Food Science & Technology · Commodity Processing Technologies · also Cereal Technology, Cereal Chemistry, Grain Science, Processing of Cereals and Millets
The structure and chemistry of cereal grains and millets and how they are milled into flour, semolina, rice and maize products, with the flour quality tests that decide what each flour can make.
In the exams: GATE XE-G Section 3 (milling of rice, wheat and maize; parboiling; waste utilisation from rice milling); ICAR AICE 35.a Unit 2 (food grain processing)
Cereals are the seeds of grasses, and with millets they provide about half of the world's dietary energy. Wheat, rice and maize alone supply the largest share; barley, oats, sorghum and the millets matter regionally and nutritionally. Cereal science explains how the structure and chemistry of a grain decide what it can be made into, and milling technology turns the grain into flour, semolina, polished rice, grits, starch and bran. The link between the two is quality: a miller must deliver flour that a baker, biscuit maker or noodle maker can use, and a rice mill must deliver whole, well-cooking grains.
The subject is examined in GATE XE-G Section 3, which names the milling of rice, wheat and maize, parboiling and utilisation of rice milling by-products, and in the ICAR AICE-JRF/SRF 35.a syllabus under food grain processing. Examiners ask for the parts and composition of each grain, the stages of wheat milling and the meaning of extraction rate, flour quality tests and dough rheology instruments, rice quality and parboiling, maize wet milling and starch conversion, malting, oats and β-glucan, millets, rice bran stabilisation, starch modification and breakfast cereal processes, with short calculations of tempering water, protein from nitrogen and yields.
This manual concentrates on product chemistry and quality. The machinery and engineering of grain drying, husking, whitening and wheat mill flow are taught in the Agricultural Engineering subject post-harvest-cereals-pulses-oilseeds, and the chemistry of starch gelatinisation, pasting and retrogradation and of proteins in food-chemistry-major-constituents; this manual refers to them rather than repeating them. Bread, biscuits, pasta and snacks made from these flours are covered in bakery-pasta-snacks.
All cereal grains share a basic structure. The fruit coat (pericarp) is fused to the seed coat, so the grain is a caryopsis. Beneath these lies the aleurone layer, botanically part of the endosperm but removed with the bran in milling; then the starchy endosperm, which is the food reserve and the source of white flour; and the germ or embryo, at one end. In wheat the bran, including aleurone, makes up about 14 to 15 per cent of the kernel, the germ about 2.5 to 3 per cent and the endosperm about 82 to 83 per cent. The wheat kernel has a deep crease along its ventral side, which makes it impossible to remove the bran by abrasion as is done for rice, and is the reason wheat is milled by grinding and sieving.
Composition differs by tissue. The endosperm is mostly starch (about 70 to 75 per cent of the kernel overall) with storage protein. The bran and aleurone hold most of the fibre, minerals, B vitamins and phytate. The germ is rich in oil, protein, vitamin E and enzymes, and its oil becomes rancid quickly in whole-grain flour. Wheat protein usually lies between about 8 and 15 per cent, depending on variety and growing conditions. About four-fifths of it is gluten-forming protein: gliadins, monomeric proteins that give viscosity and extensibility, and glutenins, polymeric proteins linked by disulphide bonds that give elasticity and strength; the high-molecular-weight glutenin subunits are closely linked to bread-making quality. The chemistry of these proteins and of starch is treated in food-chemistry-major-constituents.
Wheat types are classified by hardness, colour and season. Hardness, controlled mainly by the puroindoline genes, decides how the endosperm breaks in milling: hard wheats fracture along cell boundaries into gritty particles with more damaged starch and suit bread, while soft wheats give fine flour suited to biscuits and cakes. Common or bread wheat (Triticum aestivum) is hexaploid; durum wheat (Triticum durum), tetraploid and very hard with yellow endosperm, is milled into semolina for pasta.
Rice is harvested as paddy, in which the grain is enclosed by a hull or husk that makes up about 20 per cent of its weight. Removing the husk gives brown rice; removing bran and germ gives white rice, with a total milled yield commonly around 70 per cent of paddy. Rice protein is low, about 7 per cent, but of relatively good quality, and its starch varies in amylose content, which determines cooking texture (chapter 5).
