Traditional processing methods

Contents - Previous - Next

Processing untreated grains

Flour made by grinding whole grain is occasionally used, particularly with the smaller millets, but in most places where sorghum and millets are consumed the grain is partially separated into its constituents before food is prepared from it.

The first objective of processing is usually to remove some of the hull or bran - the fibrous outer layers of the grain. This is usually done by pounding followed by winnowing or sieving. The grain may first be moistened with about 10 percent water or soaked overnight. When hard grains are pounded, the endosperm remains relatively intact and can be separated from the heavy grits by winnowing. With soft grains, the endosperm breaks into small particles and the pericarp can be separated by winnowing and screening.

When suitably prepared grain is pounded, the bran fraction contains most of the pericarp, along with some germ and endosperm. This traction is usually ted to domestic animals. The other fraction, containing most of the endosperm and much of the germ along with some pericarp, is retained for human consumption. Retaining the germ in the flour will improve aspects of its nutritional quality, but at the same time it will increase the rate at which the flour will become rancid. This is particularly important in the case of pearl millet.

Dry, moistened or wet grain is normally pounded with a wooden pestle in a wooden or stone mortar. Moistening the grain by adding about 10 percent water facilitates not only the removal of the fibrous bran, but also separation of the germ and the endosperm, if desired. Although this practice produces a slightly moist flour, many people temper the grain in this way before they pound it. Pounding moist or dry grain by hand is very laborious, time consuming and inefficient. A woman working hard with a pestle and mortar can at best only decorticate 1.5 kg per hour (Perter, 1983). Pounding gives a non-uniform product that has poor keeping qualities.

Many pearl millet grains have an irregular indentation in the pericarp. This makes it more difficult to decorticate pearl millet than it is to decorticate most other cereal grains (Kent, 1983).

The particle size of the endosperm fraction can be reduced by crushing or grinding to produce coarse grits or fine flour. This unpleasantly hard work is almost always done by women. Traditional grinding stones used to grind whole or decorticated grain to flour usually consist of a small stone which is held in the hand and a larger flat stone which is placed on the ground (Subramanian and Jambunathan, 1980; Vogel and Graham, 1979). Grain, which should be fairly dry, is crushed and pulverized by the backward and forward movement of the hand-held stone on the lower stone. The work is very laborious, and it is hard work for anyone to grind more than 2 kg of flour in an hour. In a traditional process used in many countries of Africa and Asia, decorticated grain is crushed to a coarse flour either with a pestle and mortar or between stones. Grain is also ground to coarse or fine flour in mechanized disk mills now located in many villages.

In wet milling, the sorghum or millet is soaked in water overnight (and sometimes longer) and then ground to a batter by hand, often between two stones. Soaking makes the endosperm very soft and the pericarp quite tough and makes grinding much easier, but it gives a batter or paste instead of flour.

Processing malted grains

Malting involves germinating grain and allowing it to sprout. Typically the grain is soaked for 16 to 24 hours, which allows it to absorb sufficient moisture for germination and for sprouts to appear. However, germinated sorghum rootless and sprouts contain very large amounts of dhurrin, a cyanogenic glucoside, which on hydrolysis produces a potent toxin variously known as prussic acid, hydrocyanic acid (HCN) and cyanide (Panasiuk and Bills, 1984). The fresh shoots and rootless of germinated sorghum and their extracts must therefore never be consumed, either by people or by animals, except in very small quantities (e.g. when the germinated grain is used just as a source of enzymes). Dada and Dendy ( 1988) showed that the removal of shoots and roots and subsequent processing reduced the HCN content by more than 90 percent.

Malted sorghum has traditionally been used in several countries in Africa, but always after careful removal of the toxic parts. Hullu-murr is an important traditional food prepared from malted sorghum in the Sudan ( Bureng, Badi and Monawar, 1987). Alcoholic beverages and dumplings are prepared in Kenya from germinated sorghum and millet.

