|Blending of New and Traditional Technologies - Case Studies (ILO - WEP, 1984, 312 p.)|
|PART 2: CASE STUDIES|
* Contributed by UNIDO. This chapter is based on Keith A. Steinkraus, Applications of biotechnology and genetic engineering to African fermented food processes, UNIDO/IS.336. Vienna. 1982.
IN THIS CHAPTER the cases of four important traditional fermented foods where modern technology has been applied to produce them on an industrial scale are examined and the process changes highlighted. Based on these examples, brief general observations are offered on the question of upgrading traditional African fermented foods through the application of genetic engineering and microprocessor controls.
I. KAFFIR CORN (SORGHUM) BEER
Kaffir corn (sorghum) beer also called Bantu beer is an example of a primitive beer that is still produced as a household fermentation. It is also produced in high volumes, e.g. an estimated thousand million litres per year in municipal plants.1 Kaffir beer is an alcoholic, effervescent, pinkish brown beverage with a sour yoghurt-like flavour and consistency of a thin gruel. It is opaque because of its content of undigested starch granules, yeasts and other microorganisms. It is not hopped or pasteurised and is consumed while still actively fermenting. The essential steps in kaffir beer-brewing are malting, mashing, souring, boiling, conversion, straining and alcoholic fermentation. In the indigenous process, kaffir beer is made in large pots of 115 to 180 litre batches.
Sorghum, maize or millet grains or combinations are malted by soaking in water for one or two days, draining and allowing the seed to germinate for five to seven days until it has a distinct plumule. The sprouted grain is then sun-dried and allowed to mature for several months. It is then pulverised and slurried to form a thin gruel, boiled and cooled. A small amount of fresh uncooked malt is added as a source of amylases and yeasts for the subsequent fermentation. About equal quantities of malted and unmalted grains are mashed in cold and boiling water, and the two mashes are combined to yield a mixture at a temperature favourable to saccharification, souring and yeast fermentation. The mixture is incubated on the first day. On the second day it is boiled and cooled. On the third and fourth days, more uncooked malt is added. On the fifth day the brew is strained through a coarse basket to remove husks. The beer is then consumed.
In the indigenous process, saccharification. souring and alcoholic fermentation proceed more or less simultaneously without the addition of pure cultures. In the industrialised process,2 there are two distinct fermentations. The first is saccharification accompanied by lactic acid souring. The second is the alcoholic fermentation. Souring is achieved by holding the mixture of sorghum malt and water at 48 to 50 degrees Centigrade for 8 to 16 hours until the proper acidity, pH 3.0 to 3.3 with a total acidity of 0.3 to 1.6 per cent (average 0.8 per cent) as lactic acid is attained. This sour is about one third of the final beer volume. The souring step controls the course of the remaining fermentation, including mashing, body and alcoholic content of the beer.3 Although pure culture inoculation of lactic acid bacteria is not used, 10 per cent of each batch of sour is used to inoculate the next batch. The soured malt mixture is pumped to the cooker and diluted with two volumes of water. An adjunct, usually for maize grits, is added and the whole mash is boiled for two hours. The thick cooked mash is cooled to 60 degrees Centigrade, conversion malt is added and the mixture is held for one-and-a-half to two hours. The sweetened mash, now thinner, is cooled to 30 degrees Centigrade and inoculated with a top-fermenting strain of Saccharomyces cerevisiae. The yeast is obtained as dry yeast which is produced locally and is slurried before pitching. No yeast is recovered as it is consumed as part of the beer. The pitched mash is passed through coarse strainers - either screw presses or basket centrifuges - to remove husks. The worst is then fermented for 8 to 24 hours. Fermentation continues in the packages in which it is distributed. These are unique in that they allow escape of excess gas. Large amounts of kaffir beer are piped directly to beer parlours where it is sold as draught beer.
The municipal breweries produce about a thousand million litres of sorghum (kaffir) beer each year. An equal amount may still be produced in the home by indigenous processes. Draught sorghum beer sells for the equivalent of 8 us cents per litre. A litre of sorghum beer in cardboard cartons lined with polyethylene sells for the equivalent of 12 us cents, probably the cheapest industrially produced beer in the world.
Starch is a very important component in kaffir beer which must contain both gelatinised and ungelatinised starch to be acceptable in texture. The gelatinised starch helps keep the ungelatinised starch in suspension, makes the beer creamy and adds body.
