Dilute (1-10 vol. %) solutions of milk in hot London mains water forme
d films on the surface which increased with time. No film formed in so
lutions of milk in distilled water unless calcium chloride had been ad
ded. The mass of the surface film depended on the total mass of milk u
sed, not on its concentration. These facts are consistent with an inte
raction between calcium (and other metal) ions and milk components wit
hin the bulk solution, the product then rising to the surface. Surface
oxidation did not seem to be involved. This mechanism is quite differ
ent from that which forms films on infusions of black tea. The mass of
the milk film increased in the order skimmed milk < homogenised full
cream milk < non-homogenised full cream milk < single cream, and both
proteins and fat were involved. This was confirmed by chemical analysi
s of the films, which were also examined by infrared spectroscopy and
scanning electron microscopy. (C) 1997 Elsevier Science Ltd.