Characterisation of lactic starters based on acidification and reduction activities

The acidification and reduction activities of lactic starters have been followed by continuous measurement of the pH and the Eh during the course of milk fermentation. These measurements allowed the calculation of the maximum acidification and reduction rates, and the time, pH and Eh at which these values occurred. Nine strains of Lactococcus sp., 6 strains of Streptococcus thermophilus and 5 strains of Lactobacillus helveticus were studied. In general, the maximum reduction rate of the lactococci was six-fold higher than those of the streptococci and lactobacilli. On the other hand, the streptococci and the lactobacilli acidified with a higher maximum acidifying rate than those of lactococci. Consequently, it was observed that all the cultures with lactococci reached their final Eh before the end of the lactic acid fermentation, while acidifications with the streptococci or the lactobacilli finished before the end of the reduction phase. A principal components analysis clearly differentiates the three species on the basis of their aptitudes for acidification and reduction. This new approach might be used to select adequate starters for the manufacture of fermented dairy products.