The bacteria in butter are largely contained in the moisture droplets. Since water is not the continuous phase, as in milk and cream, migration does not take place readily, and growth is restricted primarily to infected droplets. As pointed out by various investigators, the working of butter disperses the moisture into finer droplets. In under worked butter the droplets are relatively large while in thoroughly worked butter they are smaller and well separated by fat. The number of bacteria in a portion of butter remains practically constant during working, consequently an increase in the number of water droplets through a finer dispersion of the moisture results in a decrease in the percentage of infected droplets. Decreasing the size of the droplets also decreases the nutrients available to the bacteria in the infected droplets. Accordingly, the distribution of moisture in butter is a factor tending to influence the growth of bacteria, and changes due to bacterial action should take place more slowly in thoroughly worked than in under worked butter.
Long, H. F. and Hammer, B. W.
"Bacteriology of butter VI. Effect of moisture dispersion in butter on growth of bacteria,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 22
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol22/iss246/1