The relationship between rheological parameters and whey separation in milk gels

The relation between whey separation of rennet-induced gels and rheological properties of those gels is reasonably well understood. A low fracture str ess and a high value for the loss tangent at low frequencies have been corr elated with a tendency to exhibit syneresis in rennet gels. In contrast, li ttle is known about the relationship between mechanical properties of gels and whey separation in acid-induced milk gels, such as yoghurt, although th is continues to be a major defect. In recent work, it has been found that c onditions such as high milk heat treatment, fast rates of acidification and high incubation temperatures all gave high levels of whey separation compa red with gels made from unheated milk that were incubated at low temperatur es and where the rate of acidification was slow (i.e. when bacterial cultur es were used instead of the acidogen, glucono-B-lactone). The tendency to e xhibit whey separation in acid gels made from heated milk was related to a low fracture strain and an increase in the loss tangent (observed even at h igh frequencies) during the gelation process (a high value indicates condit ions favouring relaxation of bonds). Excessive rearrangements of particles in the gel network before and during gelation were implicated as being resp onsible for whey separation and rheological conditions that appeared to ind icate this defect are described. It was also concluded that techniques that measure the spontaneous formation of surface whey should be distinguished from those that measure the expression of whey from networks under pressure as the latter tests only measure gel rigidity. (C) 2001 Elsevier Science L td. All rights reserved.