How to Re-Hydrate Yeast

Yeast, a winemaker’s best friend, and star of the show when it comes to fermenting grapes into wine. They make it all possible. With a properly hydrated yeast your fermentation will start strong and be less likely to get stuck. Which is why it’s critical to understand how to rehydrate yeast.

What is dry yeast?

Dry yeast is made up of small granules that consists of live, active yeast cells enclosed in a hard shell of dead yeast and a growth medium. In order for the live yeast cells to break free and ferment your must the shell must first be broken down.

This is where hydration comes in. Whether you re-hydrate yeast yourself or allow nature to take its course in the must what you’re doing is breaking down that outer shell.

If yeast is not properly re-hydrated the individual organisms can’t function properly. Their cell walls will not be fully permeable and they won’t be able to take in sugars and release carbon dioxide and alcohol.

How does yeast re-hydrate if we don’t do it?

In most wine kits rehydrating the yeast is not only not necessary, the directions clearly state “do not re-hydrate the yeast”. You just add it straight to the grape juice concentrate and water mixture.

By doing this we’re trusting that the dried yeast will hydrate well enough on its own. Kit makers choose the yeast strains based on many factors but one of the key factors is its ability to hydrate on its own in the must.

If you read yeast hydration instructions, however, you’ll notice that the optimal water temperature for hydration is 104-109 degrees (F). Kits instructions call for inoculation temperatures of 72-75 degrees (F).

These are less than optimal conditions for the hydration of yeast which is why kits only come with certain strains.

How Rehydrating Yeast Affects Fermentation

Rehydrating your yeast at 104-109 degrees (F) helps break down that crusty outer layer and allows the live cells within to break free and begin multiplying. In just a few minutes your yeast population is already starting to explode.

Contrast this with pitching dry yeast in a cool must. Without that heat it takes longer to break down the outer shell. This is why you only see evidence of fermentation two days or so into the wine making process. If you re-hydrate the yeast that comes with a wine kit you’ll likely see evidence of fermentation within a few hours.

The rapid population growth speeds up fermentation because there are more of the little guys sooner. Another benefit of such a rapid population growth is that the yeast can dominate the environment much more easily. Keeping undesirable strains or other bacteria from getting established in your must.

How to Rehydrate Yeast

This process can vary slightly depending on the brand of yeast and the strain. However, here is the general process I’ve followed for rehydration.

1. Heat 2 cups or so of water to 104-109 degrees (F).
2. Pour 50 ml of the heated water into a dry sanitized container.
3. Add the dry yeast to the water and stir for thirty seconds. This breaks up any clumps so all of the yeast is exposed to water.
4. Let the mixture sit for no more than fifteen minutes total. The recommended time can vary between manufacturers but in general you don’t want to exceed the recommendations for your yeast. Because plain water doesn’t have any of the nutrients the yeast needs to survive they’ll die if you wait too long.
5. After fifteen minutes add the hydrated yeast to your must. This is also referred to as inoculation.

Be Careful Not to Shock the Yeast

Yeast cells are sensitive to rapid and drastic changes to their environment. For this reason you want to make sure that there is no more than a 20 degree (F) difference in temperature between your yeast and the must.

If your hydrated yeast is at 90 degrees (F) and your must is at 60 it could kill the majority if not all your yeast when it’s added. The sudden change of temperature is too much for the little things to handle.

The change in sugar content can also shock your yeast. Sometimes hydration instructions will call for a small amount of must to be added to the warm water. This helps feed the yeast and lets them acclimate to the pH, TA, and amount of sugar in their new environment.

Conclusion

While the hydration process isn’t necessary for kit winemaking it is a good skill to develop for later on. When dealing with frozen must or fresh grapes you’ll want to hydrate your yeast for the same reasons given above.

Because frozen must and fresh grapes cost so much more you won’t want to take any chances during inoculation. You’ll want your yeast to have the most optimal conditions for fermentation so that you won’t wine up with a stuck fermentation or having some less than desirable micro-organism take over.