in poisonous cyanogens, requiring laborious and time-consuming processing for detoxification, which is almost exclusively done by women and, according to at least one anthropologist, serves to control them, “by limiting their freedom of action, in male-dominated societies.” So let’s replace those high-cyanogen, “bitter” varieties with low-cyanogen, “sweet” varieties, right?

Well, not so fast. ‘‘Interestingly enough, the women that are the custodians of this crop do not perceive the processing or the toxin to be a problem.’’ Bitter manioc is in fact better for food security for at least three reasons: it is protected against pests; immediate processing deters thieving; and the value added through processing reduces the social obligation to share. Overall, therefore, “processing bitter manioc is viewed as a useful and valorizing activity.”

But there are regular epidemics still in some parts of Africa of the paralytic disease konzo, which is “associated with several weeks of almost exclusive consumption of insufficiently processed bitter cassava.” Surely low-cyanogen varieties would be welcome there?

Again, the authors are dubious. They point out that agronomic practices, in particular careful nutrient management, together with adequate processing and a reasonably protein-rich diet are perfectly sufficient to manage the toxicity problem, and in fact normally do so. It is mainly when socioeconomic stability breaks down that konzo erupts. As they trenchantly put it: “This crop needs peace.” And reasonable rainfall. Drought stress tends to increase cyanogen levels. Plus, “[a]s they face food shortage, people may then take short cuts in processing bitter manioc.” In both cases, new, faster, more efficient detoxification methods will have a bigger impact than sweet varieties, with their greater susceptibility to pests.

In general, cassava is expected to be comparatively little affected by climate change, and is being promoted in some quarters as a “food for the future.” Fair enough, say the authors, but this

…will need to be accompanied by the necessary education for processing, as planting less bitter cultivars appears not to be a solution. Farmers have good reasons for preferring bitter varieties.

Indeed they do. So let me end with another heady quote, again from the concluding section of the paper, a little further on from the quote I started with.

We must have the humility and the broad vision to accept that our science can provide essential pieces, but only pieces, of viable solutions. ‘‘Modern’’ science can work in creative ways with folk knowledge. But no technological fix is a ‘‘magic bullet’’.

10 Replies to “Detoxifying cassava”

  1. Wow, thanks for that post! I never would have guessed bitterness would be important. It’s a good reminder for all of us not to get too ahead of ourselves in “fixing” other peoples ag problems.

  2. I knew about the value of cyanide to deter theft and pests and have also read that it’s a useful pool of nitrogen for the root, but reducing “the social obligation to share” is interesting. As I recall, “Famine in Peasant Societies” puts part of the blame for famines on the expectation that anyone who gets a little ahead will share with relatives, rather than investing in irrigation equipment or whatever. Bank accounts, even if they don’t pay interest, offer a way to hide resources from relatives so, the book claims, famines became less likely. Still, do the risks of sharing outweigh the benefits? Or is there some optimum level of sharing?

  3. There are many other fairly subtle methods to avoid sharing in “peasant” societies.

    For instance, farmers in West Africa buy cows and send them off with the Fulani, in order to hide their wealth. Also, people can avoid obligatory sharing by engaging in saving obligations, for instance by participating in a merry-go-round.

    Not shelling your maize has exactly the same effect as choosing a bitter cassava variety. It is one of the reasons why maize farmers in Guatemala don´t always adopt grain silos…

  4. You are hitting a fair few nails right on the head. The curious thing about konzo is that the symptoms are very similar to neurolathyrism, another food induced crippling affliction, caused by excessive consumption of grasspea.
    There was a meeting of cassava and lathyrus researchers in Belgium last year. Abstracts are available in the Nov 2009 CCDN newsletter. Papers are being published in Food and Chemical Toxicology 2010.

  5. Luigi,
    Thanks for calling attention to this work on cassava that documents the roles that bitterness and even toxicity play in our food. As Doyle McKey rightly notes, bitterness and toxicity is something that farmers manage in a variety of ways; selection, breeding, processing, and agronomic practices are ways to manage or regulate the phytochemistry of crops. This has been noted for many edible species in Tim John’s review of the “Chemical Ecology of Human Ingestive Behaviors”.

    What is particularly welcome in the papers by Doyle and his colleagues is the conclusion that much of this knowledge still exists and is being successfully used by traditional farmers. Yet donors and research policy continue to focus work on single factor, simplistic solutions to problems of taste, health, pest resistance and labour demands of crops. Farmers continue to keep a wide range of properties present in a crop that allow it to adapt and serve a multiplicity of purposes. Why in the face a global food and health crisis that has starkly revealed the fragility and limitations of our simplified and narrow global food system, have donors and research policymakers continued to focus on intensification to maximise yields and global trade of a few staples. Surely the experience of farmers and their management and continuing domestication of crops merits equal scientific attention.

    I begin to think that ignoring examples such as these, with lessons and clear examples that are rigorously scientific and practical is not about lack of evidence or oversight, but a policy to eliminate smallholder agriculture and the knowledge it holds. This would remove any viable alternative to the agro-industrial hegemony over our food and our diverse agrarian landscapes. I hope my fears are misplaced.

  6. Luigi, good entry — and I agree with Pablo, that R&D is often ‘overfocussed’ and thus losing users’ — growers’ — perspectives. A couple of blogs on cassava and these issues, here and here.

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