Abstract:In the early nineties the presence of flavonoids in Citrus juices began to attract the attention of a number of researchers, as a result of their biological and physiological importance. This short review will explore two different aspects. The first part will focus on analytical techniques for the characterization of juices from different Citrus fruits regarding their flavonoid content (even if present in only trace amounts), concentrating on the most widely used methods (LC-MS and LC-MS-MS). The second part analyzes data reported in the literature regarding the composition of Citrus juices. The main components that have been detected so far are flavanone-O-glycosides and flavone-O-or -C-glycosides. The presence of such derivatives in various hand-squeezed and industrial juices is discussed, with special emphasis on their correlation to different species.

A comprehensive profile of flavonoids in bergamot juice was obtained by a single DAD-ESI-LC-MS-MS course. Eight flavonoids were found for the first time, five of these are C-glucosides (lucenin-2, stellarin-2, isovitexin, scoparin, and orientin 4'-methyl ether), and three are O-glycosides (rhoifolin 4'-O-glucoside, chrysoeriol 7-O-neohesperidoside-4'-O-glucoside, and chrysoeriol 7-O-neohesperidoside). A method is proposed to differentiate chrysoeriol and diosmetin derivatives, which are often indistinguishable by LC-MS-MS. In-depth knowledge of the flavonoid content is the starting point for bergamot juice exploitation in food industry applications.

High-performance liquid chromatography-diode array detector (HPLC-DAD) coupled with electron spray mass spectrometry (ESI-MS-MS) was used to determine the flavonol profile in southern Italian red onions (Allium cepa L.). This on-line technique allowed the identification of seven flavonols in southern Italian red onion, quercetin 4'-glucoside and quercetin 3,4'-diglucoside being the most abundant components. Five minor flavonols have been recognized, offering a characteristic profile of such compounds in red onions under study. Quercetin 3-glucoside, quercetin 7,4'-diglucoside, quercetin 3,7,4'-triglucoside, and isorhamnetin 4'-glucoside have been previously reported as minor flavonoid components in Allium cepa, while isorhamnetin 3,4'-diglucoside was previously found in Allium ascalonicum. Traces of isorhamnetin 3-glucoside and free quercetin were also detected.

High-performance liquid chromatography-diode array detector (HPLC-DAD) coupled with electronspray mass spectrometry (ESI-MS) was used to detect the flavonoid profile in lemon juices obtained from the main Sicilian cultivars (Femminello comune, Monachello, and Interdonato). Significant amounts of an unusual constituent were found in the lemon juice of the above-mentioned cultivars together with eriocitrin, hesperidin, and diosmin. Following purification by preparative HPLC, the structure was assigned as 6,8-di-C-glucopyranosyldiosmetin by means of DAD-UV, ESI-MS-MS, and (1)H NMR analysis. Three other minor components were also detected. One of these presented a flavone nature, and spectral data and literature references both suggested a 6,8-di-C-glucopyranosylapigenin structure. The different contents of eriocitrin, hesperidin, diosmin, and above minor components in the cultivars allow juices to be readily differentiated.

The qualitative and quantitative compositions of chinotto juice in two different maturation periods were determined via chromatographic separation of extracted aliquots of juice of Citrus x myrtifolia Raf. by using reverse-phase LC-DAD-ESI-MS-MS. This provides a comprehensive chromatographic evaluation of 11 compounds (furanocoumarins and flavonoids C- and O-glycosides). Five flavonoids and two furanocoumarins were identified for the first time in chinotto juice: two C-glucosides (vicenin-2 and lucenin-2 4'-methyl ether), two O-glycosides (narirutin and rhoifolin), and a 3-hydroxy-3-methylglutaryl flavanone (brutieridin). Bergapten and epoxybergamottin were the primary furanocoumarins found. Overall, the juice from immature chinotto fruits is richer in bioactive compounds than that obtained from ripe fruits. The free radical and superoxide anion scavenging activities of juice from both green and ripe fruits were assessed, and results showed that the former is much more efficient in scavenging radical and superoxide species than the latter.