Onion is widely used worldwide in various forms for both food and medicinal applications, thanks to its high content of phytonutrients, such as flavonoids and volatile sulfur compounds. Fresh onion is very perishable and drying is widely applied for extending shelf-life, thus obtaining a very easy-to-use functional food ingredient. The flavonoid and volatile fractions of different onion cuts (flakes, rings) prepared through different drying cycles in a static oven, were characterized by high-performance liquid chromatography with a diode-array detector HPLC-DAD, Head Space-Solid Phase Micro Extraction followed by Gas Chromatography coupled with Mass Spectrometry (HS-SPME-GC-MS) and Head-Space Solid Phase Micro Extraction followed by comprehensive two-dimensional Gas-Chromatography (HS-SPME-GC×GC-TOF). Onion flakes showed a significantly higher flavonoid content (3.56 mg g−1) than onion rings (2.04 mg g−1). Onion flakes showed greater amount of volatile organic compounds (VOCs) (127.26 mg g−1) than onion rings (42.79 mg g−1), with different relative amounts of di- and trisulfides—disulfides largely predominate the volatile fraction (amounts over 60% on the total volatile content), followed by trisulfides and dipropyl disulfide and dipropyl trisulfide were the most abundant VOCs. HS-SPME-GC×GC-TOF allowed for the detection of the presence of allylthiol, diethanol sulfide, 4,6-diethyl1,2,3,5-tetrathiolane, not detected by HS-SPME-GC-MS, and provided a fast and direct visualization and comparison of different samples. These results highlight different nutraceutical properties of dried onion samples processed otherwise, only differing in shape and size, thus pointing out potentially different uses as functional ingredients.
Keywords: volatile compounds; onion rings; onion flakes; phenolic compounds; flavonoids; sulfur compounds; food ingredient
Allium is the larger genus of the Alliaceae family, with approx. 450 species [[
The characteristic flavor and biological properties at the basis of these uses of onion have been mainly attributed to the sulfur compounds present in the volatile fraction of onion. The volatile compounds emitted by onion also demonstrated anti-browning activity, with trisulfides as better inhibitors than disulfides [[
The type of sulfur compounds found in onions and other Allium species is strongly affected by the nature of the S-alk(en)yl-L-cysteine-S-oxide precursors (Figure 1). For example, in garlic, propiin is almost absent and alliin is the major compound, while in onion, alliin is present at a very low concentration [[
Analysis by SPME-GC-MS of fresh, frozen, freeze-dried and sterilized onions pointed out that, in the transformed samples, most of the sulfur compounds present in fresh onion (thiosulfinates and zwiebelanes) degraded to form other sulfur volatiles—mainly disulfides and trisulfides—with the consequent changes in sensorial and nutraceuticals properties [[
Besides volatile sulfur compounds, other polar compounds, as sapogenins, saponins and flavonoids have been detected in the Allium species [[
In this study, we aimed at characterizing samples prepared by different cuts (onion flakes and onion rings) and different drying cycles in a static oven of dried white onion from the Emilia Romagna region (Italy). We hypothesized that the different cuts of onion would show different chemical composition, thus providing different nutraceutical properties. The flavonoid fraction was characterized by HPLC-DAD. The volatile fraction was analyzed using optimized Head Space-Solid Phase Micro Extraction followed by Gas Chromatography coupled with Mass Spectrometry (HS-SPME-GC-MS); in addition, Head-Space Solid Phase Micro Extraction followed by comprehensive two-dimensional Gas-Chromatography (HS-SPME-GC×GC-TOF) was adopted for providing a volatile fingerprint of the two samples for a fast and direct visualization and comparison.
Two different kinds of dried onion samples were obtained by drying different cuts (onion rings and onion flakes) of Emilia Romagna's white onion (Allium cepa L.) in a static oven at 40 °C. The chemical composition of the two samples was analyzed in order to point out any variances between them and any consequent different possible nutraceutical application of these products. The volatile and flavonoid fractions of the samples were analyzed using chromatographic techniques (HPLC-DAD, HS-SPME-GC-MS and HS-SPME-GC×GC-TOF), while the total phenolic content was evaluated using the Folin-Ciocalteu method. HS-SPME was used as a well-recognized convenient sampling tool for VOCs, coupled with GC-MS for their separation and detection, techniques which are increasingly applied for analysis of foods. HS-SPME-GC×GC-TOF was thereafter applied for deep-in elucidation of the volatile profile of samples. To the author knowledge, this paper is the first report on the application of HS-SPME-GC×GC-TOF to the analysis of the volatile fraction of onion samples.
