You’ve probably had it boiled or as fine slices, deep-fried until crisp. A staple in many Kenyan kitchens, cassava is comfort food to many, but did you know it can be poisonous if prepared incorrectly?
According to Wikipedia: Cassava is classified as either sweet or bitter. Like other roots and tubers, both bitter and sweet varieties of cassava contain antinutritional factors and toxins, with the bitter varieties containing much larger amounts. It must be properly prepared before consumption, as improper preparation of cassava can leave enough residual cyanide to cause acute cyanide intoxication, goiters, and even ataxia, partial paralysis, or death.
Junji Takano, a Japanese health researcher, explains that the toxin in cassava is called “linamarin”. It is chemically similar to sugar but with a CN ion attached. When eaten raw, the human digestive system converts this to cyanide poison.
Sounds pretty scary. But what is proper preparation? How do you detoxify cassava before eating it?
Researchers have found that cyanide content of cassava differs among varieties. Once harvested, bitter cassava must be treated and prepared properly prior to human or animal consumption, while sweet cassava can be used after simple boiling.
Most cultures that traditionally eat cassava, generally understand that some processing (soaking, cooking, fermentation, etc.) is required. But most people don’t realize that the preparation is absolutely necessary in order to avoid getting sick; and not just a choice of preparation by the chef.
Main Processing Methods Used Worldwide
Here is a comprehensive comparison of different processing methods published in the Comprehensive Reviews in Food Science and Food Safety Journal.
Boiling
Boiling is not an effective method for cyanide removal (50%). The inefficiency of this processing method is due to the high temperatures. At 100 °C, linamarase, a heat-labile β-glucosidase, is denatured and linamarin cannot then be hydrolyzed into cyanohydrin.
However, using small-sized cassava pieces or increasing the volume of water in which cassava roots are boiled can increase the efficiency of the boiling method. For example by reducing cassava chip size, studies demonstrated that boiling 2- and 50-g pieces of cassava root for 30 min resulted in a 75% and 25% reduction in cyanide content, respectively.
Steaming, baking, and frying
The loss of cyanide resulting from steaming, baking, or frying is small due to processing temperatures of over 100 °C and to the stability of linamarin in neutral or weak acid conditions. These methods are only suitable for sweet cassava
Drying methods
Two kinds of drying are used for cassava: mechanical drying, such as in an oven, and natural drying by the sun. During oven-drying, an increase in drying temperature is accompanied by an increase in cyanide retention. Cyanide retention during sun-drying is lower than in oven-drying because the temperatures remain well below 55 °C. These temperatures are optimal for linamarase activity resulting in better cyanogen degradation. Generally, drying is not an efficient means of detoxification, especially for cassava varieties with high initial cyanogen glucoside content.
Fermentation
Fermentation by lactic acid bacteria is a processing method commonly used in Africa. Fermentation is done with grated or soaked cassava roots and results in a decrease in pH value. The fermentation of grated cassava roots is efficient at removing cyanogen glucosides. The process of roasting after fermentation of grated cassava, which is used for gari, is relatively efficient as free HCN and cyanohydrin are steadily removed into the atmosphere leaving little free HCN (3.4 mg/kg DW) and cyanohydrin (2.2 mg/kg DW). Cyanide content of gari further decreases during storage.
The fermentation of soaked roots in water is much more effective than that of grated roots in terms of cyanogen reduction. Indeed, more than 90% of total cyanogens were removed after 3 d of fermentation and about one-third of initial linamarin was found in the water. No significant accumulation of cyanohydrin or free cyanide was noted
Steam distillation
Studies have showed that steam distillation of fresh cassava pulp resulted in total cyanogen removal for a minimum distillate volume of 100 mL (assumed to be normalized to 1 kg). Steam distillation of fermented pulp slowly removed cyanogens.
Starch production
The process of starch extraction results in total cyanogen removal. Starch extraction involves different processing steps. First, cassava roots are grated or rasped, and then starch is extracted with a large volume of water; residues are removed by sieving. In this way, a complete hydrolysis of cyanogenic glucosides occurs, and cyanohydrins, free cyanide, and the remaining cyanogenic glucosides solubilize in the supernatant water.
Combination of several processing methods
To increase the efficiency of cyanogen removal, efficient processing techniques are usually combined with others that are less efficient.
Comparison of the different processing techniques
Methods involving grating and crushing are usually very efficient in cyanide removal because they completely rupture plant cells of cassava and allow direct contact between linamarase and linamarin. However, sun-drying and heap fermentation are less efficient because peeled roots are usually cut in half longitudinally and most of the plant cells remain intact. Hydrolysis of cyanogenic glucosides is prevented or reduced because linamarin and linamarase are located in different compartments of the plant cell. Heap fermentation retains half the cyanide of sun-drying because of the presence of microflora that can break down the linamarin during the fermentation process .
Boiling, which is relatively inefficient for removing cyanide (50%), is much more efficient than baking, steaming, or frying (15% to 20% of cyanogen removal). Even if linamarase is inactivated at high temperatures (100 °C), cyanogens are water-soluble and, therefore, they can be removed during the dewatering process.