Popular cereals like wheat, rice, corn, rye, oats, barley, sorghum and some of the millets like pearl millet, finger millet, proso millet, etc have filled up our dish with myriad of food products and are very versatile and reliable source of food. Cereals are low cost energy food with good shelf life. Normally, cereals are used as flour or in pure form. Cereal processing is a multiplex operation containing series of unit operations to convert cereals into easily digestible and consumable products such as flour, dhal, grits, or any further processed products. Different cereals follow little different processing depending up on the end products and their characteristics such as physical and biochemical properties and market demand.
Wheat flour and derivatives have huge demand in bakery and confectionery industries. The aim of wheat milling is to have white flour – the possible maximum extraction of starchy endosperm and separation of germ and bran. The outer bran layer known as wheat feed and embryonic germ are the co-products of modern wheat milling. Before wheat goes to milling, they are cleaned and tempered (application of water) or conditioned (application of heat in conjunction with water) to toughen the bran and mellow the endosperm. Due to this process, the moisture content increases in the range of 15 to 17 percent (for soft wheat, hard wheat, durum wheat, rye). Then these grains are passed to the milling section. The gradual reduction system comprising of breaking system, purification system and the reduction system, is used to produce milled wheat flour products. This system uses a series of grinding and sieving stages to yield desired quantity and quality flour. The break system is the area where maximum endosperm separation is achieved. The purification block contains three machine types: purifiers, roller millsand sifters that separate particles on the basis of differences in size, air resistance, and particle specific gravity, simultaneously.
Fig. Roller mill and set of sifters
Change in Composition from Wheat to flour
The types of wheat used for the production of different products play very important role considering its protein content, mainly gluten content. Generally, T. vulgare wheat is used to produce flour for bread making, and for cakes and cookies. T. durum is mainly ground into semolina (purified middlings) to prepare pasta instead of flour and T. compactum is more suitable for confectionery and biscuits. The modern milling technology using roller mills has made possible to extract 70 percent high quality flour from wheat berries compared to only 10 percent in the older processes using stone mills.
Whole wheat flour (before the final shipping, 100 percent extraction) can be divided into straight grade and shorts and bran. The straight grade flour is the combination of patent flour (long – about 65 percent) and low grade flour (about 7 percent). The patent flour consists of minimum ash content (least contaminated with the non-endosperm material) while the low grade flour has maximum ash content and dark in colour. The flour is passed through an impact mill to kill any insect eggs. The flour may be treated physically (e.g., air classification, fine grinding, and agglomeration), chemically (e.g., addition of vitamins and minerals, bleaching and oxidizing chemicals, and leavening agents), or biochemically (e.g., addition of α-amylase orendoxylanase) to meet consumer demands.
Numerous unit operations are performed to convert paddy into value added products like brown rice, parboiled rice, white rice, quick cooking/instant rice, enriched rice, rice bran oil, etc. Paddy has 20 percent hull, which is removed by most common highly efficient (> 90 percent) dehuller known as rubber-roll shelller. The rice hull is used as rice millfeed (mixture of 61 percent rice hull, 5 percent bran and 4 percent polish), as a bedding or litter, as a fertilizer or mulch, as a fuel, etc. Dehulling of paddy given brown rice as a main product, which has bran layer still adhered to it. It is highly nutritious than the white rice (rice without bran layer) but the higher cooking time and shorter shelf life limits its use. This brown rice is milled by pearler (where abrasion of grain to grain or grain with steel cylinder takes place) to produce the white rice and removal of bran layer. Breakage of rice occurs at this stage, so addition of water or sometimes dry calcium carbonate powder (3.3 g/kg) is added to loosen the bran layer followed by easy removal. Different fractions of paddy milling are hulls (20 percent), bran (8 percent), polish (2 percent), head rice and broken (80 percent). The head rice is the main product of process and fetch higher price than the broken, which generally goes as adjunct in rice brewing (beer making process) industry, or for rice starch isolation process. While rice bran and polish are the co-products of the process. After that the loose bran is removed by