Cereal Grains, Legumes, and Oils: Composition and Uses
Grains, Legumes, and Oils: Nutritional Significance
Grains are the most consumed food group worldwide, representing the primary source of calories for humans. In less affluent areas, they also serve as the main source of protein.
Legumes provide higher quality protein than cereals. They are rich in lysine, although they are poor in sulfur-containing amino acids. Legumes are a good complement to cereals.
Oilseeds: Their main function is oil production. They also contain protein, even more than the other two groups. These proteins are primarily used in animal feed.
Structure of Grains
- External Integuments: Rich in fiber (cellulose and hemicellulose), minerals, and phytic acid. Phytic acid is an anti-nutritional factor that can bind to minerals like calcium and iron, reducing their bioavailability.
- Germ: Rich in lipids (for oil extraction) and vitamin E, but also contains proteins.
- Aleurone: The outermost layer, rich in protein, lysine, minerals, and water-soluble vitamins.
- Albumen (Endosperm): Contains starch and protein (70% of the total), primarily located in the peripheral zone.
Wheat: Unique Bread-Making Properties
One of wheat’s main characteristics is that its flour is suitable for bread making. Rye can also be used, but the bread quality is lower.
This difference from other cereals is due to the proteins they possess. The main protein is gluten, which represents 85% of the total protein content.
Gluten allows for the production of bread with good volume and texture. It forms a structure capable of retaining the CO2 generated by yeast or chemical leavening agents.
Gluten: Composition and Function
Gluten is a protein complex consisting of two main proteins: gliadin and glutenin. They have associated polar lipids.
- Glutenin: Has a molecular weight of 106. It is composed of small units linked by disulfide bonds. Glutenin is responsible for the elasticity and mechanical strength of the dough.
- Gliadin: Provides extensibility to the dough, allowing it to change shape during fermentation and increase its volume without breaking.
Starch: Structure and Gel Formation
Starch is the reserve polysaccharide of plant tissues and the most important one from a nutritional standpoint. It is the primary source of carbohydrates for humans. Starch is found in plant cells as insoluble particles or granules. Its shape and size are characteristic of each plant. The total hydrolysis of this polysaccharide, by acid or enzymes, generates only D-glucose.
Starch consists of two components:
- Amylose: A linear chain with α-1,4 linkages. The chain length depends on its origin, typically ranging from 1000 to 5000 D-glucose units. Amylose is found in crystalline form within the granules. Hydrogen bonds are responsible for water adsorption and gel formation. The crystallinity causes swelling only at high temperatures. In aqueous solution, amylose exists as a random coil, although some areas have a short, unstable helical structure.
Gel Formation
Aqueous solutions of starch are not stable, especially upon cooling. The resulting gel is amorphous, more or less rigid, and elastic. It is formed by the thermoreversible “merging” of amylose chains at certain points, while the rest remains in a random configuration. The melting points are hydrogen bonds. Water is immobilized within the three-dimensional structure formed by the chains.