Do I need to re-hydrate dry yeast?

We all have seen or heard the horror stories about a dry yeast packet taped to the top of a malt extract can with directions that read “sprinkle yeast on top of wort.” That simple phrase invokes feelings of sadness in me as I think of the number of homebrewers that our community lost after their first batch due to the sub-par beer churned out by yeast that never had a chance. Yeast after all is the magnificent organism (Saccharomyces cervesiae) is what turns our wort into beer. Arguably the most important ingredient for making a batch of beer, yeast was the red-headed stepchild for many years. Locked in the basement and ignored. Over the the last 10-15 years however, homebrewers have brought the topic of yeast, or rather yeast health, to the forefront of homebrewing discussions. The adoption of better yeast handling practices by homebrewers has allowed us to create some of the best beers in the world. However, more people are joining our community everyday and may not always be aware of how important their yeast is to producing a fine beer. This article will address a question many homebrewers have about dry yeast: do I need to re-hydrate dry yeast? The answer to that question depends on what your goal is. If your goal is produce the best beer possible, rehydrating yeast will increase your odds of obtaining that goal. If your goal is to make beer, there is no need to rehydrate yeast. Be prepared however, for the beer to be sweet, taste like green apples, and/or a stick of butter. Our goal should always be able to brew the best beer possible, because let’s face it: good beer is more fun to drink. Yes, you should rehydrate your dry yeast. 

Why should I re-hydrate?

This is a question I hear often, because in many cases a homebrewers first experience with brewing is in the form of a kit. The instructions in the kit tell us to sprinkle the yeast on top of the wort and forget about the fermenter for 7-10 days. However, when you dig a little deeper the major yeast manufacturer’s recommend rehydrating dry yeast before pitching it into our wort. The primary reason for rehydrating dry yeast is to allow the yeast cells to draw water across their cell walls so they can restart metabolic activity. If the dry yeast is pitched to the wort without rehydrating, many of the yeast cells die because the sugar concentration in wort inhibits the ability of the yeast cells to draw water across their cell walls, i.e., inhibiting the activation of metabolic activity. Enabling the yeast to initiate metabolic activity before pitching to the wort allows the yeast to concentrate on converting sugars into alcohol, which is what we all want right?

Benefits of dry yeast re-hydration

When you pitch a properly rehydrated yeast pack you reduce the probability of a number of problems that may occur as a result of underpitching, i.e., not putting enough yeast cells into the wort. Before we go further I should point out that proper pitching rate is only one variable a brewer needs to consider for a healthy fermentation. Other factors to consider include:  ideal wort temperature (40-50°F for lagers, 60-70°F for most ales), ideal fermentation temperature (45-55°F for lagers, 60-75°F for most ales), proper wort oxygenation (minimum of 8 ppm), and nutrient availability (usually only a concern when a large number of adjuncts or simple sugars are used to produce wort). I digress though, back to the topic at hand. Rehydrating yeast allows brewers to pitch an adequate amount of healthy yeast into wort, ensuring full attenuation and reduction of off-flavors. An under-attenuated beer may taste bland, cloyingly sweet, or like malt extract. The two primary off-flavors stemming from underpitching include:

  • Acetaldehyde-green apple
  • DMS-cooked corn and/or vegetables

Both of these off-flavors usually fade over time, however, I find it easier to pitch the proper amount of yeast initially so I do not have to worry about off-flavors being present in my beer.

How do I re-hydrate?

Re-hydrating yeast is easy, the key, like most brewing techniques, is to be sanitary. The steps below detail how I re-hydrate yeast (Safale S-05 for this example) on a brew day.

  1. I take the yeast packet out of the fridge either the night before or the morning of the brew day to let the temperature of the yeast packet rise to room temperature.
  2. I will re-hydrate the yeast after I have started to boil the wort.
  3. Vessels I prefer to use for re-hydration include a beaker, Erlenmeyer flask, or glass measuring cup.
  4. Boil one cup of water and let cool to proper re-hydration temperature (74-86°F for Safale S-05*).

*Proper re-hydration temperature will vary by yeast strain and/or manufacturer. These temperatures can usually be found on the manufacturer’s website.

  1. Sprinkle the yeast on top, cover with sanitized aluminum foil and leave for 15 minutes.
  2. Return and stir the yeast with a sanitized spoon.
  3. Recover and let the slurry sit for another 15 minutes. Yeast is now ready to pitch! Remember to swirl vessel to get yeast into suspension before pitching to the wort.


One of the best favors you can do yourself as a brewer is to pitch the proper amount of healthy yeast. When using dry yeast, the best way to do this is to rehydrate your yeast packet. Pitching rate will depend on the age of your yeast and the original gravity of your wort. I like to use the tried and true Mr. Malty Yeast Pitching Rate Calculator™ to calculate my pitching rate. There are a number of pitching rate calculators out there, this one has worked best for me. Use whatever you are comfortable with and gives you the best beer. I think I’m going to have shirts made up. “Friends don’t let friends sprinkle their wort.” Additionally, if you are not already a member you should join the American Homebrewers Association which includes a subscription to Zymurgy magazine, packed full of useful homebrewing information. Cheers!

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