The maize kernel is large and flat, with a large germ, about 10 to 12 per cent of the kernel and rich in oil, and an endosperm divided into hard, translucent horny endosperm and soft floury endosperm. Maize protein, zein, is low in lysine and tryptophan, and maize-dependent diets without alkaline processing were associated with pellagra because niacin in maize is bound and poorly available. Barley and oats are usually harvested with their hulls attached; barley is valued for malting (chapter 7) and both contain mixed-linkage β-glucans (chapter 8). Sorghum and the millets, including pearl millet, finger millet and a group of small millets, are drought-tolerant cereals of semi-arid Africa and Asia and are discussed in chapter 9. Whole grains of all cereals are good sources of fibre, B vitamins, minerals and phytochemicals, much of which is lost when they are refined, which is why refined flours are often enriched or fortified.
The aim of wheat milling is to separate the endosperm from bran and germ as completely as possible and to reduce it to flour of the required fineness, with the highest yield consistent with the required quality. The mill flow begins with cleaning: removal of stones, other seeds, chaff, metal and dust by separators, aspirators, magnets, destoners and scourers, sometimes with colour sorters to remove discoloured or infected kernels. The equipment is described in the Agricultural Engineering subject post-harvest-cereals-pulses-oilseeds.
Conditioning, or tempering, adds water to wheat and holds it for a set time, commonly 8 to 24 hours depending on hardness, so that the bran becomes tough and leathery and stays in large flakes while the endosperm becomes more friable and separates cleanly. Hard wheats are usually tempered to a higher moisture and for longer than soft wheats; typical targets are in the range of about 15 to 17 per cent. The water to add is calculated from a mass balance on dry matter: water = W × (Mf − Mi)/(100 − Mf), where W is the mass of wheat and Mi and Mf the initial and final moisture contents on a wet basis. To raise 1,000 kg of wheat from 11 to 16 per cent moisture, the water needed is 1,000 × 5/84, about 59.5 kg. Warm or hot conditioning shortens the time needed.
Milling proper is gradual reduction. The break system uses pairs of corrugated (fluted) rolls running at different speeds, typically four or five break passages, to open the kernels and scrape endosperm from the bran with as little bran powdering as possible. After each break, plansifters (stacks of sieves) separate the stock into bran flakes going to the next break, coarse endosperm particles called semolina and middlings, fine flour, and intermediate fractions. Purifiers use sieves with rising air currents to separate pure endosperm particles from those still attached to bran, by size and density. The reduction system then grinds the purified middlings to flour on smooth rolls, in several passages, with sifting after each. Germ, which flattens rather than breaks, is removed as flakes in the early reduction passages.
A modern mill produces many flour streams, each with its own ash, protein, colour and damaged starch. Streams from the head of the reduction system are the purest and whitest; those from late break and reduction passages contain more bran and are darker. The miller blends streams to make products: straight-run flour combines all streams; patent flours use the best streams; clear flours are the remainder. By-products are bran, shorts or pollard and germ, used mainly for feed. The extraction rate is the percentage of flour obtained from the wheat: if 740 kg of flour is obtained from 1,000 kg of cleaned wheat, extraction is 74 per cent. White flour usually comes from extraction rates around 70 to 78 per cent; wholemeal flour approaches 100 per cent. In India, maida is refined white flour, suji or rawa is semolina, and atta is whole-wheat flour, traditionally ground in stone mills (chakki), whose high damaged starch and fine bran give the water absorption and softness wanted for chapati.
Protein content is the first measure of flour quality. It is determined by the Kjeldahl or Dumas methods as nitrogen and multiplied by a conversion factor; the factor 5.7 is conventionally used for wheat flour, rather than the general 6.25, because wheat proteins contain more nitrogen. A flour with 2.1 per cent nitrogen therefore contains 2.1 × 5.7 = 12.0 per cent protein. Results are usually expressed on a stated moisture basis, commonly 14 per cent, so that flours can be compared. Near-infrared spectroscopy, calibrated against these reference methods, gives rapid results in mills. Bread flours usually have about 11 to 13 per cent protein or more, biscuit and cake flours about 7 to 10 per cent.
