Rheological properties of stirred yoghurt as affected by gel pH on stirring, storage temperature and pH changes after stirring

The rheological properties of stirred yoghurt were studied as a function of the delay between milk heat-treatment and inoculation (0, 1 and 2 days), of pH in the acid gel on stirring (4.4, 4.7, and 5.0), of the storage temperature (4, 12, and 20 °C) for 24 h following stirring and of over-acidification (allowed or inhibited). At low pH values, the gels exhibited higher elastic modulus (G′) and fracture strength. They yielded stirred yoghurts with higher G′ and viscosity, and higher increase in G′ and viscosity during storage (“rebodying”). Rebodying was only partially explained by over-acidification and cooling. Changing the storage temperature had no impact on the evolution of G′ after stirring; hydrophobic interactions were therefore probably not involved in rebodying. Electrostatic interactions seemed to play a major role in rebodying, as pH on stirring was the significant factor.