The effect of olive leaf extract added in sous vide cooked ground beef on Listeria monocytogenes

The study was performed in three stages. At first stage, different concentrations olive leaf extract (OLE) were prepared with dimethyl sulfoxide and distilled water and then their antimicrobial activities on Listeria monocytogenes, Staphylococcus aureus, Salmonella Typhimurium, Salmonella Enteritidis, Escherichia coli O157:H7, Escherichia coli Biotype I and Bacillus cereus was tested by disc diffusion assay. Although OLE in 40% and 50% concentrations was effective on all tested pathogens, the highest antimicrobial effect was on L. monocytogenes which yielded the largest inhibitions zone. At the second stage, OLE added (0,5% and 1%) TSBYE was inoculated with L. monocytogenes (7-8 log cfu/ml) and heated at different temperatures (55, 60, 65oC). In these experiments conducted in vitro conditions L. monocytogenes counts were lower in OLE added samples than those without OLE, however these differences were not statistically significant (P>0.05). At the third stage, ground beef samples were inoculated with L. monocytogenes at the level of 7-8 log cfu/g and added with 1% OLE. Then they were vacuum packaged and sous vide cooked at the same temperatures. L. monocytogenes counts reduced by 0.96 and 1.27 log cfu/g in control and OLE added samples, respectively. On the other hand, counts of pathogen in control and OLE added samples sous vide cooked at 60oC for 20 minutes reduced by 2.79 and 3.83 log cfu/g, respectively, while reduced by 1.69 nd 2.36 log cfu/g, respectively, in those of cooked at 65oC for 7.5 minutes. It was observed that destruction of L. monocytogenes was increased by increasing temperature and OLE caused more reduction in population of the pathogen. According to these findings, it was concluded that OLE can be used as a natural antimicrobial in heat processed meat products to control L. monocytogenes. Keywords: olive leaf extract, sous vide, antimicrobial activity, Listeria monocytogenes, ground beef