The relationship between bacteria and the changes that occur in milk has been recognized since the middle of the last century. Certain of the milk abnormalities are almost characteristic of micro-organisms while others, such as abnormal flavors and odors, quite commonly result from causes other than bacterial. Recently the dairy section of the Iowa Agricultural Experiment Station in working with samples of milk from various parts of the state found that milk sometimes received low flavor scores when the number of contained bacteria was exceedingly small, while samples with high flavor scores sometimes contained large numbers of bacteria. This suggested an investigation of the number of organisms required to produce changes in the flavor and odor of milk.

The investigation of the odors and flavors produced in milk by bacteria, however, presents certain difficulties. The ordinary sterilization of milk so influences the flavor and odor that those which may be subsequently developed are largely overshadowed. Samples of aseptic milk sometimes have an undesirable flavor and odor that tends to mask other flavors that may be developed. Moreover, the accurate detection of changes in odor and taste presents difficulties because of the varying acuteness of the senses involved. Because of these difficulties much of the work herein reported deals with changes other than those in flavor and odor, such as the production of ropiness, the coagulation of milk and the reduction of litmus milk.