Table 1 shows the composition of the volatile profile of the dried onion samples when they were analyzed by HS-SPME-GC-MS either in the presence or in the absence of ascorbic acid. Overall, a total of 53 volatile organic compounds (VOCs) were tentatively identified, according to their RI and matching factor greater than 80%—30 of them are sulfur-containing compounds (3 monosulfides, 16 disulfides, 5 trisulfides, 1 mercaptane, 2 thiophenes, 2 trithiolanes and the carbon disulfide), while the remaining 23 VOCs were 15 aldehydes, 1 ketone, 3 carboxylic acids, 1 alcohol, 2 esters and the 2-pentylfuran. The reported data in Table 1 are based on the use of the internal standard, namely 4-methyl-2-pentanol and they have to be intended as a relative quantitation. The total content of VOCs detected in dried onion flakes is much higher than in dried onion rings, with values of 127.26 mg g
Sulfides—Disulfides are the most abundant class of VOCs in the volatile fraction of the samples followed by trisulfides. These two classes, together, account for the 92.6% of the total VOCs content in onion flakes and 89.2% in onion rings. In particular, the class of disulfides accounts for the 60.3% in onion flakes and for 76.9% in onion rings, with dipropyl disulfide as the definitely most abundant molecule of this class, followed by propyl trans-1-propenyl disulfide, methyl propyl disulfide, methyl cis-1-propenyl disulfide and methyl trans-1-propenyl disulfide; the dipropyl disulfide, identified in our samples as the most abundant VOC, was previously reported as linked to green notes of onion. At the same time, the class of trisulfides accounts for 32.3% in onion flakes and for 12.4% in onion rings, with dipropyl trisulfide as the most abundant molecule of this class, followed by dimethyl trisulfide, methyl propyl trisulfide and propyl trans-1-propenyl trisulfide. The presence of disulfides and trisulfides in so high percentages in the volatile profile of these dried onion samples is in agreement with previous literature [[
Other S-compounds—a total of 6 S-containing VOCs other than sulfides were identified, for a total content of 6.08 mg g
In order to detect any oxidation process affecting the composition of S-containing molecules in the applied analytical condition and to verify if the presence of an anti-oxidant molecule could inhibit these processes, further analysis was carried out adding ascorbic acid, a molecule well-known for its anti-oxidant properties, to the sample. When samples were analyzed in the presence of ascorbic acid, the total VOCs content increased from 42.79 mg g
Regarding sulfides, Table 2 shows the sum of the relative concentration of the molecules containing the 1-propenyl or the allyl moiety. According to the higher content of iso-alliin than alliin in onion, the concentration of VOCs with the 1-propenyl moiety was higher than those with the allyl moiety; however, the presence of allyl derivatives also suggests the presence of low amounts of alliin, whose transformation after onion cutting leads to allyl derivatives. The detection of certain amounts of allyl derivatives in onion processed product is in agreement with previous literature [[
Aldehydes—a total of 16 aldehydes were identified in the volatile profile of the dried onion samples, 4 of which never reported in this type of samples so far (2-methyl propanal, pentanal, octanal and furfural). The total content of aldehydes was slightly higher in dried onion flakes (1.80 µg g
Other VOCs—6 volatile compounds other than aldehydes or S-containing compounds were detected in our samples in low amounts—6-methyl-5-hepten-2-one, nonanoic acid, 2-pentylfuran, 1-octen-3-ol, isopropyl dodecanoate and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate. These VOCs showed slight differences between onion flakes and onion rings and their amount did not change when the samples were analyzed in the presence of ascorbic acid. The only exception was the nonanoic acid, which content strongly increased in the presence of ascorbic acid, which also led to the detection of hexanoic and 2-ethyl hexanoic acids.
