Antimicrobial effects of the phenolic compounds of Mentha piperita L on Streptococcus thermophilus and Lactobacillus bulgaricus and impacts on the quality of an alicament steamed yoghurt

This study contributes to the evaluation of the antibacterial effect of hy-droethanolic extract of peppermint (Mentha piperita L.) rich in phenolic compounds on the growth of reference bacterial strains Streptococcus ther-mophilus (ATCC19258) and Lactobacillus bulgaricus (ATCC11842) as well as the consequences of its addition at a dose of 6% on the quality of a steamed yoghurt. The phenolic compounds of peppermint collected in Algeria were extracted from the ground leaves of the plant in a hydroethanolic solution. The antimicrobial activity of peppermint extract and the quality of yoghurt were assessed by the following measures: bacterial growth test, disc diffu-sion test, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), pH, acidity, viscosity and the organoleptic tests (taste, odor, color and freshness). The results revealed that hydroethanolic extract of M. piperita L. had 25 mg GAE/ml of extract of total phenolic compound and a content of 8.28 mg QE/ml of extract of flavonoids. This extract had a bactericidal against S. thermophilus and a bacteriostatic inhibitory effect against L. bulgaricus. The addition of phenolic compounds of M. piperita L. at 154.02 mg GAE/100ml significantly reduced (p<0.05) acidity, viscosity, and the germ increase of S. thermophilus and L. bulgaricus in steamed yogurt prepared with 6% of hydroethanolic extract of the studied plant compared to the control. However, the fermented milk treated with the 6% of pepper-mint extract was very well appreciated by the tasting jury, just like the yo-ghurt control.