Five onion landraces belonging to
Onion (
The plant portion commonly used is the bulb, which is utilized as a food ingredient to give flavour and aroma to a great variety of dishes.
Onions are an important source of several phytonutrients as flavonoids, fructooligosaccharides (FOS), and thiosulfinates and other sulfur compounds, recognized as important elements of the Mediterranean diet [
In fact, onions contain high levels of phenolic compounds, which have antioxidant properties besides beneficial effects against different degenerative pathologies (cardiovascular and neurological diseases, dysfunctions based on oxidative stress) [
Flavonoids are the major phenolics in onions, which can be classified to different subclasses (flavones, flavanones, flavonols, isoflavones, flavanonols, flavanols, chalcones, and anthocyanins) on the basis of the degree of unsaturation and the degree of oxidation of the central ring. Flavonoids subclasses can be further differentiated on the basis of the number and nature of substituent groups attached to the rings [
Flavonols are the most abundant in onions, present as their glycosides, that is, quercetin and kaempferol [
FOS represent another source of phytochemicals in onions bulbs. They are mainly inulin, kestose, nystose, and fructofuranosylnystose. The health benefits of these carbohydrates have been widely reported in the past years due to their prebiotic effect [
In onions, sulfur compounds are responsible for typical odour and flavour and are also active antimicrobial agents [
The precursors of sulfur-containing compounds are S-alk(en)yl-L-cysteine sulfoxides (ACSOs, i.e., methiin, propiin, and isoalliin) which are hydrolysed by means of alliinase enzyme into pyruvate, ammonia, and a mixture of both volatile and nonvolatile sulfur compounds [
The concentration of pyruvate produced by alliinase activity allows assessing the pungency of onions [
The bioaccumulation of organosulfur compounds in onions depends on different factors but especially on the sulfur-based fertilization, the environment, and the genotype of the cultivars [
Onion (
Raw onions
Analytical grade chemicals, methanol, dichloromethane, trichloroacetic acid, acetic acid, acetonitrile, sodium hydroxide, Folin-Ciocalteu reagent, gallic acid, 2,2-diphenyl-1-picrylhydrazyl (DPPH), dinitrophenyl hydrazine (DNPH), quercetin, kaempferol, 2-octanol, sodium sulfate, and ferulic and chlorogenic acids, were purchased from Sigma-Aldrich (St. Louis, MO, USA).
One hundred grams of edible part of onion samples was homogenized in an Ultra-Turrax blender (T25, IKA Werke, Staufen, Germany) at room temperature. The slurry was transferred into the flask with 300 mL of distilled water and 1
The volatile compounds were determined by GC-MS (Trace MS plus, Thermo Finnigan, USA) and by GC-FID (HP 6890, Agilent), both equipped with a capillary column (SUPELCOWAX 10; 60 m, 0.25 mm, and 0.25
150 mL of methanol was added to 50 g of fresh onion tissue. The resulting mixture was homogenized and stirred for 30 minutes; homogenates were held for 15 min in an ultrasonic bath (Fungilab Ultrasound, Barcelona, Spain) and the extract was separated from the residue by centrifugation at 1900 ×g. Extraction was repeated increasing the stirring time to 60 min and to 90 min. The combined methanol : water extracts were filtered through a Whatman No. 2 filter paper and were evaporated at 40°C (rotary evaporator IKA RV-8, Staufen, Germany) to remove methanol. The extracts were redissolved in 10 mL of methanol [
Total phenols were estimated by Folin-Ciocalteu colourimetric assay [
The quali-quantitative profile was determined by HPLC (1100, Agilent, Waldbronn, Germany) according to Cinquanta et al. (2015). The polyphenol extracts were filtered through 0.45
