Fermenting Under Pressure: Process and Benefits
With the spread of pressurized fermenters (or Cornelius kegs used for this purpose), homebrewers have been able to use techniques of professional brewers: pressure and isobaric transfers and fermentations. The main advantage of these techniques is the less contact of the beer with oxygen and the possibility of packaging carbonated beer, avoiding refermentation in the bottle. Another interesting advantage is the possibility of fermenting under pressure. This technique requires that the primary fermentation phase (and maturation) takes place at pressures higher than the atmospheric one.
As we said, the main effect of increasing the pressure is to influence the concentration of carbon dioxide in the wort (the higher the pressure, the higher the dissolved CO2). The concentration of this gas in the wort / beer has an interesting effect on the yeast (based on the concentrations in which it is present): significantly reduce the production of esters. However, it does not have a significant influence on the fermentation speed and attenuation. In industrial breweries, where the speed of fermentation and maturation of beer is of enormous economic importance, overpressure is used to conduct fermentation / maturation at higher temperatures (14-20 ° C for lager beers) by decreasing the esters, obtaining low fermentation in less time and with a clean aromatic profile.
At an industrial level, there are several ways to combine temperature and pressure to speed up the fermentation and maturation of beer, maintaining a profile as neutral as possible. Generally the pressure is raised when the yeast is at 50% attenuation (generally at pressures between 1.2-1.5 bar), in combination with a proportional increase in temperatures, to speed up the metabolic reactions of the yeast and therefore the fermentation and the maturation of the beer. An excessively early rise in pressure can lead to excessive stress on the yeast cell, as well as an inhibition of the initial replication cycles and some off-flavors. An example is hydrogen sulphide, a molecule initially produced by yeast, but very volatile and generally expelled together with CO2 in the most tumultuous phase of fermentation. At higher pressures this expulsion is inhibited, and the molecule remains in higher concentrations, imparting a sulfur aroma.
The effects of pressure have mostly been studied on lager-type yeasts, as they are the most used in industrial production, as well as those that most benefit from a shortening of maturation times. Studies conducted on high fermentation strains have shown very different results from those listed above (in some cases with increased production of esters and higher alcohols), indicating that the effects of pressure depend significantly on the yeast strain used.
Unlike industrial or even artisanal productions (among which someone is using this technique), homebrewers do not have the economic need to reduce the maturation times of beer. However, some experimentation is a further opportunity to enrich the brewer's experience.