Baking soda is used in doughnuts (and many other baked goods) as a leavening agent. Leavening agents are substances that produce gas, usually carbon dioxide, which causes the dough or batter to rise, giving the final product a light and airy texture.
In the case of doughnuts, the addition of baking soda helps create a soft and fluffy texture in the dough. When baking soda is combined with an acidic ingredient (such as buttermilk, sour cream, or yogurt) and moisture, it undergoes a chemical reaction that releases carbon dioxide. This gas gets trapped in the dough, causing it to expand and rise.
The leavening process is crucial in doughnuts because it ensures that they are not dense and heavy but instead have a desirable softness and volume. The release of carbon dioxide gas creates small air pockets throughout the dough, giving the doughnuts their characteristic airy and light texture.
It's worth noting that in some doughnut recipes, baking powder may be used instead of (or in addition to) baking soda. Baking powder contains both an acidic component and a leavening agent (usually baking soda), so it can create the leavening effect without the need for additional acidic ingredients.
In summary, baking soda is used in doughnuts to provide the necessary leavening, resulting in a delightful and fluffy texture in the finished product.