The pH of fresh milk is 6. When it turns sour, the pH

Fresh milk has a pH of approximately 6, making it slightly acidic. When milk turns sour or converts into curd, the pH decreases to a value less than 6 [1]. This change is driven by the activity of lactic acid bacteria, such as Lactobacillus, which ferment the milk sugar (lactose) into lactic acid [1]. Lactic acid is a substance that dissociates in water to release hydrogen ions (H+), thereby increasing the overall acidity of the solution. As the concentration of lactic acid rises, the pH level drops accordingly. This biochemical process is why sour milk and yogurt have a characteristic tangy taste and a lower pH range, typically between 4.0 and 4.6, compared to fresh milk. Consequently, the souring process inherently involves acidification, leading to a pH value below the initial 6.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 2: The Invisible Living World: Beyond Our Naked Eye > Table 2.4: Testing for curd formation using milk in different conditions > p. 22