Types of flour: cake, all-purpose and bread flour

How flour is classified

In Korea, flour is usually sold as soft, medium or strong flour, which line up roughly with cake, all-purpose and bread flour. The names differ, but the classification rests on one thing: how much protein the flour contains. Soft flour is low in protein, strong flour is high, and medium sits between. Rough ranges often quoted are around 7 to 9 percent for soft, 9 to 10 percent for medium and 11 to 13 percent for strong, but actual figures vary with the wheat variety and milling. For the exact value, look at protein per 100 g on the nutrition label.

How protein becomes gluten

Most of the protein in flour is of two kinds, glutenin and gliadin. When they meet water and are mixed and kneaded, they link together into an elastic network called gluten. Glutenin is known to give springiness that snaps back when stretched, and gliadin gives extensibility. The more protein, the more water and the longer the kneading, the stronger the network becomes. When it traps the gas made by yeast, bread rises. When too much forms, cakes and cookies turn tough.

• More protein means stronger gluten
• Water and kneading develop the network
• Sugar and fat get in the way of gluten

Cake flour: when you want tenderness

Cake flour has little protein, so it forms only weak gluten. That makes it right for sponge cakes, cookies, muffins and tart pastry, which should be soft and crumbly. Make the same recipe with bread flour and the cake rises less and turns chewy, while cookies tend to come out hard and less crisp. Even cake flour develops gluten if you mix it too long, which is why cake and muffin recipes tell you to mix only lightly once the flour goes in.

Bread flour: the frame of bread

Bread flour is high in protein and forms strong gluten. It is used for doughs that must rise a lot through fermentation and turn out chewy, such as sandwich loaves, rolls and pizza. A tight, sturdy gluten network keeps yeast gas from escaping, so the volume grows and does not collapse after baking. Bread flour tends to absorb more water, so swapping it into a cake flour recipe can leave the batter stiff. Use cake flour in a bread recipe and the dough slumps and does not rise well.

Medium and all-purpose flour

Medium flour falls between the two and is used a lot in Korean cooking for noodles, hand-torn dough soup and savory pancakes that need moderate chew. All-purpose flour in English-language recipes generally falls in this range, but protein varies a fair amount by country and brand, so it is not exactly the same as Korean medium flour. If a foreign recipe turns out a little different, put the flour on your list of suspects. With only all-purpose flour you can manage cookies and muffins, though cakes may be less tender and bread may rise less.

Things to consider when substituting

You can swap one flour for another when the recipe's type is missing, but the result will not be identical. All-purpose in place of cake flour makes things a little firmer, and all-purpose in place of bread flour gives less chew and less volume. Replacing part of all-purpose flour with cornstarch to lower the protein ratio is a common trick, but it does not exactly reproduce cake flour. Whole wheat flour contains bran and germ that interfere with gluten and soak up more water, so it is safer to start by replacing only part.

• All-purpose for cake flour: a little firmer
• All-purpose for bread flour: less rise, less chew
• Whole wheat: replace only part and watch the water

Storage and sifting

Flour readily absorbs odors and moisture. Once opened, move it to an airtight container and keep it somewhere cool and dry. In warm months when pests appear more easily, the fridge or freezer is an option. Let chilled flour come to room temperature before use so it does not cool the dough. Sifting breaks up lumps and mixes flour evenly with other dry ingredients. If you measure by weight, it is fine to sift after weighing.

Safety and allergies

Flour is not a ready-to-eat food. Uncooked flour can carry bacteria, so it is safer not to taste raw dough or lick batter off your fingers. Wheat is a major allergen, so tell people when something you share contains flour. Another basic safety rule is not to work near an open flame while lots of flour dust is in the air. If hives, trouble breathing or similar symptoms appear after eating, contact medical services right away.