Antimicrobial Activity of Aqueous Plant Extracts as Potential Natural Additives

The 21st 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 artificial ones, and thus, the industry is looking for alternatives from plant sources. The extraction of five 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 for three extraction types, namely decoction, infusion, and cold hydroethanolic (80:20) ultrasound-assisted extraction. This optimization was carried out through a screening analysis to find the most important factors, using Design Expert, pending the analysis of phenolic compounds through HPLC-DAD-ESI/MS. The optimized variables used were temperature, time of extraction, and potency (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. The bacterial strains used were Staphylococcus aureus, Bacillus cereus, Listeria monocytogenes, Escherichia coli, Salmonella typhimurium and Enterobacter cloacae, while the fungal strains were Aspergillus fumigatus, Aspergillus niger, Aspergillus versicolor, Penicillium funiculosum, Trichoderma viride, and Penicillium verrucosum var. cyclopium. Two synthetic and widely used preservatives, namely sodium benzoate (E211) and potassium metabisulfite (E224), were also screened against these contaminants to confirm the sensitivity of the microorganisms to these known artificial preservatives. 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. viridae, where E224 showed the same inhibition capacity. Regarding the antibacterial activity, the bacteria most sensitive to the extracts was B. cereus, for which all the extracts showed the same activity as E224. Lemon balm was the stronger extract, showing the same inhibition as E221 against B. cereus and E. cloacae. Overall, this work proves that plant extracts obtained though “green” and low-cost technologies can be alternatives to artificial 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 of this work, which is ongoing, will focus on the determination of the efficacy of these extracts and their incorporation in muffins.