Proposed mechanism for the effect of polyphenols on the heat stability of milk

Caffeic acid at 5.5 mmol l(-1) markedly enhanced the heat stability of milk at 140 degrees C, the minimum was eliminated and stability increased as a function of pH; the apparent activation energy for coagulation was increased. The stabilizing effect of caffeic acid was reduced or eliminated when mill; was heated at 150 degrees C or when the air in the headspace was replaced by N-2. The stabilising effect of caffeic acid was reduced when stability was assessed as the temperature required to cause instantaneous coagulation, i.e., a maximum and minimum were evident. The thermal degradation/transformation of caffeic acid in milk was a zero-order reaction with an activation energy of similar to 40 kJ mol(-1) and Q(10) value of similar to 1.3 from 130-140 degrees C; when the air in the headspace was replaced by N-2, thermal degradation/transformation of caffeic acid was reduced. The addition of caffeic acid resulted in a reduction of the reactive lysine and sulphydryl content and inhibited the dissociation of kappa-casein-rich protein from the casein micelles in milk on heating. It is postulated that on heating in milk, caffeic acid is thermally oxidised to quinones which then interact with nucleophilic amino acid residues to inhibit kappa-casein dissociation from the casein micelle. (C) 1999 Elsevier Science Ltd. All rights reserved.