effect of temperature and ph on formulating the kinetic growth parameters and lactic acid production of lactobacillus bulgaricus

background and objectives: lactobacillus delbrueckii ssp bulgaricus is widely used in dairy industries as a starter for yogurt production. this study was designed using response surface methodology (rsm) in 12 batch ph-controlled cultures of lactobacillus delbrueckii sub-ssp. bulgaricus for determining the effect of temperature and ph on the biomass production of a native strain of l. bulgaricus, and its main metabolite, lactic acid. the performance of richards model for prediction of l. bulgaricus kinetic growth was verified using the data obtained from the experiments. materials and methods: l. bulgaricus was isolated from a plain yogurt. the medium was composed of whey and yeast extracts. rsm was used to design the experiments and quantify the effects of temperature and ph on maximum cell concentration, maximum specific growth rate, total lactic acid concentration, relative growth rate (k) and exponent parameter (d) in richards model, and product formation parameters of luedeking-piret equation (a & b). matlab software (version 7.12.0) was used to estimate the parameters of luedeking-piret equation and richards model for each experiment. results: second order model for xmax, μmax, p and k was significant but product formation parameters were almost constant. the optimum values of temperature and ph for attaining maximum biomass, maximum specific growth rate, and maximum acid production were obtained at 44 °c and 5.7, respectively. conclusions: the attained empirical mathematical correlations of rsm alongside the kinetic equations could be used to determine growth conditions under predefined temperature and ph in the fermentation process. keywords: lactobacillus bulgaricus, richards model, response surface methodology, lactic acid production, luedeking-piret model