Fish is dusted with flour before pan-frying because the flour does three things: it absorbs surface moisture to help keep the fish from sticking to the pan, browns to form a thin crust, and covers the flesh to slow drying. In Türkiye, cornmeal is the most commonly used flour for pan-fried fish; semolina gives a thicker crust, while wheat flour makes a thinner, softer surface. Flour is not a heavy coating but a thin protective layer.
The first job of flour is to dry the surface. A thin film of moisture on the fish is the only obstacle to browning; flour absorbs that moisture and gives the pan a dry surface. Its second job is to form a crust: flour browns in hot oil and creates a thin, crisp layer. Third, it protects the fish; this layer covers the flesh and slows moisture loss, so the fish stays moist longer. Together, these three effects make the piece less likely to break apart and more flavorful.
The choice of flour makes a noticeable difference. Coarse cornmeal makes the crispest, fullest crust and is the traditional choice for small fish such as anchovies, sardines, and horse mackerel. Semolina is even coarser and leaves a thick crust; it works well for squid and large fillets. Wheat flour creates a thin, soft surface and is used for fillets that will cook in sauce or when a light crust is desired.
Timing is critical. Dust the fish with flour just before it goes into the pan. If it is coated ahead of time and left to sit, the flour absorbs moisture, turns pasty, and falls off as soon as it hits the oil. First pat the fish dry, season it with salt, then coat it with flour; shake off the excess. A thick flour layer does not make a crust; it absorbs oil. Salt should not be added too early either, because it draws moisture to the surface of the fish and directly interferes with the flour sticking. Keep the flour in a wide, shallow dish; coating fish in a deep container is less even and uses more flour than necessary.
Flour forms a crust only when the oil is at the right temperature. If the oil is not hot enough, the flour absorbs oil before browning and makes the fish greasy. Heat the pan well, add the oil, and wait a few seconds; the oil is ready if a pinch of flour sizzles as soon as it is added. Do not crowd the pan or let the pieces touch; crowding lowers the temperature and prevents a crust from forming. Once the fish is in the pan, leave it undisturbed until the underside is browned. If cooking a second batch in the same oil, remove the burnt flour crumbs with a skimmer so they do not stick to the next pieces of fish.
Flour is not always necessary. Fish is not dusted with flour for grilling or baking; the skin itself forms the crust. Flour is also not used for steaming or cooking fish in sauce, because it gets wet and falls apart. You can pan-fry fish without flour, but then the pan must be very hot and the fish thoroughly dried. Transfer cooked flour-dusted fish to a wire rack; on a paper towel, the crust underneath softens in its own steam. Adjust the salt once more after cooking, because some of the salt applied earlier never reaches the flesh under the flour.