HS-SPME-GC×GC-TOF analysis was applied to better elucidate the volatile profile, thus providing a tool for the direct comparison and visualization of volatile components and pointing out the presence of molecules not identified only with GC-MS. HS-SPME-GC×GC-TOF analyses of the volatile fraction of onion products were submitted to advanced fingerprinting analysis of 2D chromatographic data.
Sulfur-containing compounds were the most abundant in samples volatile fraction. Sulfur compounds are considered to be derived from the degradation of sulfur-containing amino-acids and are associated with alliaceous, sulfuric, sweaty, onion and cabbage aromas, contributing to the characteristic flavor of raw and processed edible Alliums. Table 3 reports the list of headspace volatile compounds of the onion products, 1st D and 2nd D retention times and the average peak volume from three independent determination. A split ratio of 1:5 during SPME injection was employed for a better separation in the 2D chromatographic space of the major co-eluting molecules not evidenced in the common 1D analysis. The most intense peaks corresponded to dipropyl disulfide and dipropyl trisulfide. Diethanol disulfide and allylthiol were identified only by GC×GC, probably owing to the co-elution with peaks deriving from SPME fiber bleeding and/or other molecules in mono-dimensional chromatography. Also peaks of cis-3,5-diethyl-1,2,4-trithiolane and trans-3,5-diethyl-1,2,4-trithiolane (which probably is a peak constituted by the two co-eluting enantiomers) were simultaneous separated by GC×GC, in particular the trans and cis isomers were separated in the first dimension (29.900, 1.620 min and 30.150, 1.640 min, respectively). Another trisulfide, the propenyl propyl trisulfide, was separated in the second dimension, eluting at 30.150, 1.380 min. Tentative identification was made on the basis of non-isothermal Kovats retention indices from temperature-programming from Chemistry WebBook (Table 1) and on the presence of the ion 151 which was present only in cyclic trisulfides (cis and trans 3,5-diethyl-1,2,4-trithiolane) and not in the linear ones (propenyl propyl trisulfide). Trithiolane compounds have been identified as important aroma-active compounds in cooked onions (Allium cepa L.) showing cooked onion-like/fruity and blackcurrant-like/fruity odor impressions for the trans-enantiomers compared to the cis-isomer eliciting a meat broth-like, cooked onion-like aroma at a five- to ten-fold higher threshold [[
In Figure 2, "contour plots" from HS-SPME-GC×GC-TOF analyses of the onion products are reported—each 2D-peak corresponds to a single volatile compound. In this case, SPME and comprehensive comparative analysis of 2D chromatographic data showed visual differences between samples. The main differences that emerged between the two samples could be summarized as follows—flakes showed higher intensity of sulfides peaks as reported by GC-MS; allylthiol, 2-methyl-butenal and 4,6-diethyl-1,2,3,5-tetrathiolane were detected only in rings sample.
Data reported in literature for onion phenolic composition vary due to normal biological variations related to cultivar, growing season, environmental and agronomic conditions and, in some instances, the region of the bulb [[
HPLC analysis of our hydroalcoholic onion extracts produced two principal peaks identified as quercetin-3,4′-diglucoside and quercetin-4-glucoside by comparison of spectroscopic and literature data; they are the main compounds in our sample, data in agreement with the literature [[
Considering that in fresh onion the content of flavonoids ranges from 0.185 to 0.634 mg g
Further analysis was performed to evaluate the total polyphenol content—the values ranged from 1.49 (onion rings) to 1.62 mg
Authentic standard of rutin and Folin-Ciocalteu reagent were purchased from Sigma-Aldrich (St. Louis, MO, USA). All the used solvents were of HPLC grade purity (BDH Laboratory Supplies, Poole, UK). 4-methylpentan-2-ol used as internal standard and a mixture of linear alkanes (C9–C30) in hexane used for calculating linear retention indexes were from Sigma-Aldrich (St. Louis, MO, USA). Inert gasses (He and N
Two different dried white onion (Allium cepa L.) samples were purchased in 2017 by Officinali Agribioenergia Factory (Medicina, Bologna, Italy). The two different samples were obtained by different cuts and different drying cycles. For obtaining onion flakes, the fresh onion was cut into cubes of 8 mm sides, which were then dried in a static oven (with low air ventilation, that is, 10000 m
The dried samples were deep-frozen with liquid nitrogen and immediately chopped with mortar and pestle, until a fine and homogenous powder was obtained. The powder was used for the following analyses.
