Cassava (Manihot esculenta Crantz) is now a staple for well over 800 million people throughout the tropics. It is cultivated widely and is now about 290 million tonnes year−1 in 2020. It is grown in tropical South America, West Africa and South-east Asia. That level of cultivation is set to expand as more agricultural land becomes available. It is also an extremely resilient plant and grows very well in tropical climates. As a crop it could easily be regarded as one to resist climate change. It is the main source of cassava flour and tapioca flour and is extremely well reviewed in a series of technical books (IITA, 1990; Dufour et al., 1996; Edoh Ognakossan et al., 2016; Waisundara, 2018).
Tapioca flour is obtained from the roots of cassava but it is not the same as cassava flour. Cassava flour uses the whole root whereas tapioca flour is mainly the starchy part of the plant. In all cases the cyanogenic acid content needs to be kept to a minimum because it is a major toxin of cassava.
Tapioca flour is usually preferred to wheat flours in the making of gluten-free baked foods. Tapioca starch is also extracted for specific product development purposes.
The flour is regularly used to thicken soups, stews and sauces and is alternative to arrowroot and cornstarch. It also adds a slightly crispier and thicker texture to loaves, muffins and scones which would other wise be too springy in texture. It is a good thickening agent too for desserts especially puddings.
Nutritionally, cassava and tapioca flour is poor nutritionally except it is a source of carbohydrate as starch. Cassava contains up tp 60% starch so it is an exceptional source. They are like most flours, naturally low in any fat. the protein content is very low being much lower than potato, corn (maize), and oats and wheat.
There are different grades but HQCF is often referenced which is High Quality Cassava Flour. HQCF flour is best manufactured from sweet cassava where the cyanide level is below 100mg/kg HCN. This type of flour is never made from high CNP varieties where the level is above 100mg/kg HCN. Cassava flour has to be labelled as ‘not for food’ if the cyanide levels are too high or it was manufactured for other purposes.
In setting up a flour manufacturing facility, good hygiene through Good Hygiene Practice (GHP), adherence to HACCP principles with a quality management system in place is essential (Campos & Xavier, 2024).
Processing of Cassava To Make Flour
To make cassava palatable and safe to use requires some form of processing. Much of the interest has been devoted to reducing the hydrocyanic acid content to a level where it is negligible.
Cassava roots once harvested will start rotting quickly and they need processing soon after. Ideally this needs to happen within 48 hours of harvesting. The tubers are washed to remove dirt. They ought to be peeled before processing to remove as much of the bitterness compounds as possible, any blemishes and other extraneous matter. To extract the starch usually requires wet milling. The basic steps are:-
- washing the tubers
- peeling
- cutting of the tuber into slices for easier mashing including grating,
- dewatering and pressing, centrifugation and sedimentation
- formation of a wet cake
- breaking and sifting
- drying
- milling
- production of the dry flour
The tubers are inspected on arrival at the depot for damage, decay and any contamination. Only white, creamy white or slightly pale orange roots are used. It should also be checked for aroma and tasted just to check for any off-notes. Roots will be discarded if not suitable.
The tubers are water washed to remove dirt using brushes, tapping and root shaking. Even air blowing is used if available.
The tubers are then further sorted and graded based on their size and quality. Tubers are graded this way to make processing more straightforward.
Further cleaning occurs using high-pressure washing, scrubbing using rotary systems followed by further washing. In some cases cleaning agents are used. The intention is to remove as much dirt as possible so that a clean flour can be produced. The level of water pressure is adjusted to avoid damaging the tubers in the process. Much of the water is recycled. A final rinse occurs to get rid of cleaning agents if they had been used.
High quality cleaning is essential because at this stage any dirt present cannot be removed once mashing begins. It is also highly likely that washed tubers are dried at this stage just to remove excess moisture.
Peeling generally removes most of the cyanogens and other compounds which contribute bitterness. It also removes fibrous material that would hamper further processing and reduce the quality of the flour. Between 10 and 20% by weight of the tuber is removed at this stage.
Peeling the tubers also prevents a grey colour developing during wet storage and some processors have also seen a purple colour form as various reactions occur. This is an infamous pigmentation issue caused by oxidation of various coumarins especially coumaric acid.
The tubers are cut using mechanical knives. Grating and chipping then follows to produce particulates which can be further processed either for extraction or even left as is. It is up to the processor to decide how much further the cassava is grated. Any large lumps can be sieved off and regrated for example. Grated cassava must be processed quickly afterwards as chemical reactions can still occur.
