Biolog methodology for the antimicrobial activity evaluation of essential oils' volatile compounds against foodborne microflora of fresh-cut salad

The antimicrobial properties of six volatile organic compounds (VOCs) of essential oils were tested on foodborne microflora of fresh-cut salad by plate-count method and by Biolog methodology, with lettuce-juice as nutritive medium. VOCs examined were eugenol, limonene, cinnamaldehyde, thymol, menthol and γ-terpinene, added in 2 concentrations in the wells of Biolog MT2 microplates for metabolic analisys, and in bacteriological tubes added with lettuce broth for growth observation. Both MT2 plates and broth tubes were inoculated with salad-extracted microflora. Chloramphenicol, pimaricin, sterile broth and inoculated broth with no compounds, were used as blanks. The microbial metabolic activity was periodically measured by recording the absorbance variations in the wells at 590 nm during 144 hours of incubation. For growth measurements, the tubes were similarly incubated and periodically sampled for plate counting. The curves of absorbance temporal evolution, modelled with suitable mathematical equations, were used to express the effect of the VOCs under study on salad microbial population, and the results so obtained were compared with the ones obtained from growth measurements. A good congruence was observed between the two methods for assessing the antimicrobial effect of the VOCs under study, while Biolog metabolic profiling provided finer pieces of information, being able to kinetically describe their antimicrobial behaviour (duration and evaluation of main and residual effect) during the chosen time of incubation. Such aptitude of Biolog methodology was verified in particular for the bacterial component of salad microflora, while a poor aptitude resulted for assessing the antimicrobial activity on the fungal component. A relevant inhibition against salad bacterial component was observed for eugenol (0.5 and 3 μL/mL), cinnamaldehyde (0.2 μL/mL) and thymol (0,5 mg/mL).