Measurement of the rheological character of acid gels

Fermented milk is analysed after being kept for 24 h at +5 degrees C. After measuring the pH, the gel is stirred quickly using a domestic mixer set at low speed for 15 s. The measurement chamber of the Rheomat is filled with the stirred sample, carefully avoiding the introduction of any air bubbles. The measurement chamber used for lactic gels is a 1074 system, adapted to low viscosity substances. The equipment is set to speed modulus 7/7, which corresponds to a rotation speed interval of between 5 and 780 rpm. The measurement system is placed in the thermostatically controlled chamber and the measurement head is then fixed to the universal coupling system of the instrument. Using the 30 readings obtained for each lactic gel, it is possible to plot a rheogram, which is a graphical representation of the rheological modification of the gel during its destructuring by the rotation of the measurement cylinder at different speeds. Analysis of the rheogram enables several characteristics of the gel to be determined. We have retained three of these: (.)the apparent viscosity level, m, depends on the shearing speed, which is expressed in mPa/s; (.)the gel fragility index, Delta t, is the difference between the shear rates before and after the 5-min destructuring period at maximum speed; (.)the behaviour index, n, indicates the difference in behaviour of the gel being analysed compared to a Newtonian liquid for which n = 1. The viscosity index and behaviour index are measured during the slow-down phase of the rheogram. The repeatability of the method has been assessed with 12 commercial yoghurts from the same production batch, and the repeatability of the complete test (inoculum, fermentation, cooling and rheological measurements) was measured using a thermophile strain, with 12 repetitions. The Delta t parameter has a high CV %, but it provides a means of appreciating the susceptibility of the gels to destructuring and the ability to detect strains which are fluid. The repeatability of the measurements is better during the decreasing speed phase (slow-down phase). The three parameters retained, Delta t, m and n, calculated during the slowdown phase, have no correlation with each other.