A quantity of 250 mg of sample (onion flakes/rings) was extracted in 5 mL of 70% ethanol (pH = 3.2 by formic acid) overnight. The obtained solution was used for the determination of phenolic content by the Folin-Ciocalteu assay and for HPLC-DAD analysis.
The total phenolic content was determined using the Folin-Ciocalteu method [[
Analysis of the onion extracts was performed using an Agilent 1200 HPLC system (Agilent Technologies, Santa Clara, CA, USA). Samples were separated by a LUNA C18 column (250 × 4.6 mm i.d., 5 µm particle size) maintained at 27 °C. The flow rate was 0.8 mL/min. The mobile phase consisted of (A) water (pH = 3.2 HCOOH) and (B) 100% acetonitrile. The following binary gradient was used: 0–3 min, 5%–12% B; 3–8 min, 12%–14% B; 8–25 min, 14%–20% B; 25–35 min, 20%–35% B; 35–45 min, 35%–60% B, 45–60 min, 60%–100% B. UV/Vis spectra were recorded in the 190–600 nm range and the chromatograms were acquired at 260, 280, 330 and 350 nm. Identification of the quercetin glucosides was based on spectra, standards and literature data, while identification of other individual phenols was carried out using their retention times, spectroscopic and literature data. Quantification of individual phenolic compounds was directly performed by HPLC-DAD using a five-point regression curve (R
Volatile organic compounds (VOCs) were analyzed by both HS-SPME-GC-MS and HS-SPME-GC×GC-TOF analyses. Some trials were initially carried out aimed at optimizing sample amount and exposure time and temperature. For both the analyses, after these trials (briefly described in the supplementary file), SPME conditions were set as follow, bearing in mind that we were working with processed samples and also according to Colina-Coca et al. [[
After 5 min of equilibration at 60 °C, VOCs were absorbed exposing a 2-cm divinilbenzene/carboxen/polydimethylsiloxane SPME fiber (DVB/CAR/PDMS by Supelco) for 5 min into the vial headspace under orbital shaking at 500 rpm and then immediately desorbed at 280 °C in a gas chromatograph injection port operating in splitless mode. Samples were analyzed in triplicate.
The VOCs absorbed as described above were immediately desorbed at 280 °C in the injection port of a 7890a GC system (Agilent Technologies, Santa Clara, CA, USA), separated by a DB InnoWAX column (0.4 µm d.f. × 0.2 mm i.d., 50 m) and detected by a quadrupole Mass Spectrometer 5975c MSD (Agilent Technologies, Palo Alto, CA, USA) operating in EI mode at 70 eV. Initial oven temperature was 40 °C, held for 1 min, then raised to 220 °C at 5 °C min
Relative concentration of each identified compound was calculated according to previous literature [[
[VOC (µg/g)] = (A
where A
GC×GC was performed by a flow modulation apparatus consisting of an Agilent 7890B GC (Agilent Technologies, Palo Alto, CA, USA), with capillary flow modulator device for 2D separation, coupled with a time-of-flight mass spectrometer (TOF-DS Markes International Ltd., Llantrisant, UK)). SPME sampling was carried out at the same conditions described in 3.4 for mono-dimensional GC-MS analysis. Chromatographic separation was performed using a first dimension (1D) HP-5 column (0.18 μm d.f. × 0.18 mm i.d., 20 m) and an InnoWAX second dimension (2D) column (0.23 μm d.f. × 0.32 mm i.d., 5 m). Flow modulation was performed with a modulation period of 3 s. Helium was used as carrier gas (99.999% purity) at flow rates of 0.4 and 10 mL min
Compounds were tentatively identified comparing mass spectra of each peak with those reported in mass spectral databases; identification was confirmed by their retention index.
