Gröna små äpplen

One of my latest endeavors in home brewing is lager, Especially the German Pilsner. I’ve come to love this flavorful, bitter and crisp beer to the point where I don’t drink much else. It might be a cliché among homebrewers to regress to a state where lots of us once started, lager drinkers without much interest for dark, cloudy and bitter styles.

However, the pilsner is not an easy style to brew. It craves attention in almost every aspect of the process, from picking a yeast strain and making a big enough starter to controlling fermentation and lagering. I think this has something to do with the appeal of brewing lagers. It’s well known for its inability to hide off flavors and poor brewing. This is where I segue into this posts topic, off flavors, and as you might have guessed from the title (which I borrowed from a beloved Swedish Jazz singer) the culprit is acetaldehyde.

Acetaldehyde is a chemical compound found in an array of things. It appears in nature in places from ripe fruit to coffee.[1] It is also very often featured in web articles on hangovers which makes it difficult to find any useful information when searching for information regarding acetaldehyde and home brewing. Luckily I have something of a small library at home stocked with books on home brewing. In this post I will lay out how I go about solving my problems with acetaldehyde. Tag along and you might find something that is useful for you as well.


evaluating beer

My problem:

My lagers taste pretty good during the fermentation, they might smell like fart for a while but that is just how they do. They smell like fart. Nothing wrong with that. But in the late stages of fermentation and during lagering they take on that horrible smell of green apple. It’s a fruity and chemical odor that screams at me. FAILURE! The acetaldehyde doesn’t go away during lagering, it stays with the beer even until the point where it’s clear and beautiful. One might say the beer taste the best when just looking at it.

To find a solution I need to understand the problem. When and why does the acetaldehyde occur? In Brewing Better Beer by Gordon Strong the acetaldehyde is described as an “intermediate fermentation compound and a precursor to alcohol”.[2] This connects to the mention of the compound as a typical flavor in green beer (or young beer).  Following this thread the next step would be evaluating how long a beer needs to be conditioned in order for the acetaldehyde to disappear. As an intermediate compound acetaldehyde is what happens before something else and in yeast by Christ White and Jamil Zainasheff this “something else” is ethanol.[3] Thus acetaldehyde + an enzyme called alcohol dehydrogenase = ethanol. I hardly believe adding more of this enzyme (if it’s even possible) is the easy way to go about solving my problem. Instead I have to facilitate the best environment possible for the enzymes already occurring in the fermentation. This brings me to the question how do I do this?

The 3 phases of a lager fermentation

A lager fermentation consists of these 3 phases:

primary fermentation: main fermentation of the fermentable extract. The bulk of the CO2 and alkohol are created here

maturation: the yeast is allowed to clean up some of its byproducts like diacetyl (butterscotch flavor) and acetaldehyde (green apple flavor)

cold stabilization (lagering): the low temperature causes haze forming proteins and polyphenols come out of solution and drop out of suspension. There is also a mellowing of flavors and some formation of esters happening. The latter becomes only significant after more than 12 weeks [Narziss 2005]


– 2016-04-01

Returning to G. Strong he explains that the acetaldehyde is high during fermentation and decreases during conditioning.[4] If applied to my problem it can either be false, since the acetaldehyde is increasing during conditioning, or true which means that I rushed my beer into the conditioning phase to quickly. Since the first scenario would be useless unless I dwell on the latter I will proceed in describing my process.

After pitching the yeast on the 20th of February I let the fermentation go on until 19th of Mars. By this time the beer had reached terminal gravity and then some. A week before, the 12th of mars, I ramped up the temperature to 13-14 degrees Celsius for a diacetyl rest. On the 19th of Mars I kegged the beer following good sanitary procedures and purging the kegs with Co2. After two weeks of cold conditioning the beers were tasted and this is when the acetaldehyde was showing in its fullest and most gruesome form. Blasting my face with icky green apples. According to Strong I should condition my beer for an even longer period, thus giving the enzymes more time. This is the first step.

I do fear the time solution to be a bit too simple since a good amount of time has already been given. Therefore I will keep looking for more solutions.

White and Zainasheff presents data by White Labs where they used gas chromatography to measure acetaldehyde in beer. The perceptible amount of acetaldehyde is 10 ppm. In this test white labs fermented the same wort using an ale strain at two different temperatures, 19 degrees Celsius and 24 degrees. The one fermented higher produces higher numbers of acetaldehyde than the one fermented colder. It is not necessary that this applies to me since lager fermentations occur at much lower temperatures. On the other hand low fermentation temperatures can stress yeast cells as well. I will now explore the yeast amount and health as a possible reason for my problem. Using the Mr. Malty Yeast Pitching Calculator I calculated that I would need 5.18 liters of starter to reach the amount of cells needed. My started was approx. 5 liters and would therefore be enough. It was plenty healthy which was apparent during the starter fermentation.

Pitching a large and healthy starter might not be enough. Another factor to take into account is aeration of the wort in which the yeast is pitch. I aerate through rocking the carboy back and forth, which will give me a maximum of 8 ppm.[5] This is in the lower range of the recommended amount of oxygen. A solution to this problem would be using pure oxygen. This is the second step.

I will try these two steps in order to reduce the amount of acetaldehyde in my lagers.

[1] 2016-04-01

[2] Strong 2011, 232f.

[3] White and Zainasheff 2010, 31.

[4] Strong 2011, 233.

[5] White and Zainasheff 2010, 78f.


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