What else does a diacetyl rest do?

[menschmaschine]

OK, let's get this forum going...

We talk about diacetyl rests (primarily for lagers) and how the yeast "cleans up" diacetyl. IIRC, the yeast do this because, when aided by an increase in temperature, they turn to other compounds for "food" (in the case of diacetyl, metabolic waste that they've created). But, aside from this and possibly achieving a slightly better attenuation, are there other benefits to doing diacetyl rests?

I do them for every lager whether or not I have diacetyl (and I've only had it once). I didn't put a whole lot of thought into doing them every time, I just decided to do that instinctively because I figure there has to be more going on than just "diacetyl clean-up" that can be beneficial to making a cleaner lager.

Any thoughts or knowledge on other benefits of diacetyl rests? Are there other flavor-active compounds getting cleaned up as well?

 

[Beerrific]

In general as you increase the temperature (over a reasonable range), every enzymatic process is increased. I know there is a chart in How to Brew that shows the aging or lagering times needed at different temperatures and this time decreases at higher temperatures simply because these processes whether they are specific to yeast or not are happening faster. There is nothing specific about the diacetyl rest temperature that promotes the removal of diacetyl, it is just the increase in temperature that speeds up the chemical processes in the metabolism of yeast.

Take for example the Arrhenius equation. This asserts that for every 10C increase in temperature, the rate of a chemical reaction doubles. This is what is happening in the case of diacetyl; diacetyl will be eventually be absorbed by the yeast (assuming the yeast is healthy) at fermentation temperatures, but if you raise the temperature 10C it will happen 2 times faster. This is the case for other chemical reactions as well, some of which age the beer in a good way (removal of acetaldehyde, etc.) but will also age beer in a bad way (increase oxidation rate, etc.).

 

[menschmaschine]

That's a great explanation Beerrific. Although I am curious about the oxidation part of that (how much of an increase in temp is too much).

It seems then that "diacetyl rest" is a bit of a misnomer... maybe just a bit because the taste threshold for diacetyl is probably low, so it's the compound of most concern.

 

[Beerrific]

That's a great explanation Beerrific. Although I am curious about the oxidation part of that (how much of an increase in temp is too much).

It seems then that "diacetyl rest" is a bit of a misnomer... maybe just a bit because the taste threshold for diacetyl is probably low, so it's the compound of most concern.

I was familiar with the Arrhenius Equation, but did not think to make the connection between it and brewing until I heard the Brew Strong episode on hot side aeration. John Palmer interview Dr. Charlie Bamforth and one of the points he made was that all beer stalling processes take time and if you can keep the temperature low, this will slow this stalling. If you are at all interested in the technical side of brewing, I highly recommend that show (and all Brew Strong episodes).

The taste threshold for diacetyl is around 0.08 ppm (source), so, yeah, that is very low. I think it is a target so often becasue of the taste threshold, the fact that some people really dislike it, and it is fairly easy to get rid of if the beer is still on the yeast.

 

[BarleyWater]

I was looking for that link yesterday when talking to someone about their beer aging. That was a great episode. I like it when Jamil doesn't talk too much, there's more beer information and less dick jokes.

 

[menschmaschine]

If you are at all interested in the technical side of brewing, I highly recommend that show (and all Brew Strong episodes)

I've listened to one or two. In the few I listened to, it didn't seem like there was much chemistry between Palmer and Z. And I mean that as a compliment to Palmer. Perhaps I can get over certain hang-ups and give it another try.

 

[pjj2ba]

There's a lot more to it than just speeding up reactions. If it were merely speeding up the reactions, then a longer ferment would be all that is required, not an actual increase in temp. A rate doubling for 10 C would mean that a 2 day rest at ~68F would take only 4 days at ~48 so why bother. We know that is not the case.

Whole different sets of genes will be turned on/off as the temperature changes. We're certainly all familiar with the differences in flavors from an ale fermented at 65 F versus 75 F. The genes that are turned on/off by a 10 C change in temp are going to be different based on the food resources available (ie. fresh wort versus one at 90% fermented). It is really quite complex. Now for a lager, many of the fermentables have already been consumed by the time a diacetyl rest would be done.

Ok, I started to dig around a little in the scientific literature and found a very current article on pitching rates and the influence on diacetyl PRODUCTION (among other things). They don't talk about it's removal - That would be another paper.

Impact of pitching rate on yeast fermentation performance and beer flavour

OK, now if were going to be all geeky in this sub-forum, why don't we go for broke and discuss scientific papers/books that people come across. Sort of an online journal club. Someone finds an article - like the one above, and then provides a link to it and then people read it and then we discuss, particulary with our thoughts on how it applies to homebrewing as many of the papers I come across are geared toward large scale brewing and the emphasis of the writing reflects this.

For those who might find this kind of reading a bit daunting, the trick is to be selective in what you read. The introductions are often fairly easy to understand and often have a lot of nice background information. Plus they will point you towards other articles that have more detailed information than what the authors have summarized in the introduction. Then hopefully there is a discussion or conclussion section - read these. Often the last paragrah(s) of the paper will summarize the whole thing. One doesn't really need to read the materials and method, or even the results section to understand the point of the paper. Figures and tables are always good to look at. Sometimes the results and discussion are in one section and these can be a bit rough to read if you aren't familiar with the subject

Here is one place to search for scientific articles
PubMed

I prefer the Web of Science, but I don't know what kind of access the general public has to this search engine. I'll have to try from home.

