Application of Response Surface Methodology to Improve Fermentation Time and Rheological Properties of Probiotic Yogurt Containing Lactobacillus reuteri

Response surface methodology was employed to investigate the combined effects of inulin addition (0–4% w/w), probiotic inoculum level (1–3% v/v), and fermentation temperature (37–45 °C) on fermentation time and rheological properties of nonfat-set yogurt. The rheological characteristics were measured by dynamic oscillatory rheometery. The second-order polynomial model was fitted to the fermentation time (Tf), structure strength (A value), resistance to mechanical force or yield stress (τy), and complex viscosity (ŋ*) of runs as the responses. Analysis of variance revealed that the quadratic models are well adjusted to predict the experimental data. Lack-of-fit tests were not significant and determination coefficients (R2) were higher than 88.6%. The results showed that fermentation time decreased with increasing inulin content, incubation temperature, and probiotic inoculum level. Rheological properties significantly influenced by independent variables. Inulin addition had a softening effect on yogurt. However, the effect of incubation temperature on structure strength was positive. Medium levels of probiotic inoculum resulted in stronger gels.