Antioxidant activity was measured by 1,1-diphenyl-2-picrylhydrazyl (DPPH) solution (
The antioxidant activity was expressed according to Albanese et al. [
Pungency of onions was determined as enzymatically (alliinase) produced pyruvate (EPY) by colourimetric analysis according to Schwimmer and Weston (1961) with slight modifications. Onion landraces were sliced in half longitudinally: 50 g was homogenized by Ultra-Turrax blender (T25, IKA Werke, Staufen, Germany) with 50 mL of distilled water for the determination of total pyruvate alliinase produced, whereas 50 g of onions was pretreated with 50 mL of 5% trichloroacetic acid solution to inactivate the alliinase in order to quantify pyruvate basal level. Both mixtures were left at room temperature for 15 min and filtered with Whatman filter paper (grade 1) and 10 mL of the filtrate was diluted ten times with bidistilled water. One millilitre of sample was placed in a reaction tube with 1 mL of 2,4-dinitrophenyl hydrazine (DNPH) solution (0.0125% DNPH in 2 M HCl) and 1 mL of bidistilled water. Reaction tube was vortexed and placed in a water bath at 37°C for 10 minutes. After the incubation time, 5 mL of 0.6 M NaOH was added to the tube and allowed to stand for 5 min. The DNP hydrazine derivative of pyruvate was measured using PerkinElmer
Sugars were determined by HPLC (Hewlett Packard, mod. 79852, USA) [
1 g of fresh onion was added to distilled water up to 10 mL and homogenized in an Ultra-Turrax blender (T25, IKA Werke, Staufen, Germany) for 2 min. The samples were centrifuged at 4000 rpm for 10 min and filtered through 0.45
Analysis of parameters investigated was carried out on five different samples belonging to each onion group. Results were reported as the mean and standard deviation. The analysis of variance (ANOVA) was applied to the data. The least significant differences were obtained using an LSD test (
A total of 22 compounds (Table
Volatile compounds (mg/kg dw; mean ± SD) detected in
Number | RT | Volatile compound | mg/kg dw | ||||
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Febbrarese | Marzatica | Aprilatica | Maggiaiola | Giugnese | |||
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1 | 6.7 | Propionaldehyde | 5.30 ± 0.10 |
12.65 ± 0.26 |
72.39 ± 1.51 |
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132.51 ± 2.77 |
2 | 23.4 | 2-Methyl-2-pentenal | n.d. | 3.79 ± 0.08 |
49.65 ± 1.04 |
65.43 ± 1.37 |
38.82 ± 0.81 |
3 | 44.1 | Furfuraldehyde | 39.38 ± 1.36 |
82.25 ± 2.84 |
166.21 ± 5.73 |
97.26 ± 3.35 |
135.11 ± 4.66 |
4 | 49.9 | 5-Methyl-2-furfuraldehyde | 35.68 ± 0.87 |
13.48 ± 0.33 |
68.76 ± 1.67 |
105.06 ± 2.56 |
88.84 ± 2.16 |
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5 | 7.3 | 1-Propanethiol | n.d. | 5.42 ± 0.11 |
64.04 ± 1.34 |
127.95 ± 2.67 |
94.69 ± 1.98 |
6 | 9.7 | Propylene sulfide | 26.01 ± 0.84 |
27.83 ± 1.49 |
59.83 ± 1.94 |
29.02 ± 0.94 |
52.08 ± 1.69 |
7 | 17.1 | Dimethyl sulfide | n.d. | 22.35 ± 0.47 |
18.00 ± 0.38 |
53.87 ± 1.13 |
16.88 ± 0.35 |
8 | 25.8 | Methyl propyl disulfide | 10.04 ± 0.27 |
25.51 ± 0.69 |
5.21 ± 0.14 |
84.34 ± 2.28 |
11.59 ± 0.31 |
9 | 29.3 |
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3.67 ± 0.15 |
4.01 ± 0.17 |
23.95 ± 0.99 |
99.70 ± 4.11 |
54.23 ± 2.23 |
10 | 29.5 | 5-Methyl-1,3-thiazole | n.d. | n.d. | 21.76 ± 0.45 |
20.02 ± 0.42 |
17.75 ± 0.37 |
11 | 31.9 |
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34.98 ± 0.72 |
30.96 ± 0.67 |
232.11 ± 3.47 |
138.67 ± 0.87 |
181.23 ± 2.76 |
12 | 32.9 | 3,4-Dimethyl thiophene | n.d. | n.d. | 184.18 ± 3.85 |
40.46 ± 2.90 |
127.86 ± 3.79 |
13 | 34.3 | Methyl-2-propenyl disulfide | 4.16 ± 0.17 |
7.97 ± 0.33 |
7.62 ± 0.32 |
16.44 ± 0.69 |
102.86 ± 4.31 |
14 | 37.5 | Dipropyl disulfide | 33.42 ± 0.78 |
