Thu May 09, 2013 3:31 pm
Is this true? Because if it is, we home brewers have the whole diacetyl rest thing backwards. I've recently read that the purpose of diacetyl rests isn't to get rid of diacetyl, but it's to produce all the diacetyl you can because diacetyl is easily detected by taste and easily taken up by yeast. The theory being that if you've fermented correctly at lager temperatures, you shouldn't have a lot of diacetyl present, but you can't avoid having a large amount of the precursor, alpha acetolactate, which is completely undetectable by human senses. At the end of fermentation a lot of us will warm our lagers up to 55-60 °F to drive away diacetyl, but what we're really doing is helping to speed up the oxidation of acetolactate to diacetyl and its subsequent metabolism by the yeast back to flavorless compounds, which can still take up to 4 weeks; not 2-3 days. (People that like to do 2-3 day diacetyl rests, then rack and crash cool to 30-35F should take note). Diacetyl is a by-product of alpha acetolactate, and acetolactate can't be detected by taste. As beer ages, the reaction that converts acetolactate to diacetyl continues, but if you've already racked your beer to a keg, there's very little yeast present to convert the diacetyl back to relatively flavorless compounds so unless you perform a diacetyl rest, you end up with an unpredictable final product. So when you do one of those "diacetyl tests" where you heat the beer up to 140 and cool back to room temperature to compare to another sample of your beer, you're not really performing a diacetyl test, you're really performing an "alpha-acetolactate test", by speeding up the conversion of acetolactate to diacetyl which is easily detected by taste. When you read that diacetyl can be difficult to perceive, this is why; acetolactate is not only difficult to perceive, but it's impossible to perceive. I currently have a pilsener that had an OG of 1.052 that's sitting at 1.011. It's still fermenting a bit, and I expect it to get to 1.008-1.009. I fermented at 52 degrees with Wyeast Budvar yeast for 10 days. It tasted kind of buttery when the gravity was 1.014 so I raised the temperature to 58 degrees for the past 2 days and now it smells and tastes just like something you'd get at the movie theater. Not only that, but it even leaves a buttery coating on the roof of my mouth. I've never had a beer like this one. Has anyone ever experienced anything like this? How long does it take for the diacetyl to go away? Will it go away? I'd like to enter this beer in a competition in mid-June.
Come On Fulham!!
"...know your process, know your yeast." - Ozwald
