Opened packages of dry yeast (active dry or instant) can be stored in the refrigerator for three to four months. If your yeast is older than this, it may still be good. Proof your yeast to find out if it's still active by adding 1 teaspoon of sugar and 2 1/4 teaspoons of yeast (one envelope) to 1/4 cup of warm water. Then, wait 10 minutes. If the mixture bubbles and develops a yeasty aroma, the yeast is still good.
Want to store yeast longer? Stick it in the freezer, and it will stay fresh for years.
Substitutions to Try
If your yeast failed the test, it will need to be replaced with fresh yeast. But, if you're already in the middle of a recipe, you can put off a trip to the store by using a yeast substitute to finish your recipe.
Add equal parts baking soda and acid (either lemon juice, vinegar, or buttermilk) to equal the amount of yeast called for in the recipe.
So, if a recipe calls for 1 tablespoon of yeast, replace it with 1/2 tablespoon of baking soda and 1/2 tablespoon of acid. When the acid comes into contact with the baking soda, it'll create a chemical reaction, which produces carbon dioxide bubbles, and it's those carbon dioxide bubbles that will make the bread rise. For this substitute to work properly, you need to add your baking soda and acid after all other ingredients have been added to the recipe. Baking soda is a single-acting leavening, so it's important to get your baked good into the oven soon after you've added it.
Replace the yeast called for with an equal amount of baking powder. Since baking powder contains both baking soda and acid (cream of tartar), it contains everything needed to make your baked goods rise. Just add it in with the rest of your ingredients, and as soon as it comes into contact with whatever liquid your recipe calls for, it'll begin to bubble and make your dough/batter rise.
Baking powder is double-acting, meaning it causes two rises: the first when you add it to your recipe, and the second when it's heated in the oven. Because of this, you don't have to wait until the end to add it, as you do with baking soda. Just throw it into the mixing bowl with the rest of your dry ingredients, and continue on with your recipe.
If your recipe already calls for baking soda, baking powder, or one of the acids mentioned, your substitute needs to be added in addition to the amount already called for in the recipe.