Quantity is not quality. Two flours of the same protein content can perform very differently, because the composition of the gluten proteins differs. Wet gluten is measured by washing a dough with salt solution to remove starch and solubles, leaving a cohesive gluten ball; automated washing instruments also give a gluten index, the proportion of wet gluten retained on a sieve after centrifuging, which indicates gluten strength. The sodium dodecyl sulphate (SDS) or Zeleny sedimentation test measures the swelling of flour particles in a weak acid or detergent solution; a larger sediment volume indicates stronger gluten. Damaged starch, produced by milling hard wheat, absorbs more water and is more readily attacked by amylase; it raises water absorption and gas production but excess makes dough sticky.
The falling number test (Hagberg-Perten) measures α-amylase activity. A suspension of flour or ground wheat in water is heated in a boiling water bath and stirred, so the starch gelatinises into a viscous paste, and the time in seconds for a plunger to fall through it is recorded. High α-amylase breaks down the starch quickly, lowering viscosity and giving a short falling number. Wheat that has begun to sprout in the ear because of rain at harvest (pre-harvest sprouting) has high α-amylase and a low falling number, and gives sticky crumb and poor bread. Bread millers commonly look for falling numbers of about 250 seconds or more; very high values show little enzyme, which may be corrected with malted flour or fungal amylase. Specifications vary by product and market.
Ash is the residue after incineration and reflects the mineral content. Because minerals are concentrated in bran and aleurone, ash measures the separation of endosperm from bran and therefore flour refinement and grade; white flours commonly have ash of about 0.4 to 0.6 per cent, while wholemeal flours are much higher. Ash is reported on a stated moisture basis: 0.50 per cent ash at 14 per cent moisture equals 0.50/0.86 = 0.58 per cent on a dry basis. Flour colour, measured by reflectance instruments, reflects bran specks and yellow pigments; some countries permit bleaching agents and some do not. Other tests include moisture, particle size, wet and dry gluten, the amylograph or rapid viscoanalyser for pasting behaviour, and the bake test, which remains the final judge of flour performance.
Dough rheology tests measure how a flour-water dough behaves under mixing and stretching, which predicts processing and baking performance. The farinograph mixes flour and water in a temperature-controlled bowl and records the resistance of the dough to the mixing blades, in Brabender units (BU), against time. Water is added to bring the peak of the curve to 500 BU; that amount, expressed as a percentage of flour at 14 per cent moisture, is the farinograph water absorption. From the curve are read the dough development time (time to peak), stability (time the curve stays above 500 BU), the mixing tolerance index or degree of softening (fall of the curve after the peak) and other indices. Strong bread flours show high absorption, longer development time and long stability; weak flours develop quickly and break down quickly.
The mixograph uses a small sample in a pin mixer and records a similar curve, useful in plant breeding where samples are small. Combined instruments now measure both mixing behaviour and, by heating the dough, protein weakening, starch gelatinisation and retrogradation in one test. These tests are empirical: their results depend on the instrument and conditions, which are standardised by methods of bodies such as AACC International and ICC.
The extensograph prepares a salted dough at farinograph absorption, shapes it into a cylinder, rests it for set times (commonly 45, 90 and 135 minutes) and stretches it with a hook until it breaks, recording force against extension. The resistance to extension (height of the curve), the extensibility (length) and the area under the curve, a measure of energy, describe the dough's strength and its response to resting and to oxidising improvers. The ratio of resistance to extensibility indicates balance: bread doughs need both strength and enough extensibility.
The alveograph (Chopin) rolls a sheet of salted dough, cuts discs and inflates each with air into a bubble until it bursts, mimicking the biaxial extension of a dough around expanding gas cells. It records pressure against time and gives P, the maximum overpressure, related to tenacity or resistance; L, the length of the curve, related to extensibility; their ratio P/L, which describes the balance; and W, the deformation energy, in units of 10⁻⁴ joules, an overall measure of baking strength. Bread flours typically have higher W than biscuit flours, and biscuit flours favour low P/L and extensible doughs. The alveograph is widely used in Europe and in the international wheat trade, while the farinograph and extensograph are common elsewhere.
Rice milling removes husk, then bran, from paddy; the equipment and flow, from cleaning through husking, paddy separation, whitening and polishing, and the yields of each stage, are covered in post-harvest-cereals-pulses-oilseeds. Two yields matter commercially. Total milled rice is the white rice from a given weight of paddy, commonly around 65 to 70 per cent. Head rice is milled rice with kernels at least three-quarters of the original length; broken rice sells at a lower price. If 1,000 kg of paddy gives 700 kg of milled rice of which 560 kg is head rice, the head rice yield is 56 per cent of paddy and 80 per cent of the milled rice. Breakage arises mainly from fissures caused by rapid moisture changes in the field and during drying, as well as from immature grains, chalkiness and harsh milling. The degree of milling, how much bran is removed, affects appearance, nutrient content and storage stability.
