Modelling of the acidification process and rheological properties of milk fermented with a yogurt starter culture using response surface methodology

The simultaneous effects of fermentation temperature (FT, 36.7–43.4 °C), milk total solid level (TS, 11.3–14.7%, w/v) and total inoculum concentration (TI, 1.66–3.34 v/v) on the acidication process and the rheological properties of milk fermentation with Lactobacillus delbrueckii ssp bulgaricus Y 6.15 and Streptococcus thermophilus Y 4.10 were explored by the means of response surface methodology. Maximum storage modulus (G′max), minimum loss tangent (tanδmin), rate of gelation (IE) and onset of gelation were the rheological parameters studied, whereas maximum acidification rate (Vm), time at which maximum acidification rate was observed (Tm) and time to reach the end of fermentation (Te) characterized the kinetics of acidification during milk fermentation. TS strongly affected G′max and tanδmin; high TS resulted in a large increase in G′max and decrease in tanδmin. Increasing fermentation temperature gave a decrease in the onset of gelation, in Tm and Te, and increase in the rate of gelation (IE). Vm was mainly affected by the interaction TS×FT. Under conditions of relatively low FT (37–39 °C), high TS (around 14%) and high TI (3–3.5%), the gelation and acidification rates were at medium levels and fermentation took a longer time to finish, but the formed gels were firmer, showing higher G′max and lower tanδmin values. Under all conditions examined, a high number of viable lactic acid bacteria in the final product was detected during storage (21 days, 4 °C) for both microorganisms used in the starter culture.