Cassava – Production, Properties and Benefits

Cassava is a woody perennial shrub with tuberous roots. It belongs to he genus Manihot which is in the Euphorbiaceae family and is also called Tapioca, Mandioca, Yucca, and Manioc (French) in different languages.

Cassava tubers (Manihot esculenta, Crantz) provide an important food source. They are especially rich in carbohydrate and thus calories in Africa, parts of the Far East, West Indies and South America especially Brazil. 

Cassava is now exported to many Western countries. It is reckoned to feed up to 800 million people in tropical areas where it is nearly 50%  of the total energy source of food (Sansome-Smith, 2013). World production of cassava in 2012 was about 220 million Tonnes but is continuing to grow. It is a crop that has excellent security against famine because it is such a readily available carbohydrate source.

Cassava tubers are either processed by fermenting or left as is. The process of fermentation helps produce a more easily processed food as well as remove cyanogensides. Cassava is retted for example by steeping the roots in water for up to 4 days. The roots are softened by the action of various bacteria  including Gram-positive bacterial rods such as Bacillus, Corynebacterium and Clostridium, some of which produce pectinesterase that catalyses the hydrolysis of pectins. Other enzymes are produced that hydrolyse cell wall fragments. This also improves the nutritional quality of the tuber.

Cassava is often processed further into starch which can be refined further to tapioca and tapioca starch too.

Nutritional Composition Of The Tubers

The root contains between 62 and 80%w/w water and 35%w/w carbohydrates. The protein content is about 0.7 to 2.6%w/w and of poor quality, 0.3% fat, 1 or 2% fibre and 1% w/w minerals. 

The amino-acid content  is very weak, especially on the sulphur containing types such as methionine and cysteine (Johnson and Raymond, 1965). The leaves actually have a higher biological value in terms of their protein content.

Some nutritionists have a lower regard for the tuber than they should because compared to potato and yams, the nutritional value is not as high as some would like.

Having stated all that it is a vegetable which has retained value because of its ease of growth in many sub-tropical and tropical climates where other vegetables do not easily grow.

Cassava has a carbohydrate content of 30 to 35% in the fresh root (Montagnac et al., 2009). It also yields a fine starch (CS), which is produced at relatively low cost compared to other polysaccharides because of the abundance of the feedstock. The starchy part is also used to make tapioca flour.

Cassava starch is also hydrolysed further to produce maltodextrin and as an alternative source of bulk sugar relying on starch hydrolysis. 

Cassava Is Rich In Vitamin C

Cassava tubers are a good source of vitamin C which is important for general health especially in maintaining immunity and for natural collagen production.

A cup of cooked cassava still retains 29mg of this sunshine vitamin and that is still 39 per cent of your recommended daily intake. Taken with other vegetables, there should be enough to meet your total daily requirement.

Cassava Contains Lots Of Antioxidants

As well as vitamin C which is an antioxidant in its own right, there are a number of anti-inflammatory phenolic compounds which are worth ingesting. Cassava roots offer a diverse selection which are claimed to help with both short- and long-term health issues where inflammation management is concerned.

Fibre And Complex Carbohydrates

Cassava is well known as a source of starch but this means it is a good source of basic carbohydrate related energy. There is some resistant starch too which means it provides some dietary fibre that often gets overlooked.

About 4g of dietary fibre is found in a cup of cassava. Dietary fibre is well established as an important probiotic which kepps the gut flora and fauna ‘happy’, a regulator for sugar intake which impacts on diabetes management and on regulating blood cholesterol levels.

Toxicity Issues!

One peculiarity – the tuber contains cyanogenic glycosides. The cyanogenic acid content ranges from 10 to over 500 mg HCN (hydrogen cyanide) equivalents per kg dry weight (Siritunga and Sayre, 2004), or approx. 25 mg/kg flour and so the plant must be washed and ground under running water before consumption. A lethal dose for humans of hydrogen cyanide is 40 to 60 mg (Coursey, 1973) or roughly 1 mg/kg body weight, and there have been cases of poisoning but the traditional methods of preparation are highly effective at preventing such occurrences.