Rice quality is judged by physical and cooking properties. Physical traits include grain length, shape and chalkiness. Cooking and eating quality depend mainly on amylose content: waxy rice has almost no amylose and is sticky; low-amylose rice (about 10 to 20 per cent) is soft and moist; intermediate-amylose rice (about 20 to 25 per cent) is soft and fluffy, preferred in much of Asia; high-amylose rice (above 25 per cent) cooks dry and flaky and hardens on cooling. Gelatinisation temperature, estimated by the alkali spreading value, affects cooking time; gel consistency indicates the firmness of cooked rice. Aromatic rices such as basmati and jasmine owe their aroma mainly to 2-acetyl-1-pyrroline; basmati is also valued for long, slender grains that elongate greatly on cooking.
Parboiling is a hydrothermal treatment of paddy before milling: soaking, steaming and drying. The methods and their engineering are described in post-harvest-cereals-pulses-oilseeds. The changes it produces are the subject here. Soaking hydrates the grain; steaming gelatinises the starch, which swells and fills air spaces, cementing cracks and fissures; slow drying and tempering then set the structure. The results are grains that are harder, more translucent and resistant to breakage, so head rice yield rises; water-soluble vitamins, especially thiamine, and minerals migrate from the bran and aleurone into the endosperm, so parboiled rice retains more of them after milling; the grain becomes more resistant to insects; and cooked grains stay separate and firm. Disadvantages are a darker colour and characteristic flavour that some consumers dislike, energy and water use, and the risk of off-odours and mycotoxin growth if soaking water is not managed. Parboiled rice is the main form eaten in much of South Asia and parts of Africa.
The by-products of rice milling are husk and bran. Husk, about one-fifth of the paddy, is mostly cellulose, lignin and silica; it is used as boiler fuel in rice mills and parboiling plants, and its ash, high in silica, has uses in cement, steel and refractory industries. Bran, with germ, contains oil, protein, fibre, vitamins and phytochemicals, but is unstable, as described in chapter 10. Broken rice is used for flour, brewing, starch and snacks.
Maize dry milling separates the kernel into endosperm products, germ and bran without steeping. In the tempering-degermination process, maize is tempered to raise moisture, and a degerminator removes germ and pericarp while breaking the endosperm into large particles; the stock is dried, cooled and separated by roller milling, sifting and aspiration into grits of various sizes, meal, flour, germ and bran. Large flaking grits from horny endosperm are used for corn flakes; smaller grits for snacks, brewing adjuncts and polenta. Germ is pressed or solvent-extracted for oil. Non-degerminated whole meal is also produced by simple grinding but stores poorly because of the germ oil.
Nixtamalisation is the traditional Mesoamerican process of cooking maize in lime (calcium hydroxide) solution, steeping, washing and grinding into masa for tortillas and other foods. It loosens the pericarp, adds calcium, improves the availability of niacin, which explains why populations eating nixtamalised maize avoided pellagra, and changes flavour and dough properties. It also reduces some mycotoxins, as noted in food-toxicology-contaminants-allergens.
Wet milling separates maize into starch, protein, fibre and germ, and is the basis of the starch and sweetener industry. Cleaned maize is steeped in warm water containing sulphur dioxide, commonly at about 50 °C for 30 to 50 hours; the SO2 and the lactic acid bacteria that grow in the steep break the disulphide bonds of the protein matrix around the starch granules, soften the kernel and limit unwanted microbial growth. The steep water, rich in soluble nutrients, is concentrated and used in feed and as a fermentation nutrient. The steeped kernels are coarsely ground to free the germ, which is separated by hydrocyclones because its oil makes it lighter, then finely ground; fibre is removed by screens, and starch is separated from protein (gluten) by centrifuges and hydrocyclones, using the density difference. Products are starch, corn gluten meal (a high-protein feed), corn gluten feed (fibre with steep liquor) and germ, from which oil is extracted.
