I am still fairly new to homebrewing, and am on my second batch using this kit. The instructions given in the kit I don't think are particularly clear. It states that I should add 3.5 litres of boiling water and top up with cold water (up to 23 litres). It then states that it should be kept between 18 and 20 degrees.

However, what temperature is "cold water"? I live in Scotland. The first batch I did was in the winter; tap water is so cold it actually hurts your hand. Compare that with a warm region and you could be talking about a difference of 15 or 20 degrees C. Nonetheless, following these intructions I end up with the brew way up at almost 30 deg C.

What I am wondering is whether these kits are supposed to be kicked off at a higher temperature and then left to settle down to the ideal 18-20 zone?

It would far clearer if they replaced their nonsense about ratios of hot vs "cold" water, and simply told you what temperature to aim for!

So my question is: is the brew supposed to start at a higher temperaure? (to kick start the yeast..?) and if so, what sort of temperature should I aim for, ideally?


2 Answers 2


No, best is to start off for ale fermentation at 18° C, temperature of the wort, if possible.

The reason is that the yeast readily adapts to the temperature, and if you would start higher, and the temperature drops, then the fermentation could (would) slow down or stall.

I always try to put my fermenting beer in a closed room that keeps a more or less constant temperature.

  • 1
    Thanks for the advice. So I guess the concept of "kick starting" the yeast at an elevated temperature is a myth that I just invented. I have to rant again about the instructions I got with this thing: they are terrible! Nevermind, I'll get it right next time!
    – Inigo
    May 26, 2018 at 21:51
  • Also: once the fermentation kicks off, the temperature will rise due to the process. With my volumes (6 to 10 l) I see quickly that the temperature of the vessel is about 2° higher than the ambient temperature. This will be more for larger vessels.
    – chthon
    May 27, 2018 at 11:38

Early in the fermentation (especially with ale yeasts) diacetyl is produced. Later in the fermentation this is, to a certain extent, removed again (the yeast reabsorbs it through a process known as diacetyl uptake). Diacetyl is known for a buttery flavour.

Both diacetyl production and diacetyl uptake are increased by higher temperatures.

That means that if you start the fermentation at a higher temperature and finish at a lower temperature, diacetyl production is increased but diacetyl uptake is decreased, resulting in more diacetyl (buttery flavours) in the beer. Starting low and ending high results in low diacetyl levels.

This is a staple of lager brewing, where the fermentation ends with a "diacetyl rest" at a slightly elevated temperature to remove as much diacetyl as possible, since noticeable diacetyl levels in lagers are considered a defect.

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