In the germination process, the grain produces a-amylase, an enzyme that converts insoluble starch to soluble sugars. This has the effect of thinning paste made by heating a slurry of starch in water, in turn allowing a higher caloric density in paste of a given viscosity, since as much as three times more flour can be used when the grain has been germinated. The energy that young children can consume is often limited by the bulk that they can consume. Thus using germinated grain can make food more suitable for certain categories of young children. Flour from malted grain is consequently used quite widely in the production of children's food, but when such foods are made from sorghum, great care must always be taken to ensure that the level of cyanide is adequately low, as children are particularly vulnerable to cyanide.

In India, malted finger millet is common and is considered to be superior to malted sorghum and malted maize. Studies have shown that finger millet develops higher amylase activity than sorghum and other millets (Seenappa, 1988). Germination of grain is reported to change the amino acid composition, convert starch into sugars and improve the availability of fat, vitamins and minerals.

Pal, Wagle and Sheorain (1976) measured the changes in the constituents of sorghum and various millets (finger, pearl, prove, kodo and barnyard) during malting. The malting losses for finger millet and foxtail millet were high. Pearl millet had the highest a-amylase activity. Amylolytic and proteolytic enzyme levels in malted pearl millet were comparable to those in malted barley.

The use of only 5 percent malted sorghum or finger millet was found to reduce the viscosity of weaning foods (Mosha and Svanberg, 1983; Seenappa, 1988).

Processing grain treated with alkali

To produce a particular type of tortilla that is popular in Mexico, sorghum grains are cooked in lime water for a short time and steeped overnight, washed to remove the excess alkali and then ground to a paste (Rizley and Suter, 1977).

Wood ash is used in traditional treatments to reduce the level of tannin in brown sorghums and improve the nutritional quality. Muindi and Thomke (1981) reported the use of wood ash in the United Republic of Tanzania. Mukuru (1992) described a tannin-reducing technique used in parts of eastern and central Africa where, because of grain-eating birds, only "high-tannin" sorghums are grown. The sorghum is first soaked overnight in a slurry of wood ash in water. After draining it is left for three or four days to germinate. The germinated grains are sun-dried and pounded to loosen the adhering wood ash and to remove the sprouts, with their high levels of cyanide. The grain is then ground and used to prepare either a non-alcoholic beverage called obushara or an alcoholic drink containing about 3 percent alcohol called omuramba.

Processing parboiled grain

Parboiling is reported to help in dehusking kodo millet (Shrestha, 1972) and to eliminate the stickiness in cooked finger millet porridge (Desikachar, 1975).

Industrial processing

While there are many machines available for processing hard white sorghum, there is unfortunately no well-proven industrial process available that is entirely satisfactory for making white products from coloured sorghums and millets.

Cereal grains can be milled wet, in the form of a thin aqueous slurry, usually to produce starch, or in an essentially dry form (often suitably dampened or "tempered") which usually produces meal (coarse or fine flour). A factory in Texas, United States, for wet milling sorghum operated intermittently from the 1940s to the 1970s (Rooney, 1992) but is now closed. No millets have ever been wet milled commercially to produce starch. The following technologies are all for dry and semi-wet milling.

In industrial processing, once the grain has been cleaned, the first operation is usually the separation of offal (the portion not normally used for human consumption) from the edible portion. The offal consists of the pericarp and sometimes the germ. Offal removal is frequently called decortication or dehulling.

Following the removal of offal, the edible portion is often milled to reduce the particle size of the edible fraction. There is usually a choice of techniques and mills that may be used for particle size reduction if a finer product is desired. Some of the earliest research and development work on milling technology for pearl millet and sorghum was promoted by FAO in 1964, initially on a laboratory scale in Senegal and later on a semi-industrial scale in the Sudan. The conclusion was reached that the technology for milling wheat is not optimal for milling sorghum and millet (Perter, 1977).

Most industrial operations that can be carried out on untreated grain can also be used with grain that hats been prepared in some way, for example grain that has been germinated and then suitably dried.