aspirator and milled rice is polished by polisher (a rotating vertical cylinder with straps of leather on it). The rice bran is a source of food grade oil (15-18 percent) which is extracted using solvent. The use of rice in different products preparation depends up on the amylose content which is responsible for the cooking characteristics of the rice. Typical long-grain cultivars of rice (23-27 percent amylose content) cook to be dry and fluffy and are preferred for quick-cooking rice, canned rice, canned soups, and convenience foods containing rice. In contrast, typical short- and medium-grain types (15-21 percent amylose content) are moist and sticky after cooking and are suitable for breakfast cereals, baby foods, and brewing. Cooking quality of rice varies with the aging/after ripening (changes in rice biochemical/physico -chemical quality) process took place during the storage of rice. To produce better quality of rice, parboiling process has been in practice which comprises of soaking (50–70°C water for 2–4 h), steaming (using steam under pressure for 8–20 min) and drying (to reduce moisture from around 35 percent to 14 percent) before it is dehulled and debranned. It gives improved milling efficiency, resistant to adverse storage condition and altered cooking quality. Quick cooking rice/ instant rice is produced by cooking the rice with 60-80 percent moisture to have fully cooked rice and then drying it slowly by freeze drying or puffed drying process. This dried rice is then rehydrated while using it. Many a times, the white milled rice can be enriched with vitamin B (thiamin) to avoid the problem of Beriberi.
Maize/corn is the cereal used for human consumption, livestock, feed and fuel. Maize is processed into starch, protein, fiber, and oil-rich germ by wet milling method. Maize is processed industrially to produce maize flours and corn meals to obtain precooked refined maize flour, dehydrated nixtamalized flour (Masa), fermented maize flours, and other maize products. Maize undergoes different processing techniques such as dry maize mechanical processing to obtain whole or fractionated products, separated by anatomical features such as bran, germ, and endosperm and wet maize processing techniques which separates maize by chemical compound classification such as starch and protein. The germ is separated out using degerming process which is then used for oil extraction and deoiled germ, hull (known as Hominy) goes as feed. Generally, the corn milling process yields 70 percent endosperm and 30 percent hominy feed. The process comprises of many unit operations such as cleaning, tempering/conditioning, degerming, drying and cooling, rolling and grading to have endosperm, germ and bran as separate products. While wet milling process includes similar stages like cleaning, steeping, germ recovery, milling and fiber recovery, starch-gluten separation, etc to obtain starch and gluten as an end products.
Application of cereals
Cereals are consumed mainly as flour. The use of cereals as nonfood products’ cellulose in paper industry, wheat gluten in metal industry as core binder, rice chaff as fuel and many more. The cereals are used in many other ways besides milled products and bakery products. The cereals are used as malt. The malt is prepared from various cereals such as wheat, barley, finger millet, rye, etc. The malt is prepared using many processing steps such as cleaning, steeping, germinating, kilning, etc. The malt from cereals are used in the production of beer making industry, in the bread making industry (i.e., as an enzyme or flavor source), and in the breakfast food industry (mainly as a flavoring agent). Wheat is used for bread making using naturally leavening agent (yeast), however many other non-wheat cereals (oats, barley, finger millet, sorghum, gluten free bread, etc.) are used for making composite flour bread to have improved nutritional characteristics. Many other chemically leavened (using sodium or ammonium bicarbonates) products prepared from wheat and other cereals are also available in market. Some of these are cookies/biscuits (high in sugar and fat and low in fat), sugar wafers, crackers (contain less sugar and moderate to high fat), snack crackers, cakes, pasta and noodles (extrudate products). Utilization of cereals as a breakfast cereal has also great demand and market worldwide. The breakfast cereal products may be classified as cooked before serving and ready-to-eat products. Farina made from hard wheat often flavoured with malt or cocoa, rolled oats, grits from maize and rice are the popular breakfast cereal products which require prior cooking before consumption.
The author is 1Research Scholar, C P College of Agriculture, Sardarkrushinagar Dantiwada Agricultural University, Gujarat, 2,3Department of Processing and Food Engineering, College of Agricultural Engineering and Technology\, Junagadh Agricultural University, Junagadh – 362001, Gujarat