Chapter 7: Grain processing

Contents - Previous - Next

grain huller

In Africa, rural dedicate a great deal of time and effort to processing local cereals such as millet and sorghum. In the countries where these cereals were traditionally the basic foodstuffs of the people, an increasing tendency noted in urban areas is to substitute cereals such as wheat and rice which are already processed and ready to use. The drudgery of the task and the lack of services to hull local grain in urban areas are the principal constraints which explain this phenomenon and which create the risk, if nothing is done, that traditional cereals such as millet and sorghum will be marginalized or might even disappear.

7.1.1 Sorghum and Millet

If a longitudinal section of a sorghum or millet grain is examined, three principal parts may be identified: i) the pericarp (the external layer of the grain); ii) the endosperm (the farinaceous albumen, rich in starch) and iii) the germ (or embryo, development of which gives birth to the new plant).

Figure 7.1- Longitudinal section of a cereal grain (Source: François, 1988)

The pericarp, also commonly called the "bran", consists mainly of fibre and sometimes unpalatable compounds. Thin pericarps have a tendency to adhere firmly to the kernel, contrary to the thick pericarps and this makes hulling difficult.

Some sorghum grains are characterised by a coloured layer (brown to violet) called the testa, immediately under the pericarp. This coloration is associated with a heavy concentration of polyphenols (also called tannins) which inhibit the capacity of the human organism to digest the proteins in the grain. It is for this reason that in the traditional hulling process the operation is maintained until the testa is completely removed.

The endosperm comprises a glassy (or vitreous) portion and a farinaceous portion. The relative proportions of these two components determine the texture of the grain (an important characteristic for the quality of the flour produced from it). The proportion of vitreous endosperm determines the capability of the grain to withstand traditional pounding without being reduced to a powder. The higher this proportion, the less cracking there will be during hulling with a pestle and mortar.

The germ contains, essentially, the fat and protein. Among some sorghum cultivars, the germ is firmly anchored in the endosperm, making its extraction difficult.. Although flour containing a high proportion of germ has poor keeping quality (it becomes rancid) it is much more nutritious.

With cereals such as wheat or maize the bran may be removed by a simple milling followed by sieving the flour. In the case of millet and sorghum, on the other hand, such a process is not possible. The pericarp of a millet or sorghum grain has the peculiarity of disintegrating into fine particles when the grain is crushed. These particles of bran are then impossible to separate from the rest of the flour by sieving. For this reason, if it is necessary to remove the bran from millet or sorghum grain a separate hulling process is required.

The objective of hulling is to remove the external layers which contain mainly fibre and sometimes tannins, presence of which in the flour affects the cooking quality and the taste and texture of the food. Hulling, therefore, should effectively remove the complete pericarp and testa, at the same time minimising loss of endosperm and germ. The degree of hulling needed for this optimal result varies from one type of grain to another and even between varieties of the same type (Table 7.1).

The process of hulling still most commonly used in Africa is to pound the grain in a wooden mortar. This is a laborious task which is generally incumbent on the women. Usually, the grain is moistened (about 50g of water per kg of grain), before pounding in the mortar, then dried in the sun and winnowed to separate the bran.

In 1981 - 82 the regional project FAO/RAF/045/DEN, in collaboration with ICRISAT, mounted a study on traditional methods of grain hulling and milling in Mali (Vanek, 1986). For hulling, some results of the study are presented in Table 7.2.

Table 7.1- Relative proportions of pericarp, endosperm and germ in various types of sorghum and millet grain (Bassey and Schmidt, 1989)

Table 7.2 - The effectiveness of traditional methods of manual grain hulling in Mali (Vanek, 1986)

* Specific energy consumption was calculated from the time taken for pounding multiplied by the average power output of a human being, estimated at 75 Wan.

7.1.2 Description of the technique

In order to better understand the comparison of different methods of hulling, it would be worthwhile to recapitulate the definition of three parameters often used:

• the hulling yield measures the degree of hulling which the person supervising the operation, (that is, the woman), judges acceptable as an indication of the completeness of the operation; this is the proportion (as a percentage of the weight) of hulled grain compared with the initial weight of grain prior to hulling. Thus, for example, for the traditional method of hulling sorghum in Mali, the hulling yield is 65 - 75 % (Table 7.2).
• The theoretical yield is the proportion of grain (as a percentage of the weight) which represents the optimal consumable portion or the proportion acceptable by the consumer. For the sorghum with a thick pericarp, for example, the theoretical yield is 94% (Table 7.1).
• The hulling efficiency (in %) integrates the two previous parameters. It provides an objective measure of the efficiency of the operation and a comparison between equipment and types of grain in terms of hulling performance. It is given by the following formula:

Hulling Efficiency (%) = 100-RT /100-TE´ 100

RT = Theoretical Yield

TE = Hulling Yield

7.1.3 The Engleberg-type huller

Many initiatives have been tried to add more value to local cereals in mechanizing the processing, notably hulling. The "Engleberg" type hullers were introduced first and are still the most commonly used in Africa for hulling cereals. These machines have the advantage that they are simple and robust. That, in turn, has favoured local manufacture and a greater availability of repair parts in the marketplace.