22.92 ± 0.51 |
40.23 ± 0.90 |
40.46 ± 0.90 |
65.46 ± 1.46 |
15 | 39.8 | 1,2,4-Trithiolane | 3.51 ± 0.09 |
4.87 ± 0.13 |
160.66 ± 6.12 |
116.808 ± 3.1 |
54.74 ± 1.44 |
16 | 41.3 |
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16.63 ± 0.67 |
30.31 ± 1.23 |
94.20 ± 3.82 |
79.91 ± 3.24 |
58.25 ± 2.36 |
17 | 42.9 |
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n.d. | n.d. | 112.60 ± 2.35 |
21.32 ± 0.45 |
33.42 ± 0.70 |
18 | 45.6 | Methyl propyl trisulfide | 38.86 ± 1.01 |
46.03 ± 0.77 |
276.67 ± 7.67 |
379.52 ± 10.52 |
226.48 ± 6.28 |
19 | 55.9 | Dipropyl trisulfide | 36.58 ± 0.89 |
37.54 ± 0.86 |
158.34 ± 3.63 |
37.01 ± 0.85 |
96.68 ± 2.22 |
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20 | 51.1 | 1,2-Cyclopentanedione | 12.15 ± 0.34 |
12.90 ± 0.37 |
59.43 ± 1.69 |
72.06 ± 2.05 |
69.21 ± 1.97 |
21 | 53.7 | Butyrolactone | n.d. | n.d. | 47.56 ± 0.99 |
59.98 ± 1.25 |
64.25 ± 1.34 |
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22 | 54.5 | Furfuryl alcohol | 34.90 ± 0.73 |
18.34 ± 0.38 |
43.97 ± 0.92 |
64.81 ± 1.35 |
156.85 ± 3.28 |
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Different letters (a, b, c, etc.) correspond to significant differences (
S-compounds were the main volatile compounds of the onion profile [
The overall content of volatile compounds seems to be influenced by growth cycle and harvest month which is a function of temperature. Metabolite compositions in onions are strongly affected by climate conditions, in particular by air temperature [
For all the investigated onion landraces, quantitative analysis showed that di- and trisulfides, such as
Di- and trisulfides increased in landraces harvested in spring months (
According to Lanzotti (2006), di- and trisulfides were formed by the degradation of thiosulfinates whose biosynthetic pathway came from the condensation reactions of alk(en)yl-sulfenic acids (e.g., Z,E-propanethial S-oxide or lachrymator factor). The amount of alk(en)yl-sulfenic acids (the lachrymator factor) was closely linked to the concentration of the aroma precursors: S-alkenyl cysteine sulfoxides (ACSOs). The higher content of di- and trisulfides than other volatile compounds in onion samples cultivated during warmer spring months was probably due to an increase of S-alkenyl cysteine sulfoxides (ACSOs), according to Randle and Coolong [
About the aldehydes class, four compounds were detected in the volatile pattern of onion samples except for 2-methyl-2-pentenal which was not detected in
The other volatile compounds were two ketones and furfuryl alcohol (Table
Total phenols, quantified in the onion samples, were reported in Table
Phenols (mg/g dw; mean ± SD), antioxidant activity (EC50 mg extract/mL; mean ± SD), and total phenols (mg GAE/g dw) of
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Gallic acid | 55.66 ± 2.30 |
59.56 ± 1.10 |
61.23 ± 2.50 |
61.94 ± 1.91 |
64.90 ± 1.22 |
Ferulic acid | 1.52 ± 0.20 |
1.62 ± 0.25 |
1.67 ± 0.41 |
1.69 ± 0.19 |
1.77 ± 0.30 |
Quercetin | 6.98 ± 0.42 |
7.47 ± 0.30 |
7.68 ± 0.28 |
7.77 ± 0.30 |
8.14 ± 0.20 |
Kaempferol | 1.62 ± 0.33 |
1.73 ± 0.27 |
1.78 ± 0.15 |
1.80 ± 0.21 |
1.89 ± 0.32 |
Chlorogenic acid | 0.84 ± 0.06 |
0.90 ± 0.02 |
0.92 ± 0.08 |
0.93 ± 0.04 |
0.98 ± 0.07 |
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EC50 (mg extract/mL) | 18.80 ± 1.0 |
18.50 ± 0.50 |
20.90 ± 0.60 |
20.25 ± 0.40 |
21.27 ± 0.8 |
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4.75 ± 0.24 |
4.90 ± 0.10 |
5.14 ± 0.35 |
5.06 ± 0.28 |
5.31 ± 0.30 |
Different letters (a, b) correspond to significant differences (
The onion landraces were also studied for their specific phenols composition (Figure
Chromatograms of phenols in onion samples at 280 and 350 nm (red and blue lines, resp.).