Cyanide binds to the trivalent iron centre of cytochrome oxidase. It causes respiratory,  cardiovascular, neurological and thyroid defects, and ultimately
death.

The other componentry which might be an issue in raw cassava are the alkaloids, flavonoids, tannins, various reducing sugars and anthocyanosides. The leaves however do contain cardiac glycosides, anthraquinone, phlobatinnins, saponins and anthrocyanosides. It is recommended during processing of any sort that the leaves are not included in any process because of their apparent toxicity.

Linamarin is a cyanogen glucoside synthesized in the leaves and transported to the roots. It is released by the enzyme linamarase which is released when cells in the tuber or leaf are broken. Hydrolysis releases hydrogen cyanide which has an LD50 of 60mg for humans. A small amount of cyanide is metabolised by people into thiocyanate but not enough if the level of poisoning is high. The release of free cyanide is extremely rapid. The intermediate is an unstable cyanhydrin (Wheatley and Chuzel, 1993). It spontaneously breaks down to HCN and acetone which is rate dependent on temperature and pH. Acetone cyanohydrin then breaks down via the enzyme hydroxynitrile lyase. This enzyme is found mostly in leaves but there is little in the roots. 

Any form of juice extraction, heat application, processing etc. reduces HCN levels to a safe level.

 Lotaustraulin is another cyanide component (Westby, 2002) that is not that well understood.

Sweet Cassava

Sweet cassava is noted for its low levels of toxins which are less than 50 ppm. Sweet cassava is edible and very safe to use even as a fresh tuber and certainly when processed.

Bitter Cassava

Bitter cassava is a tuber with much higher levels of cyanide than sweet cassava.  Bitter cassava can only be used if it is comprehensively processed. The tubers can contain at least 100 mg/kg in the fresh material and in some cases up to 900 mg/kg. The crop can be classified further into moderately poisonous – 50 to 100 ppm (Hidayat et al., 2016) and then the very poisonous referenced earlier.

It is becoming an acceptable crop because such high levels of cyanide mean that it is not eaten by rodents and other pests and thieves are less likely to remove a crop that needs processing directly.

The peels in particular have high levels of these compounds and are not used highly.

Cultivars

We know there are at least 1,500 cultivars but only a few are routinely grown. growing such varieties tends to be region specific rather than many of them being available commercially for global use. A number are also grown primarily as sweet or bitter types specifically to meet processing criteria into starch rather than as a shop-bought food. There are also well over 100 species in the Manihot which can also add to identification issues as well as providing source material for hybridization.

A large number of varieties are available for growing including Kirikawadi (Somendrika et al., 2016), MU51, CARI-555 (Somendrika et al., 2017), Swarna, Shani, Muthukawadi and Suranimala. These are varieties found in Sri Lanka.

Sweet cassava cultivars include MCol 22, TMS 30572 and BRA 383. The bitter cassava varieties may have high levels of cyanide but are suited to commercial processing. These are Manihot GL 145, TME 1 and TMS 4(2)1425.

African cultivars have a strong yield and good disease resistance – these are the TMS (Tropical Manihot Species) cultivars such as TMS 98/0581 and TMS 92/0326. Nigeria produces the IITA-developed cultivars including the TME 419 and TMS 96/1632. The South American cultivars include the BRA and CM cultivars. These include BRA 9004 and CM 523-7.

The cultivars NR 87/184 and TME 419 are thought to be good for fufu and garri production because they suffer less from post-harvest deterioration (Nwadilli et al., 2024).

Most cassava varieties are diploid with 2n chromosome number = 36 (Bokanga et al., 1994).  

Growing Cassava

Tubers can be planted in a range of soils. They survive drought and some cold but not for prolonged periods. The tubers are harvested after 18 months. They can be left in the ground for a considerable period of time but will start sprouting in the new season as with many tubers.

Growing any crop benefits from crop rotation as opposed to continuous cultivation. When combined with legume cultivation, the nitrogen content of the soil increases allowing a variety of other crops to be grown. There is always some damage from pests but not as widely if rotation occurs. Hornworm is the most serious pest.

Other good farming practices such as low tillage, leaving fields fallow and adding grass-legume mixtures also seems a good idea.

