Bran is the hard outer layer of grain and consists of combined aleurone and pericarp. Along with germ, it is an integral part of whole grains, and is often produced as a by-product of milling in the production of refined grains. When bran is removed from grains, they lose a portion of their nutritional value. Bran is present in and may be milled from any cereal grain, including rice, wheat, maize, oats, and millet.
Bran is particularly rich in dietary fiber, and contains significant quantitities of starch, protein, vitamins, and dietary minerals.
Rolled oats are oat groats that have been rolled into flat flakes under heavy rollers. The oat, like some other cereals, has a hard, inedible outer hull that must be removed before the grain can be eaten. After the hulls have been removed from the bran-covered oat grains, the remainder is called oat groats. Oat groats can be used as cereal, but since the bran layer makes the grains tough to chew and contains an enzyme that can cause the oats to go rancid, oat groats are usually steam-treated to soften them and denature the enzymes. It can be argued that this process removes the enzyme phytase which would otherwise serve to break down the the high-phytate content of commercial oats which inhibits the absorption of iron by the human body. Steel-cut oats are oat groats that have been chopped into smaller pieces and retain bits of the bran layer.
Rolled oats that are sold as oatmeal usually, but not always, have had the tough bran removed. They have often been lightly baked or pressure-cooked. Thick-rolled oats are large whole flakes, and thin-rolled oats are smaller, fragmented flakes. Oat flakes that have simply had the bran removed can be cooked and eaten as "old-fashioned" oatmeal, but more highly fragmented rolled oats absorb water much more easily and therefore cook faster, so they are sometimes called "quick" or "instant" oatmeal.