Volatile and phenolic contents and antioxidant and antibacterial properties of Tunisian milk thistle and mastic oils

Abstract

The present study examined the chemical compositions as well as the antioxidant and antimicrobial activities of the vegetal oils of milk thistle (Silybum marianum) (MTO) and mastic (Pistacia lentiscus) (MO). The mastic oil extract was shown to have higher total phenolic and flavonoid contents than the oil extract of the milk thistle. The fatty acid and sterol compositions of the oil extracts were determined using gas chromatography coupled to flame ionization detection (GC-FID). The main fatty acids identified were palmitic, oleic, and linoleic acids. The main sterols found in milk thistle oil were β-sitosterol and campesterol, followed by Δ-7-stigmastenol, stigmasterol, Δ-7-avenasterol, and Δ-5-avenasterol. The most abundant sterol in mastic oil was β-sitosterol, followed by campesterol, Δ-5-avenasterol, and stigmasterol. The volatile compounds of the two vegetal oils were evaluated by headspace solid-phase microextraction (HS-SPME). In both oils, monoterpenes and non-terpenes were found to be the dominant volatile compounds in the mastic and milk thistle vegetal oils. 1,8-Cineole, methylpyrazine, 2,5-dimethylpyrazine, and hexanal were found to be the major volatile compounds of milk thistle oil, whereas myrcene and α-pinene were the major volatile compounds found in mastic oil. The antioxidant activities of the oil extracts were evaluated using various methods, such as DPPH, the ferric reducing power, the iron-chelating effect, and the β-carotene bleaching assay. In all methods, mastic oil exhibited a higher activity than milk thistle oil. Furthermore, both oils exhibited antibacterial activity, but mastic oil was the most potent, particularly against Gram-negative bacteria. Given their high total phenolic and flavonoid contents, the presence of volatile bioactive compounds, and their sterol and fatty acid compositions, the two oil extracts possess high potential for pharmaceutical and cosmetic applications and for utilization as natural preservative ingredients in food.