Monarda citriodora hydrolate vs essential oil comparison in several anti-microbial applications

The hydrolate, or hydrosol, is the residue of essential oil (EO) distillation traditionally used in perfumery. Evidence that also this product exerts anti-microbial activity promoted a study on Monarda citriodora EO and hydrolate against thirty representative strains of bacteria and fungi. Additionally, the sole hydrolate was tested in four moulds responsible for the spoilage of paper artworks. The micro-broth dilution test served to assess EO (concentrations between 0.0078% and 4%) and hydrolate (concentrations between 12.5% and 50%) suppressiveness vs untreated control. EO and hydrolate were analysed (GC–MS), resulting in thymol being the major terpenic compound of both products (19.6% and 66.4% in the respective spectra). EO suppressed all micro-organisms, although resistant Pseudomonas aeruginosa strains required up to 4% EO as minimum inhibitory concentration (MIC). Hydrolate suppressed all micro-organisms, except three P. aeruginosa strains where even the highest concentration (50% hydrolate) failed to reach the MIC. In each strain, the concentration needed to obtain 50% inhibition (IC50) was calculated by means of an exponential decay function. Median IC50 was 0.06% (EO) and 7.35% (hydrolate), while six IC50 EO data were shown to be outliers. In non-outlier data, IC50 hydrolate and EO were significantly correlated (r = 0.73**), and IC50 hydrolate was approximately a hundred times higher than IC50 EO. Despite higher concentration needed for the same inhibitory effect, the amount of terpenes supplied with hydrolate was lower than with EO. This means a higher hydrolate effectiveness that was likely due to the hydrophilic environment promoting higher terpene availability. Lastly, hydrolate exhibited promising results in the control of fungal growth on paper artworks, suppressing the four tested strains at concentrations of 25–50%.