My Warm Lagering Method

I must apologise about my poor blogging output over the past year or so, but there have been some big changes behind the scenes which have got in the way of my blogging and brewing...but I've not been completely inactive.

As my regular readers may recall, I've done some test brews using a newer "lagering" method in which lager-style beers are produced using lager yeasts fermented at ale temperatures (post 1, 2, 3 and 4). Motivated by these successes, I've brewed over a dozen lagers using this method in order to refine this process, with my last two batches produced using the same refined method. The first of these was a German-style pilsner; specifically the "Myburger" Bitburger clone from "Brewing Classic Style". Not only was it delicious, but I had trouble telling it apart in side-by-side tastings from its commercial cousin. The second beer was a doppelbock, and was everything I'd expect from the style. The take-home lesson from those two brews is that you can make very good "lagers", true to style, without the need for prolonged cold fermentation. Indeed, the Pilsner was 2 weeks grain-to-glass, and a month for the doppelbock.

All the gory details are below the fold.

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The Method:

My method is pretty straight forward, and does not differ much from the one worked out by Brulosophy.

Preparing the yeast: The single most important part of the process is a large pitch of very healthy yeast. A good pitch of healthy yeast, into an appropriately prepared wort, will provide the clean fermentation your lager requires. Get the yeast part right and the rest will fall into place.

For liquid yeast:A few days in advance prepare a large starter of yeast - for an average gravity lager you will want 1.5 to 2 million cells/ml/degree plato (AKA strong ale-to-conventional lager pitch rates). For high gravity lagers you will want 2 to 2.5 million/ml/degree plato. Your goal with this starter is to maximise yeast numbers and yeast health - use plenty of yeast nutrient, and be sure to stir vigorously enough to ensure maximum oxygenation. These starters are large, so you are going to want to decant the spent wort prior to pitching your yeast.

For repitched yeast: Simply having a lot of yeast is not sufficient - you also need it to be healthy. As such, if repitching yeast from a previous brew, make sure you run it through a revitalization starter - e.g. add the amount of yeast you intend to pitch to a 1 to 1.5L/1.040 gravity starter, made with a good dose of yeast nutrient and stirred to give a high level of oxygenation. This will not produce more yeast, but will ensure that the yeast you pitch are at peak health. Again, I would suggest decanting these starters, although the smaller volume make this less critical than when preparing a starter from a tube/smackpack/yeast bank.

For dry yeast: Dry yeast tends to be in relatively good shape, so long as it isn't too old or was stored improperly. Again, you want a lot of yeast, so for an average-gravity lager you will likely want to use 2 sachets of yeast; 3-4 sachets for a high-gravity lager. Be sure to rehydrate the yeast properly, in order to ensure the maximum number of active yeast and the best yeast health.

Preparing the beer: Nothing special needs to be done here, and no modifications need to be done to the recipe; simply brew the wort to the best of your abilities. At the end of the boil, cool the beer as cold as you can reasonably get it - ideally to 14-16C, although anything below 20C is acceptable.
Transfer the beer to a semi-sealed fermenter such as a carboy + blowoff tube. You want to avoid "open" bucket-style fermenters as these can enhance ester formation and may lead to a loss of the modest sulphur character present in most lagers. When you transfer the beer be sure to transfer a small amount of trub as this can also help to suppress both ester and fusel alcohol production. Finally, oxygenate well and pitch your yeast.

Primary Fermentation: Because we've front-loaded our brewing process with several methods to provide a clean fermentation, we can get away with a fair bit on the fermentation end. A broken temperature controller led to an accidental "lager" fermentation at 25C (77F), and the beer still turned out fine. That said, some control over fermentation temperature is simple insurance, and at a minimum, should speed the time the beer needs to age before it is keg ready. I've had the best results (or, at least, keggable beer the quickest) when I held the fermentation temperature to between 14 and 17C (57 to 62C) for the first 3 to 4 days; this is when most ester formation occurs, so by keeping things cool we can further minimise ester production. At this point I raise my fermentation temperature to 20-21C (68-70F), and hold that temperature until primary fermentation is done (~2 weeks). These higher temperatures ensure full attenuation and cleanup of off-flavours such as diacetyl and acetaldehyde.

Secondary Fermentation & Packaging: Secondary fermentation is not required for average-gravity lagers, but I've found that transferring higher-gravity lagers to a carboy with airlock for an additional 2-4 weeks can greatly help their character and get them closer to style. Once ready for packaging, crash-cool the beer and transfer to the keg/bottle (add gelatin at this time, if you use it). Carb as per usual and serve - all of my test batches but the one brewed with S-23 was fantastic on the first serve, although in about half of cases I've noticed some modest improvement over the first week or so in the keg.

