I started brewing back in may, at the beginning of a Californian summer. I quickly learned that the swamp cooler was necessary to prevent unwanted ester formation due to a too-high of temperature for a given yeast. The swamp cooler isn't good enough, however, and requires substituting frozen water bottles 3x per day. This is absolutely excellent for keeping a ceiling on the wort temperature. However, the temperature swings beneath the ceiling can be pretty severe. For my current 4 batches going, I have put the ceiling at 66, but temperatures vary widely from 57-66, 24 hours per day.

I hear from this forum and others that temperature swings are also bad (that it will stress the yeast).

What is the reasoning behind this? I.e., too high of a temperature causes ester formation. Ester = off flavor.

Do the temperature swings, even with a ceiling on temperature, cause off flavors? Why. If they don't cause off flavors, why are they bad (other than swings that would cause a decrease in attenuation for going too low)? The swings are no doubt superior to ester production from too-high of a temperature, correct?

I would imagine that with a ceiling, a swing of 20*F would be bad as it would cause a lower attenuation. So for the second part of my question, lets add a theoretical floor to whatever temperature gives the maximum effective attenuation for a given strain. In this circumstance, with a ceiling (to prevent esters) and a floor (to retain attenuation), why would swings be bad?

4 Answers 4


Short answer - it's not that bad, per se.

Long answer:

The biggest 'problem' is consistency/isolation of variables.

Particularly when all-grain brewing there are a lot of things to keep track of throughout the process. As you keep brewing, you'll want to aim to improve parts of your process, and you'll probably develop a few favorite recipes. In a non-professional setup it's difficult to create a environment wherein ALL these variables are constant (boil time, hop additions, fermentation time, amount of yeast pitched, etc), but fermentation temperature is something of an attainable aim for most amateur brewers (do a quick search on homebrewtalk for 'fermentation chamber')

On to the 'reason' people get nutty about fermentation temperature -

The beer you make with the same recipe can and will vary (sometimes substantially) in overall quality, clarity, flavor, color, etc when these variables aren't stable. So it's important to stabilize what you can in the process so more uncontrollable variables can be isolated.

Fermentation temperature is fairly high in the hierarchy of importance among these variables. Swings in temperature, as you noted, can cause off-flavors, and too high a temperature can create excessive esters and diacetyl production. More controlled swings won't cause 'off-flavors' per se - but if you're making a bitter that you've made ten times before with stable fermentation temperatures and the same yeast, and this time the temperature jumps around during fermentation you'll definitely notice a difference. Will that difference be bad? Who's to say? Beauty is in the palate of the drinker - but if you like (or HATE) the results you'll not be able to replicate (or steer clear of) it in the future. Whereas if you could control the temperature, and you know you like the results you get when fermenting at 68F, but not at 72F or 64F...

  • 1
    I disagree that temperature swings aren't bad. Having precise temperature control is probably the biggest thing you can do to improve your brewing, and not only because of the consistency, but because of the cleaner end result.
    – mdma
    Commented Sep 5, 2013 at 19:31
  • 3
    @mdma, I agree with you. but lack of temperature control won't ruin beer - it's just that temperature control can dramatically improve it.
    – dax
    Commented Sep 6, 2013 at 7:27

Positive temperature swings have been discussed - fusels and esters are the main problems. If the temperature fluctuating then you'll also have negative swings.

Negative temperature swings can cause the yeast to drop out. This is particularly true with the Belgian strains, such as WLP530 and WLP570 - even with constant ambient temperature, removing the heat that was coming from fermentation as primary nears completion is enough to cause these strains to sometimes drop out prematurely.

The yeast remain active so long as they can maintain a sufficient metabolic rate via food sources and temperature - temperature is key since it affects how quickly the enzymatic reactions proceed. The yeast have various metabolic pathways available, activated based on temperature and available metabolites. A negative temperature swing can reduce the rate of metabolism via the current metabolic pathway to below the active threshold, causing the yeast cell to go dormant.


My understanding is that swings within a range are not a problem, as dax said. It may stress the yeast somewhat, but unless you are attempting to get perfect replication from brew to brew, or are trying to really nail down a flavor profile, it should be fine. I suspect that attempts to keep fermentation temperature at a rock steady level are as much an attempt to exercise maximum geekery, as anything else. Which is fun itself!


As a new brewer you should not try to be proficient beyond your ability. Maintaining consistency early and learning what to expect, not only in temperature but yeast quality, etc., allows you to better make modifications and corrections in future brews. Also, I suggest you use a note book and keep meticulous notes about such issues as temperature, hop additions (type and time), and as many other items of interest as you can think of. Temperature should be one carefully monitored item. When you brew a great batch you can go back and grade that batch, as well as bad ones. But at first.....consistency. Sometimes you may find other common factors recorded that make a good/bad brew.

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