It appears that growing cassava in low altitude areas produces crops with a higher level of cyanogen glycosides whereas those grown higher up have less amount (Oluwole et al., 2007).

Cassava plants are prone to viral attack. the main culprit is African cassava mosaic virus which attacks the leaves turning them brown and stunting their growth. The virus is especially prevalent in Africa. A few of the cultivars have been developed with viral resistance using hybridization. There is a bacterial blight caused by a Xanthomonas spp. and most seriously of all a fungal pathogen which is related to Phytophera. .

Storage Of Cassava Roots

As a root crop, cassava is extremely poor at retaining its quality upon harvesting. It is highly perishable and cannot be eaten after 24 to 72 hours. There is rapid cellular destruction where polyphenols are released and start reacting. These polymerize forming blue, brown and black complexes usually made up of condensed tannins along with coumaric acid. It is in part, the coumaric acid which oxidises and blackens the root tubers making them inedible. This will be due in part to polyphenol oxidase (PPO) but the rapid rate of reaction is suggestive of a bruised soft fruit. PPO converts polyphenols released to quinones and then onto polyphenolic complexes (Tanaka et al., 1983).

The physiological disorder is called vascular streaking or the blue-black vascular discolouration. This is followed by microbial spoilage. The condition is also called Postharvest Physiological Deterioration (PPD) phenomenon (Beeching et al., 1997).

As well as discolouration, the roots start to taste bitter along and there is a distinctively unpleasant aroma that replaces the sweeter aroma of freshly harvested tuber. It is more than likely that the bitterness comes from cyanide release but there will be contributions from other compounds.

The coumarin polyphenol called scopoletin is generated which forms very rapidly after harvest. So scopoletin (6-methoxy-7-hydroxycoumarin) was found as the major accumulating polyphenol in cassava PPD, along with other phenolic coumarin sugar compounds in cassava roots such as scopolin  (6-methoxy-7-hydroxycoumaroyl-7-beta-D-glucoside), esculin (6,7,-dihydroxycoumaroyl-6-beta-D-glucoside, proanthocyanidins, (+)-catechin, (+) gallocatechin and rutin (Beeching et al., 2002).

The coumarins disappear after 1 week storage of the tubers. The losses are due to enzymic destruction using peroxidases. A black precipitate is generated which can be washed out of the macerated tuber. The streaking is probably due to this streaking event in the vascular tissue of stems. To support this view, there is also a rise in hydrogen peroxide concentration in the storage tissue (Buschmann et al., 2000; Bayoumi et al., 2008).

To overcome this rapid deterioration, the harvested rhizomes are treated to prevent any oxidation. This can take the form of waxing, freezing. Cultivar development has also been aimed at reducing this level of blackening. An alternative is to use dips to serve as reducing agents and also preserve with chemicals such as thiabendazole. The amino-acid proline is also claimed to have some protective effect (Tang et al., 2024) but simple chemical dips would work well. the other method is storage using low oxygen environments but not making it entirely anaerobic.

The biosynthesis of hydroxycoumarins may well involve a few different pathways. Esculetin and scopoletin are synthesised from ferulic and caffeic acid. It could also be the case that one hydroxycoumarin is a modified form of the other. Uarotta et al., 2014 used metabolomics along with chemometric tools such as PCA, PLS-DA and others to screen tubers for deterioration on storage. The metabolomic approach allows for the parallel assessment of the levels of a broad range of metabolites. It has been used to great effect in both phenotyping and diagnostic analyses in plants (Fernie & Schauer, 2008).

Post-harvest treatment could include use of ethanol ( >20%), sodium sulphite (10%), sodium dithiocarbamate (10%), saturated sodium chloride, benomyl (500 ppm) and dicloran (1000 ppm). These are said to delay the onset of PPD for several days (Booth, 1976) but there are significant issues with their use, not least their own toxicity. Dips based around ascorbic acid, calcium ions and strong antioxidants could also sequester or remove prooxidant molecules from the system. Chitosan sprays are claimed to improve wound healing and stimulate disease resistance as well as reduce deterioration (Wang et al., 2023).

Much of the effort in reducing the cyanide content of cassava is through the ‘wetting method’ where the flour is mixed with water to make a think paste similar to wallpaper paste and then dry it fully in the sun where it stands for five hours at 30C in the shade.

