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Distribution and Growth of Bacteria in Butter

Department of Dairy Industry, Cornell University, Ithaca, N. Y.

ABSTRACT

1. The authors believe to have established the fact that a considerable portion of the moisture of cream becomes sterile in churning by being divided up into very small droplets. Even in sour cream butter there are more than 100 droplets to one bacterium. In this case about 40 per cent of the moisture, mostly the smaller drops, will become sterile; with pasteurized sweet cream butter more than 80 per cent of the moisture remain free from bacteria.
   
2. The amount of moisture shut off from bacteria depends upon two factors: i.e., the number of bacteria in the cream at the moment of churning and the degree of dispersion of the moisture in butter. In overworked butter more moisture is made sterile.
   
3. All experiments show uniformly that the formation of acid in the moisture within the butter is much slower than if the same amount of moisture is freed from fat and in the form of a continuous liquid. But it seems that the percentage of acid formed in butter slowly increases above the theoretical value computed from the amount of moisture which remains infected. This can be accounted for only by a diffusion of acid from the infected droplets to those free from bacteria. This diffusion is probable because the distance between the moisture droplets averages not more than five microns and diffusion of salt into unsalted butter has been established. Overworked butter shows less diffusion.
   
4. To prevent deterioration of butter by bacteria it seems advisable to churn pasteurized sweet cream with as few bacteria as possible and work it as much as possible without making it salty. Experiments by Guthrie of Cornell University (unpublished) demonstrated that of 10 churnings in 1926 and four churnings in 1927 the highly worked butter scored higher after five months of cold storage than the butter which was worked but very little. However, the distinctly overworked butter scored no higher than the butter with very little working. This last effect might be due to air worked into the butter.
   
5. Washing decreases acid formation much more than would be expected from the fairly large amount of protein and lactose left in washed butter. This result can be accounted for by the assumption that washing does not change the concentration of the smaller droplets but dilutes the larger drops to almost pure water. As the large droplets are infected, while most of the smaller ones contain no bacteria, the separation between bacteria and food is much more complete than the chemical analysis of the butter will show.
   
6. These deductions and computations probably will not hold true for molds which have the ability to force their way from one droplet to the other.