And that's it - pitch lots of healthy yeast, maintain some level of control over your fermentation, and you should end up with a nice lager...in two (or so) weeks.

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Why Does It Work?

The key feature separating lagers from ales is the relative absence of esters and other compounds such as diacetyl. In addition, lagers tend to have a modest sulphur note that is absent in ales. This unique set of  flavour characteristic is a product of how these beers are fermented - cool temperatures suppress ester formation, and sulphur-producing yeasts give the sulphur character which is preserved by the slower fermentation that occurs at cooler temperatures. But this colder fermentation also comes with some costs - namely, a tendency for certain metabolic intermediaries such as diacetyl and acetaldehyde to accumulate in the beer. Traditionally, these undesired off-flavours were dealt with through prolonged ageing, during which time the yeast slowly metabolise these intermediaries, eventually leaving behind the clean crisp beer we expect of lagers.

The "warm" method achieves the same ends - low esters, some sulphur and minimal diacetyl or other off-flavours, but does so through different means:

Right Yeast: Not all yeasts are created equal, and as such, not all lager yeasts will work as well with this method as others. In my hands W34/70 is the king, producing clean lagers under nearly any condition I threw at it. Other good yeasts (again, in my experience, this list is not comprehensive) includes Wyeast 2112, WLP800, and WLP833.

Wrong Yeasts: As you can image, when there are right yeasts there are also wrong yeasts. S-23 is the wrong yeast as at warm temperatures it makes your beer taste like fruit salad...although, now that I think about it, it is a generally horrible yeast and should be condemned to the fiery depths of hell.

Reduced Esters: The relative absence of esters are the primary feature that separates lagers from ales, and therefore reducing ester formation (and other off-flavours such as fusel alcohols and acetaldehyde) are key to a successful warm-fermented lager. Esters are a product of an alcohol being reacted with an acid; for conventional brewers yeast the most common esters are made through the reaction of ethanol (the alcohol) with metabolic intermediaries such as acetyl-CoA; in this particular case producing the ester ethyl acetate (apple-like flavour). Ester production in beer is dictated largely by the metabolism of yeast, with any factor that increases the production of metabolic intermediaries, or which prolongs their presence in the fermenting beer, leading to more esters.

Many of the intermediaries are produced at their highest levels during cell division and during cell stress, while the presence of oxygen can aid in the rapid removal of these compounds. So by maximising cell numbers and health, and oxygenating well, we can minimise ester production (as well as the production of fusels, acetaldehyde and diacetyl). A small amount of trub present in the fermenting wort has also been demonstrated to help reduce esters, which is why I make sure to transfer a small amount of trub to the primary fermenter.

Likewise, keeping the first few days of fermentation as cool as possible should help - that said, I've failed to do this on a few of my test batches without noticeable defects in the resulting beer. In my opinion, if you do everything else right, keeping the early stages cooler is simply a bit of insurance rather than a necessity.

Low diacetyl and acetaldehyde: Diacetyl (buttery) and acetaldehyde (green apple) flavours are common flaws in lagers. Both of these flavours tend to be present in modest quantities early in lager fermentation, and traditional lager fermentations require prolonged ageing to reduce them to appropriate levels. Diacetyl and acetaldehyde both form as intermediaries in the breakdown of sugars by yeast. Given enough time, the yeast will reconsume and remove these compounds...and do so more quickly at warmer temperatures. For your average-gravity fast lager, the two week fermentation is sufficient for this; for higher gravity lagers an additional 2 or 3 weeks in secondary (again, at a warmer temperature) will lead to the reduction of these compounds.

Sulphur: A modest amount of volatile sulphur compounds are a normal constituent of a lager's flavour and aroma profile. Lager yeasts tend to produce a lot of volatile sulphur compounds (hence why lager fermentations tend to smell like flatus), and again, the traditional long ageing of lagers gives time for this to off-gas. Ironically, maintaining an appropriate level of sulphur compounds in the warm-fermented beer can be difficult as the more vigorous fermentation tends to drive these compounds off. In my experience using a semi-sealed fermenter (carboy + blow-off tube) maintains enough of this character to stay in-style; using a more open fermenter such as a pail may lead to excess loss of these compounds, and an off-style finish.