|MadSci Network: Chemistry|
Four things limit the pH of fermenting milk: 1) Sugar: The amount of sugar limits the amount of lactic acid produced. One molecule of lactose (disaccharide in milk) produces four molecules of lactic acid. Add more sugar, e.g. sucrose or glucose, and you'll have more acid production. Lactose is 4.8 to 5.2% of milk. Try adding another 5% sucrose and check the pH. Lactose This is the main limiter of the pH around pH 5.0. Three other effects are: 2) Weak acid: The primary acid produced in milk fermentation is lactic acid. It's a weak acid so the pH drop isn't always directly proportional to the amount of acid (unlike a strong acid e.g. HCl). The pKa of lactic acid is 3.86 which means that close to pH 3.86, it will take a lot of lactic acid to move the pH a little. Thus, the "buffering" effect of lactic acid isn't a main effect at pH 5.0. 3) Buffering: In addition to lactic acid, the proteins in milk also act as weak acids and bases and thus "buffer" the effect of additional lactic acid. Actually, not the protein but the reactive groups of the amino acids that stick out from the peptide chain. For more information on weak acids and "buffering" see: Acid-Base Titrations Weak Acids 4) The fermenting cultures tolerance for low pH: Some bacterial cultures e.g. lactobacilli can drop the pH quite low - if they have sugar to convert; other cultures stop fermenting around pH 4.0.
Try the links in the MadSci Library for more information on Chemistry.