Potential sources and methodologies for the recovery of phenolic compounds from distillation residues of Mediterranean aromatic plants. An approach to the valuation of by-products of the essential oil market-A review

This review compiles the available information on potential sources and methodologies for the recovery of phenolic compounds from the residues (solid residue, wastewater or hydrolate) of the essential oil distillation of a total of 47 cultivated (native and exotic) or wild-collected aromatic plants of the Mediterranean region. Special attention has been paid to the new and promising techniques of distillation and processing of residues in the sense of a more efficient recovery of phenolic compounds with a reduced use of solvents and a time and energy saving. However, it has been observed that technique used for the distillation of the essential oil as well as the subsequent processing of the residue clearly affect the composition, and hence the biological properties, of the phenolic fraction. The most abundant compounds (42) were phenolic monoterpenes (thymol and carvacrol) and diterpenes (carnosic acid, carnosol and methyl carnosate), hydroxybenzoic acids (p-hydroxybenzoic, protocatechuic, vanillic, gallic, ellagic and catechol), phenylpropanoic acids (p-coumaric, caffeic, ferulic, rosmarinic chlorogenic, cryptochlorogenic and neochlorogenic), phenylpropenes (eugenol), coumarins (coumarin and herniarine), flavanones (naringenin, naringin, eriocitrin and hesperidin), flavones (apigenin, genkwanin, apigetrin, luteolin, cynaroside, luteolin 7-glucuronide, scolymoside, cirsimaritin and salvigenin) and flavanols (catechin, kaempferol methylethers, astragalin, quercetin, isoquercetin, hyperoside, miquelianin and rutin). The feasibility of recovering some of these phenolic fractions/compounds is discussed on the basis of the availability of the residue, the potential cost of the implementation by the industry, and in terms of sustainability like the use of less pollutant solvents as water or ethanol.