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-   -   Mysterious acetaldehyde problem in high-gravity lagers (http://www.homebrewtalk.com/f163/mysterious-acetaldehyde-problem-high-gravity-lagers-271612/)

stoutaholic 09-29-2011 03:10 AM

Mysterious acetaldehyde problem in high-gravity lagers
I've brewed a about 35 batches, everything from Belgian Dark Strongs to American Lagers to Scottish Ales, and have had success with every style except high-gravity lagers. Of the 5 high-gravity lagers I have brewed, all have contained a distinct acetaldehyde character; that is to say, an acetic-cider, or green-apple type character, with the attendent astringency.

None of my other beers have ever had this character; in fact, my medium-gravity lagers, such as my Munich Dunkel, German Pilsner, and Oktoberfest, have all won medals. So I don't have a generalized problem with brewing beer or lagers. This is not a sanitation issue; it is not lactic acid that I am tasting, and it wouldn't make sense for ONLY the high-gravity lagers to become contaminated (fermentation starts as quickly as my other lagers).

So I am really struggling to understand what I am doing wrong. I have read that acetaldehyde can be due to over-pitching and over-oxygenation, and I am pitching at the upper range of the guidelines, but somehow this over-pitching and over-oxygenation (if that is the problem) only manifests itself in high-gravity lagers, because I pitch and oxygenate my regular gravity lagers in exactly the same fashion.

My technique, for all lagers, is to pitch at about 1.5 million cells per mL per degree Plato. So, for 6 gallons of a 1.070 Bock, that would mean pitching about 581 billion cells. I have obtained this pitching level in various ways for the different batches; either by creating a large continuously aerated starter (using Mr. Malty's pitching rate calculator) or by directly pitching 6 - 7 Wyeast Activator packs (depending on the calculated viability of those packs, based upon their packaging date), or by a combination of a starter and direct pitching. Actual pitching rates have probably varied between 1.45 and 1.9 million cells per mL Plato, due to the impreciseness of viability and starter growth calculations.

My oxygenation procedure uses a standard disposable oxygen tank (the red kind from the hardware store) and a regulator attached to a 2 micron stone. Prior to pitching, I open up the regulator all the way and oxygenate for 1 - 2 minutes. Tests with my dissolved oxygen meter have indicated that this provides about 14 - 17 ppm of oxygen.

So, according to what I have read of the brewing literature, these pitching and oxygenation rates SHOULD be ideal. And they work wonderfully for my normal gravity lagers.

I pitch at 46 - 48 F and allow the temperature to rise to 48 or 50 F during fermentation.

Exposure of the beer to oxygen after ethanol has been produced can also produce high acetaldehyde levels, as ethanol can be re-oxidized to acetaldehyde and acetic acid. However, this cannot be the issue because I am using a sealed stainless conical fermenter and actually draining the wort (instead of siphoning) directly into a CO2 purged keg. The keg is purged by filling with sanitizer and blowing it all out with CO2.

Another possibility would be that I am separating the beer from the yeast too soon, or crash cooling before fermentation has finished, or something of that nature. However, I typically raise fermentation temps to 55 - 58 F once the ferment has reached 2 points of final gravity (as measured by a hydrometer, not by the bubbling of an airlock or guestimation) and hold that temp for a week before cooling.

Long warm conditioning is also supposed to reduce acetaldehyde. So I tried letting these beers sit at about 65 F for 3 weeks after the end of fermentation. Though the acetaldehyde character was somewhat reduced by this conditioning, it was still very present. I even tried initiating a secondary fermentation by adding corn sugar to the keg and allowing it to completely ferment out. I've also tried krausening. My hope was that the re-activated yeast would then convert the acetaldehyde. Again, no luck.

Could this character be something else other than acetaldehyde? If so, what compound would make sense? And why would this character reliably show up in every high-gravity lager I have brewed, but in none of the other 25 batches? Apart from this acetaldehyde character, the beers have no other off-flavors, as would be expected in the case of contamination. It is an unmistakably appley, acetic character that is very apparent in the aroma and first few sips, but then becomes less noticeable (but still apparent) as you continue drinking the beer. My wife had the same impression; she assumed I was trying to brew an apple beer.

mhenry41h 10-06-2011 02:53 PM

I see nobody has chimed in on this one and I fear I may have just had the same problem with my Oktoberfest. Today, after a 4 week primary, I racked it to secondary for lagering, when I smelled the empty primary fermenter...bang...Acetylaldehyde. Now, this is particularly alarming to me due to the fact that just last week I pulled a sample and it tasted great...not a speck of green apple. I'm hoping upon all hope that perhaps I noticed it when racking due to stirring up some yeast cake and the beer is just too green. I sure hope that after a 4 week lagerig period it clears up and the beer is fine. This is my first lager so I'm not terribly familiar with the stages of taste and conditioning these styles. The million dollar question is: how can the beer be perfect tasting last week yet taste like Acetylaldehyde today?

ayoungrad 10-06-2011 03:03 PM

Acetylaldehyde, to my understanding, is a precursor of EtOH. So, most often, it is present in young beers or in those beers that were separated from the yeast too early, before acetylaldehyde was converted to EtOH.

Did you guys do a diacetyl rest? I have only done one lager, so take it for what it's worth. But such a rest should solve diacetyl AND acetylaldehyde issues at the same time. This was the case for my batch which had both green apple and buttered popcorn flavors prior to the rest. Again, my experience is based on an of 1 but that's all I can offer.

It makes sense though. Yeast work faster at higher temperatures and reduction of acetylaldehyde to EtOH is a yeast process.

So my advice based on limited experience is to prolong the time on the yeast and/or do a diacetyl rest.

ayoungrad 10-06-2011 03:15 PM

I now see the OP did a rest. The rest I did, based on a lot of reading, was at 68 degrees and I started it when the batch was 2/3-3/4 of the way to predicted FG.

But, it does sound like there are many on here who don't do rests and don't have off-flavors so there must be something earlier in the process that can prevent acetylaldehyde rather than doing a rest to fix it after the fact. Wish I could help.

remilard 10-06-2011 03:21 PM

Try a different yeast strain. Sounds like everything else is correct.

AmandaK 10-06-2011 04:49 PM

I'm with remilard. What yeast strain are you using? If it's the same as your other lagers, it could be stressed in the high alcohol environment and create off flavors.

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