Combined effects of culture conditions and storage time on acidification and viscosity of stirred yogurt

This work aims to study the effects of culture conditions and storage time on stirred yogurt acidity and texture and to determine relationships in order to better control product viscosity. A four-factor experimental design was carried out to display the simple and combined effects of the ratio of two Streptococcus thermophilus strains, incubation temperature, final fermentation pH, and storage time on bacterial concentrations, acidification activity (determined by the time corresponding to the maximum acidification rate), texture (determined by the flowing time coefficient measured with a modified Posthumus funnel), and postacidification. The development of viscosity was influenced by all four factors studied and by some two-factor interactions. The acidification activity was affected by temperature and strain association. Greater viscosity was correlated to lower acidification activity, thus indicating that fermentation time affects the development of texture. As a consequence, linear relationships between the flowing time coefficient and final fermentation time were established. Finally, the most viscous yogurt was obtained by low temperature and fermentation at a low final pH with a texturing strain of S. thermophilus, which required the longest fermentation time.