Rheological properties of milk gels formed by a combination of rennet and glucono-δ-lactone

The effects of heat treatment of milli, and a range of rennet and glucono delta-lactone (GDL) concentrations on the rheological properties! at small and large deformation, of milli gels were investigated. Gels mere made from reconstituted skim milli at 30 degrees C, with two levels each of rennet and GDL. Together with controls this gave a total of sixteen gelation conditions, eight for unheated and eight for heated milli. Acid gels made from unheated milks had ion; storage moduli (G') of < 20 Pa. Heating milks at 80 degrees C for 30 min resulted in a large increase in the G' value of acid gels. Rennet-induced gels made from unheated milk had G' values in the range similar to 80-100 Pa. However, heat treatment severely impaired rennet coagulation: no gel was formed at low rennet levels and only a very weak gel was formed at high levels. In gels made with a combination of rennet and GDL unusual rheological behaviour was observed. After gelation, G' initially increased rapidly but then remained steady or even decreased, and at long ageing times G' values increased moderately or remained lon. The loss tangent (tan delta) of acid gels made from heated milk increased after gelation to attain a maximum at pH similar to 5.1 but no maximum was observed in gels made from unheated milli. Gels made by a combination of rennet and GDL also exhibited a maximum in tan delta, indicating increased relaxation behaviour of the protein-protein beads. We suggest that this maximum in tan delta was caused by a loosening of the intermolecular forces in casein particles caused by solubilization of colloidal calcium phosphate. We also suggest that in combination gels made from unheated milk a low value for the fracture stress and a high tan delta during gelation indicated an increased susceptibility of the network to excessive large scale rearrangements. In contrast, combination gels made from heated milli formed firmer gels crosslinked by denatured whey proteins and underwent fen-er large scale rearrangements.