Steel-cut oats (pinhead oatmeal) are oat groats that have been chopped into smaller pieces before any steaming and thus retain bits of the bran layer. Since the bran layer, though nutritious, makes the grains tough to chew and contains an enzyme that can cause the oats to go rancid, raw oat groats are often further steam-treated to soften them for quicker cooking time (modern "quick oats") and to denature the enzymes for longer shelf life.