Volatile compounds of bovine milk as related to the stage of the estrous cycle

Previous reports on behavioral assays with trained dogs suggested that milk samples from cows at diestrus, proestrus, and estrus had different odors. To identify the odor differences, volatile compounds in milk were isolated and analyzed by gas chromatography and mass spectrometry. About 80 peaks were detected in each chromatogram, of which 59 were present in all samples, and 23 were tentatively identified. The major identified compounds included the following six structurally distinct classes: ester, aldehyde, ketone, alcohol, fatty acid, and lactone. Although no unique peaks were found to be specific to samples taken at diestrus, proestrus, or estrus, 36 compounds exhibited significant differences in concentration among the three reproductive stages. These quantitative differences may account for the variation of milk odors during the estrous cycle. In order to investigate the quantitative differences systematically, multivariate discriminant techniques were used to relate the gas chromatographic profiles with the three stages of the estrous cycle. Stepwise discriminant analysis indicated that 15 of the 59 peaks in each chromatogram could best be used to reveal the differences among milk samples taken at diestrus, proestrus, and estrus stages. The discriminant function based on the 15 key peaks could classify all of the samples into their original categories at a total accuracy of 97.9%. Canonical analysis indicated that milk samples from different stages were clearly separated from each other in a two-dimensional space.