In our samples, this flavonol was found to be between 6.98 and 8.14
All the other phenols increased in landraces harvested in spring months. Our results highlighted the notion that the quantity of phenolic pool may change not only with the cultivar as reported in the literature [
The antioxidant activity varied from 19.00 to 21.27 mg of extract/mL (Table
It is generally accepted that there is a high correlation between levels of enzymatically produced pyruvate (EPY) present in onions and the perception of pungency [
Herein, all onion landraces had high pungency values expressed by EPY amount, ranging from 9 to 14
In order to define the sensory profile of onion, the sugars content was investigated. Three soluble sugars, sucrose, glucose, and fructose, were detected in the five onion landraces (Table
Organic acids (mg/100 g fw; mean ± SD), sugars (g/100 g fw; mean ± SD) content, and pungency (
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Malic acid | 60.43 ± 6.53 |
66.33 ± 6.61 |
61.85 ± 4.50 |
57.61 ± 3.34 |
78.94 ± 2.10 |
Citric acid | 63.32 ± 5.90 |
57.05 ± 3.24 |
54.55 ± 5.43 |
34.46 ± 5.38 |
19.70 ± 3.95 |
Tartaric acid | 13.93 ± 2.75 |
6.10 ± 1.21 |
16.15 ± 1.25 |
25.88 ± 4.57 |
12.02 ± 2.85 |
Oxalic acid | 30.43 ± 2.97 |
19.93 ± 1.51 |
13.60 ± 1.41 |
12.21 ± 1.90 |
10.68 ± 2.92 |
Ascorbic acid | 4.63 ± 0.85 |
4.85 ± 0.15 |
21.65 ± 2.80 |
17.69 ± 1.96 |
14.80 ± 1.73 |
Succinic acid | 11.55 ± 1.04 |
9.28 ± 2.72 |
19.40 ± 0.89 |
23.35 ± 2.21 |
14.34 ± 0.82 |
Pyruvic acid | 1.07 ± 0.44 |
1.18 ± 0.66 |
0.35 ± 0.09 |
0.76 ± 0.21 |
0.50 ± 0.10 |
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Fructose | 1.74 ± 0.42 |
1.66 ± 0.62 |
2.11 ± 0.23 |
2.00 ± 0.60 |
2.26 ± 0.70 |
Glucose | 1.09 ± 0.18 |
0.88 ± 0.43 |
1.97 ± 0.09 |
1.79 ± 0.54 |
2.00 ± 0.27 |
Sucrose | 0.90 ± 0.11 |
0.81 ± 0.21 |
1.76 ± 0.36 |
1.04 ± 0.27 |
1.21 ± 0.24 |
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Different letters (a, b, c, etc.) correspond to significant differences (
The total content of soluble sugars in
Seven organic acids were identified and quantified in onion samples (Table
Malic acid was the most abundant organic acid for all onion samples except for
In our study, we highlighted the differences, of the most important quality parameters, existing among five onion landraces belonging to
The authors declare that there are no competing interests regarding the publication of this paper.
This research was financially supported by “Ministero dell’Istruzione, dell’Università e della Ricerca” and Regione Campania as part of project “AGRIGENET—Network per la Salvaguardia e la Gestione delle Risorse Genetiche Agro-Alimentari Campane” (PSR 2007–2013, Misura 214).