Cassava And Cyanogenic Glycosides In Relation To Human Health

L L Daisy, A K Faraj and J M Matofari

Abstract

Kenya is experiencing severe drought and acute food shortages, and as a result the country is coming up with foods that can be used for food security, including cassava. It is an indigenous crop that is fast growing and is well adapted to the dry environment. The starch filled roots can be used like potatoes or ground into flour, while the leaves can be used like spinach. The long term effect of increased cassava production is reduced famine during droughts, increased economic output, and improved living conditions for Kenyans. Cassava on the other hand is associated with toxins. These toxins present usually pose a threat to the overall health of individuals consuming the tuber. The effect of the toxins can even lead to death depending on the concentrations consumed. This review paper provides a greater understanding of the toxicity of the different cassava varieties, effects of  cyanide on human health, factors affecting the cyanide content in cassava, and various methods used in the detoxification process of cassava.

Introduction

Cassava (Manihot esculenta Crantz), also known as tapioca or manioc, is one of the major tuber crops grown in more than 80 countries of the humid tropics. It is a woody shrub of the Euphorbiaceae family, native to South America. It is extensively cultivated as an annual in tropical and subtropical regions for its edible starchy tuberous roots. It is also a high energy food obtained with low inputs and little effort. To the people in the tropics, it is either a main or a secondary staple food. Most of the world’s production of cassava is used for human consumption in tropical countries; the other main uses are for animal feeds and the starch industry.

Health Benefits of Cassava

1.    Cassava has nearly twice the calories of potatoes, perhaps highest for any tropical starch-rich tubers and roots, with 100 g root providing 160 calories. Their calories mainly come from sucrose forming the bulk of the sugars in tubers, accounting for more than 69% of the total sugars.  Amylose is another major carbohydrate source (16-17%).

2.    Cassava is lower in fats and proteins than cereals and pulses, although it has more protein than that of other tropical food sources like yam, potatoes, and plantains.

3.     As in other roots and tubers, cassava is free from gluten. Gluten-free starch is used in special food preparations for celiac patients.

4.    Young tender cassava leaves are a good source of dietary proteins and vitamin K. Vitamin K has a potential role in bone mass building by promoting osteotrophic activity in bones. It also has established role in the treatment of Alzheimer’s disease in patients by limiting neuronal damage in the brain.

5.    Cassava is a moderate source of some of the B-complex group of vitamins e.g. folate, thiamin, pyridoxin (vit. B6), riboflavin and pantothenic acid.

6.    The root is the chief source of some important minerals like zinc, magnesium, copper, iron and manganese for many inhabitants in the tropic belts. It also has adequate amounts of potassium (271mg per 100g or 6%RDA) which is an important component of the cell and body fluids that help regulate heart rate and blood pressure.

7.    Dried cassava leaves are a good source of proteins, minerals and vitamins. Hence the leaves offer vast scope as a protein ingredient in compounded feeds for livestock and poultry.

8.    Cassava leaves are superior to many other leaves as there is less crude fibre and a high concentration of fats and carbohydrates.

Table 1 gives the nutrient values of the various nutrients present in cassava.

Table 1, Cassava root (Manihot esculenta(L.) Crantz), raw, nutritional value per 100g,

Toxins Associated With Cassava

A toxin is a poisonous substance, especially a protein that is produced by living cells or organisms and is capable of causing diseases when introduced into body tissues. They are also capable of inducing neutralising antibodies and antitoxins. All cassava varieties produce toxins.

A food safety problem with cassava is that cassava roots contain considerable quantities of toxins (cyanogenic glycosides). These glycosides exist in more than one form, i.e. primarily linamarin and a small amount of lotaustralin (Uyoh et al., 2007). The leaves also have a considerable amount of the toxins. Cassava leaves contain more linamarin than do the plant’s roots. It is believed than linamarin is transported from the leaves to the roots early in a plant’s life. In relation to the toxicity of cassava, there are sweet and bitter cassava varieties. The sweet cassava roots contain less than 50 mg per kg hydrogen cyanide on a fresh weight basis, whereas that of the bitter varieties may contain up to 400 mg per kg.

