The primary role of yeast in baked goods is so that they rise to give that soft dough at the end. It is
usually added to a mixture of flour and water.
After addition, yeast breaks the large starch molecules into simple sugars. While doing this, carbon dioxide and ethyl alcohol are created, and they make the air bubbles that cause the bread dough to grow bigger.
When you run out of yeast, there are multiple ways to replace it. Although these are not the optimal option, it is cheap, and a lifesaver when not having yeast is unprecedented.
The secret is combining baking soda and acid. You have to swap in the right amount of baking soda and acid. To make the acid, you can use lemon juice, buttermilk, or milk combined with an equal part of vinegar.
Add all the baking ingredients according to the recipe. Then, add equal parts baking soda and lemon juice to equal the amount of yeast required in the recipe.
For example, if the recipe requires that you use two teaspoons of yeast, you need to add a teaspoon of baking soda and a teaspoon of lemon juice.
Buttermilk can also be used as a substitute in this soda and lemon juice mixture. Also, using equal parts of milk and vinegar works.
Using these mixtures works to produce carbon dioxide, much like regular yeast would do. However, you must know that, following the application of a yeast substitute, you should pop the dough in the oven immediately for the best results.
These substitutes limit wastage when you have no yeast and have already mixed your ingredients; however, they don't produce optimum results.