Gas production in dairy products is a subject of considerable importance, since it is responsible for certain defects in milk, cream, cheese and occasionally condensed milk. Four groups of micro-organisms are concerned, namely, yeasts, lactic acid bacteria, coliform bacteria and spore-forming anaerobes, and of these the fixs two produce carbon dioxide only while the last two produce a mixture of carbon dioxide and hydrogen (with some anaerobes there may be methane as well). Where both gases are evolved the effectiveness of the two as regards ability to produce effervescence or to develop pressure in a closed container or in a hard-pressed substance like cheese differs by virtue of the fact that carbon dioxide is soluble in water (its solubility depending on the pressure and pH of the medium) while hydrogen is insoluble. Until the medium in which such micro-organisms are growing becomes saturated with carbon dioxide the effective gas production is therefore due to hydrogen. On the other hand, in some media the hydrogen may be involved in secondary reactions with some of the constituents while in the nascent condition and will therefore not form gas.