Volatile Compounds in Citrus Essential Oils: A Comprehensive Review

Abstract

The essential oil fraction obtained from the rind of Citrus spp. is rich in chemical compounds of interest for the food and perfume industries, and therefore has been extensively studied during the last decades. In this manuscript, we provide a comprehensive review of the volatile composition of this oil fraction and rind extracts for the 10 most studied Citrus species: C. sinensis (sweet orange), C. reticulata (mandarin), C. paradisi (grapefruit), C. grandis (pummelo), C. limon (lemon), C. medica (citron), C. aurantifolia (lime), C. aurantium (bitter orange), C. bergamia (bergamot orange), and C. junos (yuzu). Forty-nine volatile organic compounds have been reported in all 10 species, most of them terpenoid (90%), although about half of the volatile compounds identified in Citrus peel are non-terpenoid. Over 400 volatiles of different chemical nature have been exclusively described in only one of these species and some of them could be useful as species biomarkers. A hierarchical cluster analysis based on volatile composition arranges these Citrus species in three clusters which essentially mirrors those obtained with genetic information. The first cluster is comprised by C. reticulata, C. grandis, C. sinensis, C. paradisi and C. aurantium, and is mainly characterized by the presence of a larger abundance of non-terpenoid ester and aldehyde compounds than in the other species reviewed. The second cluster is comprised by C. junos, C. medica, C. aurantifolia, and C. bergamia, and is characterized by the prevalence of mono- and sesquiterpene hydrocarbons. Finally, C. limon shows a particular volatile profile with some sulfur monoterpenoids and non-terpenoid esters and aldehydes as part of its main differential peculiarities. A systematic description of the rind volatile composition in each of the species is provided together with a general comparison with those in leaves and blossoms. Additionally, the most widely used techniques for the extraction and analysis of volatile Citrus compounds are also described.

Keywords: analytical methods; citrus essential oil; flowers; leaves; rind; volatile compounds.

FIGURE 1

Relative frequency of each group of compounds present in the 10 most common Citrus species. Left, all the compounds described; right outer circle, compounds up to date detected in only one species (one-species only); right inner circle, compounds detected in all 10 species (common). Orange bars indicate separation between mono-, sesqui-, and other terpenoids, and non-terpenoid compounds. The qualitative data used is available in Supplementary Table S4.

FIGURE 2

Heatmap showing the relative frequency of each group of compounds is presented for each of the most common Citrus species. Data represent the number of compounds reported in each category divided by the total number of compounds reported in each particular species, according to the scale below: red, high frequency; black, intermediate frequency; green, low frequency. The qualitative data used is available in Supplementary Table S5.

FIGURE 3

Hierarchical clustering of the 10 most common Citrus species according to their volatile profile as described in the scientific literature. For each species, data were normalized by dividing the number of times a particular compound was reported by the total number of reports of all the compounds in that species. The statistical parameters used were as described in Rambla et al. (2015). The qualitative data used to create HCA is available in Supplementary Table S4.