Novellie4 reports that the content of thiamine, riboflavin and niacin in kaffir beers has tended to decrease in recent years. This may be due in part to a decrease in the proportion of sorghum to one part maize. Traditional kaffir brewing may use 4.9 parts sorghum to one part maize while municipal breweries may use 1.2 parts sorghum or less to one part maize. This represents a serious loss of nutrients which has occurred with modernisation. It would be even more serious nutritionally if attempts were made to produce clear beers like those used in the Western world.
The most important processing changes that have occurred in the industrialisation of kaffir beer are the more careful malting of the grain which is thoroughly precleaned, washed and watered during malting.5 Division of the process into two distinct steps, that is, souring and alcoholic fermentation, makes it possible to control both steps better. Souring is carried out at 48 to 50 degrees Centigrade, optimum for thermophilic lactobacilli which then complete the souring in front for 8 to 16 hours. Inoculation of each new batch with sour from a previous batch also helps control this step in the process. Souring is carried to the desired pH 3.0-3.3. Amylolytic conversion is conducted at 60 degrees Centigrade which is favourable for conversion of the starch to produce the desired viscosity and sugars used by the yeast for alcohol production. A selected strain of Saccharomyces cerevisiae is inoculated into the mash at a temperature of 30 degrees Centigrade, favourable for the alcoholic fermentation. All these modifications of the traditional processing are desirable. They could not be used in the indigenous processing of kaffir beer. Unfortunately, the use of less sorghum in the industrialised process has resulted in a decrease in the nutritive value of the product.
II. NIGERIAN OGI (PAP)
Nigerian ogi is a smooth-textured, sour porridge with a flavour resembling yoghurt made by fermentation of corn, sorghum or millet. Ogi is a natural fermentation and a wide variety of microorganisms - molds, yeasts and bacteria are initially present. The essential microorganism appears to be Lactobacillus plantarum.6 Lactobacillus plantarum is able to use dextrins after the initial sugars are fermented. Aerobacter cloacae has been isolated and may be responsible in part for increases in the content of riboflavin and niacin in ogi. Corynebacterium sp. is reported to be able to hydrolyse starch and produce organic acids. Saccharomyces cerevisiae and Candida mycoderma contribute to the flavour.7 Banigo et al.8 suggested the use of a mixed inoculum rouxii. Ogi is obviously a complex fermentation. The essential microorganisms involved have not as yet been completely characterised.
Traditional and industrialised methods for manufacturing Nigerian ogi are compared in Figures 14.1 and 14.2. Traditional ogi preparation is a batch process carried out on a small scale two or three times a week. The cleaned maize kernels are steeped in pots for one to three days. During this time, the desirable microorganisms which are responsible for souring develop. The grain is then wet-ground with a stone slab or mortar and pestle. In the improved process, the grinding is done more efficiently by hammermills. The ground material is slurried with water and passed through a fine wire sieve (aperture 300 to 800 microns).
The unfiltered coarse material is washed with several lots of water. Alternatively, the slurry may be washed through a cloth filter tied over a pot. The filtered slurry settles and ferments for one or two days at ambient temperature. The fermented sediment is ogi which is boiled either in water or in the ogi water (supernatant) to give ogi porridge (pap). The uncooked ogi is sold wrapped in leaves after removal of excess water. Shelf life is less than 30 hours unless refrigerated.
In the industrialised process (Figure 14.1) the maize is dry-milled to a fine flour and subsequent inoculation of a flour/water slurry with a mixture of lactobacilli and yeast. This gives a more reliable fermentation. A further improvement is the manufacture of soy-ogi. Maize is cleaned, soaked wet-milled and sieved in the traditional manner. Soybeans are similarly cleaned, dehulled, cooked, wet-milled and sieved through a vibroscreen (72 mesh). The two slurries are mixed, fermented, sweetened and spray dried using a Niro Atomiser. The dried product is flavoured, enriched with vitamins and minerals and packaged in polyethyne bags for sale. Addition of soy improves the protein content and the nutritive value.
