Chapter 3 - Storage and processing

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When sorghum or millet is stored in developing countries, it is usually stored in small quantities in traditional containers, often on the farm. Large quantities are seldom accumulated and bulk storage is uncommon.

Processing involves the partial separation and/or modification of the three major constituents of the cereal grain - the germ, the starch-containing endosperm and the protective pericarp. Various traditional methods of processing are still widely used, particularly in those parts of the semi-arid tropics where sorghum and millets are grown primarily for human consumption. Most traditional processing techniques are laborious, monotonous and carried out by hand. They are almost entirely left for women to do. To some extent, the methods that are used have been developed to make traditional foods to suit local tastes and are appropriate for these purposes. Traditional techniques that are commonly used include decorticating (usually by pounding followed by winnowing or sometimes sifting), malting, fermentation, roasting, flaking and grinding. These methods are mostly labour intensive and give a poor-quality product. Sorghum and millets would probably be more widely used if processing were improved and if sufficient good-quality flour were made available to meet the demand (Eastman, 1980).

In general, industrial methods of processing sorghum and millets are not as well developed as the methods used for processing wheat and rice, which in most places are held in much higher regard than sorghum and millets. The potential for industrial processing of sorghum and millets is good, and attempts to develop improved industrial techniques have been made in several countries. Custom milling has had a significant impact in several African countries where it has recently been introduced. In Nigeria alone, where about 80 percent of sorghum and millets is now custom milled into whole flour, over 2.5 million tonnes of sorghum have been processed in this way (Ngoddy, 1989).

To some extent for storage, but particularly for processing, the type of sorghum - brown, white or yellow - is important. The outward appearance is no indication of the variety's type; all three types can appear white, yellow, brown, red or purple, although brown sorghums generally have darker seed-coats than yellow and white sorghums. (Subsequently in this chapter the more widely recognized term "white" is used for both white and yellow types.) The important difference is whether there is a testa. The testa is usually brick red, and even a small amount of red testa left in the flour will give it a pronounced pinkness, which many people find objectionable. If the variety contains tannin, most of it will be found in the testa. Tannin is objectionable for two reasons: it competes for available protein and it has a bitter taste. However, this bitter taste is also a major advantage, because it makes graniverous birds dislike high-tannin sorghums. For this reason these varieties are widely grown in places where bird damage to white sorghum is severe.

The presence of a testa is controlled by two dominant genes, B1 and B2 (Hulse, Laing and Pearson, 1980). Wild sorghum will usually contain some of these dominant genes, so open pollination of white hybrids will tend to degenerate them to brown varieties. Repeated replanting of harvested seeds is often accompanied by increasing occurrence of seeds with a testa. Seeds with a testa are much harder to mill than seeds without a testa.

Brown sorghums tend to be softer than white sorghums and are more susceptible to insect damage under storage than white sorghums. However, they are markedly less susceptible to fungal damage in the field and in storage.

It is in processing that brown sorghums present the most difficulty, for the following reasons.

The best way of separating the testa of a brown sorghum from the endosperm is to cut the endosperm from the inside of the pericarp, as happens in roller milling. However, this is not possible using traditional methods. It is for these reasons that brown sorghums are usually only used in the production of beer, where some bitterness and some colour are not only acceptable but often preferred.

Storage

The objective of storage is to preserve as much as possible of the value of the grain for its intended future use. This means either retaining as high a proportion of viable seeds as possible for planting at the next harvest or preserving as much as possible of the food value of the grain for as long as possible. Several factors lead to the loss of both viability and nutrients, but globally the main causes of loss are the depredations of pests (insects, birds and rodents) and mould damage. Germination of the grain (sprouting) also results in losses, but on a smaller scale. Grain is stored by consumers and by processors for future consumption. It is also stored by commercial traders for resale, usually on the home market but occasionally for export.

Moisture in the grain and the temperature of storage are the most important physical factors that contribute to losses (FAO, 1970b). Most activity that causes losses occurs more rapidly as the temperature increases. With even minor changes in temperature, moisture will migrate and accumulate in certain areas, either near the top of the container or in places that are cooler than the rest. This often allows microbiological activity to occur in comparatively dry grain. Microbiological activity usually produces heat, and in unventilated stores, moist areas can get so hot that charring can occur. At this stage the grain is ruined. It may even burst into flames when it is exposed to air.

Storage bins are best filled early in the day when the air is cool and the humidity is often at its lowest. The grain should be packed as tightly as possible to allow insects the minimum space to move around and to breed. Sand is sometimes mixed with the grain to reduce the free space further.

TABLE 14: Damage and weight loss of sorghum and millets under home storage, India

Storage period

Percent damage

Percent weight loss Increase in uric acid (mg/100 g)
  By weight By number    
Sorghum  
1 month 3 3 0.2 0.0
5 months 5 6 1.5 4.3
9 months 9 11 2.4 5.4
Pearl millet  
1 month 0 0 0.1 0.0
5 months 2 2 0.2 3.3
9 months 2 4 1.0 3.6
Finger millet  
1 month 0 1 0.0 0.0
5 months 0 1 0.0 1 4
9 months 0 1 0.1 1 6

Source: Pushpamma et al., 1985.

Studies conducted in Senegal showed that when properly dried and threshed sorghum and millets were mixed with 30 percent sand, storage losses were reduced.

