Hydrophobic Amino Acid Content in Onions as Potential Fingerprints of Geographical Origin: The Case of Rossa da Inverno sel. Rojo Duro

Abstract

In this study, we were interested in comparing the amino acid profile in a specific variety of onion, Rossa da inverno sel. Rojo Duro, produced in two different Italian sites: the Cannara (Umbria region) and Imola (Emilia Romagna region) sites. Onions were cultivated in a comparable manner, mostly in terms of the mineral fertilization, seeding, and harvesting stages, as well as good weed control. Furthermore, in both regions, the plants were irrigated by the water sprinkler method and subjected to similar temperature and weather conditions. A further group of Cannara onions that were grown by micro-irrigation was also evaluated. After the extraction of the free amino acid mixture, an ion-pairing reversed-phase high performance liquid chromatography-evaporative light scattering detector (IP-RP HPLC-ELSD) method allowed for the separation and detection of almost all the standard proteinogenic amino acids. However, only the peaks corresponding to leucine (Leu), phenylalanine (Phe), and tryptophan (Trp), were present in all the investigated samples and they were unaffected from the matrix interfering peaks. The use of the beeswarm/box plots revealed that the content of Leu and Phe were markedly influenced by the geographical origin of the onions (with *** p << 0.001 for Phe), but not by the irrigation procedure. The applied HPLC method was validated in terms of the specificity, the linearity (a logarithm transformation was applied for the method linearization), the limit of detection (LOD) and limit of quantification (LOQ), the accuracy (≥90% for inter-day Recovery percentage), and the precision (≤10.51 for the inter-day RSD percentage), before the quantitative assay of Leu, Phe, and Trp in the onion samples. These preliminary findings are a good starting point for considering the quantity of the specific amino acids in the Rossa da inverno sel. Rojo Duro variety as a fingerprint of its geographical origin.

The Relationships between Somatic Cells and Isoleucine, Leucine and Tyrosine Content in Cow Milk

“…The chromatogram of an extract is exemplarily shown in Figure 1. As expected [18,21], the selected IP-RP-HPLC-ELSD method, produced the co-elution of Ala/Glu and Ser/Asn (data not shown). Moreover, interfering matrix effects were found in the first 18 min of each sample analysis, thus impairing the detection of certain amino acids (data not shown).…”

“…A direct ion-pairing reversed-phase method based on the use of the evaporative light-scattering detector (IP-RP-HPLC/ELSD) was applied for the analysis of the milk extracts. The method was previously developed in our laboratory and profitably employed for the direct analysis of amino acids in complex food matrices [18,21]. Detailed information on the method is reported in the caption to Figure 1.…”

“…The IP-RP-HPLC/ELSD method was validated using two Leu external control solutions as fully described in Reference [21]. In the following paragraphs, a brief summary of the results is reported.…”

“…An appreciable precision of the method was obtained in the short-period, with RSD% values ranging from 1.05 to 9.5%, thus producing evidence on the good stability of the employed analytical method. The resulted rather high RSD% value (that is, 9.5%) can be ascribed to the so-called "detector fatigue" [21,33]. Indeed, a progressive reduction in the output stability after a prolonged use is a rather common issue with ELSDs.…”

Correlation between Quality and Geographical Origins of Cortex Periplocae, Based on the Qualitative and Quantitative Determination of Chemical Markers Combined with Chemical Pattern Recognition

“…A growing number of studies have demonstrated that geographical sources could significantly affect the quality of these herbs, since climate and environment influence biosynthesis and the accumulation of secondary metabolites in organisms [8,9,10]. Cortex Periplocae is widespread in Shanxi, Henan, Hebei, and Shandong provinces in China [11,12], and there are no standard planting grounds for it [13].…”