Effects of tea, coffee and cocoa extracts on the colloidal stability of milk and concentrated milk

Addition of freeze-dried extracts of green or black tea or commercial samples of cocoa and instant coffee at concentrations of 0.4 or 0.8 g 100 mL(-1) increased the heat stability of skimmed milk at 140 degrees C, especially in the pH range of the minimum (pH 6.8-7.1), increased the rennet coagulation time of skimmed milk, especially that of reconstituted low-heat milk powder not supplemented with CaCl2 and slightly increased the alcohol stability of skimmed milk in the pH range 6.1-7.1. Chelation of Ca2+ does not appear to be the exclusive mechanism involved, at least in the case of heat stability. All extracts contain a high concentration of polyphenols and related compounds, which are highly reactive and may be responsible for enchanced casein micellar stability. (C) 1998 Elsevier Science Ltd. All rights reserved.