Cereal Grains: Composition, Properties, and Processing
Cereal Grains: Composition and Properties
Grasses: Cereals are cultivated grasses; members of the monocotyledonous family Poaceae. Do not confuse them with pulses, from legumes. Fruit (grain) is developed after pollination. Pseudo-cereals: quinoa, amaranth.
Cereal Grain Structure
Embryo: 2 to 10%; embryonic axis (EA) and scutellum (S). EA: primordial roots and shoots with leaf initials. S: a secretory and absorptive organ; means to connect with endosperm; hormones and enzymes during germination. Plumule and radicle developed from there.
Endosperm: 75 to 90%; starchy endosperm (SE) and aleurone (A). SE: starch, protein content at periphery. A: one to three layers; high protein content, rich in lipids; cells remain alive after harvest. Transfer cells and cells adjacent to the embryo.
Seed coats: Nucellus, remains: where endosperm and embryo developed after fertilization; it may also include cuticle on the outer surface. Testa: the outermost tissue; it reduces permeability and accumulates pigments; cuticle that regulates water and gas exchange.
Pericarp: Dry at maturity stage; mainly empty cells. It serves to protect and support the growing endosperm and embryo. The inner epidermis; tubes cells >> Cross cell layers >> mesocarp, not found as a true layer >> epicarp, cuticle. // In some cereal grains, such as oats, barley, rice, hulled wheat species and some millets, the lemma and palea are not removed by threshing.
Nutritional Composition of Cereal Grains
Carbohydrates: 65 to 75% DM; mostly starch. Polymers of glucoses: amylose, linear chain with ⍺-(1-4) links; amylopectin, branch points with ⍺-(1-6) links. Occurs as solid granules, 30 μm; clear bodies but white due to light scattering in starch-air interface.
Protein: 8 to 11% DM; types: albumins and globulins (soluble); prolamins and glutenins (insoluble). Soluble fractions: aleurone and embryo tissues of cereals; enzymes; account 20%. Insoluble fractions: endosperm, particularly deficient in lysine, high content of proline and glutamine. Wheat gluten: glutenins and gliadins (prolamins). Gliadin fractions have little elasticity and are less cohesive than glutenins; they contribute mainly to the viscosity and extensibility of dough. Glutenin fractions are both cohesive and elastic, and are responsible for dough strength and elasticity. A proper mixture (2:1) of the two is essential to give desirable dough and bread properties.
Lipids: 1 to 17% DM; stored as triacylglycerols. The highest levels occur in aleurone and scutellar tissue; about 10% store in cereal endosperm. Lipids are rich in unsaturated fatty acids, linoleic (18:2); some saturated fatty acids, palmitic (16:0). Oats 6 to 8% lipids content, oleic acid (18:1). Key factor for dough liquor/gas cell interface in baking.
Specific Cereal Grains
Maize: Sweetcorn and popcorn types are used primarily as human food. Starch, from wet milling process. Corn syrups, as sweeteners. A feed grain & biofuel. The main deficiencies: lysine and tryptophan content.
Quinoa: Fruits are disc-like and 1–3 mm in diameter. Quinoa pericarp is rich in saponins, which are considered to be antinutritional. Twice protein content of rice or barley; all essential amino acids are presented; able to growth on poor quality soil conditions.
Dough Making Process
Dough making:
- Formation of a gluten network, air bubbles within it.
- To turn the gluten network into a foam by CO2.
- Gas retention and dough expansion.
- To stabilize, structure coagulation by heating.
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Gluten, protein hydration provides elasticity, strength and stability. • Gassing properties: high amylase and damaged starch to yield sugars. Moisture content -14% safe storage. Compressed, cream, pellets or active powders. Quantity depends on fermentation parameters: time and T. Effects: providing CO2, bubble expansion, lowering pH, alcohol production, aroma and flavor. Sodium bicarbonate as source of CO2 gas, which may be liberated by the action of sodium acid phosphate, monocalcium phosphate, sodium aluminum phosphate or glucan delta lactone. Salt to develop flavor. Because it slows down fermentation, it might be added until dough is partly fermented. Fat improves loaf volume, reduces crust toughness and gives thinner crumb cell wall. Sugar to develop a sweet flavor.
Water absorption: 50 to 60 parts per 100 parts of flour to achieve standard consistency, it differs by protein, starch granules content.
Fermentation: The starch to maltose by the amylase enzymes, the maltose to glucose by maltase, glucose and fructose to CO2 and alcohol by the zymase complex.
Other Important Cereal Grains
Rice: Types: long-, short-grain; perfumes. INIAP varieties: 14, 17. Mainly eaten cooked as whole grains after removal of hulls (as brown rice); as white, or polished, rice, to remove pericarp. Rice flour; feed grain.
Wheat: Protein content varies considerably among types. Hard or soft: the way the starchy endosperm behaves during milling. Degree of adhesion between the starch granules and their surrounding protein matrix.
Material Handling Equipment
Bucket conveyors consist of endless chains or belts to which buckets are attached to transport bulk material in horizontal, inclined and vertical paths. The buckets remain in the transport position until they are tilted to discharge the material. Several unloading mechanisms are available.
Screw conveyors consist of rotating shafts with continuous or broken spiral blades that operate inside a casing. Powered by an electric motor and a suitable gear, the screw conveyor generally operates in one direction only to move fine bulk material such as flour, seeds and coal.