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

The effects of heat treatment of milli, and a range of rennet and glucono d elta-lactone (GDL) concentrations on the rheological properties! at small a nd large deformation, of milli gels were investigated. Gels mere made from reconstituted skim milli at 30 degrees C, with two levels each of rennet an d GDL. Together with controls this gave a total of sixteen gelation conditi ons, eight for unheated and eight for heated milli. Acid gels made from unh eated milks had ion; storage moduli (G') of < 20 Pa. Heating milks at 80 de grees C for 30 min resulted in a large increase in the G' value of acid gel s. Rennet-induced gels made from unheated milk had G' values in the range s imilar to 80-100 Pa. However, heat treatment severely impaired rennet coagu lation: 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 u nusual rheological behaviour was observed. After gelation, G' initially inc reased rapidly but then remained steady or even decreased, and at long agei ng times G' values increased moderately or remained lon. The loss tangent ( tan delta) of acid gels made from heated milk increased after gelation to a ttain a maximum at pH similar to 5.1 but no maximum was observed in gels ma de from unheated milli. Gels made by a combination of rennet and GDL also e xhibited 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 cau sed by solubilization of colloidal calcium phosphate. We also suggest that in combination gels made from unheated milk a low value for the fracture st ress and a high tan delta during gelation indicated an increased susceptibi lity of the network to excessive large scale rearrangements. In contrast, c ombination gels made from heated milli formed firmer gels crosslinked by de natured whey proteins and underwent fen-er large scale rearrangements.