Use of response surface methodology to evaluate some variables affecting the growth and acidification characteristics of yoghurt cultures

The fermentation temperature, fat and non-fat dry matter content of milk, inoculum size and the initial ratio of cocci/rods were varied according to a Central Composite Design with the aim of assessing the effects of these variables and their interactions on the acidification rate and the growth of Lactobacillus delbrueckii subsp. bulgaricus ST56 and Streptococcus thermophilus DN21 strains used in yoghurt production. The technological and microbiological parameters selected to evaluate the fermentative behaviour, i.e. variation of pH and increase in the number of lactobacilli and streptococci, were analysed in order to develop predictive polynomial quadratic equations. The models obtained emphasized the role attributable to a variable such as the initial ratio of cocci/rods and to its interactions with the other variables. A cocci/rods ratio of 1.5:1 was found to be optimum for all parameters evaluated. An analysis of the response surfaces permitted the identification of the operational conditions needed to optimize the performances of the tested strains. The methodology used in this search was very efficient and the model obtained was found to be valid and accurate for the evaluation of a complex biotechnological process such as yoghurt production.