SORGHUM: DRY AND WET MILLING (MALTING)

NTRODUCTION

Sorghum (Sorghum bicolar (L.) Moench) is one of the important cereal crops and major staple food in most part of the world with more than 40.5 M hectares and production figures of 55.7 M tonnes, respectively with Asia as a leading continent, followed by Africa in which Nigeria cultivate 4.7 million hectares. However, area cultivated in Africa was 24.8 M hectares with the production quantity of 20.9 M tonnes and average yield of 0.8 M tonnes per hectare (FAO. 2010).

The land area put into cultivation of sorghum in Nigeria was 4.7 M hectares while the production figures were 4.8 M tonnes, respectively. The average yield of the Nigerian farmers was 1.0 tonnes per hectare.

The guinea sorghum is the most widely cultivated and adaptable race which are common in the savannas ecological zone.

Sorghum is mainly used in the form of flour or paste processed into two main dishes. “Tuwo” a thick porridge and OGI or KAMU a thin diet or porridge. Other dishes that are sometimes made from sorghum include a number of deep-fried snacks, steamed dumplings and other boiled or roasted snack foods.

Processing inputs

The processing inputs are divided into traditional and modern. The traditional ones include machete, knives, ropes, empty sacks/tarpaulin, mortar and pestle, beating stick, winnower/calabashes/bowls and grinding stones while the modern ones are different type of threshers, dryers, dehuller, milling machines, etc.

Sorghum Processing Technology

Processing of sorghum entails applying suitable grain, milling and malting procedures, which will not only maintain nutritional value, but also lead to minimal grain losses with improved marketability of the end products. The process goes through primary processing which involves cleaning, dehulling (decorticating), pounding and milling. Then comes the secondary processing, which involves turning material into food, i.e. cooking, blending, fermentation and roasting. Both traditional and industrial processing methods are employed, which may involve the partial or complete separation and/or modification of the three major constituents of the cereal grain, i.e. the germ, the starch-containing endosperm and the protective pericarp (or testa). Industrial methods of processing sorghum though not well developed as that of other cereals have had significant impact on the food security of the country. Industrial processing is in most cases, geared towards production of grit, malt, meal/flour and sometimes germ, with bran and germ-cake as by-product for feeding animals. Two main methods usually employed in sorghum processing are dry-milling and wet-milling.

Cleaning

In traditional systems, grain cleaning is achieved by winnowing, while washing in water will remove most dust and stones. In mechanised systems, forced air (aspiration) is used to remove lighter materials, while most stones, dust and other material are removed as the grain passes over a series of screens. Ferrous metal are removed by a permanent magnet placed in the flow path of the grain.

Small Scale Dry milling of Sorghum

The production of virtually all sorghum foods first comprise of two major operations: 1) Debranning

(dehulling)- removal of the unpalatable, sometimes tannin-rich and highly pigmented bran and the rancidity causing fat-rich germ; 2) Size reduction (grinding)- converting the endosperm into meal or flour. Traditionally in Africa, sorghum milling has been done using a pestle and mortar for dehulling and saddle quern for grinding or size reducing the grain, methods still used in many African communities

(Fig. 14.2).

Today, mechanised milling is becoming the norm, creating a milling industry and the opportunity for manufacture of more versatile sorghum food products to meet growing world food demands. Probably the most common method of mechanised sorghum milling in Africa is by abrasive debranning (also known as decortication or dehulling), followed by hammer milling of the endosperm material.

Large scale Milling

The sequence of operations in sorghum dry milling is as follows:

 Cleaning: the sorghum is weighted and conveyed to a separator consisting of two sets of sieves equipped with an air aspirator. Stones, sticks and other coarse and fine materials are removed.

 Conditioning: The sorghum grain is passed into bins and at the same time conditioned by the addition of water (cold ot hot) or steam so that the moisture content is raised to 19-22%, at which condition it is best suited for miling. The conditioned grain rests for about 24 hours before milling.

 De-germing and miling: When optimum conditioning has been achieved, the sorghum is passed through a worm-conveyor from where it is dropped into a degerming machine consisting of a conical rotor revolving inside a conical stator. Both parts are fitted with studs which break up the grain. The grains then passed through successive fluted-rollers with decreasing fineness where they are reduced to grits, coarse, middlings, flour and bran.

