Of all the post-fermentation steps, perhaps none is as important as carbonation. Carbona-tion provides sensual appeal by enhancing mouth feel, flavor, body, and foam (or head).The CO2 preserves the beer by reducing pH and the oxidation-reduction potential (Eh), such that various aerobic, acid-sensitive spoilage organisms are inhibited. Carbonation of beer can occur naturally, via a secondary fermentation, or mechanically, by directly adding CO2 after the conditioning and filtration steps.
As described earlier, beer can be conditioned via several procedures. In the case of traditional ales, conditioning is commonly done in casks, whereas traditional lagers are conditioned, or lagered, in enclosed tanks. Some specialty beers (and those made by home-brewers) are conditioned directly in bottles. In all of these processes, the beers become naturally carbonated due to a secondary fermentation that yields CO2 (as well as additional ethanol).
Several requirements are necessary to induce a successful secondary fermentation. First, there must be sufficient fermentable sugar present in the beer to serve as substrate for the yeast. The cask, tank, or bottle must be able to withstand the pressure that accumulates as a result of CO2 gas formation. Finally, either naturally present yeasts must remain in the beer, even after flocculation, or more yeast must be added. For lagers, in particular, the krausening technique can be used to initiate a secondary fermentation. Recall that the krausen is wort obtained during the most active phase of the fermentation and that it contains sugars as well as log phase yeasts. The CO2 bubbles that form by krausening are believed to be "finer" compared to those produced by other procedures. However, the krausening technique is now only infrequently used.
The amount of CO2 that forms during a secondary fermentation and is retained in the beer depends on several factors.The amount of available sugar present or added to the beer directly influences CO2 production, but rarely is the sugar a limiting factor. Rather, the correct amount of carbonation is more a function of the counter-pressure and temperature in the tank. Obviously, the more back pressure applied, the greater the dissolved CO2 in the beer. Likewise, as the temperature decreases, solubility of CO2 increases. Proper adjustment of temperature and back pressure, therefore, is critical to achieving the desired level of carbonation in the finished beer.
Typically, ales have less carbonation that lagers. In the United States, for example, lagerstyle beers contain about 2.6 volumes of CO2 per volume of beer (that is, if the dissolved CO2 was completely dissipated and collected, it would have a volume 2.6 times that of the "flat" beer).Traditional ales, in contrast, contain about half as much CO2.
It should be noted that beers that have undergone a secondary fermentation still need to be clarified or filtered (or both) to remove cells and haze-forming material, but without losing any of the accumulated CO2. Traditional cask-conditioned beer can be treated with fining agents to enhance sedimentation, leaving the sediment in the bottom of the cask (where it remains even during dispensing). In contrast, lagers or tank-conditioned beer must be processed under pressure to retain the CO2.
In the absence of a secondary fermentation, mechanical carbonation provides a convenient and more controllable means for introducing CO2 into the beer. Mechanical carbonation is now widely used in the beer industry, not just by large breweries, but by small and even micro-breweries. There are essentially two sources of CO2. As noted earlier, the CO2 that evolves during the primary fermentation can be collected, cleaned and purified, and then added back to the beer.This process is not inexpensive, however. Therefore, the more common means of mechanical carbonation is simply to pump CO2 from pressurized tanks directly into the beer. As for when a secondary fermentation occurs, the CO2 back pressure and temperature dictate the necessary volume of CO2 pumped into the beer. In some cases, CO2 is even added to beer that has undergone a secondary fermentation to achieve the desired level of carbonation.
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