Effect of Traditional Cultures of Yoghurt Starter and the Probiotic Starter Cultures of Bifidobacterium on the Pathogenic Bacterium Salmonella

The study was conducted on three samples of yoghurt, the first produced by traditional starter cultures (Streptococcus and Lactobacillus), the second by Bifidobacterium Bb-12, and the third by Bifidobacterium Bb-46. Each sample was divided into two parts and each part inoculated with Salmonella at 1750 and 350 cells per gram and stored at 4 ± 1°C for 15 days to determine the effect on the viability of this pathogen. The pH of the samples, as well as their contents of lactic and acetic acids and the counts of Salmonella were determined. The pH of the yoghurt produced by the traditional cultures was found to be the lowest, followed by that of the yoghurt produced by Bifidobacterium Bb-12, and then the one produced by Bifidobacterium Bb-46. The yoghurt produced by Bifidobacterium contained more acetic acid, while the one produced by traditional cultures contained more lactic acid. Disk diffusion tests using permeate from yoghurt samples as antibiotic reveled that the
traditional culture was more inhibitory to Salmonella (clear zone diameter of 27 cm) than Bb-46 and Bb-12 (clear zones diameters of 22 and 23, respectively). The counts of Salmonella decreased in all samples during storage, with more inhibition found in the traditional cultures. Complete inhibition of all Salmonella cells occurred within 5 days in traditional yoghurt samples inoculated with 1750 cells per gram and within 7 days in samples inoculated with 3500 cells per gram. Salmonella in Bb-12 and Bb-46 samples inoculated with 1750 cells per gram was completely inhibited within 10 and 15 days of storage, respectively, while the samples inoculated with 3500 cells per gram contained some viable Salmonella cells after the 15 days storage period. The study showed that both types of yoghurt, traditional or probiotic, were inhibitory to Salmonella. Traditional yoghurt had higher inhibitory effect, followed by the Bb-12 and then the Bb-46 yoghurts. The inhibitory effects of these yoghurts result from their low pH and their high contents of the antimicrobial organic acids.