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 degreesC), 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 (tandelta(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,) characterized the kinetics of acidification during milk fermentation. TS strongly affected G'(max) and tandelta(min); high TS resulted in a large increase in G'(max) and decrease in tandelta(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 TSxFT. Under conditions of relatively low FT (37-39 degreesC), 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 tandelta(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 degreesC) for both microorganisms used in the starter culture. (C) 2003 Elsevier Ltd. All rights reserved.