Why do steel-cut oats take so much longer to cook than old-fashioned oats?
The difference between steel-cut oats and old-fashioned (“rolled”) oats lies in the manufacturing process. Steel-cut oats are full oat groats (whole oat kernels with the hull, or outer shell, removed) sliced into small pieces with steel blades. This leaves much of the kernel intact and helps give steel-cut oats their signature chewy texture. Rolled oats are oat kernels that have been steamed and flattened using large disks, which makes the oats thin, flaky, soft, and able to readily absorb liquid. Quick-cooking and instant oats are processed the same way that rolled oats are, but they are rolled thinner and are sometimes precooked and then dried so that they cook even more quickly.
Because steel-cut oats are physically more dense, they require more liquid during cooking and typically take about 25 minutes to cook (although soaking them in water overnight will shorten their cooking time); rolled oats take about 5 minutes to cook.
The bottom line: Since steel-cut oats are less processed than old-fashioned rolled oats and aren't flattened, they take longer to absorb liquid and therefore take longer to cook than old-fashioned rolled oats.