Undiluted Milk Research Articles

The Development of Acidity in Milk

The development of acidity in milk is a biological phenomenon which offers many very interesting but obscure problems for those to solve who are interested in scientific dairying. Milk fresh from the cow has an amphoteric action on litmus papers but is distinctly acid to phenolphthalein solution. The acidity of milk is usually estimated, using 10 cc. of undiluted milk, by titration with sodium hydroxide solution with phenolphthalein as indicator, and the acidity is then calculated as lactic acid. By this means Koning has shown that the acidity of fresh milk from individual cows varies from 0.10 to 0.21 per cent (1). This agrees very well with the work of other investigators; but on the question of the average acidity of fresh milk writers disagree a little; it varies apparently with locality and other factors. The cause of the natural acidity of milk has been the subject of some experiment. It is evidently not due to lactic acid (2). Commercial milk in Cardiff has usually an acidity between 0.16 and 0.20 per cent and it may contain from 20,000 to 1,500,000 bacteria per cubic centimeter (counted on ordinary agar plates after three days at blood heat). When milk is retained for some time the acidity increases due to the production of acids from the milk sugar by bacteria. This increase does not always immediately become apparent. The milk may show no increase of acidity for a period of time which has been called the phase. This incubation lasts longer the lower the temperature and seems to be associated in some way with a bac~ricidal phase during which the initial bacteria, or some of them, decrease in number. The acidity of the milk may actually decrease for a time (3). Finally, then, the acidity increases and when it reaches abou~ 0.3 per cent the milk will curdle on boiling. When it reaches