The antimicrobial and anti-biofilm forming effect of thyme (Thymus vulgaris) essential oil (TEO) with different compositions was evaluated. Normally the main component in this TEO is thymol, but in 2014 we found that the proportions of -terpinene and p-cymene (the precursors in thymol biosynthesis) increased and that of thymol decreased. This altered composition led to changes in the antimicrobial and anti-biofilm forming capacity of the essential oil depending also on the type of microorganisms. In the case of bacteria, minimal inhibitory (MIC) and minimal bactericidal (MBC) concentrations either decreased or increased. In the case of yeasts, minimal fungicidal concentrations (MFC) increased 4-and 8-fold for TEO containing p-cymene as the main component. On the contrary, MIC values decreased for all the tested moulds. Anti-biofilm forming activity of TEO containing p-cymene as its main component decreased in almost all cases and P. fluorescens biofilm forming capacity was even enhanced.

The effect of cinnamon, clary sage, juniper, lemon and marjoram essential oil (EO) vapours was tested on growth, aflatoxin production and sporulation of Aspergillus parasiticus. In reversed Petri-dish method the sub-lethal EO vapour concentrations ranging from 0.05 to 0.42 mg/cm3 air were used and growth rates (mm/day) and antifungal indices (%) were calculated from the growth curves of the fungus. Aflatoxin production was determined by HPLC and spores were counted in a Burker chamber. Cinnamon, clary sage and marjoram EOs showed concentration dependent growth inhibition. Antifungal index and aflatoxin production using the weak antifungals, juniper and lemon EO, increased in parallel. The same trend was found using cinnamon and clary sage EO vapours up to 0.11 mg/cm3 concentration, and marjoram EO up to 0.21 mg/cm3, while higher concentrations caused a sharp decrease in aflatoxin production. Applying sub-lethal concentrations of EOs might induce stress response in A. parasiticus leading to increased aflatoxin production. Only EO concentrations with strong growth and sporulation inhibitory effect were suitable to inhibit the aflatoxin production of A. parasiticus..

Shelled walnut (Juglans regia) kernels are prone to rancidity during storage. In this study we examined the preservation effect of cinnamon and clove essential oil (EO) vapors compared to cold storage and vacuum packaging by measuring the hexanal content, indicating rancidity, in stored walnut kernels. Odor and taste of stored shelled walnut was investigated by sensory evaluation and by measuring residues of the main EO components in the kernels. During storage under EO vapors, cinnamaldehyde and eugenol were absorbed on the surface of walnuts in a time-dependent manner changing the odor and taste of the kernels. Clove (Syzygium aromaticum) EO prevented rancidity and EO treated kernels were rated as acceptable by the sensory panel while cinnamon EO treatment increased rancidity compared to the other treatments and the control samples. ABSTRACT clove essential oils rancidity walnut KEY WORDS Volume 62(2):141-145, 2018 Acta Biologica Szegediensis