Although conceived for a continuous function, the machine is generally used for hulling small batches of about 5kg on a rental basis. Therefore, the machine is not used to its full capacity, and the specific energy consumption increases with the small batches hulled (Table 7.3).

The grain is often moistened by adding water just before hulling. This has the effect of softening the surface of the grain and facilitating detachment of the pericarp under the actions of friction and pressure inside the machine. Typical hulling yields of wet grains are of the order of 6570% which correspond to the levels obtained by manual hulling in a mortar.

Table 7.3 - Performance of Engleberg Hullers m Mali (Vanek, 1986)

7.1.4 Abrasive hullers

The operating principle of these hullers is essentially based on the friction between the grain and an abrasive surface, which has the effect of removing the external coat of the grain. The friction of grain against grain also contributes to the hulling to a certain extent. The abrasive surface may be stationary or rotating.

Stationary cone hullers

In the case of the huller produced by FAO (Foundries of the Western Workshops), the abrasive surface is a stationary cone. The grain is moved and rubbed on the cone through the action of rotating rubber vanes. The capacity of the machine varies between 200 and 275 kg/in, for a hulling yield of about 78 % and a specific energy consumption of 75-95kJ/kg. The machine includes a system for separating the bran and hulled grain. Unfortunately, this huller which was introduced in Senegal in the 1970s for hulling millet and sorghum is too complex for use in a rural environment. This explains, perhaps, the fact that its use did not spread throughout the country.

Abrasive disk hullers

Abrasive disk hullers are simple in concept. They consist essentially of a mild steel casing with a rounded base, inside which is a rotating set of abrasive disks fastened on a shaft (Figure 7.2). In operation, the quantity of grain loaded up to the level of the shaft depends on the volume of the casing. The friction between grain and abrasive disks and between the grains themselves brings about the removal of the pericarp.

The first prototype of the abrasive disk huller was introduced in the mid-1970s in Botswana, Senegal and Nigeria for research station trials with the assistance of the International

Development Research Centre (IDRC). This first machine, then called the "PRL Huller" was conceived for continuous operation, with a capacity varying between 250 and 500kg/h depending on the type and variety of grain. The minimum quantity of grain that could be hulled was 20kg.

The PRL/RIIC Huller was a modified and improved version of the preceding one. It was manufactured locally and spread widely among the private millers in Botswana.

Figure 7.2 - The abrasive disk huller or PRL/RIIC huller (Source: UNIFEM, 1988)

7.2 Case study on rice hulling in Mali

The region of Ségou is among the most productive rice producing regions in Mali. Since the 1970s the Office du Niger (Niger Valley Development Authority) has been charged with promoting irrigated rice cultivation and intensification of production (double cropping). Production more than tripled between 1981 and 1994 (Table 7.4). The office du Niger had a monopoly in the rice sector up to 1984. At present, 44,000ha (30-50% of national production) are under the control of the Office. The producers (about 10,000, each cultivating an average of 4ha) are not obliged to sell their paddy to the Office.

Since 1979, The Netherlands had assisted the Office du Niger through Project ARPON (Improvement of Peasant Rice Cultivation in The Office du Niger) with the objective of increasing production and productivity. In 1989, Project ARPON, evaluating the needs of the villagers, gave support to the Female Economic Interest Groups (GIEF) in the mechanization of rice hulling. This project, entitled "Action Hullers", was to serve as a motor of socioeconomic development in the region in creating new employment opportunities and income for the women and in improving the financial situation and status of the GIEF. In effect, the mechanized hulling permits women to add value to their produce, in selling it as rice rather than as paddy, as well as saving time and avoiding the drudgery of manual pounding. Also, a by-product, the bran and pulverized hulls can be used to feed animals.

Table 7.4 - Rice production levels in the "Office du Niger" Region

7.2.1 Introduction of technology

The project team. in collaboration with the interested users chose the VOTEX huller from among existing models for various reasons:

• its low cost compared with other hullers;
• its ease of use, maintenance and repair; and
• the possibility of local manufacture by the Farm Machinery Assembly Workshop (AAMA).

The introduction of this equipment was made possible thanks to the "Village Development Fund which provided credit at 9%, repayable in two years.

