Various defects of butter are due to growth of bacteria in the product. A number of the species involved belong to the genus Pseudomonas, and this genus includes certain species, such as Pseudomonas putrefaciens and Pseudomonas fragi, that are very objectionable from the standpoint of butter deterioration. Pseudomonas organisms and related types commonly are found in moist soil and in water, which suggests that contaminated water used in butter plants may be a source of the organisms in butter.

The general characters of the Pseudomollas organisms indicate that they are important in butter deterioration. A number of them actively attack protein, fat or both. Growth temperatures are relatively low and include the range in which butter often is held; various species are so typically psychrophilic that they do not grow at 37° C. and may even be killed by continued exposure. While the organisms generally are sensitive to salt, the protective action of salt has been reduced or eliminated by the demand for lightly salted and unsalted butter.