Linamarin

This is the main toxic principle which occurs in varying amounts in all parts of the cassava plant. Linamarin is a cyanogenic glycoside which is converted to toxic hydrocyanic acid or prussic acid when it comes into contact with linamarase, an enzyme that is released when the cells of the cassava roots are ruptured. Otherwise, linamarin is a stable compound which is not changed by boiling the cassava.

The structure of Linamarin is shown in Figure 1.

Figure 1.

Lotaustralin

This is another form of the cyanogenic glycoside present in cassava. Its structure is as shown in Figure 2:

Figure 2.

Hydrocyanic acid (HCN) is a volatile compound. It evaporates rapidly in the air at temperatures of over 28^oc and dissolves readily in water. It may easily be lost during transport, storage and analysis of specimens. The normal range of cyanogen content of cassava tubers falls between 15 and 400  mg HCN/kg fresh weight (Coursey 1993). The concentration of cyanogenic glycosides increases from the centre of the tubers outwards (Bruijin 1973). Generally, the cyanide content is substantially higher in cassava peels.

Effects of Cyanogenic Glycosides on Human Health

Ingestion of cassava can trigger several toxic manifestations due to the release of HCN from cyanogenic glycosides. The incidence of acute poisoning from consumption of cassava is relatively rare since the amount ingested is often low. However, chronic intake of cassava can lead to toxic conditions as the person is exposed to sub-lethal doses of cyanide for a prolonged period. The toxicity of cassava is due to the release of HCN in vivo which is a potent cytotoxin exerting a wide range of biological effects.

The following are human diseases associated with cyanide.

·       Leber’s Optical Atrophy: In humans, two neurological syndromes resulting from chronic exposure to cyanide have been recognised, i.e. Leber’s Optical                        Atrophy and tobacco amblyopia. These defects arise due to impairment in the conversion of cyanide to thiocyanate.

·      Tropical Ataxic Neuropathy: Epidemiology and clinical studies conducted in tropical areas where cassava forms a staple food item have shown that             tropical neuropathies characterised by nerve deafness, optical atrophy and ataxia are common in these areas. Osuntokun et al. (1968) described the chronic degenerative disease called tropical ataxic neuropathy which occurs in several parts of Africa. It is characterised by myelopathy, bilateral optical atrophy and perceptive deafness.

·        Endemic Goitre and Cretinism: Although the goitrogenic properties of plants were known long ago, the goitrogenic effect of cassava was suspected for the first time in 1966 in East Nigeria. The frequency of goitre was so high in these areas that iodine deficiency alone could not account for this high incidence. The goitrogenic action of cassava was originally attributed to a thionamide compound. Further studies showed that the antithyroid action of cassava was due to the thiocyanate produced from it. Thiocyanate inhibited iodine uptake by increasing the velocity constant of iodine efflux from the gland.

·      Tropical Calcifying Pancreatitis.

Tropical calcifying pancreatitis and pancreatic diabetes are symptoms often reported from Kerala, India. Histopathological studies conducted in rats administered cassava for 18 months showed pancreatic changes like dilated ductules, papillary infoldings, atrophic acini, and round cell infiltration.

Factors Affecting the Cyanide Content of Cassava

Cultivars

All cassava cultivars contain cyanogenic glycosides. However, a wide variation in the concentration exists among different cultivars. This can range from 1 to 2000 mg/kg (Cardoso et al., 2005, CIAT 2007). Cultivars with <100 mg/kg hydrogen cyanide are called sweet while those with >100 mg/kg are called bitter (Wheatley et al. 1993)

Climatic Conditions

Cyanide content of cassava tends to increase during periods of droughts and / or prolonged dry weather due to water stress on the plant (Bokanga et al. 1994). Splittstoesser and Tunya (1992) reported that cassava grown in wet areas contains relatively lower amounts of cyanide than those grown in dry areas.

          Age of Cassava at Harvesting

Cyanide content is higher in younger roots than in older roots. This is in accordance to the study by Hidayat et al. (2002). According to the study, the cyanide content was higher in younger leaves compared to older leaves, suggesting that cyanide potential in roots desreases as the plant ages.

          Post harvest Practices

There are a series of activities done to cassava after harvesting which are aimed at not only to increase palatability, but also to decrease its cyanogenic potential. These activities consist of different combinations of peeling, chopping, grating, soaking, drying, frying, boiling and fermenting.

          Harvesting

During harvesting, the roots might be injured. This increases the rate of postharvest deterioration and it also has an effect on the cyanide content.

