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The Gospel of John states that dry yeast should be hydrated at a temperature warmer than what you would typically start fermentation. But John also states that yeast should be pitched from cold to warm, i.e. the yeast slurry should be colder than the cooled wort.

An example of ale yeast:

  • recommended rehydration temperature: 24°C
  • recommended fermentation temperature: 16-20°C (I want to start at 16°C)

So...

  • How do I hydrate my yeast at 24°C and then cool it to below 16°C without shocking the yeast?
  • Is it OK to let it cool down?
  • Over what time period (It should be pitched about 30min after hydration)?
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I think the jury is still out on whether or not you need to rehydrate your yeast at all. You can find people on both sides of the argument. For example here is an article detailing an experiment between rehydrated and dry sprinkled yeast. Personally, I don't like to rehydrate due to the extra work and potential for contamination, but that's just my own preference.

If you do decide to rehydrate, in general you want to have your yeast slurry close to your wort temperature. The way you can accomplish this is to slowly add a little bit of wort to your rehydrated yeast. Each time you add some wort, your yeast slurry will cool and get closer to the wort temperature. Once your yeast temperature is within a few degrees of your wort temperature, you can pitch it. This document from Lallemand explains a little more.

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  • If you don't make a yeast starter, make sure you pitch enough yeast or the size of your wort, it is an important part of the process. Most brewers (both pro and homebrewers) will tell you that a yeast starter is important to ensure good pitch rate. – JeanMi Oct 26 '18 at 9:03
  • Definitely agree on pitching the right amount of yeast, but the generally accepted position seems to be that you do not need to make a starter with dry yeast - if you are pitching the right amount. – Nate Parsons Oct 26 '18 at 12:04
  • i agree for the pitch rate, my point was more like, if you have only 1 yeast package or don't want to buy a large amount of it, you should make a starter to ensure correct pitch rate :) – JeanMi Oct 26 '18 at 16:47
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Once you finish you boil phase and cool your wort, depending on the material you use, it would be quite time consuming to go down to 16C especially if your room temperature is above that, Considering that,

  • I would say that you should pitch your yeast around 20-22C (temp after boil wort cooling) and then place your wort in your temperature controlled environnement (at 16C). That way you will not shock your yeast (a couple of degrees doesn't do much). Your fermentation will start after few hours and by that time your wort would be at desired temperature.

  • Cooling down your yeast could also be a solution, the only precaution is to do it slowly (not submerge your yeast in ice).

  • Concerning your point about pitching 30min after rehydratation, I would not do that. To ensure good fermentation, It is always recommended to make a yeast starter (from 0.5L to 2L depending on your needs) at least one day in advance. You can find good yeast starter calculators online and good technical documentation.

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  • I live in England and keep my fermenter in the garage which is rarely over 16°C (except May-Aug). And I can keep fermenter in a fridge if necessary. I also use a wort chiller after the boil and tap water temp here is almost always below 20°C. – Scotty.NET Oct 24 '18 at 10:38
  • Then slowly cool down your yeast preparation to the temperature of the wort, knowing that a 2C difference doesnt really make any change all things considered (precision of measurement, yeast stress...) – JeanMi Oct 26 '18 at 9:05

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