Grating is ideal for high cyanide (bitter) cassava and the preferred method of choice. Chipping is not! This latter process can only be conducted with low cyanide tubers. Chipping of bitter cassava roots can be conducted if the product is not intended for human consumption.
In some cases extraction of starch occurs from the grated cassava. Extraction tends to be highly variable with all sorts of processors. Some processors use different ratios of water to solids to extract the starch ranging from 2:1 to 10:1. Some processors also need to tackle the hydrogen cyanide and hydrocyanic acid early on and will use sodium metabisulphite (SMS) or other sulphur-containing water to do so. The use of SMS also reduces microbial growth. It is at this stage that fermentation is followed as an alternative to remove hydrogen cyanide and also help develop particular flavours.
Fermentation involves addition of starter cultures and using well-established strains (Kouamé et al., 2020). These are lactobacillus cultures. Grated cassava has often been fermented in a type of solid-state system using a framework of plantain leaves, metal grids etc. to support the chipped cassava. The vats are generally substantially large. It has to be a short process of up to 2 days in many cases.
Fermentation ends when the cassava is heat treated to kill bacteria or when it is dewatered and dried. Ensuring heating is controlled properly is crucial to avoid burning or destruction of the microbes.
However the starch has been processed, grated cassava is dewatered rapidly. It could either be fermented or non-fermented material but the process remains consistent. Dewatering usually involves pressing – mechanical screw pressing, hydraulic etc. The other processors use sedimentation and decanting/centrifugation. Sedimentation is less energy intensive compared to centrifugation. Any type of dewatering occurs within 10 minutes of the end of the preceding process which will most likely be mashing. The product at this stage is a pressed cassava cake. The moisture content at this point is between 45 and 50%w/w.
The cake needs to be pulverised using a rough mill to break up particulates for the next phase which is drying. It is not the conventional milling phase however because the water content is too dry. Any press liquor generated has to be treated to remove toxins.
Drying – by whatever means, the moisture content of the flour needs to be carefully monitored to check that the desired moisture level is achieved. Drying most often is solar but it can also be mechanically dried in ovens and spray driers if the flour is fine enough. Belt dryers are also routinely used. Commercial processors have moved to mechanical systems. Excess drying will damage the starch granules.
Sun drying requires the flour to be spread out on clean surfaces with turning to ensure regular moisture loss. The wet starch is dried in the sun or oven dried at 35–40 °C for 12 hours.
Drying extends shelf-life by reducing microbial contamination. Over-drying produces a loss in quality. Depending on the type of flour required and wet cassava granules are dried to about 10-12%w/w. There is a further sieving process to refine the flour further as drying also produces lumps.
The flour is cooled. It is then milled using pin mills and/or hammer mills. Some processors still rely on manual grinding and milling.
Milled flour is further sifted and sieved to remove lumps, coarse particles and generate a consistent product of desired texture and refinement. Most flour producers use vibrating sieves and angled sieves are especially useful. A typical final mill size is between 250 and 500 microns. Clearly, the finer the milling the better the quality of flour.
The flour is then packed in air-tight containers before final portioning to reduce pick-up of moisture.
If processors use fermentation the manufacturer should label it as fermented cassava flour.
Quality Control Parameters
The following parameters are:-
- Moisture content – less than 10%w/w
- Starch content – 65 to 70%
- Total ash on a dry matter basis – 3%
- Acid insoluble ash – 0.15%
- Total titrateable acid usually quoted in lactic acid units – less than 0.25%.
- total cyanogens (CNP) <10 mg/kg HCN
- pH >5.8
- sieve size – at least 90% by mass passes through a 0.6mm sieve
- Cook paste viscosity – 740 BU
- Crude fibre – dry matter basis – 2%.
Check for mycotyoxins, yeast, mould and bacteria counts including Salmonella, Vibrio cholerae, Staphylococcus and Coliforms.
Starch content is analysed using Krochmal and Kilbride’s method.
Cyanide testing
We have discussed methods for monitoring cyanide elsewhere but there are some highly specific methods available which include:-
- Qualitative testing based on Quignard’s test. This uses picric acid which works like pH indicator paper. The roots are tested to see if the picrate infused paper turns orange. The rate at which this occurs is a good indicator of the relative level of cyanide in the sample.
