Effects of Ocimum basilicum and Salvia sclarea essential oils on Listeria monocytogenes and Aspergillus flavus in Iranian white cheese

Mold growth on cheese is a common problem for cheese manufacturers during ripening and curing, as well as for the retailer and consumer during refrigerated storage. Listeria monocytogenes has gained increasing attention as a pathogen of public health importance owing to large numbers of foodborne outbreaks of listeriosis. Ingested by mouth, Listeria is among the most virulent of foodborne pathogens with up to 20% of clinical infections resulting in death. Various types of foods, mostly dairy products such as cheese, have been associated with these outbreaks and there is considerable interest in stopping this upward trend. As a result of the negative consumer perception of chemical preservatives, attention is shifting towards natural alternatives. A technique that has been used since ancient times to prevent fungal growth on foods such as cheese involves physically rubbing the product with certain herbs or spices or their oils. Therefore, regarding to the harmful effects of synthetic preservatives on consumers’ health, there is an increasing attention, both in food industry and authorities, to medicinal and aromatic plants as natural preservatives in food products. In this research, the effect of salvia and basil essential oils (EOs) on Listeria monocytogenes and Aspergillus flavus in Iranian white cheese has been investigated.
Commercially available EOs from basil and salvia were used in this study (Pranarôm International, Ghislenghien, Belgium). EOs were analyzed by gas chromatography mass spectrometry (GC-MS). A broth microdilution assay was employed to determine the lowest concentration (MIC) in which visible growth of the bacterium is inhibited. The concentration of EO present in those wells that yielded plates with no visible colonies was considered to be the minimum bactericidal concentration (MBC). The effects of EOs on radial growth of fungal mycelium were assayed using an agar dilution method. The lowest concentration which inhibited the growth of the fungus was considered to be the minimum inhibitory concentration (MIC) whereas the lowest concentration of EO which killed the fungus (no growth observed on fresh medium) was taken to be minimum fungicidal concentration (MFC). Iranian ultra-filtered white cheese was produced in a commercial factory with different concentrations of EOs and the effects of EOs on bacterial and fungal growth in cheese during shelf-life were determined. Also, eight trained panellists performed sensory analyses. The panellists scored for colour, odour, flavour, overall acceptability and texture using a 9-point hedonic scale (1, dislike extremely to 9, like extremely).
Main components of salvia EO included linalyl acetate and linalool and of basil EO consisted of linalool and α-cadinol. The MIC and MBC of salvia were obtained %0.015 and %0.02 and of basil %0.05 and %0.06, respectively, against L. monocytogenes. In the current report, L. monocytogenes was neither eliminated nor completely inhibited by basil EO, but salvia EO was able to inhibit its proliferation in cheese. The effect was even more pronounced with 1% salvia oil compared to 0.5% or 0.75%. Basil EO at a concentration of 1% caused a 7-day delay in the growth of L. monocytogenes. A growth delay of this type is particularly useful in terms of food safety for the short-term storage of products but not on prolonged storage as Listeria may reach high levels in foods over longer periods. MIC and MFC of salvia were %0.5 and %0.65, and of basil % 0.6 and %0.8, respectively, against A. flavus. At %0.35 and %0.5 the sporulation was inhibited by salvia and basil EOs, respectively. In the present study, no growth of A. flavus was observed in the presence of 1% of salvia EO, and colony diameter attained less than 5mm by the 21st day of cold storage in cheese samples treated with 1% basil EO. Also, the bacterial growth reduced up to 6 log cfu/g of cheese. EO of basil showed weaker antimicrobial effect compared to salvia EO. Cheese samples with different concentrations of EOs were evaluated and compared to the control sample to ascertain consumer acceptability for it. Significant differences were detected among samples containing EOs and the control sample in odour, color and texture, but the samples containing 0.75% and 1% of salvia EO were significantly impaired in both odour and taste as compared with the other samples. With regard to the overall acceptability, the cheese sample containing 0.75% of basil EO was the highest acceptable sample. Generally, it is well known that in complex systems such as cheese, several ingredients interact with each other and affect the sensory properties.
In conclusion, the EO of salvia showed the greatest effect on limiting microbial growth. Both EOs could have potential for commercial use in improving the preservation of these products without the need for propionates or other synthetic additives. Further research could examine the utility of the combined application of basil and salvia EOs in different dairy products such as different types of cheese, as well as the use of different quantities/ratios for optimization of their antimicrobial effects.