Evaluation of milk protein interactions during acid gelation using a simulated yoghurt model

Caseins and denatured whey proteins exhibit acid gelation behaviour when ex amined individually. In a typical dairy acid gel such as yoghurt, heat-indu ced interaction between the 2 major protein groups is also taking place. Th e extent to which such interactions affect acid gelation was investigated u sing a model yoghurt milk system which combined native phosphocasein and wh ey protein isolate in the approximate ratio that occurs in milk. All protei n dispersions containing heated whey protein commenced gelation in the rang e pH 5.8-6.2. This was followed by a rapid rise in gel strength according a s pH declined to 5.3. Similar rheological patterns were obtained for (i) ca sein/preheated (78 degrees C, 30 min) whey protein and (ii) preheated (90 d egrees C, 10 min) casein/whey protein solutions. However, further heat trea tment (90 degrees C, 10 min) of (i) gave an additional increase in consiste ncy of the resulting acid gel. Two important phases were identified during acid gelation of a combined casein/whey protein system: (i) the dominant ef fect on initial gelation pH exerted by complexation (even under ambient con ditions) of pre-denatured whey proteins with casein, and (ii) the synergist ic effects on gel strength arising from casein aggregation effects that tak e over as acidification progresses.