SIMULTANEOUS EVALUATION OF SHEAR RATE AND TIME DEPENDENCY OF STIRRED YOGURT RHEOLOGY AS INFLUENCED BY ADDED PECTIN AND STRAWBERRY CONCENTRATE

A methodology was developed to evaluate simultaneously the shear rate and time dependency of stirred yogurt rheology. The rheograms, obtained using a rotational viscometer, included an upward linear ramp shearing (0-300 s-1 in 3 min), followed by a stress decay at a steady shear rate (10 min at 300 s-1) and a second downward linear ramp shearing (300-0 s-1 in 3 min). Results were comparable to those of earlier studies based on independent evaluation of shear rate and time dependency. The study confirmed thixotropic and stress decay behaviour of stirred yogurt under all conditions. The study further indicated that the rheology of flavored yogurt was influenced by both pectin (0-0.5%) and concentrate (64-degrees-B; 0-10%) and that the desired product viscosity could be obtained by post-fermentation mixing of stirred yogurt with pectin and fruit concentrate.