Growth and fermentation activity of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in milk: a calorimetric investigation

Cultures of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus were studied with a calorimetric approach at various temperatures, as well as with more traditional techniques. The use of the Gompertz function was improved to account for the microbial decline that follows the exponential growth and the plateau phase of the microbial population. Special calorimetric cells allowed the simultaneous record of heath flow and pH accompanying the microbial growth. these data were matched with determinations of the D-and L-Lactic acid concentration (within medium) and plate count results. The calorimetric approach confirmed the expected behaviour, namely, (i) the enhanced growth in mixed culture conditions, (ii) the optimal temperature for yogurt production (42 degrees C), (iii) the earlier onset of Streptococcus growth and (iv) the best tolerance of Lactobacillus in the acid medium. Furthermore it allowed the overall energy to be split into a couple of thermal effects related to cell duplication and metabolic activity, respectively.