Quantitative data (Table 1) are expressed as the mean of three determinations. One-way ANOVA and F-test (p < 0.05) were applied using Microsoft Excel statistical software for evaluating statistical differences between samples. Fisher's LSD test was then used for comparing means (DSAASTAT excel
This study deals with the characterization of two different types of dried onion, obtained by different cuts of an onion from Emilia Romagna (Italy) dried in a static oven at 40 °C. The flavonoid fraction was analyzed by HPLC-DAD and the volatile fraction was analyzed by both HS-SPME-GC-MS and HS-SPME-GC×GC-TOF, with particular attention to the sulfur volatile compounds. The analysis was mainly focused on characterizing the samples and pointing out any differences in their quali-quantitative volatile and flavonoid profiles. The study also pointed out the presence of some volatile molecules never before reported in the volatile profile of dried onion samples—allyl propyl sulfide, 1-propenyl propyl sulfide, allyl isopropyl disulfide, allyl cis-1-propenyl disulfide, allyl trans-1-propenyl disulfide, 1-(1-(methylthio)propyl)-2-propyl disulfide, methyl 1-(propylthio)propyl disulfide, 1-(cis-1-propenylthio)propyl propyl disulfide, 1-(1-trans-propenylthio)propyl propyl disulfide. Some analysis, carried out in the presence of high amount of the antioxidant ascorbic acid, allowed confirming that in the volatile fraction of these dried onion samples, nonoxidized volatile S-compounds as thiols are absent or present in negligible amounts. To the author knowledge, this study is the first one about the use of HS-SPME-GC×GC-TOF to deepen the characterization of the volatile fraction of dried onion samples.
The flavonoid content was higher in onion flakes (3.56 mg g
In light of the obtained results, highlighting different volatile and flavonoid profiles, different uses for the different parts of dried onion can be proposed by dried onion producers to the industry of food ingredients and beyond.
Graph: Figure 1 Typical S-alk(en)yl-L-cysteine-S-oxide found in different Allium species.
Graph: Figure 2 Comprehensive two-dimensional chromatography–mass spectrometry (GC×GC-TOF) color diagram and comprehensive template matching fingerprinting with the main identified volatile compounds of onion flakes and rings.
Table 1 Relative concentration (µg g
Compound RIref RIcal Identification Ions Dry Onion (µg g−1) Dry Onion with Ascorbic Acid (µg g−1) Flakes Rings Flakes Rings dimethyl sulfide 729 729 62, 47, 35 0.07 b 0.03 a 0.06 b 0.03 a allyl propyl sulfide 1137 1113 116, 87, 73 0.20 b 0.04 a 0.16 b 0.04 a 1-propenyl propyl sulfide - 1138 41, 116, 74 0.05 b 0.02 a 0.03 a 0.02 a dimethyl disulfide 1105 1081 94, 79, 45 0.14 b 0.03 a 0.11 b 0.02 a methyl propyl disulfide 1263 1244 80, 122, 43 2.56 b 0.51 a 2.34 b 0.73 a methyl 1298 1278 73, 120, 45 2.09 b 0.20 a 1.60 b 0.42 a methyl allyl disulfide 1322 1293 41, 120, 45 0.05 b 0.02 a 0.05 b 0.04 a,b methyl 1322 1302 73, 120, 45 2.81 c 0.29 a 2.47 c 0.80 b isopropyl propyl disulfide - 1331 150, 43, 108 0.04 b 0.01 a 0.03 b 0.01 a dipropyl disulfide 1413 1391 150, 43, 108 49.34 b 28.53 a 49.77 b 33.35 a propyl 1450 1427 148, 106, 41 6.69 b 0.61 a 6.05 b 1.38 a allyl isopropyl