Yes, kosher salt and flake salt differ in their crystal structure, production methods, and often, their mineral content. Kosher salt typically has larger, irregular crystals, while flake salt consists of thin, delicate flakes.
Kosher salt gets its name from its use in the koshering process of meat, where its larger crystals are effective at drawing out moisture and blood. It's usually produced through evaporation or the Alberger process, resulting in coarse, uneven grains. Because of its larger size, it's easier to pinch and distribute evenly when seasoning. Most kosher salt brands do not contain iodine.
Flake salt, on the other hand, is made by evaporating brine, often using a slow, careful process that allows thin, delicate flakes to form. These flakes have a higher surface area than kosher salt, which means they deliver a more intense burst of flavor when they dissolve on the tongue. Flake salt is often used as a finishing salt, sprinkled on food just before serving to add texture and flavor.
The mineral content can also vary. Some flake salts, particularly those harvested from specific regions, may contain trace minerals that contribute to their unique flavor profiles. Kosher salt is generally purer sodium chloride.
Don't substitute flake salt directly for kosher salt in recipes that call for precise measurements by volume. Because of its lower density, you'll need more flake salt to achieve the same level of saltiness. It's best to use flake salt as a finishing salt to enhance the flavor and texture of your dishes.