Figure 14.1. Traditional Nigerian ogi manufacture
Nigerian gari is a granular starchy food made from cassava (Manihoc esculenta) by acid fermentation of the grated pulp of the tuber followed by a dry-heat treatment (garification) which gelatinises, semi-dextrinises and dehydrates the pulp. Gari is of unusual interest because it is made from cassava - a major source of food for the worlds poor. Its protein content generally is less than 1 per cent and it cannot, by itself, provide sufficient protein for adequate nutrition. For consumption, gari is added to boiling water to produce a semi-solid, plastic dough. During cooking, the volume increases by 300 per cent. This places gari in the position of being starch with unusual functional characteristics. It may very well be used as an ingredient in other foods.
Figure 14.2. Improved Nigerian ogi manufacture, maize (or millet or sorghum)
Corynebacterium sp. and Geotrichum candidum are the important microorganisms in the fermentation. There are five genera in the gari fermentation, namely, Leuconostoc; Alcaligines; Corynebacterium, Lactobacillus and Candida. Only Leuconostoc and Candida appeared to be the essential microorganisms in the gari fermentation. These studies reveal how difficult it is to identify the essential microorganisms in mixed natural fermentations. Often there is a sequence of essential microorganisms as in the case of sauerkraut.
Gari is primarily consumed in the form of a meal called eba. This is prepared by soaking gari in boiling water to swell the starch and by working the mixture in a wooden mortar and pestle into a semi-solid, plastic dough. Boiled yams may be added to the dough to enhance the flavour. The stiff porridge is rolled into a ball of about 10 to 30 grams wet weight with the fingers and is dipped into a stew containing vegetables, palm oil, and meat or fish. The amount of nourishment obtained depends upon the quality of the stew. Gari is the staple diet of the majority of low-income persons, who consume it regularly (two or more times daily). It is estimated that 90 per cent (over 30 million) of the Nigerians living in the southern states consume gari regularly at least once or twice daily. It contributes up to 60 per cent of the total calorie intake in Western Africa, the rest being derived from other sources like yams, rice and maize.9 An average adult consumes 300 grams of gari in a meal. A related product, cassava flour or lafun, is made by soaking whole tubers in water for a few days, then peeling, cutting, drying to 13 per cent moisture content, grinding and sieving.
Traditional and pilot plant processes for Nigerian gari fermentation are compared in Figures 14.3 and 14.4.
The utensils required for household production are a knife to peel off the outer layers, grater to reduce the roots to fine particles, a bag to squeeze out liquid from the grated pulp, and a pot to fry the partially dried pulp. The major substrate for gari production is the enlarged root of the cassava plant. The central inner fleshy region of the cassava root is the portion which is eaten. The two outer coverings, the brown, external paper-like skin and the inner leathery whitish covering are removed with a resultant loss of 30 per cent of the total solids by weight. The central fibrous region is grated along with the fleshy portion.
Traditionally, gari is made in the villages by women in the home, from cassava roots bought or grown locally, using a time-consuming, unhygienic process (Figure 14.3). Roots not used for 48 hours after harvesting are no longer suitable for gari processing due to bio-deterioration.
The roots are peeled with sharp kitchen knives to remove the inner cortex, which may develop a mauve colour. Peeled roots are grated into a fine pulp using aluminum sheets perforated with nails and fixed on wooden frames. Sometimes grating is done by a pulping machine in a central place in the village. Grated pulp is placed in Hessian sacks which are left outside for up to four days to allow the mash to drain and ferment.
The fermented pulp is semi-dry (about 60 per cent moisture) and harsh. Using sieves locally fabricated from palm fronds, coarse fibres are removed and discarded; the finer grains are then toasted in shallow iron pots heated to about 120 degrees Centigrade on an open fire. A piece of calabash is used to turn the toasting pulp to prevent stocking. This temperature is sufficient to semi-dextrinise the starch and to dry the mash to about 20 per cent moisture. People in some parts of the country prefer yellow gari which is made by adding a small amount of palm oil during the toasting process. After the gari is cooked, it is sieved again and stored in open enamel basins to await sale to middle-men.
Figure 14.3. Traditional production of Nigerian gari
Figure 14.4. Nigerian gari pilot plant process
Because of the increasing tendency of both husband and wife to work, the difficulty of collecting and transporting sufficient cassava root to meet the demands of a rapidly growing urban community and the subsequent escalation in prices, it became increasingly apparent that the whole production system must be modernised. The Federal Institute of Industrial Research (FIIRO) pioneered research in the fermentation of cassava with subsequent development of a pilot gari processing plant, which was a model for larger plants in other parts of the country. The Projects Development Agency (PRODA) in Enugu, Anambra State in Nigeria, also developed a pilot plant. Basically, the method adopted in the gari processing pilot plant is an upgraded village method (see Figure 14.4).
