Yeast Is Too Old
If you have a must that has started fermenting very slowly or hardly at all, one of the many things you need to ask yourself is, how long have you had the wine yeast, and how was it being stored during that time.
Wine yeast is a living organism that has a limited life-span just as any other living thing would. When you purchase yeast it comes conveniently packaged in a suspended, freeze-dried state. When the dried wine yeast is put in a liquid it rejuvenates itself back to its original, natural state.
While the wine yeast is in freeze-dried form it can become old, however it does not do so all at once, but rather, slowly over time. The average 5 gram package of dried wine yeast contains roughly 150 billion yeast cells. More than enough to produce a healthy, vigorous fermentation in a 5 or 6 gallon batch. Even 75 billion active yeast cells would be more than sufficient to ignite a very active fermentation.
A portion of these cells die every year, every month, and even every day. If you store your packets of wine yeast at room temperature, it will be active enough to use for at least 1 year. If your yeast is being stored above 80 degrees F., then its useful life-span will be shortened accordingly.
If you store your yeast in the refrigerator, your yeast will be fine for at least two years. It is important to note here though that you should never store your yeast in the freezer. Doing so damages the cell walls of the yeast with freezer-burn. The cell walls of the yeast actually become damaged from the effects of freezer-burn.
So, the whole point here is to be aware of the age of yeast packets you have on hand. Understand that these packets of wine yeast will not be usable forever and may be one of many possible reasons for having a fermentation that will not start.
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