Comparative study of some of the methods of determining the quality of milk

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Virginia Agricultural and Mechanical College and Polytechnic Institute


In this work an attempt has been made to show the merit of some of the more rapid tests of milk quality, as compared with the plate count method. Results secured from the tests studied have not been compared with the actual keeping quality of the samples of milk used but with the results of the official plate counts secured from these same samples. This was taken as a basis for comparison because it is the recognized official method of determining quality. Though the official method, it be remembered that it is by no means a perfect method. As brought out by Ward (17) it is subject to many wide variations. It is the opinion of the writer that in many cases where lack of relation was shown between plate counts and some other tests, especially the methylene blue reduction test, that the lack of relation was due more to the inadequacy of the plate count method than to inadequacy of the other test.

The methylene blue test showed good correlation with the plate count and is especially adapted to indicating milk of very poor quality as shown by results of its use under summer conditions.

The writer is convinced by this study that the reductase test is just as good a test of quality of milk for fluid milk purposes as is the plate count. It is many times cheaper, quicker, and more adaptable to the use of the small milk plant and to inspection service.

The pH test and catalase test showed little merit from results of this study. The catalase test seems to be of value in locating diseased udders when run on individual samples.

The fermentation test proved valuable in showing the nature of bacterial flora present and is, for this reason, indispensable for judging quality of milk for cheese making.

It seems that no one test yet in use will prove adequate to judge quality of milk for all purposes. Combinations of tests are often necessary; for bacterial numbers the methylene blue test and for cheese milk the methylene blue test and the fermentation test.