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another question for lager brewers
A couple quotes from Palmer and link to his page with the same quotes.
http://www.howtobrew.com/section1/chapter10-4.html
Start of third paragraph:
"Some brewers pitch their yeast when the wort is warmer and slowly lower the temperature of the whole fermenter gradually over the course of several days until they have reached the optimum temperature for their yeast strain. This method works, and works well, but tends to produce more diacetyl (a buttery-flavored ketone) than the previous method."
That previous method is pitching cold.
End of paragraph
"Some yeast strains produce less diacetyl than others; a diacetyl rest is needed only if the pitching or fermentation conditions warrant it."
How many lager brewers in the homebrewing world are CRITICALLY tasting their beers and determining if a D-rest is necessary. Probably not many, most are just applying a D-rest because that is conventional wisdom.
brewchez wrote:
The d-rest is also part of the "pitch warm" then chill practice with lager brewing.
When the yeast get started at warmer temperatures there is more diacetyl produced.
Diacetly production is dependent upon available oxygen (off which there is less in colder wort) and the size of the yeast pitch. It seems to me, that proper yeast pitching rates and cold temperature on its own, greatly limit diacetyl production in the first place. The use of a D-rest then becomes less critical after a cold pitch.
Well, at warmer temps yeast do in fact produce more diacetyl but at the same time these warmer temps are more conducive to reduction of diacetyl, hence the reason for doing the D-rest. So, I really don't see how pitching at higher temps can really contribute that much more diacetyl when the yeast would be reducing it at the same time. In fact, diacetyl reduction occurs very slowly at cold temps, so it would seem that only exposing your fermenting lager to cooler temps would result in a beer with higher amounts of diacetyl than if the same beer were raised to D-rest temps. I wonder if there is data out there on the difference in diacetyl levels in warm vs cold pitched beer that is fermented at lager temps?
Yes, diacetyl production is affected by O2 levels, but your statement about there being less dissolved O2 in colder wort is just not true. Henry's law states that as a liquid cools is holds more dissolved gas. Diacetyl production actually increases if there are low amounts of dissolved O2 (<0.8mg/L O2), whereas O2 levels above this and below 8mg/L tend to decrease the level of diacetyl production. Any homebrewer who properly oxygenates their wort will attain O2 levels somewhere between these levels. This information can be obtained from the George Fix article you posted.
I think that the reason that most homebrewers do a Diacetyl rest is to prevent any issues that might arise when making a lager, regardless of whether or not their yeast strain is a high diacetyl producer. It's better to be on the safe side than to ruin a beer, in my opinion.
This is a very interesting topic, though, and warrants further research and investigation. I'll try to see if I can find data about temperature dependence of diacetyl production and reduction.
I thought that I had also read on one of these sources that the amount of diacetyl produced at a lower pitching temperature is at or below the detectable falvor threshold. Hence the rest isn't necessary, and extended lagering cleans it up. Albeit at a slow rate.
1n1m3g, I am with you; this is an interesting topic. Personally as a lab guy I prefer to look at what the literature says about these things, then form my own hypothesis. I personally don't take much of any of these things I read as being gospel. Because most of these guys aren't chemists, their jsut reading and reiterating. (Also, just because Palmer writes it, I never get to see his reference so where is he getting his information???) I'd much rather test it for myself. But also being a lab guy I'd rather rely on a chemical analysis of the diacety in a warm pitch/cold pitch sample, than on my taste buds.
BTW, I concede the O2 solubility. I later reread that statement and was like "Am I on crack?" I think I just got typing and hung up in the moment. I was thinking along the lines of a poorly oxygenated wort, with a big pitch of lager yeast, wouldn't require much O2 for growth phase, because you put plenty of yeast in there. Hence the lower O2 would limit some diacetly production
I found an interesting paper on PubMed about diacetyl formation in fermenting wine (there seems to be a lack of good info about diacetyl formation in fermenting beer): The 'buttery' attribute of wine--diacetyl--desirability, spoilage and beyond
Here is an interesting quote from this review paper:
"However, little information is available on
the effect of temperature on diacetyl accumulation in
wine. De Revel et al. (1989) noted a higher concentration
of diacetyl in the medium incubated at 18 C rather
than 25 C, at both days 7 and 16, during MLF in a
chemically defined medium lacking in ethanol. The
difference may reflect a slower rate of remetabolism of
diacetyl at the lower temperatures, however, more work
is needed to confirm the effect of temperature."
Here MLF refers to malolactic fermentation... (read more about this in the paper if you are interested).
This paper really looks at how malolactic bacteria are involved in formation of diacetyl during wine fermentation and doesn't consider how Saccharomyces cerevisiae (or brewing yeast) can contribute. Just knowing how the biochemistry of amino acid synthesis works (both bacteria and yeast use similar enzymatic pathways), though, I would suspect that temperature would have the same effect on diacetyl formation by yeast. The authors do state that little is known about how temperature influences diacetyl formation and that more research is required.
So, the question remains... Do you pitch warm or cold for lagers and is a diacetyl rest necessary? Given the lack of information, I think I would just continue to use the tried and true method of pitching cold and doing a diacetyl rest after fermentation is complete, at least until someone can show me data otherwise from a properly controlled experiment.
1n1m3g wrote:
So, the question remains... Do you pitch warm or cold for lagers and is a diacetyl rest necessary? Given the lack of information, I think I would just continue to use the tried and true method of pitching cold and doing a diacetyl rest after fermentation is complete, at least until someone can show me data otherwise from a properly controlled experiment.
Agreed.
I think the point that also needs to be conceeded here is that we won't switch in the absence of data. Yet, we do what we are doing in the absence of the same well controlled data.
What makes Standard Practic standard?
But I fully agree that a D-rest is the best insurance against the presence of diacetyl. I don't agrue the accepted metabolism that yeast provide in eliminating diacetyl.
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