Science in the Kitchen by E. E. Kellogg

eighteen or twenty-four hours after it is drawn, of small, indigo-blue spots, which rapidly enlarge until the whole surface is covered with a blue film. If the milk be allowed to stand a few days, the blue is converted into a greenish or reddish color. This coloration of the milk is due to the growth of microscopic organisms. The butter made from 'blue milk' is dirty-white, gelatinous, and bitter.