A living organism, yeast is essential in baking bread and fermenting beer. Purchasing large packages of yeast can save money, but because you normally only need small quantities, it can be tough to figure out how to store it well. Freezing yeast ensures longevity, and the yeast will still be usable, as long as it's done properly.
Temperature can kill yeast, but not the temperatures in a freezer. Low temperatures make yeast go dormant while high temperatures will kill it. When freezing yeast, moisture is more of an issue. Moisture will activate dormant dry yeast, but because yeast is a living organism, it also needs food to sustain it. Yeast will die if it is reactivated and not fed the sugars necessary for its survival, so keeping moisture out of yeast is vital.
Active dry yeast used for baking is coated with dead yeast granules to protect the living yeast inside. This type of yeast is the least susceptible to a little extra moisture and is your best bet for freezing. Instant yeast is finer and not covered in a protective coating, making it more sensitive to moisture. Keep instant yeast properly sealed and stored in the back of the freezer if you choose to use it. Brewing yeast can also be frozen with no adverse effects.
Freezing may be one of the best methods for storing dry yeast. To ensure that moisture doesn't start a reaction in the freezer, keep the yeast in an airtight container and store it at the back of the freezer so it doesn't constantly warm up and accumulate condensation when the freezer is opened. You can also store yeast in an airtight container at the back of the refrigerator.
Using Frozen Yeast
You can add baking yeast directly from the freezer to your recipe. Because the amount of yeast used is proportionally much smaller than the other ingredients in a recipe, it will warm up almost immediately once it's added. Don't take the entire container out of the freezer and allow it to warm up -- this will cause extra condensation. Stored properly, frozen yeast should function well past the expiration date.
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