Tunisian essential oils as potential food antimicrobials and antioxidants and screening of their element profile

Abstract

The potential employment of 18 commercial Tunisian essential oils (EOs) as natural food preservatives was investigated. Their antimicrobial activity was evaluated against two food-borne bacteria (Escherichia coli O157:H7 and Klebsiella pneumoniae) by the agar-well diffusion method and, subsequently, by determining the minimum inhibitory concentration (MIC); while their antioxidant activity was assayed by the DPPH test. Also, the main volatile constituents and inorganic elements were elucidated and discussed in relation to the biological activities. Excluding the wormwood EO, all EOs inhibited the growth of at least one reference strain. In particular, spearmint, sage and rosemary EOs had the lowest MICs against E. coli O157:H7 (0.09, 0.09, 0.07 mg mL−1, respectively); whereas sage EO was effective against K. pneumoniae at 0.41 mg mL−1. All EOs displayed also a promising antioxidant activity. Specifically, spearmint, bigroot geranium, and sage EOs showed the lowest IC50 values (0.024, 0.046 and 0.052 mg mL−1), and a behavior similar to that of butylated hydroxytoluene. EOs exhibited variegated and peculiar element profiles. Among minerals, Mg and K were in the range 1.72–14.12 mg Kg−1 and 0.57–20.90 mg Kg−1; while Cu and Fe were generally the most abundant trace essential metals (0.07–1.02 mg Kg−1 0.20–2.98 mg Kg−1, respectively). Very low and safe levels of heavy metals were also revealed. A statistical correlation analysis pointed out a significant positive correlation between some elements (K, Cu, and Fe) and the biological activities displayed. In conclusion, the selected Tunisian EOs had a marked potential applicability as antibacterial and antioxidant additive in food industry.