Can Pressure Improve Lager Brewing?

In recent years, there has been a great deal of chatter within homebrew communities regarding the use of pressure during fermentation to modify flavor outcomes in beer. This is touted as a way to produce beers at warmer fermentation temperatures without the need for costly equipment such as glycol chillers.

Lager character defined by yeast

Lager as a beer style is mainly categorized by the use of specific cold-tolerant strains of yeast now called Saccharomyces pastorianus and previously known as Saccharomyces carlsbergensis. The style is characterized by its crisp, clean flavor. To the uninformed palate, it could be described as relatively flavor neutral. Certainly, lager is of low bitterness and can have muted hop and malt characteristics depending on the style. However, much of the taste profile of lager comes from production of various flavor compounds by yeast during fermentation. Specifically, you get fruity, solvently characteristics from the common acetate esters, ethyl acetate, and isoamyl acetate. For a U.S. style light lager, these esters can make up much of the flavor profile, and it is important that the level in a commercial product remains consistent.

Tall tanks and hydrostatic pressure

The latter half of the 20th century saw consolidation among regional breweries around the world. One of the results was that brewery sites were closed and amalgamated to make efficiency gains, and brewhouse and fermentation volumes began to grow. However, there was an unintended consequence. As vessel volumes increased towards 12000 HL, the ester profile of the beers created dropped, fermentation time became sluggish, and the time to diacetyl reduction increased.

Why could this be? In any fermenter or tank, pressure is exerted by the force of gravity acting upon the liquid present within it. A pressure of 1 bar would correspond to a fermenter height of 10 m, so some of the tallest fermenters ever built at 21 m would exert a hydrostatic pressure of 2.1 bars, were it filled to the top. According to Henry’s Law, as pressure rises, the solubility of a gas within a liquid increases. Therefore, high pressures, or in very tall fermenters, the amount of carbon dioxide dissolved within a fermenting wort or beer is increased. This increased level of dissolved carbon dioxide is the cause of the reduced levels of esters seen in very tall or pressurized fermenters. Why is this the case?

The chemistry of pressure and flavor

As it happens, many of the reactions that take place within a yeast cell along the pathway to ester and higher alcohol formation are decarboxylation reactions. That is, they rely on the enzymatic removal of carbon dioxide to transform one chemical into another. With high levels of dissolved carbon dioxide, these reactions cannot occur to the same degree, and ester formation is inhibited.

In the case of tall fermenters, this effect was undesirable; reducing the ester concentration of your core lager brand tends to get noticed on a large scale. However, the same effect has been used commercially to reduce ester production in beers that were required to be brewed at higher temperatures or gravities to meet production targets for shorter fermentation times. Work done by Miedaner at Weihenstephan in Germany recommended a head pressure equal to the fermentation temperature in degrees centigrade divided by 10. Using this rationale, a lager fermented at 20°C ought to utilize a head pressure of 2 bars. At these increased pressures, ester production is significantly reduced, typically dropping 50% or more compared with ambient pressure.

Practical tips for fermenting under pressure

Yes, you can use increased head pressure to reduce levels of esters and higher alcohol production in beer. However, there are other factors to consider. While it has been shown that alcohol production is relatively unaffected at pressures up to 4 bars, at pressures above 2.5 bars, cell division is completely inhibited, fermentation time is increased, and yeast crop and viability are reduced.

If fermenting under pressure, make sure that you use a sufficient pitch of high viability yeast.

In addition, decarboxylation reactions are also involved in the diacetyl reduction pathway. It is not entirely clear that inhibition of these reactions is the cause, but higher levels of diacetyl have been reported in beers fermented under pressure, along with poor uptake of certain amino acids. When fermenting under pressure, it is important to do a proper diacetyl rest and lagering period to avoid possible off-flavors.

In summary, CO₂ pressure can be used to control ester production in lager beer. The original recommendation from Miedaner of P = T/10 was published in 1978. As a rule of thumb, it still stands. If I were set a challenge to produce a lager with low esters, I would use 2 bars as a maximum pressure target. Anything above this could lead to poor yeast health and raised levels of diacetyl, which would probably outweigh any of the gains from reduced ester formation.