Pushpamma et al. ( 1985) found in India that the storage loss of sorghum over seven months was greater than that of pearl millet, which was in turn greater than that of finger millet (Table 14). They also found that the moisture content of all the stored grains increased and the levels of niacin and protein fell (Table 15). Rao and Vimala ( 1993) showed that pretreatment of sorghum grain with 2 percent tricalcium phosphate reduced the development of rancidity during storage.

The influence of seed moisture (relative humidity), temperature and the surrounding atmosphere on sorghum germination was studied by Bass and Stanwood (1978). Sorghum seeds were stored in sealed metal cans in six different atmospheric conditions (air, nitrogen, carbon dioxide, helium, argon and a vacuum) at three different moisture levels and at five different temperatures over a 1 6-month period. Temperature was the only parameter that affected the rate of germination, which was lowest at -12C.

TABLE 15: Chemical composition of sorghum and millets stored for different periods (moisture-free basis)

Storage period Number of samples Moisture (%) Protein (g) Non protein nitrogen (mg ) Thiamine (mg) Riboflavin (mg) Niacin (mg)
Sorghum  
1 month 26 10.4 8.5 326 0.32 0.18 2.3
5months 26 10.4 8.2 240 0.31 0.16 2.1
    (0) (-3.5) (+1.7) ( 3.1) (-11.1) (-8 7)
9months 22 11.1 7.6 246 0.24 0.16 2.0
    (+67) (-106) (+4.3) (-25.1) (-11.1) (-13.0)
Pearl millet  
1 month 18 9.3 10.0 282 0 33 0.21 2.4
5months 18 11.0 9.9 285 0.29 0.21 2.4
    (+18.3) (-1.0) (+1.1) (-12 1) (0) (0)
9months 12 10.7 8.9 297 0.20 0.21 2.0
    (+15.1) (-11.0) (+5.3) (-39.4) (0) (-16.7)
Finger millet  
1 month 7 10.9 7.6 193 0.37 0.19 1.3
5months 7 10.9 7.4 216 0.33 0.18 1.3
    (0) (-2 6) (+12.0) (-10.8) (-5.3) (0)
9months 7 11.6 7.2 275 0.21 0.17 1.1
    (+6 4) (-5.3) (+42.5) (-43.2) (-10.5) (-15.4)

Note: Figures in parenthese indicate percentage decrease (-) or incrrease (+) from the values at tile initial (one - month) sampling.
Source: Pushpamma et al., 1985.

Methods used for storing grains are influenced by the value of the crop, the quantity stored and environmental conditions. Compared to other cereal crops, sorghum and millets are not widely traded internationally, and within those developing countries where they are grown for human food there is usually a balance between local production and local demand. Farmers and rural householders in developing countries store most of what is grown in small storage structures. There is not much need for bulk storage of these crops.

Storage containers vary from small traditional on-farm or domestic containers to silos which are sometimes found on large farms. In many countries, small granaries are made by weaving plant materials such as bamboo, stalks, bark and small branches and then sealing any gaps with mud or dung. These structures may be built directly on the ground or raised off the ground on platforms or stilts.

Storage practices in Africa

In some countries in West Africa sorghum and millet grains are mixed with wood ash and stored in clay pots (Vogel and Graham, 1979). In Nigeria sorghum and millets are stored as unthreshed heads in a solid walled container called a rumbu. For short-term storage, bundles of sorghum and millet heads are arranged in layers in the rumbu. For long-term storage of three to six years, the heads are laid out individually rather than in bundles. Some farmers spread the leaves of gwander daji (Anona senegalensis) on the bottom of the rumbu and between each layer of grain. When a rumbu is full, the mouth is sealed with clay.

In Uganda, sorghum is threshed and stored in gunny sacks, whereas millets are stored unthreshed. In the Sudan, pits holding 2 to 5 tonnes of grain are used as underground stores.

Storage practices in India

Most of the sorghum and millets grown in Andhra Pradesh are grown for personal consumption. Pushpamma and Chittemma Rao ( 1981 ) described the various ways these grains are stored there. Occasionally sorghum and millets are stored on the ground, usually unthreshed. The earheads are heaped in a pile (either indoors or outdoors) and covered with straw. As the grain is needed, earheads are removed and threshed. More often, grain is stored in gunny sacks, which are stacked either on the floor or on raised wooden platforms. Underground pits, which may be located underneath the house or outside, are also used. The pit is lined with paddy straw or sorghum straw. When it is full of grain the grain is covered with straw and soil. For longer-term storage, the top is plastered over with mud. Storage jars, silos and bins are made from a number of different materials. On the smallest scale, grain is stored in clay pots. Larger containers are made from wood, brick or stone or from bamboo made into a basket which is then sealed with clay or dung. When these containers are kept indoors they are sometimes left uncovered, but when they are kept outdoors they are covered with either a lid or a thatched roof. If the grain is to be stored for a long time, the top of the bin is plastered over with mud or dung. Occasional exposure to sunshine is the most commonly used measure for preventing insect infestation.

Storage of flour

Flour is usually produced as it is needed and is not often stored for long periods because it tends to turn rancid. This is particularly evident with pearl millet flour, because of its very high tat content. Sorghum and millets, particularly pearl millet, are therefore best stored as whole grain.


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