Cereal Grains: Structure, Composition, and Dough Making
Grasses: Cereals are cultivated grasses, members of the monocotyledonous family Poaceae. Do not confuse them with pulses, which are legumes. The fruit (grain) develops after pollination. Pseudo-cereals include quinoa and amaranth.
Grain Structure
Embryo: 2 to 10% of the grain. It consists of the embryonic axis (EA) and scutellum (S). The EA contains primordial roots and shoots with leaf initials. The S is a secretory and absorptive organ that connects with the endosperm and releases hormones and enzymes during germination. The plumule and radicle develop from the EA.
Endosperm: 75 to 90% of the grain. It consists of starchy endosperm (SE) and aleurone (A). The SE is rich in starch, with protein content at the periphery. The A is one to three layers thick, has a high protein content, is rich in lipids, and its cells remain alive after harvest. Transfer cells and cells adjacent to the embryo are also present.
Seed Coats: The nucellus remains where the endosperm and embryo developed after fertilization, and may include a cuticle on the outer surface. The testa is the outermost tissue, which reduces permeability, accumulates pigments, and has a cuticle that regulates water and gas exchange.
Pericarp: Dry at maturity, mainly composed of empty cells. It protects and supports the growing endosperm and embryo. It consists of the inner epidermis, tube cells, cross cell layers, mesocarp (not a true layer), epicarp, and 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.
Grain Composition
Carbohydrates: 65 to 75% DM (dry matter), mostly starch. Starch is a polymer of glucose, including amylose (linear chains with α-(1-4) links) and amylopectin (branch points with α-(1-6) links). Starch occurs as solid granules, approximately 30 μm in size, which appear clear but white due to light scattering at the starch-air interface.
Protein: 8 to 11% DM. Types include albumins and globulins (soluble) and prolamins and glutenins (insoluble). Soluble fractions are found in the aleurone and embryo tissues of cereals and include enzymes, accounting for about 20% of the total protein. Insoluble fractions are found in the endosperm and are particularly deficient in lysine but high in proline and glutamine. Wheat gluten consists of 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 for desirable dough and bread properties.
Lipids: 1 to 17% DM, stored as triacylglycerols. The highest levels occur in aleurone and scutellar tissue, with about 10% stored in cereal endosperm. Lipids are rich in unsaturated fatty acids, such as linoleic acid (18:2), and some saturated fatty acids, such as palmitic acid (16:0). Oats contain 6 to 8% lipids, including oleic acid (18:1). Lipids are a key factor for the dough liquor/gas cell interface in baking.
Specific Grains
Maize: Sweetcorn and popcorn types are used primarily as human food. Starch is extracted from maize through wet milling. Corn syrups are used as sweeteners. Maize is also used as a feed grain and for biofuel production. The main deficiencies are lysine and tryptophan content.
Rice: Types include long-grain, short-grain, and perfumed varieties. INIAP varieties include 14 and 17. Rice is mainly eaten cooked as whole grains after removal of hulls (as brown rice) or as white, or polished, rice, to remove the pericarp. Rice flour is also used, and rice is used as a feed grain.
Wheat: Protein content varies considerably among types. Hard or soft wheat is determined by how the starchy endosperm behaves during milling, specifically the degree of adhesion between the starch granules and their surrounding protein matrix.
Quinoa: Fruits are disc-like and 1–3 mm in diameter. The quinoa pericarp is rich in saponins, which are considered antinutritional. Quinoa has twice the protein content of rice or barley, contains all essential amino acids, and can grow on poor-quality soil conditions.
Dough Making
The process of dough making involves:
- Formation of a gluten network with air bubbles within it.
- Turning the gluten network into a foam by CO2 production.
- Gas retention and dough expansion.
- Stabilizing the structure through coagulation by heating.
- Gluten protein hydration provides elasticity, strength, and stability.
- Gassing properties: high amylase and damaged starch yield sugars.
- Moisture content < 14% ensures safe storage.
- Yeast can be compressed, creamed, pelleted, or in active powder form.
- Yeast quantity depends on fermentation parameters: time and temperature.
- Yeast effects: providing CO2, bubble expansion, lowering pH, alcohol production, aroma, and flavor.
- Sodium bicarbonate can be used as a 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 is added to develop flavor and slow down fermentation; it might be added until the dough is partly fermented.
- Fat improves loaf volume, reduces crust toughness, and gives a thinner crumb cell wall.
- Sugar is added to develop a sweet flavor.
Water Absorption: 50 to 60 parts per 100 parts of flour are needed to achieve standard consistency. This varies by protein and starch granule content.
Fermentation: The amylase enzymes convert starch to maltose, maltase converts maltose to glucose, and the zymase complex converts glucose and fructose to CO2 and alcohol.