PubMed gave me 27 hits for Diacetyl and Beer versus 97 for the Web of Science. Although many of the hits for the latter are in journals I don't have electronic access too

Should I start a new thread with the above paper?

 

[Kaiser]

home brewers tend to think that a diacetyl rest is necessary and the special thing in lager fermentation. And there is confusion about when to hold it and how long.

But in most cases the primary fermentation is actually long and slow enough for the diacetyl reduction to happen during the primary fermentation phase. That's why you don't notice it at the end. But I sill recommend warming the beer towards the end unless you know that it doesn't need it for proper attenuation. Mainly because you can speed up the fermentation and reach for desired FG before you crash the beer to lagering. Even if you plan to lower the temp slowly to lagering you want to get close to your FG since it is really difficult to not shock the yeast. I have made several attempts at this and always stalled the fermentation.

The diacetyl rest (also called maturation in the industry) can be held above, at or below primary fermentation temps. The higher the temp the faster the process of diacetyl reduction. But it is commonly agreed upon that a lower temp is better for quality. The latter only applies if you are able to hit your FG with the lower temp. If you stall the fermentation then you will be worse off compared to holding a higher temp rest or doing an extended primary fermentation ("diacetyl" rest done at primary fermentation temps)

Kai

 

[Mike123]

After everything I have read I've concluded that I'll simply leave my lager alone for a couple of weeks at fermentation temperature (50 F) after signs of fermentation are gone (no bubbles in airlock). I'll taste it after that and if I detect diacetyl then I'll let it sit another week and taste again. When I cannot detect diacetyl via tasting I'll lower the temperature to 40 F and lager my beer 4 weeks. Anybody see an issue with that approach?

 

[ajdelange]

Diacetyl is usually produced after the fermentation when the precursor, alpha acetolactate, a byproduct of valine synthesis, is is oxidized extra cellularly in the finished beer. Therefore, the goal is to be sure that any acetolactate in the finished beer either does not get oxidized to diacetyl in the package or that any which does subsequently gets reduced. The modern way to handle the problem is with Maturex, a commercial enzyme (alpha acetolactate decarboxylase) which takes acetolatate straight to acetoin (CH3HCOHCOCH3) which has a much higher taste/aroma threshold than diacetyl (CH3COCOCH3). The traditional ways of dealing with diacetyl was to allow it to form and then subject the beer to the reduced conditions associated with active yeast. This was done either by dosing in active yeast (kreusening) or by gradually reducing the temperature of the nearly finished beer (1 - 2 °P remaining) to lagering temperature. In either case, diacety is reduced first to acetoin and them to 2,3 butane diol (CH3HCOHHOCHCH3) which also has high taste and aroma thresholds. Intermediate between the Maturex and traditional long lagering methods is the diacetyl rest method in which the temperature is raised to kick the yeast into high gear so that they will go after the acetolactate more quickly.

If you have time and someplace to store beer cold then a diacetyl rest is unnecesary. Traditional lagering does the job and results in a better beer (if you want to start a discussion that statement will probably be the one that kicks it off). Clearly a modern brewery does not want to be lowering temperature over three weeks and then store near freezing for months before they can serve a beer but a couple still do it (including, I am happy to say, my local).

The other jobs of lagering (and thus the diacetyl rest) are to allow phenol/protein complexes to at least form which will settle (traditional) or be filtered (modern), to allow the reduced state to reduces acetaldehyde to ethanol, to allow volatiles (Jungbuket) to escape to allow adjustments to the protein spectrum and CO2 "melding" (traditionally lagered beers have finer head) and, of course, flavor maturation.

 

[wailingguitar]

I am doing a happy dance here... I have been telling people this stuff for years, but there is this mantra "lager beer, must diacetyl rest" ... I have never done a diacetyl rest, either at home or commercially. It has been my experience that leaving the yeast to do what it does naturally JUST WORKS... and that colder is better where lagers are concerned. Different yeasts strains behave a bit differently of course, so you have to get to know the yeast you're using. What is it's low-end point of active ferment? etc.

 

[SmokeNbrew]

I believe the biggest importance for a Diacetyl rest is because during aging, Diacetyl tends to increase. So if you have a beer at bottling that tastes fine with a slight buttery flavor, that flavor may be overpowering later as it ages. The rate of this increase is affected by temperature as an earlier poster mentioned.

 

[ajdelange]

Diacetyl increases during aging if there is residual alpha acetolactate in the package. The object of the diacetyl rest (or proper lagering or the addition of Maturex) is to see to it that there isn't. The higher temperature accelerates the conversion of AAL to diacetyl and the reduction of diacetyl to acetoin. If the conditioning (by any of the three methods) is managed properly there should be no (or below threshold) diacetyl and no AAL awaiting conversion to diacetyl.

 

[mabrungard]

pH also has an affect on AAL conversion rate and the ultimate rate of diacetyl clearing from a beer. The AAL conversion rate falls with increasing pH of the beer. Making sure that your wort pH is in the proper range should help bring the pH of the finished beer into range also. That will aid in the clearing of diacetyl from a beer.