Kinetics and equilibria of tea infusion, Part 14. Surface films formed in hard water by black tea brews containing milk

Added milk greatly increased the mass of film formed on the surface of black tea brewed in hot London mains water. The amounts of film collected after adding different types of milk (skimmed, homogenised full cream, non-homogenised full cream, and UHT single cream) to black tea brews increased in the sequence listed. No film formed when distilled water was used, and very little even in hard water if the tea concentration was large. Chemical microanalysis and Fourier transform infrared examination of the films indicated that they were composed of milk constituents modified by tea polyphenols and aggregated by calcium ions. The role of the tea polyphenols is consistent with their being sequestered in casein micelles as well as bound to the protein membranes of milk fat globules. The presence of milk prevented the formation of most of the dark tea scum observed on black tea infused in hard water.