Three types of processors can be used to mill sorghum and millets on a commercial scale: abrasive decorticators, which abrade the pericarp away, i.e. progressively remove offal from the outside; machines that rub (rather than abrade) the pericarp off the endosperm; and roller mills, which cut the endosperm from the inside of the pericarp.

Abrasive decortication

Abrasive decorticators work by abrading away the fibrous pericarp. Obviously, the outer layers of the seed-coat are abraded away first and the innermost layers, which in many varieties contain those factors that most need to be removed, are the last to be abraded away. If all parts of all grains could be abraded away at the same rate, abrasive decortication would be an efficient way of removing the pericarp. However, different parts of individual grains are abraded away at very different rates, and there is some loss of endosperm (particularly from damaged grains) even when the grain is only lightly abraded. Also, non-spherical seeds, e.g. pearl millet grains, tend to abrade away much more quickly at some points than at others.

When hard white sorghum grains, uncontaminated with seeds with a red testa, are decorticated in an abrasive decorticator, any pericarp left on the grain is hard to see, and when the pearled grain is milled, the presence of pericarp goes largely unnoticed. However, the ability of abrasive decorticators to produce an adequately white product falls sharply with increasing levels of contamination from seeds with a coloured seed-coat. When the contaminating seeds have a red testa (which is deeply coloured and is practically the last layer to be abraded away) a decorticator's ability to produce an acceptably white product falls even more sharply. The problem is compounded by the fact that many contaminating seeds are comparatively soft and their exposed endosperm is ground away quickly. As a result, milling yields often fall to unacceptably low levels.

Decorticators produce what is visually a very acceptable product in a good yield from grain well suited to abrasive decortication. However, if the grain to be ground is not always going to consist of a very high proportion of hard, white, spherical seeds of fairly regular size, a very careful analysis of the economics of operating an abrasive decorticator should be made on the basis of recovery rates derived from trial runs.

Even though decorticators are well suited to small-scale operations, these machines have often proved to be too large for the system into which they were introduced. In many cases they have been introduced less successfully than originally hoped, either because of a lack of supplies of the high-quality grain that is needed for them to work properlyor because of insufficient local demand for the product. Very small units are likely to be run less efficiently than larger ones.

Most decorticators are based on a prototype put out by the Prairie Regional Laboratory (PRL) in Canada. This type of decorticator has the enormous advantages of being relatively inexpensive to install and relatively simple to maintain and operate. Bassey and Schmidt (1989) described the development of this type of decorticator and its use in Africa. More recently it has been introduced in India.

In 1976, a prototype decorticator was established in Maiduguri, Nigeria. A larger unit to process 5 to 10 tonnes of sorghum per day was installed at Pitsane in southern Botswana in 1978 but the demand for the product was inadequate to keep the equipment running at full capacity. The Centre national de recherches agronomiques (CNRA) in Bambey, Senegal, began to use a PRL decorticator to decorticate sorghum and millet in 1979. The capacity of this decorticator also exceeded the demand for the product.

FAO supplied the Food Research Centre (FRC) in the Sudan with a pilot plant including a decorticator manufactured in Germany after FRC had compared decorticators made by several different manufacturers. FRC is currently decorticating white sorghum for a local urban market. The centre has also produced pearled sorghum as a substitute for rice (Bad), Perten and Abert, 1980); although the product has to be cooked much longer than rice, it was well accepted. Of the five most popular varieties of sorghum grown in the Sudan, two (Feterita and Mayo) are unsuitable for abrasive decortication.

James and Nyambati (1987) described the industrial preparation of pearled brown and white sorghum in Kenya using a decorticator that could mill sorghum in batches or continuously, but they found it was difficult to obtain sufficient sorghum suitable for processing. The product was sold at 60 percent of the price of rice and consumer acceptance was very good. Flour was also produced from the pearled grain.

