Corn: Structure and Properties

The Structure and Properties of Corn
The physical properties of corn are important in the design of handling equipment and storage facilities. These properties of course, are affected somewhat by moisture content.

Compared to other grains, corn has a unique shape and low specific gravity, but many other properties are similar to other cereals.

Corn kernels are the largest cereal seed, weighing 250 – 300 mg each. They are flat seeds due to pressure during growth from adjacent on the cob.

The corn kernel has a blunt crown and pointed conical tip cap. The corn kernel is botanically classified as a caryopsis (dry, indehiscent, single seeded fruit) and is attached to the cob by the pedicle.

The kernel contains a complete embryo and all the structural nutritional and enzymatic functions required for growth and development into a plant.

The kernel is composed of four anatomical parts; the tip cap, which provides the point of attachment to the cob; the bran, which is the protective outer covering; the germ or embryo, which becomes the new plant; and the endosperm, which is the reservoir of nutrients to support germination.

Each anatomical part of the corn kernel has a different composition.

The largest fraction of the kernel is the endosperm which is largely composed of starch, the reserve energy supply for a germinating embryo.

Endosperm cells are packed with starch granules embedded in a combination matrix of amorphous protein.

Also embedded in this matrix are protein bodies composed almost entirely of the storage protein zein.

Corn is often harvested as soon as the moisture content drops below 28%. Unless quickly dried, high moisture corn is subject to raid deterioration, especially by mold infestation.

However, high temperature drying, which is occasionally used during the busy harvest period to speed drying operations, can adversely affect wet milling properties and air inlet temperature exceeding 50 degree C should be avoided.

Any hygroscopic material material like corn losses (desorbs) or gains (adsorbs) moisture depending on whether its water vapor pressure (water activity) is greater or less than that of its environment.

Equilibrium moisture content for corn is complicated by the hysteresis effect, in which the equilibrium moisture content depends on whether the grain is desorbing or adsorbing.

Also each component has a different affinity for moisture, so it is not surprise that different corn fractions have different equilibrium moisture isotherms.

Because different types of corn have slightly different composition, their moisture sorption isotherms may vary accordingly.

The critical moisture content for “safe” storage of corn is generally to be 15%, but lower moisture contents are required for corn to be stored for long periods at warm temperatures as occur in the tropics.
Corn: Structure and Properties

Basic structure of cell wall

The cell is the basic structural unit if all plant tissues. These cells are surrounded by cell walls that provide an elastic support for retaining the contents of the cell.

Strength and elasticity are gained by use of several cross-linked polymers of different elastic properties, with an adhesive cementing structures together.

The cell also has a membrane layer, which is located just inside the cell wall and which controls the passage of liquids into and out of the cell. The cell is filled with jelly-like substance, termed the cytoplasm, which is composed of protein, sugars, salts and other substance dispersed in water.

Mature cells also contain vacuoles, which are separate compartments filled with a fluid, cell-sap and which are composed of dissolved sugars, salts, organic acids, pigments and other materials.

Also located within the cytoplasm are separate inclusion bodies, called plastids, which contains the pigment of chlorophyll. These plastids are only both 4 to 10 nm in diameter. The plant cell wall plays important roles in controlling cell differentiation.

The thin cell wall of a young cell, called the primary cell wall, contains many enzymes including not only glycosyl hydrolases, which simply disassemble cell wall polysaccharide, but also cell wall modifying enzymes such as expansin, endoxygloglucan transferase and pectinmethyl esterase.
Basic structure of cell wall