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How to Rehydrate Yeast

Rehydration defined | Why you should rehydrate yeast
How to Rehydrate Yeast | References

Despite being one of the most inexpensive supplies in the winemaker's arsenal, wine yeast is is arguably the most important ingredient you can add to your must. Since yeast is responsible for converting the sugar in the must into alcohol and CO2, it makes sense to ensure that the yeast is properly prepared so it can do its job - or else your wine may not ferment properly.

For home vintners (winemakers), the rehydration of wine yeast is especially important to ensure thorough fermentation.* After having gone to the trouble of picking your fresh fruit (grapes, berries, etc.) and preparing it for making wine, skipping a simple 5-step process in order to "save time" is self-defeating and can mean the difference between a good wine and a great wine.

By definition, rehydrating active dry wine yeast is the process of adding water back to the dried yeast cells.

This is done by combining yeast and water in a certain way that facilitates uptake of water into the yeast cells. See how this is done by reading the simple rehydration steps below.

Why Should You Rehydrate Yeast?
Yeast is a living organism, and like you, depends on water to live. When active dry wine yeast is made in the laboratory, the manufacturer removes the excess water - both inside the cell and out - effectively putting the yeast in a desiccated state of "hibernation" until we're ready to use it. In order to be functional again, we must add back the water that was removed during the drying process. Without this water, the yeast cannot process the sugars properly due to inefficient oxygen and nutrient transfer to the cells.

That being said, here are the reasons why you should rehydrate yeast:

  1. To ensure cellular integrity of the yeast. We want to make sure that the cell walls and all the organelles have all the water they need to do their jobs properly.
  2. Rehydrated yeast performs fermentation more efficiently. More active yeast cells means better, more complete fermentation.
  3. Must can contain high levels of SO2 or residual fungicides that can kill the yeast cells during the rehydration phase. Once rehydrated, the yeast cells do a better job of resisting SO2 and low levels of fungicides, but not during water uptake.
  4. To ensure dispersement. Yeast works better if it is widely dispersed throughout your must/wine, and yeast cells that have been rehydrated are dispersed more easily.

There are things that can go wrong when rehydrating yeast, and you should avoid these pitfalls at all costs:

  1. Avoid extremely high or low temperatures when rehydrating yeast. Temperatures that are too high or too low can kill the yeast cells before water uptake can occur.
  2. Avoid "shocking" the yeast during inoculation. A shock occurs when there is a big difference in temperature between the yeast slurry and the must. If you desire to ferment at low temperatures (59°F / 15°C or below), adjust the yeast slurry temperature by slowly adding a small amount of must to the suspension mixture and stirring for about a minute before inoculating.
  3. Avoid impatience. Don't be in a rush to inoculate your must. Allow sufficient time for the water uptake to occur (15 to 30 minutes).

How to Rehydrate Yeast
The steps to rehydrate wine yeast are straightforward and simple:

  1. Make sure must is ready to inoculate and that you have taken and recorded temperature and hydrometer readings. Starting fermentation temperatures of the must should be between 50°F to 86°F (10°C and 30°C).
  2. Pour the contents of the yeast packet into 50 ml (about 1/3 cup) of 104°F (40°C) clean, chlorine-free water.
  3. Stir the mixture lightly.
  4. Allow the mixture to stand for at least 15 minutes - but no more than 30 minutes - then stir again to ensure the entire mixture is wet.
  5. Add the yeast suspension (sometimes called "slurry") to the must. This step is also known as inoculation.

Even if the proper yeast is used, most experienced vintners know that wine yeast is pretty particular when it comes to fermenting wine to dryness - the proper environmental conditions (such as cleanliness and temperature) must be met, and nutrients (such as a balanced source of DAP [diammonium phosphate], amino acids, minerals, and vitamins) need to be available for the yeast to continue their hard work. See this related article on preventing and treating stuck fermentations.

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*Note: These instructions for rehydrating active dry wine yeast are mainly intended for winemakers who make their own wine from scratch, and not necessarily for those who make wine from kits (although rehydrating yeast properly will not hurt under any circumstance). Many wine kit manufacturers, including the suppliers of wine kits to, do not mention rehydrating yeast in their kit directions. For instance, RJ Spagnols (the makers of Vino del Vida and Cellar Classic wine kits) instructs vintners to merely sprinkle the wine yeast on top of the must when ready to inoculate. We contacted RJ Spagnols and asked about this, especially since the manufacturer of the wine yeasts provided in their kits (Lalvin) recommends rehydration. Our contact at RJ Spagnols explained that their manufacturing process produces a consistent environment that is conducive to supporting yeast cells, so the possibility of unintentionally killing yeast colonies upon inoculation is minimized. To further support this, the staff at, whenever making wine from a kit, has always sprinkled yeast on top of the must and has never experienced adverse results.