Antimicrobial Activity of Aqueous Plant Extracts as Potential Natural Additives

The XXI century has brought along many changes in how consumers look at food and perceive their diets. There is an increasing awareness towards what goes into manufacturing these foods, with an important concern being drawn towards food additives. While it is known that additives are needed to preserve or change attributes of food, it is also widely understood that consumers prefer natural additives to synthetic ones, and thus, the industry is looking for alternatives from plant sources. Here, the extraction of 5 different plants (oregano (Origanum vulgare L.), rosemary (Rosmarinus officinalis L.), salvia (Salvia officinalis L.), lemon balm (Melissa officinalis L.) and basil (Ocimum basilicum L.)), was optimized from three extraction types, namely decoction, infusion and cold hydroethanolic (80:20) ultrasound extraction, carried out through Response Surface Methodology (RSM) using Design Expert, pending the analysis of phenolic compounds through HPLC-DAD-ESI/MS. The variables used for RSM were temperature, time of extraction and watts (ultrasound). The extraction with the highest phenolic content for each plant was then selected and screened for its antibacterial and antifungal activity, relying on the microdilution method against foodborne pathogens, before using these extracts as natural food additives in yogurts and muffins. The bacterial species used were Staphylococcus aureus, Bacillus cereus, Listeria monocytogenes, Escherichia coli, Salmonella typhimurium and Enterobacter cloacae, while the fungi species were Aspergillus fumigatus, Aspergillus niger, Aspergillus versicolor, Penicillium funiculosum, Trichoderma viride and Penicillium verrucosum var. cyclopium. Two synthetic and widely used preservatives were also screened against these contaminants, namely sodium benzoate (E211), and potassium metabisulfite (E224), while also confirming the sensitivity of the microbes with known antibiotics. Overall, the plant extracts showed a high inhibition of fungi, with all extracts showing lower minimum inhibition concentrations than both the synthetic preservatives, except for P. verrucosum, where potassium metabisulfite showed the same inhibition capacity. Regarding the antibacterial activity, the most sensitive bacteria to the extracts was S. aureus, where all showed the same activity as potassium metabisulfite. Lemon balm was the best extract of all, showing the same inhibition as sodium benzoate against B. cereus and E. cloacae. Overall, this work proves that plant extracts, obtained though “green” and cheap technologies can be alternatives to synthetic food additives, due to showing the same, or, in some cases, better antimicrobial activity. Furthermore, a mixture of these extracts can result in synergistic effects and improve the antimicrobial activities. The next step in the determination of the efficacy of these extracts is their incorporation in yogurts and muffins, which is ongoing.

Acknowledgments

Foundation for Science and Technology (FCT, Portugal) for financial support through national funds FCT/MCTES to CIMO (UIDB/00690/2020). S. Heleno and M. Carocho thank FCT for their individual research program-contract (CEECIND/00831/2018; CEECIND/03040/2017), while L. Barros thanks FCT for her institutional scientific employment program-contract.