Changes in the rheological properties of stirred acid milk gels as induced by the acidification procedure

The aim of this study was to compare the rheological properties of stirred acid milk gels, acidified using two different acidification procedures. Pre-heated milks were acidified either by means of glucono-delta-lactone (GDL gel) at 20 degrees C or by incubation with lactic acid bacteria (lactic gel) at 38 degrees C. The latter procedure gave a harder acid set gel with a more heterogeneous structure. Once formed, the gels were filtered and stirred, then immediately characterised using the low-amplitude dynamic oscillation method for 20 h at 4 degrees C or creep measurement at 4 degrees C. The stirred gels were stored at 4 degrees C for 28 d, during which the pH and the viscosity at 64 s(-1) were measured. After stirring, the lactic gel made at 38 degrees C had a higher instantaneous elastic modulus, a higher viscosity and showed a greater increase in the elastic modulus over 20 h compared with the stirred GDL gel. The viscosity of both stirred gels progressively increased during storage at 4 degrees C, but that of GDL acidified gels increased 3.5 times more than that of lactic gels. The procedure used to prepare the set acid gels had notable effects on the interactions formed within 20 h and 28 d of storage of stirred gels.