Various modifications have been made to the PRL design to suit specific conditions. A variant of the PRL decorticator was developed in the early 1980s by Palyi and tested in Canada. The Palyi-Hanson BR 001-2 can mill 3 tonnes per hour. Under local management in the Gambia a PRL decorticator processed 50 tonnes of pearl millet over a one-year test period, after which modifications were made to the design. In 1986 the Rural Industrial Innovation Centre (RIIC) introduced a modification that enabled the machine to handle small quantities of grain (Bassey and Schmidt, 1989). By 1989, about 35 RIIC decorticators had been installed in Botswana, but for one reason or another, not all of these machines are still being used for milling sorghum or millet. In turn, local agencies in some of the main countries to which the RIIC design has been exported (e.g. Zimbabwe, Senegal) have deemed it necessary to modify the RIIC design for improved operation for local grain.

In Zimbabwe, decorticators were placed in five rural locations for evaluation. A local research group, Environment Development Activities, produced a modified version that can process one tonne of grain in eight hours. In Senegal, a local modification was evaluated in ten villages. Decorticators based on a second local design (called the mini-SISMAR/ISRA), which can mill about 600 kg of grain in eight hours, were then introduced.

Equipment of RIIC design was introduced at Morogoro, United Republic of Tanzania, in 1982. Although the first unit was unsuccessful, four pilot systems were established locally for evaluation. In 1982, a mill with an RIIC decorticator was established in Ethiopia, but the supplies of grain for it were inadequate because of the drought.

There has also been an intensive effort to introduce RIIC decorticators in Andhra Pradesh. Decortication improved the quality of the flour from sorghum and millets so that it could be used in new ways (Geervani and Vimala, 1993).

High-yielding sorghums introduced in Mali were soft and could not be decorticated in PRL-type decorticators (Scheuring et al., 1983).

A number of large decorticators have been installed around the world with capacity ranging from 1 to 2.5 tonnes per hour. Typically, they are vertical axis units with abrasive disks that have been carefully selected for the optimal degree of abrasion. The grain is first cleaned to separate sand, dust, coarse material and other impurities. An aspirator removes the abraded bran through a screen. The bran is sometimes further separated into fine bran (mostly pericarp) and a mixture of germ, broken grain and coarse bran. A l-t/hour decorticator manufactured in Switzerland was run for several years in Zimbabwe, preparing coarse sorghum flour that was introduced into a wheat flour mill. A 2.5-l/hour unit manufactured in Germany was installed in the Sudan. Other large units are reportedly in operation in Nigeria. As with small units, high-quality sorghum is needed to produce an acceptably white product in these larger decorticators. Sufficient quantities of high-quality sorghum to keep large mills running at full capacity are not often available.

Rubbing techniques

Munck, Bach Knudsen and Axtell (1982)describedanew industrial milling process developed in Denmark, which does not involve abrasive milling. Decortication is achieved by a steel rotor rotating the grain mass within a generally cylindrical chamber. When the grain is properly tempered, the pericarp is rubbed off by the movement of one seed against another. However, when the grain is too dry, as was the case in a factory in the Sudan, abrasion of the internal components of the mill becomes severe. The hulls and the endosperm fragments are separated in a cyclone and the endosperm particles are milled in a proprietary mill. These units have a capacity of 2 tonnes of sorghum per hour. The system was reported to yield 80 percent flour with whiteness comparable to traditional milling, but to do this it requires grain with specifications similar to those required for efficient abrasive decortication.

Roller mills

Most wheat is milled in a type of mill called a roller mill. Roller mills are the most efficient mills for separating the constituents of cereals. Two types of rollers are used: rollers with axial grooves, which cut the endosperm from the pericarp (effectively cutting it away from the inside), and smooth rollers, which progressively crush the endosperm pieces into finer and finer flour. Normally the grain is passed through a number of roller mills, often 20 or more. Wheat milling technology is suitable for milling large quantities of grain, but it requires a large investment and experience in operating and maintaining the equipment. For all these reasons, it is therefore not suitable for milling sorghum and millets in very small-scale operations. However, roller mills are very efficient in separating the edible portion of cereals from the offal and can do so with sorghum and millets regardless of the physical characteristics of the grain; it does not matter if the grain is soft, coloured or broken. Roller milling might therefore have a place where high-quality products are required from comparatively large quantities of grain of poor or indifferent quality, particularly where there is spare capacity in an existing wheat mill.