disulfide - 1443 57, 148, 106 0.46 c 0.14 a 0.47 c 0.25 b propyl 1473 1452 148, 106, 41 11.30 b 1.53 a 10.87 b 3.34 a allyl 1464 1480 146, 41, 105 0.05 b 0.01 a 0.04 b 0.02 a allyl 1533 1500 146, 41, 105 0.08 b 0.03 a 0.08 b 0.04 a 1-(1-(methylthio)propyl)-2-propyl disulfide - 1876 89, 61, 73 0.15 ab 0.07 a 0.19 b 0.18 b methyl 1-(propylthio)propyl disulfide - 1985 117, 75, 41 0.44 a 0.65 b 0.48 a 0.82 c 1-( - 2075 115, 81, 73 0.19 b 0.09 a 0.20 b 0.14 a,b 1-( - 2080 115, 81, 73 0.37 b 0.17 a 0.29 a,b 0.20 a dimethyl trisulfide 1403 1399 126, 111, 79 6.01 c 0.49 a 7.16 c 1.97 b methyl propyl trisulfide 1576 1553 154, 112, 43 6.56 c 0.88 a 6.45 c 1.92 b dipropyl trisulfide 1713 1695 182, 43, 75 23.28 b 3.32 a 22.08 b 5.37 a allyl propyl trisulfide 1797 1753 115, 180, 73 0.13 b 0.04 a 0.10 b 0.03 a propyl 1770 1768 180, 74, 116 5.10 c 0.57 a 5.90 c 1.92 b carbon disulfide 745 716 76, 44, 32 0.07 b 0.02 a 0.03 a 0.02 a 1-propanethiol 845 816 76, 47, 43 0.21 a 0.14 a 0.84 c 0.46 b 2,4-dimethylthiophene 1197 1199 112, 111, 97 0.42 b,c 0.16 a 0.46 c 0.26 a,b 3,4-dimethylthiophene 1253 1264 112, 111, 97 3.57 b 1.16 a 5.80 c 2.55 b 1775 1807 180, 74, 151 0.77 c 0.35 a 0.84 c 0.63 b 1795 1826 180, 74, 151 1.04 c 0.62 a 1.12 c 0.91 b propanal 784 764 58, 29, 28 0.20 b 0.11 a 0.23 b 0.21 b 2-methyl propanal 800 788 72, 43, 41 0.03 a 0.03 a 0.03 a 0.03 a 2-methyl butanal 916 917 41, 86, 57 0.09 a 0.06 a 0.07 a 0.07 a 3-methyl butanal 914 921 44, 71, 45 0.36 c 0.31 b,c 0.21 a 0.23 a,b pentanal 978 985 44, 86, 58 0.02 a 0.02 a 0.06 b 0.05 b hexanal 1084 1088 56, 72, 82 0.30 b 0.23 a 0.47 c 0.31 b 2-methyl-2-butenal 1104 1106 84, 55, 39 0.05 a 0.03 a 0.04 a 0.04 a 2-methyl-2-pentenal 1185 1170 98, 41, 69 0.20 c 0.07 a 0.14 b n.d. heptanal 1186 1192 70, 86, 96 0.07 a 0.06 a 0.11 b 0.13 b octanal 1296 1298 84, 100, 110 0.08 a 0.06 a 0.63 c 0.35 b ( 1339 1336 83, 112, 55 0.10 b,c 0.06 a 0.13 b,c 0.08 a,b nonanal 1394 1402 98, 57, 114 tr tr tr tr furfural 1471 1470 96, 95, 39 n.d. 0.06 a 1.00 b 1.55 c decanal 1515 1507 112, 82, 95 0.16 a 0.13 a 0.49 c 0.28 b benzaldehyde 1529 1542 106, 105, 77 0.14 c 0.08 a 0.10 b 0.08 a 6-methyl-5-hepten-2-one 1338 1340 126, 69, 108 0.04 a 0.03 a 0.03 a 0.03 a hexanoic acid 1838 1846 87, 60, 73 n.d. n.d. 0.62 a 0.55 a 2-ethyl hexanoic acid 1950 1951 88, 73, 116 n.d. n.d. 3.26 b 1.85 a nonanoic acid 2173 2167 73, 158, 129 0.02 a 0.07 a 1.42 c 0.92 b 2-penthylfuran 1236 1235 138, 81, 82 0.05 a,b 0.02 a 0.07 b 0.05 a,b 1-octen-3-ol 1451 1444 57, 72, 99 0.27 b 0.20 a 0.22 a 0.32 a isopropyl dodecanoate 1832 1835 200, 102, 183 0.27 b 0.17 a 0.29 b 0.19 a 2,2,4-trimethyl-1,3-pentanediol diisobutyrate - 1885 71, 83, 111 0.57 b 0.26 a 0.52 b 0.23 a
Table 2 Sum of the concentration (µg g
Flakes 28.73 0.97 Rings 3.52 0.28
Table 3 Main volatile organic compounds detected by HS-SPME-GC×GC-TOF in the different onion samples. Data are the mean of three determinations. First dimension retention time in minute (1st D RT). Second dimension retention time in second (2nd D RT). Response of the two-dimensional peak (volume). Relative amount (%) calculations were based on the ratio between the peak volume of each compound and the sum of volumes of all selected compounds. Not detected (n.d.).
