This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Freshly secreted milk is nearly neutral to litmus. The reaction varies slightly but has an approximate pH of 6.6. The freshly secreted milk contains carbon dioxide. The amount of this gas in the milk decreases during milking and the subsequent handling of the milk, while the percentage of oxygen and nitrogen increase. For this reason the titratable acidity decreases for a time in milk exposed to the air. Confined milk does not show as great a decrease in titratable acidity as the exposed milk, for the percentage of carbon dioxide lost is smaller.
Effect of heating milk on acidity. When milk is heated at the boiling point or at temperatures above or near the boiling point the titratable acidity at first decreases owing to the loss of carbon dioxide, and then increases. Whittier and Benton report that the hydrogen-ion concentration increases continuously. They find the hydrogen-ion increase and the later increase in titratable acidity is due to the formation of acids from constituents of the milk. The amount of acid produced depends upon the time and temperature of heating, a greater amount of acid being produced with a longer heating period and with higher temperatures. From their experiments they conclude that the acid is produced from the lactose of the milk. They have shown that, the greater the concentration of lactose present, the greater the amount of acid formed at a definite temperature and for a definite time.