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BackgroundFlavonoids have shown to exert multiple beneficial effects on human health, being also appreciated by both food and pharmaceutical industries. Citrus fruits are a key source of flavonoids, thus promoting studies to obtain them. Characteristics of these studies are the discrepancies among sample pretreatments and among extraction methods, and also the scant number of comparative studies developed so far.ObjectiveEvaluate the effect of both the sample pretreatment and the extraction method on the profile of flavonoids isolated from lemon.ResultsExtracts from fresh, lyophilized and air-dried samples obtained by shaking extraction (SE), ultrasound-assisted extraction (USAE), microwave-assisted extraction (MAE) and superheated liquid extraction (SHLE) were analyzed by LC–QTOF MS/MS, and 32 flavonoids were tentatively identified using MS/MS information. ANOVA applied to the data from fresh and dehydrated samples and from extraction by the different methods revealed that 26 and 32 flavonoids, respectively, were significant (p≤0.01). The pairwise comparison (Tukey HSD; p≤0.01) showed that lyophilized samples are more different from fresh samples than from air-dried samples; also, principal component analysis (PCA) showed a clear discrimination among sample pretreatment strategies and suggested that such differences are mainly created by the abundance of major flavonoids. On the other hand, pairwise comparison of extraction methods revealed that USAE and MAE provided quite similar extracts, being SHLE extracts different from the other two. In this case, PCA showed a clear discrimination among extraction methods, and their position in the scores plot suggests a lower abundance of flavonoids in the extracts from SHLE. In the two PCA the loadings plots revealed a trend to forming groups according to flavonoid aglycones.ConclusionsThe present study shows clear discrimination caused by both sample pretreatments and extraction methods. Under the studied conditions, liophilization provides extracts with higher amounts of flavonoids, and USAE is the best method for isolation of these compounds, followed by MAE and SE. On the contrary, the SHLE method was the less favorable to extract flavonoids from citrus owing to degradation.

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Eighty four metabolites (32 flavonoids, 15 amino acids, nine carboxylic acids, six coumarins, six sugars, five phenolic acids and 11 unclassified compounds) have been tentatively identified in a polar extract from lemon, without reference standards, based on their liquid chromatography-quadrupole-time-of-flight MS/MS spectra and the comparison with databases. Despite information in databases for some families of plant compounds is poor, tentative identification based on MS/MS information (mass of the precursor ion and their fragments, together with neutral mass loss) was possible with the help of known fragmentation patterns for the given families of compounds. Both positive and negative ionization modes and at least two collision energies were always applied to obtain as much information as possible from each molecular entity, thus helping for identification. As the tentatively identified metabolites are the same regardless of the organism they belong, their fragmentation patterns are useful for identification with independence of the sample nature.

Citrus fruits possess a high content of phenolic compounds; however, few studies have focused on the changes occurring during fruit growth. In this study, the changes in the concentration of 20 flavonoids, 4 phenolic acids, and their biosynthetic precursors phenylalanine and tyrosine have been evaluated during fruit maturation (14 weeks). Extracts from all samples, obtained by ultrasound assistance, were analyzed by liquid chromatography coupled to tandem mass spectrometry with a triple quad system (LC-QqQ MS/MS). In general, the concentration of flavanones, which represented over 70% of the studied phenols, and flavones increased during fruit growth, reaching their maximum concentration around week 12. In general, flavanols and phenolic acids exhibited their maximum concentration at week 5 and then decreasing significantly during the rest of maturation. Phenylalanine and tyrosine showed a sinuous behavior during fruit growth. Partial least-squares showed a clear differentiation among fruits belonging to different maturation stages, coumaric acid derivatives being the most influential variables on the projection.

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