          Fertiliser

Application of fertiliser does not significantly affect cyanide content (Rolinda et al. 2008). The amount of nutrients in the soil does not considerably contribute to the cyanogenic character of the cultivar.

Effects of Cassava Processing on Cyanide Levels (Detoxification Methods)

Cassava tubers are traditionally processed by a wide range of methods, which reduce their toxicity, improve palatability and convert the perishable fresh root into stable products. These methods consist of different combinations of peeling, chopping, grating, soaking, drying, boiling and fermenting. While all these methods reduce the cyanide levels, the reported loss in cyanide content differs considerably due to analytical methods, the combinations of methods and the extent to which the processes are carried out. The specific effects of various processing techniques on the cyanide content of the cassava are discussed below:

Peeling

The cassava peels contain higher cyanide content than the pulp. Removal of peels therefore reduces the cyanogenic glycosides content considerably. This can reduce the cyanide content by at least 50% in cassava tubers.

Grating

This process takes place after peeling and is sometimes applied to whole tubers. Grating of the whole tubers ensures the even distribution of cyanide in the product and will also make the nutrients contained in the peel available for use. In the grated product, the concentration of the cyanide depends on the time during which the glucoside and the glucosidase interact in an aqeous medium. Grating also obviously provides a greater surface area for the fermentation to take place.

Soaking

Soaking of cassava roots normally preceeds cooking or fermentation. It provides a larger medium for fermentation and allows for greater extraction of the soluble cyanide into soaking water. A very significant reduction in total cyanide is achieved if the soaking water is routinely changed over a period of 3-5 days.

Boiling/cookingAbout 90% of free cyanide is removed within 15 minutes of boiling fresh cassava chips. As with soaking, the free cyanide of cassava chips is rapidly lost in boiling water. Cooking destroys the enzyme linamarase at about 72^oC thus leaving a considerable portion of the glucoside intact.

Fermentation

Importance is based on its ability to reduce the cyanogenic glucosides to relatively insignificant levels. It is believed that some cyanidrophilic/cyanide tolerant micro-organisms, effect breakdown of the cyanogenic glycosides.

Ensiling

The ensiling process causes the disintegration of the intact glucoside via marked cell disruption, drop in pH of ensiled medium and intense heat generation. Ensiling cassava chips reduces the cyanide content to 36% of the initial value after an ensiling period of 26 weeks (Gemez and Valdiviesco1988). It was also noted that about 98% of the free cyanide was lost by ensiling cassava roots with poultry litter for 8 weeks.

Drying

Since cassava roots contain about 61% water, coupled with the solubility of its cyanogenic glucoside component, the dehydration (dewatering) process results in a substantial reduction in the content of this toxin in the pressed pulp. Drying is carried out using solar radiation (sun dying) or drier (electric or fuel) depending on economic availability.

Sun drying results in greater loss of total cyanide compared to laboratory oven drying at 60^oC for 48 hours, tends to produce greater loss of bound cyanide due to slower drying rate relatively to oven drying, allows a longer contract period between the glucosidase and the glucoside in the aqueous medium., facilitates the continuation of the fermentation process, and is cost effective but slow and often encourages the growth of mould and other micro-organisms.

Conclusions

In so far as cassava is a very nutritious food, every knowledge concerning its toxicity should be obtained from whatever sources available. Care should be taken during the consumption of cassava because of the various toxins associated with it. These toxins, as discussed, have some detrimental effects to the health of individuals and it can as well lead to death.

It is hence important to consider the variety used for food, and also the quantities to be consumed.

The various processes applied to the tuber before consumption should be done carefully. This will help to reduce the spread of the toxin to the whole tuber. It will also help to reduce the concentration of the toxins in the cassava and this will at least make the tuber safer for consumption. In addition, cassava should be consumed together with other foods rich in other nutrients, e.g. proteins, whose contents in the roots are very minimal, so as to provide a balanced diet.

References

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LL Daisy (corresponding author, e-mail: daisy.lanoi@gmail.com), AK Faraj (e-mail: ab_faraj@hotmail.com) and JM Matofari (e-mail: jmatofari@gmal.com) are with the Department of Food Science and Technology, Egerton University, PO Box 536-20115, Egerton, Kenya.