Viability and acidification activity of pure and mixed starters of Streptococcus thermophilus CNRZ 404 and Lactobacillus delbrueckii subsp. bulgaricus CNRZ 398 at the different steps of their production
Résumé
Three types of lactic starters of Streptococcus salivarius ssp. thermophilus 404 and Lactobacillus delbrueckii ssp. bulgaricus 398 have been produced by controlled pH batch cultures: pure starters, mixings of pure starters and mixed starters. They were compared according to their viability and their acidification activity after the different steps of their production and during storage. At the end of the batch cultures, the acidification activity, characterized by the maximum acidification rate in milk, was 40% higher for the starters issued from the mixed cultures or the mixings of pure cultures with regard to the pure starters. During the concentration, freezing and freeze-drying processes, tea significant alteration of the activity of the starters was observed. The viability of the starters, during 24 wk of preservation after freezing (-75°C) or freeze-drying (4°C), decreased continuously. This decrease was slightly faster in the case of the starter obtained from a mixed culture. The starters were more resistant during storage in a frozen state than in a freeze-dried state. Finally, thanks to the use of a well normalized procedure, the acidification activity was not altered during the storage of each type of starter.