Sunday, November 29, 2009

Another Extreme Beer

News broke this week of another entry into the race to create the strongest and most extreme beer on the commercial market. Scottish brewer BrewDog, currently the largest independent brewery in Scotland, recently announced the release of a 32% ABV imperial stout aged in whisky casks quizzically named Tactical Nuclear Penguin.

Aside from bragging rights used in marketing, high-gravity extreme beers are quite an accomplishment and a testament to the skill of a brewer. Alcohol is a toxic substance (hence the root word for intoxication), especially to microorganisms. Even though their entire existence is spent converting sugar to alcohol and carbon dioxide, most yeast strains cannot survive as their environment approaches 10% to 12% alcohol by volume.

So how are such high-gravity beers made? The first step is in the choice of yeast used, and some brewers have developed their own robust yeast strains in their house laboratories. Even so, these specialized species cannot achieve the record-setting levels of 25% ABV or above without a lot of prodding from the brewer.

Such a batch begins with making a very large starter, which is a type of mini-brew used just to increase the cell count of the yeast before introducing it to the wort. Homebrewers will commonly make a starter out of nothing but sugar, yeast and water, allowing it to grow for hours or days before brewing. The greater number of yeast cells that can begin fermentation immediately when introduced forgoes a lot of other problems as well.

The second consideration is to use highly fermentable grains, that is, grains with abundant starch content that can be fully converted and fully attenuated by the yeast. Likewise, fermentation is given another starting boost by highly oxygenating the wort once the yeast has been pitched, generally by agitation or even bubbling pure oxygen through the liquid.

Fermentation will naturally proceed quite vigorously at first and will begin to slow after a certain point, eventually coming to a halt once the consumable sugar content has been depleted. For that reason, brewers of high-gravity beers will continuously make small additions of sugar to feed the yeast and keep it alive and active, sometimes for weeks or months.

Eventually, any ale or lager yeast will reach its limit of survivability as the alcohol levels continue to rise. Thus, brewers will sometimes switch to other strains of yeast in later stages to finish off the fermentation to the target levels. Champagne yeast is a favorite alternative, as it is highly tolerant to alcohol and neutral in flavor.

However, BrewDog used none of these methods in brewing their record-setting beer, instead relying on a bit of a brewing cheat. Although water freezes at 0°C, ethanol does not freeze until −114°C. By freezing the wort after fermentation, the water content will freeze into a solid block of ice that can be easily removed, leaving behind the unfrozen alcohol in a much stronger beverage.

This method of freezing and removing the water is used in producing the German beer style eisbock, strong beers of about 16% ABV so named after this process. BrewDog did not actually brew a 32% beer but instead made a 10% imperial stout that was held at −20°C for three weeks. The result may be the same in the final product but the achievement is far less impressive than actually coaxing and manipulating yeast to perform far beyond their limits.

1 comment:

Anonymous said...

They produce a straight fermented strong stout called Tokyo*, This is at 18.2% abv. Thats good going for a brewing yeast.