Compound 1st D RT 2nd D RT Rings Flakes Rings Flakes propyl mercaptan 6.05 0.42 1,724,317 2,979,593 5.0 5.8 allylthiol 6.20 0.62 816,521 n.d. 2.4 n.d. 2-methyl-2-butenal 8.65 1.04 677,649 n.d. 2.0 n.d. 2,4-dimethylthiophene 12.85 1.06 562,705 787,731 1.6 1.5 3,4-dimethylthiophene 13.80 1.24 1,170,022 6,773,463 3.4 13.2 methyl propyl disulfide 14.80 1.04 416,658 2,247,811 1.2 4.4 methyl 1-propenyl disulfide 15.15 1.28 149,034 1,541,084 0.4 3.0 dimethyl trisulfide 16.40 1.72 133,334 1,679,770 0.4 3.3 dipropyl dilsulide 21.60 0.94 11,529,581 14,500,819 33.4 28.3 propenyl propyl disulfide 21.95 1.10 1,365,546 3,030,564 4.0 5.9 diethanol disulfide 23.40 1.38 740,611 3,813,741 2.1 7.4 unidentified sulfide mw 152 23.95 1.68 254,301 2,194,284 0.7 4.3 decanal 25.15 0.92 145,059 1,481,756 0.4 2.9 dipropyl trisulfide 29.50 1.08 8,135,894 5,312,160 23.6 10.4 trans-3,5-diethyl-1,2,4-trithiolane 29.90 1.62 1,341,744 220,925 3.9 0.4 cis-3,5-diethyl-1,2,4-trithiolane 30.15 1.64 2,565,177 912,354 7.4 1.8 propenyl propyl trisulfide 30.15 1.38 1,970,506 3,730,800 5.7 7.3 4,6-Diethyl-1,2,3,5-tetrathiolane 38.40 2.22 809,467 n.d. 2.3 n.d.
Table 4 Quali-quantitative flavonols content, expressed as mg
RT (min) UV Absorption (nm) Onion Flakes Onion Rings (mg g−1) (mg g−1) Quercetin-7,4′-diglucoside 22.3 252–366 0.05 0.04 Quercetin-3,4′-diglucoside 25.2 265–344 1.76 0.94 Isorhamnetin-3,4′-diglucoside 27.3 266–344 0.11 0.01 Quercetin-3-glucoside 33.7 256–350 0.05 0.02 Quercetin-4′-glucoside 36.9 253–365 1.52 0.87 Isorhamnetina-4′-glucoside 38.0 252–366 0.03 0.13 Quercetin 42.9 255–370 0.04 0.04
Conceptualization, A.R., L.C.; Data curation, F.I., L.C., N.M. and P.V.; Formal analysis, L.C., F.I and P.V.; Project administration, A.R.; Resources and Supervision, A.R. and N.M.; Writing—original draft, F.I., P.V. and L.C. All authors have read and agree to the published version of the manuscript.
This research received no external funding.
The authors declare no conflict of interest.
The research was developed with the support of the Officinali Agribioenergia factory (Medicina, Bologna, Italy) and the BIOMAVO business network (Val d'Orcia, Siena, Italy). The authors are grateful for supporting the research to Mario Massai and SRA instruments (Cernusco sul Naviglio, Milan, Italy) for technical support.
The following are available online, file S1: Brief description of preliminary trials aimed at optimizing sample amount, and exposure time and temperature.
By Lorenzo Cecchi; Francesca Ieri; Pamela Vignolini; Nadia Mulinacci; Annalisa Romani and Henryk Jeleń
Reported by Author; Author; Author; Author; Author; Author