- Quantitative determination can be based on an alkaline titration method. It relies on steam distillation of the sample into a sodium hydroxide solution and then addition of potassium iodide. Titration of a sample is against silver nitrate and the end-point is the detection of turbidity.
Mycotoxins
Mould growth is an issue if the processing environment is not kept scrupulously clean. The moisture content needs to be below 10%w/w on storage so that further mould contamination does not progress. Cassava chips produced during processing are more susceptible to contamination. Mycotoxins such as patulin, penicillic acid, tenuazonic acid, stergmatocyatin etc. have been detected
A great deal of research has been conducted on cassava starch (Chisenga et al., 2019). Amylose is believed to be largely found in the amorphous lamellae. Both types of flour have little if any flavour which makes them ideal bases for food processing. They are also excellent for flavour inclusion complexes.
In Southern Africa such as Malawi, two types of flour are produced including kondowole and ntandaza.
Can Cassava Flour Replace Tapioca Flour?
Cassava flour would be a natural substitute but because it has a higher fiber content, tends to make it a much more effective thickening agent. In any product development application, the levels of other thickeners such as gums and other starches can be reduced so that cassava flour is able to exert its additional thickening power.
Some consumers find tapioca flour has a less nutty flavour compared to cassava flour in baked foods.
Both are usually obtained from health-food shops although some supermarkets now stock it.
Alternatives
Cornstarch is a common cupboard essential whereas tapioca starch is much more difficult to obtain.
Arrowroot is a gluten-free flour with very little taste and comes from the Maranta arundinacea plant. It is used as a substitute for both cassava and tapioca flours on a like for like basis. It is a very effective thickening agent in sauces and gravy but must be combined with other starches like potato to obtain the same texture benefit in baked goods.
If you compare muffins made for example from both types of starches, the ones prepared from cassava flour have the chewy texture.
Rice flour is also an effective alternative to tapioca and cassava flours.
Adulteration of other flours using cassava flour is becoming widespread. Corn flour has also been used to adulterate cassava starch A recent article discussed methods for checking flour quality including the following:-
- DNA probes
- optical-techniques
- FT-IR spectroscopy
- gel electrophoresis.
- Spectral imaging methods.
A hand-held MicroNIR device has been used to check for different flour quality. It operates in the 1150-2150nm range (Tao, 2021).
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References
Campos, G., & Xavier, S. P. (2024). Application of hazard analysis and critical control points technique in a cassava flour processing unit. Brazilian Journal of Production Engineering, 10(3), pp. 1-13.
Chisenga, S. M., Workneh, T. S., Bultosa, G., & Alimi, B. A. (2019). Progress in research and applications of cassava flour and starch: a review. Journal of Food Science and Technology, 56, pp. 2799-2813.
Dufour, D., O’Brien, G.M. Best, R. (1996) Cassava Flour and Starch: Progress in Research and Development. CIAT Publication No. 271 ISBN 958-9439-88-8
Edoh Ognakossan, K., Kouakou, J., Mazalo Pali, A., Nanga Nanga, S., & Plagne-Ismail, C. (2016). Cassava: Production and Processing. CTA.
International Institute of Tropical Agriculture. (1990) Cassava in Tropical Africa: A Reference Manual.
Kouamé, A. K., Bouatenin, M. P., Djéni, T. N., & Dje, K. M. (2020). Identification of hazards and critical control points during attiéké (a fermented cassava product) process in Côte d’Ivoire. Letters in Applied Microbiology, 70(2), pp. 87-94.
Su, W. H., & Sun, D. W. (2017). Evaluation of spectral imaging for inspection of adulterants in terms of common wheat flour, cassava flour and corn flour in organic Avatar wheat (Triticum spp.) flour. Journal of Food Engineering, 200, pp. 59-69.
Uchechukwu-Agua AD, Caleb OJ, Opara, U.L. (2015). Postharvest handling and storage of fresh cassava root and products: a review. Food Bioprocess Tech. 8(4) pp. 729–748.
Udoro, E. O., Anyasi, T. A., & Jideani, A. I. O. (2021). Process-induced modifications on quality attributes of cassava (Manihot esculenta Crantz) flour. Processes, 9(11), pp. 1891.
Zhu F. (2015) Composition, structure, physicochemical properties, and modifications of cassava starch. Carbohydr Polym. 122(2) pp. 456–480 .
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