Dear Mr. Wizard,

I just tried my first all-grain brewed pale Pilsner malt beer. Bottled about 12 days ago, it had a very “unripe” somewhat sour taste to it. Kind of a “green” peach or something. Will this subside? Does it just need more time to age? I used 3–4 ounces (84–112 g) of Saaz hops at three different stages and a fresh hops flavor is present. The beer itself tastes fresh as well . . . but a little too fresh — like it needs to age and mellow out. Please help! Will time fix this? If so, will it take two weeks . . . six months?

Dave Morrow
via email

Mr. Wizard replies:

One thought comes to mind and that is acetaldehyde. Almost all homebrewers know about diacetyl and are often taught to dislike diacetyl with vigor. I am one of those brewers who really detests diacetyl in most beers. Oddly enough, I like a big buttery red wine or an oaky chardonnay with detectable diacetyl coming from malolactic fermentation. Acetaldehyde and diacetyl are two flavor compounds frequently associated with insufficient aging time and both flavors are easily reduced if a rest period is allowed between fermentation and cooling. (Diacetyl is also associated with bacterial contamination and time will not cure that problem!)

Acetaldehyde is a metabolic intermediate and is leaked from the yeast cell during fermentation. After vigorous fermentation is complete, yeast cells will absorb acetaldehyde and reduce this “green” aroma compound to ethanol. Most commercial lager brewers monitor the level of diacetyl, diacetyl precursor (alpha acetolactate) and acetaldehyde before cooling beer and moving forward in the process. When beer is prematurely separated from yeast and crash cooled (which effectively halts yeast activity) the likelihood of having a beer smelling of apples and butter increases.

If there is sufficient yeast in your bottled Pils the problem may subside. I would suggest taking a few bottles of your green beer, gently rolling them around to re-suspend the yeast and storing them warm for a week or two. If the flavor intensity does not drop in this timeframe you are probably stuck with it. In the future, especially when fermenting lagers, make sure you hold the beer at fermentation temperature for at least four days prior to cooling.