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Dr Roger weighs in on the conversation…

The rehydration of dry yeast is often discussed amongst craft brewers; many people direct pitch, even though the manufacturers recommend and describe the method for rehydration. Here is some background:

The recommended pitching rates for dry yeast are 50 to 120 g/hl, with the higher rates applicable to colder temperatures, lagers, sours, and higher gravity brews. Brewers tend to follow these recommendations pretty closely – with actual pitching rates around 25 to 80 g/hl for ales, and 50 to 100 g/hl for lagers.  When pitching really cold, some manufacturers even recommend pitching up to 300 g/hl – but I am yet to hear of a craft brewer pitching dry yeast at this rate!

Manufacturers usually recommend rehydration prior to pitching, with instructions similar to this:

  1. Sprinkle yeast on 10 x weight of sterile tap water or wort at 25 – 30°C.

[So, for a 500g brick, you would add this to 5 L of water or wort. The manufacturer often neglects to mention it, but it should be in a sterile container with a large surface. A stainless-steel bucket works well for this purpose.]

  1. Let sit for 15 minutes
  2. Give it a mix
  3. Let sit for another 5 minutes, gradually bringing the temperature down to within 5°C of the main wort temperature.
  4. Pitch

Many brewers tell us they don’t see a need to rehydrate, for the following reasons:

  • There is no noticeable benefit from rehydration
  • Direct pitching reduces the risk of contamination
  • less preparation is required with direct pitching.

I think we would all agree with the last two points. But what about no noticeable benefit from rehydration? Yet it makes sense because the majority of dry yeast is pitched into wort with a pH of about 5.2 and specific gravity of roughly 1.050. These are actually very mild conditions for Saccharomyces cerevisiae. It is only when the specific gravity is high (>1.060) or you have a sour mash wort that you probably need to rehydrate. You can see this from the practices of wine and cider makers. They are usually starting with a juice pH of about 3.4 and specific gravity of 1.065 or more, and they always rehydrate their dry yeast. So, unless you are making high gravity beers or sours, and you aren’t having fermentation problems with your other brew, then perhaps it is ok to keep on direct pitching that dry yeast?

WHAT DO YOU THINK?

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