Most US breweries use mechanical/physical techniques to carbonate their beer, rather than relying on carbon dioxide naturally generated by a secondary fermentation. However, the advantages in time saved and greater efficiency achieved by artificial carbonation are countered by an increased risk of oxidation. During secondary fermentation in a covered vessel, the carbon dioxide liberated from the beer fills the headspace, forcing any air present out through the bunging vent and preventing oxygen from dissolving in the beer, but if artificially carbonated beer is stored without proper measures being taken to exclude air from the vessel, oxygen can dissolve in it. An understanding of the physical laws governing the behaviour of gases and their dissolution in liquids (which are summarised in this paper) is essential if this is to be prevented. Tables show the equivalent values of pressures measured in lb/sq in, mm and inches of mercury, and atmospheres, and of temperatures in degrees C, K and F, the proportions of carbon dioxide, nitrogen and oxygen in different mixtures of carbon dioxide and air, the solubility coefficients for air, nitrogen, oxygen and carbon dioxide under Henry's law at different temperatures, the solubility of these gases in water, and the relationship between the proportions of these gases in headspace gas and dissolved in beer. The importance of preventing air from dissolving in the beer during storage before packaging is stressed. The author considers that some means of monitoring the purity of carbon dioxide in storage tank headspace is essential if oxidation is to be prevented.
Keywords : air beer carbon dioxide headspace physics solubility storage