Dean's mood was souring by the minute.
The lactic acid bacillus, always present in unboiled milk (to which the souring of milk is due), is easily destroyed by heat; but the bacillus mesentericus, often found in it, forms spores, which are not destroyed by ordinary boiling, and germinate when the milk is kept at a moderately warm temperature, producing a brisk fermentation whereby a large volume of gas is liberated.
Schuppan has shown that it is possible to produce good butter from Pasteurized or even sterilized cream, and Weigmann introduced the plan of artificially souring cream by means of pure cultures of B.
W.) No milk is free from bacteria, because the external orifices of the milk-ducts always contain them, but the forms present in the normal fluid are principally those which induce such changes as the souring or " turning " so frequently observed in standing milk (these were examined by Lord Lister as long ago as 1873-1877, though several other species are now known), and those which bring about the various changes and fermentations in butter and cheese made from it.
Without further purification it is also used for "souring" in bleaching, and in tin and lead soldering.