A methanol extract of green tea was fractionated on Sephadex LH-20.
The compounds eluted were identified by thin layer chromatography as catechin–epicatechin,
gallocatechin, epigallocatechin, epicatechin gallate and epigallocatechin
gallate. When added to milk at 2·0 g/l, these polyphenols, apart from the
catechin–epicatechin mixture, increased the heat stability of skim milk, particularly
in the region of the minimum (pH 6·8–7·1).
When added at 0·4 g/l, green tea
polyphenols also increased the heat stability of concentrated milk. The effects of
other phenolic compounds on the heat stability of milk were also examined.
Chlorogenic acid, guaiacol, thymol, vanillin, butylene hydroxyanisole, propyl gallate
and butylene hydroxytoluene did not affect the heat stability of milk or concentrated
milk. Quinic acid markedly reduced the heat stability of skim milk. Pyrogallol,
catechol, tannic acid, ellagic acid, phloroglucinol and gallate converted a type A heat
coagulation time–pH profile to a type B profile. Ferulic acid and vanillic acid
increased heat stability in the region of the maximum, with little effect on the
minimum, and stability did not recover at pH values on the alkaline side of the
minimum. Caffeic acid increased the heat stability of milk while the related non-phenolic compounds 2,5-dimethoxycinnamic acid and 3,4-dimethoxycinnamic acid
had no effect.