 Separation of the milling components (sifting): The milled grain is then passed through a rotary drier to reduce its moisture content to about 15%. It is then passed through a cooling worm into two rotating cylidrical sieves which removes the sorghum meal from the grits, germ and bran.

 Packaging: The milled products are packaged in bags of 10, 25 and 50 kg weights for distribution to market outlets. In most cases however, the grits are bulk transported to secondary production plants for use in infant foods and confectioneries.

Wet-Milling

Like other cereals, sorghum is wet-milled to obtain starch, oil, animal feeds (gluten feed, gluten meal, germ cake) and the hydrolysis products of starch (i.e. liquid and solid glucose syrup). Dark-coloured

sorghum varieties are not suitable for wet-milling as the colours leaches out in to the steep unless a means for removing the colours is employed. The process which can also be carried at cottage level yield products like starch, germ, fibre and sorghum gluten. The steps involved in sorghum wet-milling are basically the same at both the small and large scale levels.

a. Steeping. The sorghum is cleaned and steeped for about 24-72 hours at room temperature [or

24 – 48 hours in warm water (about 50 oC)] containing sulphur dioxide (0.03%).

b. De-germing. The steep water is drained off and the soaked grain is run through attrition mills to break it and free the germs. The slurry of ground sorghum is allowed to stand: the germ floats and is skimmed off for use in oil extraction and animal feeds.

c. Milling. The de-germed slurry after straining is finely ground using attrition mill, and the hulls and fibre which are not finely ground are separated from the protein and starch using fine mesh screens.

d. Separation of starch from protein. By subjecting the slurry in a high speed centrifuge, the starch which is heavier is separated at the outer region of the centrifuge while the lighter protein fractions migrate to the centre. The fractions of starch and protein are then dried to safe moisture levels.

e. Oil extraction (at large scale only). Oil can also be extracted from the germ by either hydraulic press, screw press or solvent extraction using normal hexane Main products obtained from sorghum wet-milling are starch,gluten (protein) and Germ (for oil)

Quality criteria for Sorghum

 Traders and stockists want dry, clean grain, neither infested nor damaged.

 Millers want clean grain in homogeneous batches, not too hard for grinding and giving a high yield of milling products.

 Processors want hulled or broken products of homogeneous size, free of sand or other impurities and without parasitic odours or infestation.

 Consumers are sensitive to the colour, the texture, the aroma and the taste of the product after final cooking.

These quality criteria, reflected in the price, are always present even if the current quality standards are not always respected.

General standard for export

1. The standard applies to sorghum for direct human consumption.

2. Grains shall not have abnormal odour or taste.

3. Grains may be white, pink, red, brown orange or yellow or may be a mixture of grains.

4. Grains must be sound, clean and free from living insects.

5. Moisture content should not exceed 14.5 percent; ash not more than 1.5 percent on dry matter; protein not less than 7 percent on dry matter basis.

6. Tannins: For whole grains - not to exceed 0.5 percent on dry matter. For polished grains - not to exceed 0.3 percent on dry matter basis.

Packaging:

Threshed sorghum is packaged in sacks of 100, 200, 300, 500 and 750kg weights for distribution to market outlets, warehouse, and stores. The sacks are often re-used and in case of re-using, care should be taken to prevent re-infestation of clean grain by boiling sacks in water and thorough drying.

Storage

The goal of good storage is to be able to deliver grain from store in good quality and with no loss in quantity. This is achieved by preventing the deterioration caused by: Adverse climatic conditions (temperature, relative humidity, etc), Contamination by extraneous material; Grain germination; and Pest infestation.

Traditionally, unthreshed heads are store in a solid walled container called a rumbu, bins, silos, warehouse etc. For short-term storage, bundles of sorghum 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. While threshed grains are stored in bags in small quantities for immediate consumptions or for seed are store in clay pots, tins, or calabash. In large farms and markets grains are stored in warehouse or in large silos.

Ensuring that the storage environment is clean and tidy and in a good state of repair, makes a major contribution to the quality control during storage, but it is insufficient to prevent losses by pests.