1. Root Preparation
The bitter variety of cassava (which contains more than 100 milligram hydrogen cyanide per kilogram of pulp) is the substrate. One- to two-year-old cassava is preferred. Within 48 hours after harvesting, the roots are processed by removing the ends and chopping the remainder into short pieces (about 15 to 20 cm long) with sharp knives. The roots are then fed into a peeler.
The cassava peeler is a rotating concrete mixer-like eccentric drum with an abrasive lining. By means of a large feed chute with a sliding gate at the bottom, chopped cassava is fed into the peeler. A megator water pump provides the required water pressure for the peeler. Peeling is accomplished within three minutes through the combined action of the abrasive lining and the cassava roots rubbing one another as the drum revolves at 40 rpm. The water washes the peel away from the roots. Peeling loss based on the weight of roots is 25 to 30 per cent but can be as high as 40 per cent if the process is unduly prolonged. Peeled roots are discharged onto wheeled inspection trays by gravity where incompletely peeled roots are finished by hand.
The peeled roots are fed into a grating machine with revolving blades of 2.5 cm impact cross section. The resulting mash when dewatered to about 50 per cent moisture content, should have at least 70 per cent of its weight retained on a 0.058 cm aperture sieve mesh but should pass through a 0.25 cm aperture. Cassava liquor from a three-day old fermented mash is premixed with the grated pulp at the same time in an Adelphi Mixer at the rate of 1 litre of liquor to 45 grams of pulp. Inoculating the pulp in this way reduces the normal fermentation time from four days to one day.
The fermentation of cassava is one of the most important steps in gari preparation. The grated pulp is transferred to a cylindrical silo made of fibreglass with a smooth inner surface of bonded plastic. A conical bottom with an adjustable gate facilitates withdrawal of mash for process control and degassing.
The fermentation is anaerobic. The three-day-old cassava juice used for seeding contains microorganisms in their early stationary phase. When the pH of the mash reaches 4.0 + 0.15 with about 0.85 per cent total acid (as lactic), the desired sour flavour and characteristic aroma will be attained.
The fermented mash is transferred by hand to a 53 cm basket centrifuge which reduces the moisture to form a cake of 47 to 50 per cent moisture. Alternatively, the pulp is placed in nylon bags and dewatered in a hydraulic press. Experience has shown that the basket centrifuge is not very efficient. About 50 kilogram mash might require 10 to 15 minutes to process, depending on the age of the cassava. Generally, cassava older than 18 months is difficult to free of water. A continuous screw press might reduce handling and operating costs.
The filtered cake is disintegrated in a continuous sieve-type granulator with a BS 10 sieve to remove the trash which is collected separately, dried, and sold along with the sun-dried peels as animal feed-stuffs.
Garification involves toasting the cake in a rotary kiln externally heated by a jacket of hot air. The cassava mash is partially gelatinised when the core temperature in the kiln has reached 250 to 280 degrees Centigrade. The garifier is a stainless steel tube with a rotary rake which dislodges the gelatinising bed of cassava pulp from the garifier wall to prevent sticking and burning. The gelatinisation process requires high heat and low mass transfer. The garification stage is critical for proper swelling of the gari. The moisture of the pulp is about 40 per cent and the gelatinising temperature (80 degrees Centigrade) should be attained within 15 minutes before surface drying of the gari particles occurs.
The gelatinised mash falls via a vibrator into a directly fired louvre dryer, 1 m long x 0.78 m wide. The drying requires low-heat and high-mass heat transfer, as opposed to the garifier. The hardened cake has a moisture of 8 per cent when cool.
9. Milling and Packaging
Cool gari is fed into a disc mill, ground and subsequently sieved through BS No. 14. The fines (flour) going through BS No. 18 are packaged separately as gari flour, which is usually eaten by blending into cooked kidney beans in palm oil stew.
Control of Processes
The most important stages of gari production are fermentation and toasting. These process stages must be controlled in order to obtain an acceptable product. Iron metal discolours the pulp; therefore, plastic, stainless steel, or aluminum is necessary.