Starch is converted into syrups by hydrolysis. Acid hydrolysis was the original method; enzymatic processes now dominate. Liquefaction with thermostable α-amylase at high temperature breaks gelatinised starch into dextrins; saccharification with glucoamylase produces glucose syrups; glucose isomerase converts part of the glucose to fructose, giving high-fructose syrups, commonly with about 42 per cent fructose, enriched to 55 per cent or higher by chromatography. Dextrose equivalent (DE) expresses reducing power as a percentage of that of pure glucose: starch has a DE near zero, maltodextrins below 20, glucose syrups 20 or above, and pure dextrose 100. Higher DE syrups are sweeter, more hygroscopic and brown more readily. Starch hydrolysates are also fermented to ethanol and organic acids. Other starches, from cassava, potato, wheat and rice, are produced by similar wet processes adapted to each raw material.
Malting is the controlled germination of cereal grain, stopped by drying, to develop enzymes and modify the endosperm. Barley is the main malting cereal because its husk protects the shoot during germination and later forms a filter bed in brewing, and because it develops high enzyme activity. Wheat, sorghum, finger millet and other grains are also malted.
The process has three stages. Steeping raises grain moisture to about 42 to 46 per cent, usually by alternating periods of immersion in water and air rests, to supply water and oxygen for the embryo. Germination follows at about 15 to 20 °C for about four to six days, with turning and humidified air. The embryo releases gibberellins that stimulate the aleurone layer to synthesise hydrolytic enzymes, including α-amylase, β-glucanases, proteases and, to some extent, β-amylase, which is also present in the ungerminated grain. These enzymes break down cell walls and the protein matrix of the endosperm, a change called modification, making it friable. Gibberellic acid is sometimes added to speed the process where permitted. Kilning dries the green malt to a low moisture with gradually increasing temperatures, stopping growth, preserving enzymes in pale malts and developing colour and flavour through Maillard reactions, more strongly in darker malts. Rootlets are removed and used as feed. Malting loss, from respiration and rootlets, is commonly several per cent of the dry weight.
Malt quality is measured by extract (the soluble material obtained in a standard mash), diastatic power (combined starch-degrading activity of α- and β-amylase), modification (assessed by friability, β-glucan content and the difference between fine and coarse grind extracts), soluble nitrogen, colour and moisture. Brewing malts need high extract, adequate diastatic power to convert starch in the mash, low β-glucan to avoid filtration problems and haze, and appropriate protein. Barley for malting is selected for plump, uniform kernels, high germination energy and moderate protein.
Malt is used in brewing and distilling, in malt extracts and malted milk beverages, in breakfast cereals and confectionery, and as a source of enzymes in baking, where small amounts of malted wheat or barley flour correct low amylase activity in high falling number flours (chapter 3). In India, malted finger millet (ragi) flour is a traditional weaning and health food, and malted grains are used in commercial infant foods and nutrition drinks. Sorghum malting is the basis of traditional African beers and of opaque and clear lager beers made without barley; sorghum malt has lower β-amylase and needs adapted mashing.
Oats are harvested with hulls, which make up roughly a quarter to a third of the grain weight and are removed to give groats. Oat groats differ from other cereals in having a higher lipid content, commonly about 5 to 9 per cent, and active lipase and lipoxygenase enzymes. If groats are damaged or milled without treatment, lipase releases free fatty acids within days, giving a bitter, soapy taste. Oat milling therefore includes a heat treatment, kilning or steaming, that inactivates these enzymes before further processing, and also develops a toasted flavour.
The process includes cleaning and grading, dehulling, usually by impact dehullers that throw the grain against a hard surface, aspiration to remove hulls, kilning or steaming, and then cutting or flaking. Steamed whole groats are flattened between rollers to give rolled oats; groats cut into pieces first give thinner, quicker-cooking flakes; instant oats are thinner still and may be pre-cooked. Oat bran is the outer fraction of the groat, concentrated in β-glucan. Hulls are used as fibre, fuel or feed. Oats contain avenins rather than gliadins; most people with coeliac disease tolerate oats that are free from contamination with wheat, barley or rye, and national rules differ on whether such oats can be labelled gluten-free.
