When grains are subjected to heat and liquid, the membrane, or covering, of the grain becomes porous so water can enter the grain. Then the membrane of starch granules inside the grain breaks down. The starch then absorbs water and forms a gel, so the grains become softer and are more palatable. Grains also have protein, but are mostly incomplete proteins; that is, they don't have all of the amino acid molecules human beings need to use protein in the body. Combining grains can make complete proteins; many vegetarian recipes include grains and beans, or pasta and beans, or peanut butter on wheat bread. Quinoa is the only grain that IS a complete protein. Rice is also a grain; for info on rice, see Rice Science.
To cook grains properly, first rinse them, then follow package directions. Generally, use twice as much liquid as grains. Bring to a boil, then cover the pan tightly, reduce heat, and simmer until the grains are tender. Drain if necessary, then return grains to heat and shake for a few seconds over low heat to remove excess liquid and to fluff the grains. Then the pan can sit, covered, off the heat for a few minutes or the grains can be served immediately.
There are many different types of grains; here's a brief overview.
Amaranth is a seed that is mostly gluten-free. It is very rich in iron and fiber, and has twice the calcium of milk. Amaranth flour is used in gluten-free recipes. The seed can be steamed and eaten as a pilaf or cereal, or popped like popcorn.
Barley is one of the oldest grains known to man. Whole barley includes the bran, and takes longer to cook. Pearled barley has the brain removed and has been steamed and polished. I like to add barley to soups instead of pasta; simmer for the length of time specified on the package.
Buckwheat is a seed of a fruit, similar to wheat, but it is not wheat. It's a three-cornered tan colored seed. Buckwheat groats are the hulled kernels, with a lighter color. Kasha is roasted buckwheat. Buckwheat can also be purchased as flour or grits. It is steamed or boiled in liquid to eat as a pilaf or as cereal. Groats are usually coated with egg and toasted before cooking to retain the grain consistency and texture.
Bulgur is made from whole wheat grains that have been cleaned, parboiled, dried, ground into particles, and separated into different sizes to cook quickly. It makes an excellent pilaf, and is the base for the Middle Eastern salad Tabbouleh.
Corn is a grain, although many people think of it as a vegetable. It is very high in Vitamin A and beta-carotene. It is best cooked as soon as possible after harvesting, because the sugars in the grain begin to turn to starch immediately after harvest. Popcorn is a different variety of corn that is mostly starch, so it pops, or explodes, when exposed to dry heat.
Cornmeal is dry, ground corn kernels. You can cook cornmeal and eat it as a cereal, use it in baked products to add flavor, nutrition, and crunch, or make polenta out of it. Cook's Illustrated says that medium grain cornmeal makes the best polenta.
Hominy is dried corn with the hull and germ removed. And grits are made from ground hominy.
Oatmeal is a very good source of soluble fiber, which has been shown to reduce blood cholesterol. Steel cut oats are actually oat groats (part of the oat kernel) which have been cut into only a few pieces. Instant oats have been precooked and dried, and are simply rehydrated before eating. Rolled oats have been steamed, cooked, and flaked so their membranes are broken down and absorb water and liquids easily. And quick oats are thinner than rolled oats, so they cook more quickly.
Quinoa is yet another grain-like food that is really a seed. It contains all of the amino acids, or proteins, necessary for human life. Quinoa has been cultivated for thousands of years, mostly in Peru and South America. Quinoa has a high oil content, so it should be purchased in small quantities and stored in an air-tight container in a cool, dry place. Rinse it thoroughly to remove the sticky coating. This coating has a bitter flavor from saponsins, which protect the seed from being eaten by birds. Cook it according to package directions.