A natural fermentation takes at least four days to reach the requisite acidity (0.85 per cent as lactic acid) but the time is reduced to 24 hours when an inoculum of three-day-old fermented mash liquor is used. The process proceeds best at a temperature of about 35 degrees Centigrade; sunlight and frequent mixing of the pulp accelerate the fermentation. Cassava produces its own liquor during fermentation, therefore no water should be added. Where ambient temperature is outside the range of 25 to 35 degrees Centigrade or where fermentation tanks are very large, there should be temperature control of the mash. Allowance should also be made for degassing the mash. Most of the gasses, HCN, H2, and CO2, are believed to escape through the conical spout from which some of the juice continuously drains. There should be adequate ventilation in the building to prevent cyanide poisoning.
The garification and drying stages determine the swelling capacity, as well as the shelf life. Gari should expand in cold water to at least 300 per cent of its original volume.
Mahewu (magou) is a traditional, sour, non-alcoholic maize beverage popular among the Bantu people. It is made by traditional, spontaneous fermentation in the villages. It is also produced on a large scale by industrial concerns and mining companies for consumption by their labourers. As consumed, mahewu contains about 8 to 10 per cent solids and has a pH of about 3.5 with a titrateable acidity of 0.4 to 0.5 per cent (lactic acid). Reduction of its bulk by producing it in concentrated form or as a dry powder has the advantage of easier distribution and marketing.
The traditional, spontaneous mahewu is made by mixing maize meal and water in a ratio of approximately 450 grams maize to 3.8 litres water (8 to 10 per cent solids), boiling until the porridge is cooked (approximately one and a half hours), cooling, and adding a small quantity of wheat flour or meal (about 5 per cent of the weight of maize meal). The wheat flour/meal serves as a source of inoculum and of growth factors for the spontaneous fermentation. It is the major difference in separating ogi and mahewu which are otherwise similar products. Following inoculation, the mahewu is incubated in a warm place for about 36 hours at which time the desirable sour flavour develops.
An improved method is now available for producing mahewu under controlled conditions (see Figure 14.5). Wheat flour is added to the diluted maize porridge as a source of growth factor but the mixture is then inoculated with either Lactobacillus delbruckii or Lactobacillus bulgaricus and incubated at 45 degrees Centigrade to insure a rapid and uniform fermentation.
Hesseltine10 describes recent industrial production of mahewu. Coarsely ground white maize is used as substrate. The inoculum consists of a mixture of pure cultures which have been isolated from traditional mahewu and cultures on a coarsely ground whole wheat flour. The maize meal slurry with about 9 per cent solids is cooked by boiling for one hour and holding for an additional 45 minutes. The thick maize slurry is cooled to 47 to 52 degrees Centigrade and inoculated with the starter. The fermentation then proceeds in 4,500 litre tanks in which temperature is not controlled for about 22 to 24 hours during which time the pH falls to between 3.65 and 3.95. The fermented mahewu is mixed with defatted soybean meal, sugar, whey or buttermilk powder and yeast. The additives are incorporated to improve the nutritional value. The mixture is then spray-dried to a moisture level of 3.5 to 4 per cent and has a keeping quality of at least one year. It is prepared for consumption by mixing the dried powder with water (about 9 per cent solids).
Production of Concentrated Mahewu (25 Per Cent Solids)
If the mahewu has to be distributed over long distance, it is advantageous to reduce the water content by preparing a more concentrated form. This concentrated mahewu can then be adjusted to the normal solids content at the place where it is to be consumed merely by mixing it with the requisite amount of water. Alternatively, mahewu can be dried into a powder.
Several conventional drying methods can be used for mahewu, but only two of these appear to be practicable for large-scale production, namely, spray drying used for milk, or drum drying used for the drying of mashed potatoes and similar pastes. Drying in circulating hot air tray driers has the disadvantage that the layer of mahewu must be broken by mechanical means during the process, drying is slow and the mahewu becomes brown even if the temperature is kept at 50 degrees Centigrade.
No technical problems arise in the spray or the roller drying of mahewu (8 per cent solids) if it is homogeneous. This condition can be easily achieved by passing it through a colloid mill.
Nito laboratory spray drier has been used in drying experiments; intake air temperature was 190 degrees to 210 degrees Centigrade and exhaust temperature, 90 degrees to 110 degrees Centigrade. The capacity of this drier was approximately 2 litres per hour for drying mahewu containing 8 per cent solid.