Cereal β-glucans are mixed-linkage polysaccharides of glucose joined by β-(1→3) and β-(1→4) bonds, found in the cell walls of the endosperm and aleurone of oats and barley. They form viscous solutions, which explains both their technical effects (viscosity in porridge, gums in brewing) and their physiological effects. In the gut, viscous β-glucan slows absorption and increases excretion of bile acids, which lowers blood LDL cholesterol, and it reduces the rise in blood glucose after a meal.
These effects are the basis of approved health claims in several jurisdictions. The US FDA authorised a claim in 1997 linking soluble fibre from whole oats to reduced risk of coronary heart disease, based on an intake of 3 g of β-glucan per day, and later extended it to barley. The European Union has authorised claims that oat and barley β-glucan contribute to maintaining normal blood cholesterol and to reducing the post-prandial blood glucose rise, under conditions set in the claims register. The molecular weight and solubility of β-glucan, which processing such as heat, enzymes and freezing can reduce, determine its viscosity and physiological effect, so processing must preserve them for claims to hold. Claims and conditions differ by country, as discussed in food-law-regulatory-affairs.
Millets are small-seeded cereals adapted to dry, hot climates and poor soils. The major millets are sorghum (jowar) and pearl millet (bajra); finger millet (ragi) is important in eastern Africa and southern India; and the small millets include foxtail, proso, kodo, little, barnyard and browntop millets. The United Nations declared 2023 the International Year of Millets at India's proposal, and India promotes them as nutri-cereals, also called Shree Anna.
Nutritionally, millets compare well with refined rice and wheat. Finger millet is notably rich in calcium, about 300 to 350 mg per 100 g, far more than other cereals. Pearl millet is relatively rich in iron, zinc and fat. Most millets supply dietary fibre, B vitamins and polyphenols, and are gluten-free. They also contain anti-nutritional factors, such as phytate, tannins in some coloured sorghums and finger millets, and goitrogenic flavonoids in pearl millet, which reduce mineral availability; processing reduces many of them, as described for anti-nutrients in food-toxicology-contaminants-allergens.
The main technological problem is shelf life. Pearl millet flour becomes rancid and bitter within days of milling, because grinding brings its relatively high fat into contact with active lipase. Treatments that inactivate the enzyme before or after milling, such as hydrothermal treatment, dry heat, blanching of grain, or acid treatment, extend shelf life considerably, as does decortication, which removes much of the lipid-rich germ and bran.
Traditional and modern processes adapt millets for convenience and acceptability. Decortication (dehulling) removes the outer layers by abrasion, improving colour and cooking quality but removing some fibre and minerals; it is essential for small millets, whose grains have tightly adhering husks. Milling gives flours for flatbreads such as jowar and bajra rotis and ragi mudde. Malting, described in chapter 7, is applied especially to finger millet for weaning foods. Popping and puffing, by rapid heating of moistened grain, give ready-to-eat snacks; flaking gives quick-cooking flakes; fermentation is used in porridges, beverages and the batter foods of southern India and Africa, and improves mineral availability by degrading phytate.
Newer products include extruded snacks and breakfast cereals, pasta and noodles made from millet blends, bakery products in which millet flour replaces part of the wheat, multigrain flours and ready-to-cook mixes. Because millets lack gluten, breads and noodles made largely from millet need hydrocolloids, pregelatinised starch or blending with wheat, as discussed in bakery-pasta-snacks. Barriers to wider use include the labour needed to dehull small millets, short shelf life of flours, limited processing machinery at small scale, and consumer perceptions of millets as coarse grains; public procurement, inclusion in public distribution and school meal programmes, and product development are aimed at overcoming them.
Rice bran, with the germ, makes up roughly 8 to 10 per cent of paddy and contains about 15 to 20 per cent oil, together with protein, dietary fibre, B vitamins, tocopherols and tocotrienols, and γ-oryzanol. In the intact grain its lipids are separated from its lipase, but milling brings them together. Free fatty acids then accumulate rapidly, within hours to days depending on temperature and moisture, making the oil expensive to refine because of high refining losses, and making the bran bitter and unfit for food. Lipoxygenase adds oxidative rancidity. Stabilisation inactivates these enzymes immediately after milling, so that bran can be stored and used for edible oil and food.
