Effects of Oregano Methanolic Extract on the Chemical, Microbial, and Sensory Properties of Yogurt

The demand for antimicrobial compound alternatives to replace synthetic additives is on the rise. In food commodities, use of synthetic antimicrobials based on herbal extracts is attracted attention. The present study aimed to assess the chemical, microbiological, and sensory properties of the yogurt samples treated with oregano extract (0%, 0.75%, 1%, and 1.5%) during storage for 30 days at refrigerated temperature.  The in-vitro antibacterial effect of oregano extract was evaluated using agar disk-diffusion assay. Natural yogurt was prepared using a combination mixture of whole milk and 4% skimmed milk powder. Various concentrations of the oregano extract (0%, 0.75%, 1%, and 1.5%) were added to the samples. During 30 days of refrigerated storage, various parameters were evaluated, including titratable acidity, pH, mold and yeast counts, total coliforms, and sensory attributes (overall liking, appearance, and aroma).  The descending order of the in-vitro antibacterial effect of the oregano extract was as follows: Listeria monocytogenes>Staphylococcus aureus>Bacillus subtilis>Bacillus cereus>Salmonella typhimurium>Escherichia coli O157:H7. Differences were observed in the counts of mold and yeast in the samples over time. Accordingly, bioactive yogurt had lower mold and yeast counts (1-1.5 log CFU/g) after 30 days of storage compared to the control (P<0.05). In addition, significant differences were observed in the hedonic scores of aroma and appearance between the samples containing the oregano extract compared to the control group (P<0.05).  According to the results, oregano extract could be used as a natural compound to improve the shelf life of yogurt at refrigerated temperatures for 30 days. Furthermore, oregano extract is considered to be an effective inhibitory compound against L. monocytogenes, S. aureus, B. subtilis, B. cereus, S. typhimurium, and E. coli O157:H7.