In experiments with roller driers, a small, single drum drier with a diameter of 30 centimetres was used.
The drum is dipped into a vat and revolved at a speed of 1.2 revolutions per minute. With a steam pressure of 14 kilograms psi, the capacity of the drum drier for mahewu containing 8 per cent solid is 7 litres per hour.
Because the mahewu has a high degree of acidity it can only be dried in an apparatus made of acid resistant, non-corrodible material. Owing to the high viscosity of concentrated mahewu, 25 per cent solids was the highest concentration which could be dried on the laboratory spray driers. Ordinary double drum driers such as those used for milk powder cannot be used on a highly concentrated mahewu. On a laboratory roller drier the maximum concentration was 9 per cent solids. Mahewu powder prepared from spontaneously soured mahewu has an unsatisfactory flavour and the shelf life of the powder is very limited (two days).
In the spray drying process, where the temperature of the product does not exceed 45 degrees Centigrade, enzymatic changes such as fat-splitting can take place and lead to rancidity of the product. Apparently, enzymatic action is inhibited by the use of a buffer as the buffered mahewus do not become rancid after one years storage.
Figure 14.5 Industrial Production of mahewu
A series of steps are suggested for the improvement of fermented foods in Africa south of the Sahara. These steps or stages are as follows:
(i) isolation and identification of the microorganisms involved; (ii) determination of the role played by each of the microorganisms; (iii) selection and genetic improvement of the essential microorganism(s); (iv) improvement in processing control of the fermentation; (v) improvement of the raw substrates; (vi) laboratory production of the fermented food; (vii) pilot plant scale-up of the laboratory process; and (viii) industrial production of the fermented foods.
If these steps are followed, indigenous fermentations can be vastly improved. The quality and nutritive value will be better. Fermentation time is likely to be shortened. Studies of the synthetic capabilities of the essential microorganisms may reveal some with unusual ability to produce enzymes, essential amino acids or vitamins or antibiotics or other products with potential commercial value. Studies of this type also stimulate microbiological research which lays the ground work for future genetic engineering studies related to Africa.
Opportunity to apply microelectronic and microprocessing controls arises as soon as laboratory studies are begun on an indigenous fermentation. The simplest, of course, are measurements of pH, followed by control of pH in the fermenting substrate. Temperature can also be easily monitored and controlled. Oxygen monitoring electrodes can be utilised to advantage if aeration is important. As a fermentation moves into the pilot plant stage, micro-electronic control becomes essential at each step. Depending on the type of fermentation, it is sometimes desirable to monitor the oxygen concentration and the carbon dioxide concentration as well as pH, temperature, humidity (of solid state fermentations), etc. Electronic controls become very valuable in industrial production.
1. C.W. Hesseltine: Some important fermented foods of mid-Asia, the Middle East, and Africa, in Journal of American Oil Chemists Society, No. 56, Electrochemical Society, Champaign, Illinois, 1979.
2. L. Novellie: Kaffir beer brewing ancient art and modern industry, in Wallerstein Laboratories Communications, No. 31, New York, 1968.
3. L. Novellie, Biological ennoblement and kaffir beer in Food Technology, No. 20, Institute of Food Technologists, Chicago, 1966.
5. L. Novellie: Kaffir corn malting and brewing studies, XI, XII, XIII, in Journal of the Science of Food and Agriculture, No. 13, Chemical Society, London, 1962.
6. E.O.I. Banigo and H.G. Muller: Manufacture of ogi (a Nigerian fermented cereal porridge): Comparative evaluation of corn, sorghum and millet in Journal of the Canadian Institute of Food Science and Technology, No. 5, Willowdale, Ontario, 1972.
7. I.A. Akinrele: Fermentation studies on maize during the preparation of a traditional African starch-cake food, in Journal of the Science of Food and Agriculture, No. 21, Chemical Society, London, 1970.
8. E.O.I. Banigo, J.M. deMan and C.L. Duitschaever: Utilisation of high-lysine corn for the manufacture of ogi using a new, improved processing system, in Cereal Chemistry, No. 51, St. Paul, Minnesota, 1974.
9. W.O. Jones: Manioc in Africa, Stanford University Press, California, 1959.
10. Hesseltine, op. cit.