Stabilisation methods include dry heat, for example in rotary dryers or by extrusion cooking, in which bran is heated rapidly to well above 100 °C under pressure and then released; moist heat or steaming followed by drying; microwave and infrared heating; ohmic heating; and chemical treatments such as acidification, which lower lipase activity but may leave residues. Heat treatment must be severe enough to inactivate lipase effectively, since partial inactivation allows activity to recover during storage, and moisture and temperature during later storage still matter. Parboiled rice bran is partly stabilised by the parboiling process itself, which is one reason why oil from parboiled bran has lower free fatty acids.
Rice bran oil is extracted, mainly by solvent extraction, from stabilised bran and refined. It is valued for its high smoke point, balanced fatty acid profile and content of γ-oryzanol, a mixture of ferulic acid esters of sterols and triterpene alcohols that has been associated with cholesterol lowering, and of tocotrienols. Refining requires care, because oryzanol is lost in alkali refining and the oil's waxes must be removed by dewaxing; physical refining is often preferred. India is among the largest producers of rice bran oil.
Defatted rice bran is used as animal feed and as a source of protein, dietary fibre, phytic acid and phytin; rice bran protein concentrates and fibre ingredients are made for food use. Full-fat stabilised bran is used in health foods, bakery products and snacks. The husk, as noted in chapter 5, is a fuel for boilers and power generation; burning it in controlled conditions gives rice husk ash, rich in amorphous silica, valued as a pozzolana in cement and concrete and in other industries. Broken rice is used for rice flour, brewing adjuncts, rice starch, snacks and noodles. Using every fraction of the paddy is the basis of the economics of a modern rice-processing complex, and is examined in GATE under waste utilisation.
Native starches have limitations in food processing. Their pastes may break down under shear, acid or prolonged heating; they may retrograde and weep water (syneresis) on cooling and freezing; they may need cooking to develop viscosity; and their gels may be too rubbery or too weak for a given product. Modification changes the structure of the starch granule or molecules to give properties a product needs. The structure of amylose and amylopectin, and the processes of gelatinisation, pasting and retrogradation, are described in food-chemistry-major-constituents; this chapter covers how they are altered.
Physical modifications use heat and water without chemicals and are labelled simply as starch in many jurisdictions. Pregelatinised starch is cooked and dried, often on drum dryers, so that it swells in cold water, as used in instant puddings and some bakery mixes. Heat-moisture treatment, heating starch at limited moisture above its gelatinisation temperature, and annealing, holding it in excess water below the gelatinisation temperature, reorganise the granule, raising gelatinisation temperature and stability and, in some cases, increasing resistant starch. Such 'clean label' starches, which mimic chemically modified starches functionally, are increasingly used.
Chemical modifications are regulated as food additives, listed with INS numbers in the 1400 series, and their permitted reagents, levels and degrees of substitution are specified. Acid thinning partially hydrolyses starch, lowering hot paste viscosity so that high concentrations can be used for gum confectionery. Oxidation, for example with hypochlorite, gives low-viscosity starches that form clear, soft gels, used in coatings and batters. Cross-linking, by reagents such as sodium trimetaphosphate or phosphorus oxychloride, forms bridges between starch chains, strengthening the swollen granule so that the paste resists acid, shear and high temperature, as needed in canned and retorted foods and acidic fillings. Substitution, by esterification (acetylation) or etherification (hydroxypropylation), adds groups that hinder chain association, reducing retrogradation and syneresis and improving freeze-thaw stability, as needed in frozen foods. The degree of substitution (DS) is the average number of hydroxyl groups substituted per glucose unit, which has three available; food starches have low DS. Many commercial starches combine cross-linking with substitution. Starch octenyl succinate, made by reaction with octenyl succinic anhydride, has both hydrophilic and hydrophobic groups and is used as an emulsifier and encapsulating agent.
Enzymatic modification produces maltodextrins and syrups (chapter 6), cyclodextrins, cyclic oligosaccharides formed by cyclodextrin glycosyltransferase that encapsulate flavours, and products with increased resistant starch, such as debranched and retrograded starches, which act as dietary fibre. Native starches from different sources also differ markedly: waxy starches, almost all amylopectin, give clear, non-gelling pastes with low retrogradation, while high-amylose starches gel strongly and are sources of resistant starch.
Ready-to-eat breakfast cereals grew from health-food movements in the late nineteenth century United States and are now produced worldwide. The main types are flaked, puffed, shredded, extruded and granola-type products, with many coated or fortified variants. In the traditional corn flake process, large maize grits are pressure-cooked with sugar, malt, salt and flavourings until gelatinised, dried to a suitable moisture, tempered so that moisture equilibrates, flaked between heavy rolls and toasted, which dries, blisters and browns the flakes. Wheat and rice flakes are made similarly from whole or cut grain.
