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

Abstract

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 (tan δ min), rate of gelation ( I E) and onset of gelation were the rheological parameters studied, whereas maximum acidification rate ( V m), time at which maximum acidification rate was observed ( T m) and time to reach the end of fermentation ( T e) characterized the kinetics of acidification during milk fermentation. TS strongly affected G′ max and tan δ min; high TS resulted in a large increase in G′ max and decrease in tan δ min. Increasing fermentation temperature gave a decrease in the onset of gelation, in T m and T e, and increase in the rate of gelation ( I E). V m 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 tan δ min 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.