Effects and Potential Mechanisms of Mixed-Salt Substrate on Yield, Hydrodistillation Kinetics and Essential Oil Composition of Flowers of Tree Peony ‘Taiping Hong’ (Paeoniaceae)
The use of a mixed-salt substrate (sodium chloride-choline bromide-1-ethyl-1-methyl-pyrrolidinium bromide; molar ratio: 1:1:1) has been proposed for hydrodistillation of essential oils. Tree peony ‘Taiping Hong’ (Paeoniaceae), a cultivated essential oil-bearing plant, was investigated. After using the salt substrate, distillation time decreased from 140 to 40 min, essential oil yield increased from 0.48 % to 0.69 % and the pseudo-first-order rate constant increased from 0.0233 to 0.123 min-1. Potential mechanisms are (1) osmosis effect and hydrogen-bond interactions, which improve cell wall rupture; (2) the increase in polarity of aqueous distillation media, which prevents emulsification. The essential oil composition was influenced by an increase in oxygenated compound content (32.9 % to 49.0 %) and a decrease in hydrocarbon content (65.3 % to 49.6 %). Furan linalool oxide (cis and trans), linalool and α-terpineol contents increased by 400 %, 250 % and 200 %, respectively. (E,E)-α-farnesene, α-muurolene and octadecane contents decreased by 50 %, 46 % and 38 %, respectively. Possible mechanisms are: (1) improved cell rupture, which significantly facilitates extraction of oxygenated compounds; (2) hydrogen bonding with oxygenated compounds, which enhances their transfer from the inside of cells to bulk solution; (3) salting-out effect, which decreases dissolution of oxygenated compounds and improves their distillation. The essential oil obtained was mainly composed of phloroglucinol trimethyl ether (21.9 %), nerol (10.2%), tricosane (9.1 %), nonadecane (9.0 %), α-muurolene (5.0 %) and furan linalool oxides (3.0 %). Target prediction analysis indicated that it has potential applications in aromatherapy. In conclusion, the use of the mixed-salt substrate combined with hydrodistillation can result in efficient isolation of essential oils.
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