Purpose: The objective of this study was to evaluate the ability of bacteriocin-producing strain Streptococcus thermophilus ACA-DC 0040 to inhibit Clostridium sporogenes C22/10 spores outgrowth under conditions prevailing during Gruyere cheese production and ripening. This species was used as a model for C. botulinum, since the fermentation products of its toxigenic strains are indistinguishable from those of C. sporogenes strains cultured in the same medium.
Methods: Two fermentations in skim milk were used under conditions mimicking cheese production and ripening. S. thermophilus ACA-DC 0040 (bac+) and S. thermophilus ACA-DC 0004 (bac-) were used in fermentations A and B respectively, and 103 cfu ml-1. C. sporogenes spores were used to inoculate the fermentor for both fermentations. Fermentations were carried out in a 2.5-L glass fermentor with temperature and O2 control. Samples were drawn every hour during fermentation and after 1, 5, 15, 30, and 60 days of ripening at 18°C for bacteriocin activity determination, microbial populations enumeration and organic acid profile detection by HPLC chromatography.
Results: In fermentation A with the bac+ strain, Thermophilin T was produced during the initial stages of fermentation reaching up to 2560 AU ml-1, which was reduced up to 320 AU ml‑1 during the ripening period. Neither vegetation nor outgrowth of Clostridium spores were detected during the whole ripening time. On the contrary, in fermentation B high outgrowth of Clostridium spores was counted after 15 days of ripening and high amounts of acetate (2.93 mmol) was detected from the 15th day and afterwards.
Significance: The quality and safety of Gruyere cheese was ensured in a biopreservation model.