We all understand the importance of the Diacetyl rest toward the end of fermentation by raising the fermentation temperature. So my question: how close to the end of fermentation and how much should the temperature get raised?
I have always waited until fermentation is within 3-4 points away from target and I then raise the temp 6* F for about 3-4 days, or until my SG is unchanged for 3 days. But are these normal parameters? I cannot recall reading anything specific so I thought I would tap some of the knowledge in this forum.
There is not really a one-size-fits-all answer to this. It varies by strain, fermentation temp, yeast vitality, pitch rate, etc. You don’t have to raise the temp of the beer, although it may take longer to clear diacetyl if you don’t (mostly for lagers). If your goal is to turn around fermenters as fast as possible, you should do a forced VDK test on your beers after the FG has been stable for a day or two.
There’s a lot of literature indicating that the production of flavor compounds is complete by the time the fermentation is 2/3 complete So that initial period is where you want to be fermenting at a temperature chosen to optimize your flavor profile. We know that raising the temperature reinvigorates the yeast to reduce compounds we don’t want. So based on this, anywhere after 2/3 of the way to FG (usually, that is, ~50% AA) temperature can be raised. This is basically the modern “Narziß” lager fermentation program. Your program is another traditional practice in lager brewing. (White and Zainasheff summarize all this; the standard textbooks do address temperature cycles.) But it’s true there’s no one single right way. It is indeed dependent on the yeast and the individual fermentation, as well as what is convenient given your means of temperature monitoring and control. My practice is this, for ales and lagers. While I do track attenuation, my real clue is: when, at the same ambient temperature, the temperature in the ferment has peaked and starts to drop – that is, the yeast are no longer generating as much heat – I take it as a sure indication of transition from the primary to the secondary phase. At this point I allow a free rise to my chosen final ambient temperature. Listen to the yeast. When they’re having trouble keeping themselves warm, turn up the thermostat. Works well for me because I monitor fermentation temperature, but control ambient in the chamber.
First, check for diacetyl. If there’s none, then you don’t need the rest. However, if you want to do a diacetyl rest anyway or just aren’t sure, then it won’t hurt anything to just do one anyway either. Aim for when the gravity points are half of what you started with. For example, when 1.054 OG gets to half of 54 or 27 or 1.027, then that’s a good point for the D rest. Anytime after that point is fine too.
RC - I’m not sure I know what a “VDK” test is. Perhaps you could explain?
Robert - your answer makes perfect sense. However, I don’t have the luxury of monitoring my fermentation temperature. So, my question to you is: how are you monitoring this temperature; what device are you using and does it record?
Dave - easy, simple and very logical. However, if I decide to do a D rest, how much should the temperature be raised? I’m currently raising 6° F. Is that enough or too much?
I appreciate the help! Great information! Thank you!!
I did at one time have a fermenter equipped with a thermowell. Since I now ferment in a 10 gallon corny I’m back to an old fashioned, stick on liquid crystal “fermometer.” It’s accurate enough to see temperature trends from observation to observation, and it records the way everything in my brewery does. Data passes through me to a notebook.
If you think about the purpose of a d rest, the answer to your last question becomes clear…it doesn’t matter and there is no clear number of. All you’re trying to do is make the yeast more active, so any temp that does that is fine. And I totally agree with Dave bout the need for one. In 536 batches, I don’t think it’s been necessary for me more than a dozen times.
If sufficient yeast is pitched and it is a well attenuative strain, I simply don’t think that a diacetyl rest will be needed - at least most of the time. In instances where I have spunded, I will do a d-rest on occasion, just for the sake of certainty, but that’s without testing and when using a closed loop transfer, since testing would be a hassle for me inthat circumstance…cheers!
Diacetyl is formed when the precursor is oxidized outside of the yeast cell. The Diacetyl is absorbed by the yeast. Warmer temps make that chemical reaction happen quicker. Source
RC makes a good point though. Even if diacetyl is not a problem, raising the temperature will hasten things along if you want to turn around fermenters. This is usually my reason for a temperature rise, as I’ve never really had a diacetyl problem either, and after the first 2/3 of attenuation there’s no adverse effect. Just keeps the pipeline moving.
While a D rest may or may not be required, I raise the temp of every beer regardless of yeast used Ale or Lager as a simple matter of consistency. It’s just too easy to raise the temp of the beer by 5*F. If it doesn’t reach the initial temp +5 by itself, I use a heating pad so it does. I do it primarily to help get the last couple FG points. (Sometimes it needs it sometimes it doesn’t …but I do it anyway.) As a side effect my routine performs a D rest (required or not).
Likewise, I cold crash/lager/condition every beer regardless of Ale or Lager at 34*F for 6 weeks as a matter of routine.
That routine does require planning but I do it the same way every time as a matter of standardization. It just makes scheduling easier if I treat them all the same. As a result, I am brewing two months before I plan to drink the beer. For example, I brewed a Scottish Ale the 1st week of Dec to tap late Jan/early Feb.
FWIW, on my Kolsch recipe, I start ferment at 60* for 2 days then lower to 55* until I get about 10 points away from FG. Then I let the temp free rise to 68*, set temp controller, where it will finish out and FG is steady for 3 days. Cold crash at 35* and let it sit at that temp for at least 10-14 days. I like the slight fruity taste I get from 68* Maybe I don’t need a diacetyl but the process works for me.
If you’re raising the temperature to 68°F at that late point, the yeast are no longer producing flavor compounds but reducing them, so that’s not where you’re getting fruitiness. If 60°F is on the warm end for your yeast, that critical first two days is probably giving you the fruit. But as you say the whole program gets the result you like, so that’s what counts.
Robert,
I use WLP029. Temp range is stated as 65*-69*
I don’t know enough about yeast/fermentation in regards to the flavor compounds to formulate a argument for it.
So, with my fermentation schedule as posted for this yeast, at what degree of attenuation do you propose a boost in temp?
White labs states that 50% attenuation happens within 30 hours. I’ll have to reference my notes and see if thats the case with my process because that seems entirely to fast.
If what you’re doing works, I’d say keep doing it. But in general, fermentation schedules are planned around either a rising or steady temperature rather than lowering it in the middle. The concern is that lowering the temperature prematurely will shut down the yeast to some degree, and I’m not sure what it would be intended to do. If you want a flavor profile characteristic of either warm or cool (for the strain) fermentation, that is, fruitier or cleaner, the early stages are where that will be influenced. But again, it’s all about what works for you. You could try a batch starting at the lowest temperature you find acceptable and let the temperature rise to 68° after the 2/3 of the way, or ~50% AA, I’ve mentioned, and see if it’s any different, or more or less convenient. If nothing else, it’s less complicated.
On a side note, the genomic research has shown 029 is an English yeast in the Whitbread B family. In that context, 30 hours to 50% sounds about right. Probably for most ale yeast. But I’m not sure what conditions they used to determine that. (If fermentation data for a given strain don’t include fermentation temperature I’d default to assuming they use the high end, maybe 68°, so that the analyses of fermentation products are worst case values.)