To withstand the stresses of roller milling, the pericarp of sorghum and millets has to be much moister than that of wheat. Early efforts to roller-mill sorghum and millets always ended in failure because the grain was dry when it was milled. It would shatter, the pericarp breaking into small pieces that were too brittle to allow separation of the endosperm. Using conventional tempering techniques, Perten (1983) was unable to achieve efficient separation of the offal of either sorghum or millets from the endosperm. He concluded that sorghum and millets are more difficult to grind than wheat and that they produce a coarser and much darker flour which contains high levels of fat and ash.

The use of moisture levels much higher than those used for milling wheat was first reported by Abdelrahman, Hoseney and Varriano-Marston ( 1983 ) for milling pearl millet and by Cecil (1986, 1992) for milling other millets and sorghum. The term semi-wet milling was adopted for this technique. For millets, about 10 percent water must be equilibrated in the grain for four hours before it is ready for milling; for sorghum, about 20 percent moisture must be added and the grain conditioned for six hours. The damp material flows almost as easily as normally tempered wheat products do, and no holdup problems were encountered in several hours of running 2 tonnes per hour of red sorghum in a commercial mill. In early experiments, comparatively low yields of fine flour were obtained, but subsequent work produced low-fibre, low-tannin grits from red sorghum in a commercial mill with six roller passes with a yield of 72 percent (compared with typical wheat recovery of 70 percent). In a laboratory mill with three milling passes, 84 percent yield of grits was obtained from commercial white sorghum from Botswana and 83 percent from white sorghum from Lesotho. All the grits contained very low levels of fibre.

Semi-wet milling has several advantages, including the excellent separation of the offal from the edible portion and the opportunity for using well-tested existing commercial wheat-milling equipment without the need for any changes in the set-up of the mills. White flour with practically no tannin, which tastes better, looks better and is nutritionally better than flour that contains tannin, can be produced from high-tannin coloured varieties. Mixtures of sorghum or millet varieties, soft varieties, misshapen seeds and mixtures of sorghum with other grains (including wheat) can all be milled together if necessary. Moistening the endosperm softens it to such an extent that very little energy is needed to mill it. Semi-wet milling of pearl millet, unlike abrasive decortication, may also help eliminate substances that cause goitre (Klopfenstein, Leipold and Cecil, 1991).

Redundant or underutilized wheat mills can be used with minimal additions and the mill can be reverted to milling wheat within a few minutes. Alternatively, any type of sorghum can be milled together with wheat. For a period of about five days, 0.6 tonnes of red sorghum and 14 tonnes of wheat per hour were milled together without difficulty in a commercial mill in Zimbabwe.Semi-wet milling has some disadvantages. Although it would not be difficult or very expensive in a commercial system to dry the products of semi-wet milling, they are usually too damp for long-term storage. In semi-wet milling, microbiological growth might be more vigorous than in conventional milling of wheat, but reasonable attention to hygiene will minimize this problem. Semi-wet milling is not suitable for very small operations. Finally, although it has been shown that sorghum can be milled semi-wet without any difficulty in commercial equipment, the technique has not yet been proved over an extended period of operation.

Size reduction

Many mills can be used to reduce the size of the particles obtained by decortication, but the type that is usually used (and is also probably the simplest to use and the cheapest to install) is the hammer mill. Hammer mills are available in all sizes. They consist of blunt blades rotating rapidly in an enclosed cylinder with an outlet covered by a screen. The size of the holes in the screen determines the size of the particles of flour, but small holes will reduce the throughput of the mill, and if they are too small overheating may result.

If roller mills are used for separating the endosperm from the offal, the particle size is usually reduced in roller mills with smooth rollers.

Contents - Previous - Next