Surface films formed by milk in hard water

Dilute (1–10 vol. %) solutions of milk in hot London mains water formed films on the surface which increased with time. No film formed in solutions of milk in distilled water unless calcium chloride had been added. The mass of the surface film depended on the total mass of milk used, not on its concentration. These facts are consistent with an interaction between calcium (and other metal) ions and milk components within the bulk solution, the product then rising to the surface. Surface oxidation did not seem to be involved. This mechanism is quite different from that which forms films on infusions of black tea. The mass of the milk film increased in the order skimmed milk