The project was organized through several groups:

• the work teams (2 or 3 women sometimes assisted by a machine operator) charged with the hulling, the maintenance and the administration of the equipment. These teams received training in operation and maintenance and a remuneration of 7-10% of gross revenue, and
• the management committees trained to keep the accounts and to administer and organize the production teams.

During group meetings, general questions (working hours, price of the services, results of the activity, re-investment of funds) were discussed among all the women of the group.

7.2.2 Evaluation of results

Since the start of the project in 1989, 78 huller units have been installed in 76 villages. The operation has processed 10,000 tonnes of rice with total net profit of US$133,000 (US$1 = 300FCFA) for the 76 villages. However, not all the units have achieved the same success rate. In 1992, only 56% of the huller units had been able to pay off the whole of their debt while 50% of the units had been obliged to cease hulling activity either temporarily or permanently.

Hulling appeared to be a profitable activity at the start, however profitability declined rapidly afterwards for various reasons:

• rice producers, seeing the financial success of the GIEF in 1989 bought their own hullers, thus becoming major competitors. The GIEF as a collective body, found it very difficult to react in a rapid and direct manner to the competition.
• the technical choice of the competitors was the hullers made in India and China (Engleberg type with cast hulling element) with higher capacity (300 to 400kg/h) producing a whiter rice more appreciated by consumers. In comparison, the VOTEX hullers has a capacity of 150kg/h and a hulling yield of 81 to 87% but it produces brown rice (= not white) whereas the preference of the consumers os for white rice. The advantage of the machine's mobility was not exploited in offering a door-to-door service.
• the maintenance and repair service should have been provided by the technicians of the Office du Niger. In fact the latter gave priority to the private hullers with whom they had signed service contracts.
• the competition between rental services caused the price of hulling to drop by half between 1989 and 1993. At the same time the price of fuel and repair parts was increasing. This situation led to a serious loss of motivation among the women of the GIEF.
• the lack of a spirit of enterprise on the part of the GIEF sums up the difficulties encountered in "Action Huller". In practice, the majority of women did not react as "producer groups" (joint responsibility and collective ownership) but as individual consumers considering the huller as a simple domestic tool. Taking little interest in the development of the activity, the management or the administration, the members of the GIEF could not adapt to a new competitive situation (adjustment of price, response to consumer need). (Zoomers, 1994)

7.3 Case study on sorghum porridge

In the Sudan-Sahel sometimes, sorghum is consumed by thirty to forty million people in the form of "porridges", fermented, neutral (Cameroun and North Togo), acid (Burkina Faso) or alkaline (Mali). The name most commonly used in the Sahel for these porridges is "to".

The quality criteria demanded by the consumers are:

• Texture: the porridge should be firm but should not stick to fingers or teeth.
• Conservation: the porridge should maintain its texture until the following morning without sweating.
• Colour.
• Taste: this varies by ethnic group and seems partly masked by the taste of the accompanying sauce.

The conclusions of the study on the quality criteria for porridge are:

• Vitreosity and hardness have no influence on the texture of the porridge.
• The lower the ash and protein content and the higher the starch content in the hulled grain, the firmer the porridge.
• Hulled grain with about 1% mineral matter content gives a firm enough porridge.
• 70% of the texture of the porridge is due to the characteristics of the starch (solubility and swelling at 85°C, amylose content). A porridge is as firm as the amylose content is high.

Contrary to the local varieties of sorghum, the improved varieties with good agronomic qualities (yield, resistance) produce a porridge which is often soft and unattractive to consumers.

An African regional standards project has been drawn up by the Codex Alimentarius FAO/OMS. For sorghum grain (entire or hulled) the level of moisture, ash and protein, cellulose and fat, impurities and hulling, conditions of hygiene and packing have been fixed. The application of these standards requires a serious effort on the part of the countries and operators concerned. For sorghum flours the ash content should be between 0.9% and 1.5% (dry matter) and the granularity below 0.5mm for a fine flour or 1mm for a medium flour. The application of these standards is not obligatory but it does permit the processor and user to characterize and position the product.

Since the devaluation of the CFA Franc, the consumption of local cereals has grown in urban areas and the quality of this supply should be encouraged. The three criteria which should be respected at the processing and milling stage are the choice of varieties, the cleanliness of the grain and the granularity of the milled product. The various institutes or food technology laboratories directly concerned with processing cereals have a role to play in the promotion of the concept of quality through training, price incentives and choice of processing equipment.

In the marketing and distribution stages, other quality criteria intervene, in particular the packaging and conservation of the products, subjects not treated here. At markets and in certain shops traditional cereal products are found, packed in plastic sachets, sometimes with instructions for their use. This important development should be followed up.

Contents - Previous - Next