Sensory Evaluation

Once the cyanide level has been checked in samples and found to be below the threshold limit, then is it feasible to begin sensory evaluation. Cassava and its products are assessed for all sensory cues including colour, aroma, texture and acceptability.

Common methods include using a nine-point Hedonic measure (Dada et al., 2018).

Fermentation

Cassava can be deliberately fermented to generate new flavours which are palatable but can also present an issue. Fermentation is a suitable means of removing cyanogenic glucosides from cassava. Usually, the roots should be peeled before fermentation although it is not entirely clear why. A three-day fermentation removes 100% of the cyanide for most cultivars. If any residue is left it is in the form of cyanogenic glucosides which has not been accessible to the fermenting microorganism.

Flour produced from any fermented tubers should be clean, white and free of any smell so fermentation of tubers should not be acceptable. The powdered root is also the basis of beverages such as cassareep which is made from bitter huava.

Cooking With Cassava

Cassava is processed into various food ingredients. It is often used in place of potato. The starch is used for making various types of bread.

• Fufu

(i) The sweeter cultivars are washed, peeled, boiled and then pounded or mashed to make a paste or dough which is then consumed with soups and broths.

(ii) The fresh roots are also washed with the fine bark removed and peeled off. Sieving in water removes remaining dirt. these roots are boiled in a number of changes of water to remove dissolved cyanides. The roots are always pounded or mashed before eating.

• Gari

A fine and gritty flour known as gari is prepared by peeling off the bark of the tuber, washing and then grating the root. The roots are usually placed into bags before fermenting and then macerating to a fine consistency. Sieving and roasting over an open fire in shallow pots or pans improves the use of the flour. palm oil also helps improve consistency. This is known as gari and is eaten as a cereal with milk or prepared into fufu it is prepared with hot water.

Yuca Or Cassava Starch

Cassava starch is a valuable material for creating edible coatings where it effectively reduces the respiration rate and water loss, especially when applied to minimally processed fruits (Chiumarelli et al., 2010; Garcia et al., 2012).

Cassava Fibre

Cassava pulp is a valuable by-product from starch manufacture and contains 50 to 70 %w/w starch on a dry weight basis  with 20 to 30% being fibre. That high level could be put to further use in product development as it is normally used as fertilizer or animal feed (Pandey et al., 2000; Sriroth et al., 2000). More could be made of this fibre for food use!

Off-Flavours In Cassava

(a) Cyanogenic Glycosides

• Compounds: Cassava contains two key cyanogenic glycosides, mainly linamarin and lotaustralin. When cassava is processed improperly, these compounds can break down into hydrogen cyanide (HCN), which can impart a bitter taste and is toxic in high concentrations.
• Off-flavors: The presence of these compounds can lead to a bitter or metallic taste. The bitterness is more pronounced in “bitter” varieties of cassava, which contain higher levels of cyanogenic glycosides compared to “sweet” varieties.

(b) Saponins

• Compounds: Saponins are natural compounds found in many plants, including cassava. They can impart a slightly bitter taste and astringency.
• Off-flavors: The presence of saponins can contribute to a lingering bitterness and a somewhat soapy taste.

(c) Browning Reactions

• Compounds: Cassava contains polyphenols, which can undergo enzymatic browning reactions when the plant tissue is damaged or cut, leading to the formation of melanin-like compounds. 
• Off-flavors: These browning reactions can sometimes produce bitter and astringent flavors, particularly if the cassava is not processed or cooked properly.

(d) Fermentation By-products

• Compounds: When cassava is fermented, various microbial activities can produce a range of by-products, including organic acids, alcohols, and volatile sulfur compounds. The process of fermentation is necessary for lafun production.
• Off-flavors: These by-products can sometimes result in sour, tangy, or sulfurous off-flavors, depending on the fermentation conditions and microbial population.

(e) Spoilage

• Compounds: If cassava is stored improperly, it can undergo spoilage due to microbial growth, leading to the production of undesirable compounds.
• Off-flavors: Spoiled cassava can develop off-flavors, including sourness, mustiness, or rancidity.

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