Lagering - what goes on?

[Cheshire-cheese]

I understand that lagering produces a "cleaner" flavour but I would like to know what goes on. I have read that it decreases the amount of diacetyl, acetaldehyde and sulfur compounds, but with no explanation how. Any ideas?

[steve_flack]

Yeast is responsible for cleaning up acetaldehyde, diacetyl, hydrogen sulfide, after it has produced them, exactly how is one for the scientists.

It's pretty straightforward for the carbonyl compounds (acetaldehyde, diacetyl). Yeasts are very good at reducing the carbonyl group in these compounds down to the corresponding alcohol. Fortunately, these alcohols usually have much higher flavour thresholds than the carbonyls so you don't taste them.

[mysterio]

Chill proofing, basically. Protein-tannin complexes precipitate at the lower temperatures and are hopefully left behind in the aging tank. How much protein and tannin in the beer is going to depend on how much malt and hops were used. IMO much of this occurs in about 2 - 3 weeks at near freezing temperatures, but can be enhanced with finings. Removing these compounds won't only make the beer clearer visually, but it will also taste better, because they don't taste very good. This is why hoppy beers can taste quite rough when they are green, hops contain a lot of tannin, and would probably benefit from some lagering even if it is an ale.

This is what most homebrewers and I suspect most lager breweries are doing anyway, traditional lagering implies that there is still some fermentable extract left in the beer and the yeast are slowly working away reducing diacetyl, off-gassing hydrogen sulfide, producing esters and so on. I suspect most brewers are doing diacetyl rest, that is to say raising the temperature towards the end of fermentation to perform this maturation quickly, then dropping the temperature for chill-proofing.

I read on another forum some guy was making an Oktoberfest and aging it from March until October, and pretty much telling everyone that you cannot make a true Octoberfest unless you age it for 7 months in this way. Now i'm sure this beer tasted great, but this is a fallacy that pops up quite a lot in homebrewing lore, that of translating historical processes and practices into modern applications. Usually most of the important detail is lost on the way. Just google how to make a traditional IPA. You'll be left thinking you can't make it unless you take your cornie keg on a tall ship sea voyage to India. The IPA police and the lager police are roaming HB forums looking for unsuspecting schmucks to pull over for not aging their lagers for at least 5 weeks, you have been warned! God help you if you use crystal malt in your traditional IPA.

[Cheshire-cheese]

Thank you both.

[Cheshire-cheese]

I am now considering introducing a diacetyl rest into my lager brew. With a lager yeast with an operating temperature of 11 -24 C but a recommended temp of 11 - 15 C, shall I just bring it up to room temperature and for how long?

[edit1now]

One day is fine for a diacetyl rest, then back down to cool fermentation temp until you think it's finished, then drop the temp right down for two or three weeks.

[Cheshire-cheese]

Ta!

[mysterio]

Remember to taste before you crash down - beer should be free of any acetaldehyde and diacetyl and should taste good

[Cheshire-cheese]

Perhaps I'm being dim but what do you mean by crash down?

[mysterio]

Sorry, that wasn't clear, I meant crash cool from fermentation temp (or diacetyl rest temp) down to lagering temp.

[Cheshire-cheese]

Ah, thought that'd probably be it.