The antimicrobial effect of essential oil and extract of Ziziphora clinopodioides L. at concentrations of 125, 250, 500, 1000, 2000 and 4000 µg/L was studied on the growth of 9 bacterial strains (three Gram-positive strains and six Gram-negative strains). Minimum inhibitory Concentration (MIC) was determined using dilution method and minimum bactericidal concentration(MBC) was taken from the concentration of the lowest dosed test tube showing no growth on subcultured. All tested organisms were inhibited by essential oil of Ziziphora clinopodioides except Pseudomonas aeroginosa and Shigella dysenteria. The MIC and MBC for Gram-negative bacteria, including Enterobacter aerogenes, Escherichia coli, Klebsiella pneumoniae and Salmonella enteritidis were 250, 500, 125 and 250 mg/l respectively. The MIC for Gram-negative bacteria, including Listeria monocytogenes, Staphylococcus aureus and Bacillus cereus were 125, 1000 and 125mg/l respctivily. The MIC and MBC of Ziziphora clinopodioides extract for Gram-negetive bacteria, including Enterobacter aerogenes, Escherichia coil, Klebsiella pneumoniae, Salmonella enteritidis and Shigella dysenteria were 1000-2000 µg/L. The MIC and MBC for Gram-positive bacteria, including Listeria monocytogenes, Staphylococcus aureus and Bacillus cereus, were 1000-4000 µg/L.
According to the results of in vitro stage, three concentration of essential oil (125, 250 and 500 µg/L) and extract (1000,2000 and 4000 µg/L) were selected to evaluating their effects against four food spoilage and pathogenic bacteria (Escherichia coil, staphylococcus aureus, Enterobacter aerogenes, and salmonella enteritidis) in yoghurt during the storage time. Results showed that the essential oil at 125 µg/L had inhibition effect on staphylococcus aureus and salmonella enteritidis which caused in reduction at least 4 and 5 log10 of those organisms during 2 and 6 days respectively. Such inhibition was also observed on E. coli at 250 µg/L which found to reduce 4 log10 after 4 days. No inhibition was detected on Enterobacter aerogenes at all concentrations. The extract of Ziziphora clinopodioides at 1000 µg/L had the most inhibition effect against all of tested bacteria in yoghurt, which caused in reduction 3.7, 4.2, 4.7 and 3.2 log10 of staphylococcus aureus, E. coli, salmonella enteritidis and Enterobacter aerogenes, respectively.
Viability of starter culture was investigated during the storage of yoghurt at 4°C at different time intervals. The results showed that the number of starter culture in all samples decreased during storage. Effect of the essential oil of Ziziphora clinopodioides was not significantly different (P<0.01) from the control sample. The extract of Ziziphora clinopodioides at high concentration (4000 mg/L) significantly (P<0.01) decreased the viability of starter culture after 19 th day of yoghurt production. Yoghurt samples with extract of Ziziphora clinopodioides at 2000 mg/L had the best consistency, appearance and flavour between all of treatments and control samples.
The essential oil and extract of Ziziphora clinopodioides examined in this study have potential for application as flavouring agent and preservative in foods to increase shelf-life or promote safety.