Puffed cereals are made by sudden release of pressure or by rapid heating: in gun puffing, grain is heated in a sealed chamber until internal steam pressure builds, and the chamber is opened suddenly, so that water flashes to steam and expands the grain; in oven puffing, rice that has been cooked and dried expands in a hot oven. Shredded wheat is made from whole wheat that is cooked, tempered and passed between a smooth roll and a grooved roll to form strands, which are layered, cut and baked. Granola and muesli are mixtures of flaked grains, nuts and dried fruit, baked with sugar and fat in granola, raw in muesli.
Extrusion cooking has largely replaced batch processes for many cereals and snacks. A blend of flours, starches and minor ingredients at relatively low moisture is fed into a screw extruder, where mechanical shear and heat raise the temperature well above 100 °C under pressure within seconds to minutes, gelatinising starch and denaturing protein to form a viscous melt. As the melt leaves the die, the sudden drop in pressure makes the superheated water flash to steam, expanding the product, which is then cut, dried or toasted and often coated. Expansion and texture depend on feed moisture, temperature, screw speed and configuration, die geometry and the composition of the feed, especially starch type and the amounts of fibre, protein, sugar and fat. Specific mechanical energy, the mechanical work per unit mass of product, is a key control variable. Extrusion also inactivates enzymes and anti-nutrients and reduces microbial load, but heat-sensitive vitamins are lost and Maillard reactions reduce lysine availability, so vitamins are often added after extrusion by coating. Extruded snacks are covered in bakery-pasta-snacks.
Instant and quick-cooking cereal products are pre-cooked and dried so that they need only rehydration or brief cooking: instant oats, instant rice (cooked and dried so that the grain is porous), instant porridges and infant cereals, which are often made on drum dryers or by extrusion from cereal flours, sometimes with milk solids and malted flour, and fortified with iron and vitamins. Fortification is common in breakfast cereals and is regulated as to nutrients and levels, with labelling under nutrition labelling rules. The high sugar content of some cereals marketed to children has led to front-of-pack labelling and marketing restrictions in several countries, discussed in food-law-regulatory-affairs.
A student uses this subject to learn grains from the inside out: which tissue holds which nutrient, why wheat is milled by grinding and rice by abrasion, what each quality test measures and how to read a farinograph or alveograph. The calculations are short but recur: tempering water, extraction rate, head rice yield, protein from nitrogen with the 5.7 factor, and moisture-basis conversion of ash. Visiting a flour mill and a rice mill, and running a falling number and a gluten wash in the laboratory, fixes the ideas.
Five years into a flour mill, rice mill, starch plant or breakfast cereal factory, a technologist sets tempering conditions for each wheat mix, blends flour streams to customer specifications, runs and interprets protein, falling number, gluten and rheology tests, investigates complaints such as sticky dough or poor biscuit spread, controls head rice yield and parboiling quality, keeps rice bran stabilised for the oil mill, and adjusts extrusion settings to hold product density and texture.
A senior milling or technical manager with twenty-five years' experience buys grain against quality and price, designs mill flows and expansions, sets product ranges and specifications for bakery, biscuit and noodle customers, manages by-product value chains such as bran oil and husk power, and leads product development in whole grains, millets and fortified foods, while meeting food law and export requirements.
A researcher at this level works on the grain itself and its processing: genetics of gluten proteins and hardness, starch structure and its digestibility, resistant starch and β-glucan for health, reduction of breakage and energy use in rice processing, stabilisation and valorisation of bran, millet processing for shelf-stable convenience foods, and modelling of extrusion. Such work links plant breeding, chemistry, engineering and nutrition.
This manual focuses on grain chemistry and product quality; mill machinery, drying, husking and whitening equipment and their engineering calculations are in post-harvest-cereals-pulses-oilseeds, and starch and protein chemistry in food-chemistry-major-constituents. It does not give national grain grading standards, flour specifications or exact falling number and ash limits, which vary by country, customer and product. Pulses, oilseeds and their milling are covered in pulses-oilseeds-plant-proteins. Detailed extruder engineering and brewing technology are only introduced.
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