Buttermilk, made by adding culture to milk, much like yogurt, is most commonly used for baked goods, including quick breads, such as biscuits and muffins. Buttermilk has a very pronounced tangy taste and thick texture, which comes from the presence of lactic acid. It is this sourness that makes buttermilk such an important ingredient in foods, helping keep baked goods moist and light.
Leavening Ability
The acidity in buttermilk is important for helping leaven quick breads, which are made without yeast, and depend on an acid-base reaction to keep breads light and fluffy. The acid found in buttermilk reacts to bases — most commonly baking soda — creating carbon dioxide gas. The gas presents itself as small bubbles throughout the batter or dough, giving the quick breads their light, airy structure. However, because this chemical reaction is not long lasting, goods made with buttermilk as a leavening agent need to be cooked soon after mixing; otherwise, the bubbles will deflate, leading to heavy, dense-textured foods.
Softens Gluten
Buttermilk’s high acid content also helps keep baked goods moist and tender by breaking down gluten. A protein found in wheat flour, gluten gives baked goods their structure. A lot of gluten is not good for quick breads, as the protein adds chew and density to foods. The presence of buttermilk in these recipes helps to soften the tough strands. This leads to breads with a softer texture.
Flavor, Moistness and Color
Buttermilk also adds a distinctive taste to foods — the sourness of the liquid is mellowed out by the other ingredients, but it adds a welcome note of brightness. It also helps keep foods moist, as the high viscosity of the liquid means it takes longer for moisture to evaporate during the baking process. The viscosity also creates a creamy texture in foods. In turn, the acidity of buttermilk helps prevent some foods, namely cherries and walnuts, from developing a blue tinge in muffins and other baked goods.
Substitutes
If you do not have buttermilk at home, replace it with a soured dairy product for a similar taste and effect, although the results will not be exactly the same. You can water down yogurt and sour cream to the consistency of buttermilk. You can also add acid to regular milk, making sour milk. Use white vinegar, apple cider vinegar or lemon juice, adding a small amount to milk and letting it stand for several minutes until the milk has curdled and solidified slightly. While yogurt and sour cream will provide a slight tang to your food, soured milk will not add much in way of flavor, although it will still help with leavening.