Kinetic reaction monitoring of acidified milk gels with a quartz resonator: effect of temperature and GDL quantities

The behavior of weak gels during their formation particularly attracts the attention of yoghurt factories. Our study is related to an acidified pre-heated milk gel formation fairly often used to produce soft cheeses and yoghurts. These formations require a tight control of the kinetic reaction parameters. To achieve the measurement, a thickness shear mode quartz resonator technique has been used in the MHz frequency range. This method is singularly effective to ensure a complete and reliable follow up of the viscoelastic parameters (the storage G\´- and the loss -G"-shear moduli) of gel materials. Using a suitable model, this technique gives access to a new accurate characteristic time t_vs (named viscoelastic time). This time corresponds to the instant from which the material is no longer a Newtonian liquid. Using the Arrhenius and modified Flory-Stockmayer relationships, the activation energy is extracted from a study of the gelation process at different temperatures. The experimental results of the G\´ and G" moduli are discussed in this paper, on one hand versus temperature, and on the other versus glucono-delta-lactone (GDL) added to milk.