Antimicrobial activity of extracts of Callistemon citrinus flowers and leaves against Listeria monocytogenes in beef burger

This study compared the antibacterial activities of freeze-dried extracts of Callistemon leaves (CL) and flowers (CF) against the growth of cryopreserved Listeria monocytogenes. Agar well diffusion method was used to monitor inhibitory effects of the extracts on the L. monocytogenes in vitro conditions. The extracts were added to the formulation of beef burgers to test their antimicrobial activity against L. monocytogenes at 1 and 2% concentrations. After the preparation, the beef burgers were stored at +4 A degrees C and -18 A degrees C for 6 days and 3 months, respectively. Results showed a dose-response inhibitory pattern against L. monocytogenes from methanolic extract of CL and hydromethanolic extract of CF. For example, the highest concentration (10%) of CF yielded an inhibition zone of 35.0 mm, while the lowest concentration (1%) of CL gave an inhibition zone of 21.0 mm against the growth of L. monocytogenes in vitro conditions. Control group (without extracts) had the highest L. monocytogenes counts within a month in refrigerated conditions, and those treated with 2% CL had the least L. monocytogenes counts after the three months of frozen storage. In comparison with CF, the greatest reduction in L. monocytogenes counts were observed in the groups treated with the highest concentration of CL in vitro conditions. The growth of L. monocytogenes in both refrigerated and frozen beef burgers was highly inhibited by 2% of methanolic extract of CL after a week of refrigeration and 3 months of frozen storage. As a result, it might be concluded that the extracts of Callistemon could be used as an antimicrobial agent for meat preservation purposes.