558 STATE BOARD OP AGRICULTURE. 



THE COLORIMETRIC HYDROGEN-ION DETERMINATION AS A 



MEANS OF vSTUDYING BIOLOGICAL CHANGES 



IN DAIRY PRODUCTS. 



Technical Bulletin No. 52 



BY L. H. COOLEDGE 



The method usuallj^ appHed in a study of the sanitary condition of dairy 

 products is the bacteriological count. This is determined by the plating 

 method, which gives an approximation of the number of individuals or groups 

 of live bacteria present in the sample, or by the microscopic method which 

 gives an approximation of the total number of bacteria, both dead and living 

 which are present. Either of these methods is a valuable aid to the sani- 

 tarian in judging the care which has been exercised in producing the milk, 

 and the precautions observed to prevent the growth of any bacteria present. 



The Ijacteriological count fails to indicate with a sufficient degree of ac- 

 curacy the actual ability of the bacteria present to produce the changes 

 which would make the dairy product unfit for consumption, and it is often 

 the case that the sample of milk \^dth so high a bacterial count tliat it would 

 be condemned by most of our city ordinances, is actually a good grade of 

 milk when judged as to keeping quality and type of fermentation which 

 develops when held at the temperature commonly used in the home. 



To find a method which would give a reading more nearly representing 

 the actual ability of the bacteria and enzymes present to produce the changes 

 of interest to the j^roducer and consumer, Cooledge and Wyant (1) made a 

 study of the ability of bacteria and enzymes present in milk to cause a change 

 in hydrogen ion concentration of a broth solution to which the milk was added. 

 The object of this paper is to call attention to the possibilities of this method 

 when applied not only to market milk but to other dairy products, and in 

 other phases of bacteriological investigation and to serve, as an introduction 

 to work which is under way upon each of the subjects considered. 



REVIEW OP LITERATURE 



The work of Clark and Lubs (2) and of a number of other investigators 

 shows that the hydrogen ion determination furnishes a delicate means for 

 measuring certain biological changes in solutions and that the same may be 

 determined with a fair degree of accuracy by colorimetric methods. 



Baker and VanSlyke (3) have devised a method for determining the keep- 

 ing quality of milk in which the indicator Brom-cresol-purple, is added to 

 the whole milk to be tested, and the color noted after 24 hours at room tem- 

 perature. Four stages of progress are thus noted depending upon the color 

 of the milk indicator solution. The objection to this method is the length 

 of time required to get results. It is probable that the samples held could 

 be more accuratel}^ classified after 24 hours by simply tasting. The opacity 

 of the milk makes the detection of slight changes in color difficult. 



