Thanks Mark, I was just about to clarify this misconception, but you've covered it pretty well
As mentioned, Alpha-Acetolactat is a flavourless and odourless compound that is formed during fermentation.
(It is formed as a downstream "byproduct" out of Pyruvat during the synthesis of Valin (an alpha acid). What is Pyruvat? One of the key compounds or stepping stones on the path from C6H12O6 (Glucose) ----> down to ---> C2H5OH (Ethanol) + CO2 + Energy! (anaerobic fermentation)
or --> via aerobic fermentation to a much more substantial breakdown to H20 + CO2 + significantly more energy!
Pyruvat (C3H4O3) is, in the presence of Oxygen, worked through the extremely complicated Citrat-Cycle, where amino-acids, fatty-acids, and numerous compounds that are essential to new cell production are built, broken down and transformed.
One of these amino-acids is Valin. The amount of produced valin has a direct influence on the amount of alpha-acetolactat.
As mentioned, alpha-acetolactat is odourless and flavourless. So what's the problem? The problem is that it is very reactive to the chemical "decarboxilisation" (splitting off of CO2) in beer. Alpha-acetolactat minus CO2 = Butandi
on. a.k.a Diacetyl.
Our well known "butter" flavour/smell.
However, Diacetyl is then taken in by the yeast, where it is further processed down to Acetoin --> then 2,3-Butandi
ol. (Which is flavourless and odourless). And the rate at which this happens, is of course related to the temperature.
So, what can/should the brewer take out of this understanding of how the yeast works?? -> A few things indeed.
1. Diacetyl isn't some mystical magical evil compound that just inconviently presents itself to annoy brewers. Brewers who understand the fermentation process DO know how much Diacetyl is going to end up in the constumer's glass, and they know when/where/how/why it's produced and how to steer these factors in the direction they want.
Diacetyl isn't necessarily evil. In a bavarian pils, it's considered a "fault" above 0.1mg/L. In bohemian pils, it's well above this level, and is desired as such.
There are lots of strongly hopped beers out there that are loaded with diacetyl, however most consumers don't notice it due to less experience and a shittonne of hops.
2. An increase in the amount of alpha-acetolactat = an increase in the amount of diacetyl (which we then want the yeast to "clean up")
So, to minimise the amount of diacetyl, these two things need to be considered. How do we produce less a-acetolactat (let's call this 2a.)? And how to we optimise the "break-down" (<- I really dislike this word in this context, as it's quite misleading. But my english is alarming rusty these days :/ ) of diacetyl into 2,3-Butandiol? (let's call this 2b.)
2a. Again, why is a-acetolactat produced at all? It's a downstream product of the synthesis of (one of many) amino-acids. Why does the yeast produce more amino-acids??? (and this is extremely important, as it's also directly related to the amount of higher alcohols and other unpleasants in beer)
------> amino-acids are essential building blocks for cell growth/reproduction. The wort already has quite a lot of amino-acids in it, however when/if they aren't sufficient....it has to make it's own. (Note here, this is slightly misleading because these cycles are almost inevitable at least to some extent)
So, two big points here: factors that stimulate growth, also stimulate this process. And a lack of amino-acids in the wort exaggerate this further! (Ever seen the term "FAN" --> Free Amino Nitrogen. That is = amino-acids)
Factors that stimulate growth?? We already know those as brewers: oxygen and temperature are the big ones. (Pitch rate does also play a role here, but's better just to stick to pitching at a usual advised rate and varying the other factors. Because, you can't change the pitch-rate on day 3 or 5 of fermentation, but the temperature you can
)
In short, cooler temps during the initial stages of fermentation reduce the level of a-alcetolactat produced.
And, oxygen also increases the amount. But don't let this confuse you. This doesn't mean "aerate the wort less". During the first 12-24 hours, the yeast reproduces itself exponentially. It needs a shittonne of amino-acids. There's a reason why we aerate, and this is a big part of it! However, aerating after this stage is going to stimulate the citrat-cycle, producing more amino-acids, than are needed (the wort is already getting low on sugar....the yeast doesn't need to keep reproducing so much)
In short, aerate once, and do it well.
2b. The reaction of Diacetyl -> flavourless/odourless 2,3-Butandiol can also be influenced.
Via: temperature, yeast strain, movement/agitation, PRESSURE, and again pitch-rate. (We'll leave pitch-rate aside for the same reasons as previously)
That means, once the yeast has made all of the a-acetolactat that it's going to make (i.e. once the wort is exhausted of oxygen and mostly of energy sources too), slightly warmer temps accelerated the reduction of diacetyl. Hence the popularly coined "Diacetyl rest".
(As an aside, I'm not a fan of the term, as it neglects the bigger picture. This "rest" accelerates the maturing and rounding up of numerous compounds/flavours in the beer. Not just diacetyl)
Interestingly..... pressure has a
negative affect on the "break-down" of diacetyl.
In summary, this is why bavarian/german breweries brew their lagers at the temps they do. Lots of them still don't raise the temp for "conditioning", and opt for more time in the name of tradition. But either way, there's a good understanding of what's happening in the beer. (If there's not, they don't pass the state exam, and are then not a "brewer")
All of this in summary also sheds light on Goose's dilemma with "increasing" diacetyl.
To further clarify my earlier statement: Diacetyl will and can only increase in *BEER that has properly finished fermentation and conditioning, if some other non-brewers-yeast diacetyl producing organisms have entered and/or already were in the "young-beer".
* Beer isn't "beer" until fermentation and conditioning are complete. Until then, it's actually "young-beer".
If conditioning has properly taken place, all alpha-acetolactat has been reduced to diacetyl and then further to 2,3-Butandiol.
And not just diacetyl. Like I keep saying, there are tonnes of compounds that need to be conditioned here. Diacetyl is taken as a benchmark of checking if the whole process has gone far enough, largely because it is detectable by humans in such low quantities. If it's gone (enough), then it's safe to say that conditioning is pretty well done.
Reading your old post, Goose, it jumped out at me that you pitched at 12°C, and fermented at 10°C.
I personally wouldn't have thought that this would result so extremely in the problems you seem to be having....but in light of how the yeast works, you should have better results adjusting your fermentation in the other direction
12°C is pretty damn warm for a lager.
Sorry about the long post again. But I wanted to clear up some very commonly misunderstood ideas
