2020 Volume 26 Issue 1 Pages 167-175
In this study, we investigated the antioxidant activities and taste qualities of fresh onions produced in the Minamishimabara city, Nagasaki, Japan. Samples were ‘Super-Up’ (SU), a conventional cultivar, and ‘Kazusa No. 13’ (K13), a new cultivar, and these were compared with ‘Shippowase No. 7’ (S7), a general-use cultivar. The total oxygen radical absorbance capacity of K13 was higher than that of S7. The 1, 1-diphenyl- 2-picrylhydrazyl free radical-scavenging activity and the total quercetin concentration were higher in SU and K13 than in S7. Umami and saltiness values (determined using the Taste Sensing System) and total free amino acid concentrations in SU and K13 were lower than those in S7, with the differences in total free amino acid concentration contributing to the difference in taste responses between the onion cultivars. Total sugar concentration in SU was lower than in S7. These results suggest that the advantages of SU and K13 were in terms of high levels of antioxidant activity and quercetin concentration, rather than in terms of taste or flavor. Thus, onions of cultivars SU and K13 are expected to be new regional brand foods of Nagasaki, targeted at health-conscious consumers.
The onion (Allium cepa L.) is a popular foodstuff that is used frequently in menus around the world. Onion is used in vegetable salads, stir-fried dishes, soups, and grilled dishes in Japanese, Chinese, and Western-style dishes. Onion contains high levels of dietary fiber, potassium, and vitamin C (i), and high concentrations of flavonoids such as quercetin (Tsushida and Suzuki, 1995). Quercetin is an antioxidant that has anti-inflammatory in vivo and in vitro (Boots et al., 2008). In Japan, onions are sown in spring or autumn, to be harvested from summer to autumn or in spring, respectively. In the former case, the onions are harvested from August to September, whereas, in the latter case, the onions are harvested before April (very early-season variety), April to May (early-season variety), May to June (medium-late-season variety), and after June (late-season variety). Generally, the onions harvested before May are called fresh onions (shin-onion) in Japan, that are often eaten raw, because they taste juicier and less pungent than do late-season variety onions.
In Japan, a number of functional, nutrient-and palatability-rich foods in some regions are known as the regional brand food. For example, Mikkabi tangerine (Citrus reticulata L.), produced in Shizuoka prefecture, has functional claims because it includes high level of β-cryptoxanthin (ii), whereas Yubari melon (Cucumis melo L.) produced in Hokkaido, have a delicious image (Morishima, 2013). These are examples of regional brand foods that are expensive, because they have added value such as functionality or good taste. Such foods have received considerable attention from the health-conscious consumers or the gourmets, so the consumption of these foods should increase. Therefore, it is believed that the regional brand food will contribute toward regional revitalization.
The Nagasaki prefecture of Japan is a production region for a number of agricultural and horticultural crops, such as loquat (Eriobotrya japonica), orange (Citrus unshiu), strawberry (Fragaria × ananassa), potato (Solanum tuberosum), and onion (iii). Minamishimabara is a city in Nagasaki prefecture, where some agricultural crop production is carried out. Fresh onions are produced in Minamishimabara city, principally cultivars such as ‘Super-Up’ (SU) (a conventional product) and ‘Kazusa No. 13’ (K13) (a new product). The characteristics of SU include a low incidence of bolting and splitting, so that this onion is capable of problem-free harvesting. K13 is an improved cultivar that can be harvested earlier than SU, and it has been reported to exhibit higher antioxidant activity than SU (Yuasa et al., 2018). However, the popularity of these fresh onions is not very high, and they are sold only in Nagasaki prefecture. If the advantages of these fresh onions can be confirmed, they may contribute, like other regional brand foods, to the revitalization of Nagasaki prefecture. The antioxidant activities and taste characteristics of fresh onions of these two cultivars SU and K13 were evaluated in a previous study (Yuasa et al., 2018), but these traits were not compared with onions of a general-use fresh onion variety. In addition, the nutritional and taste characteristics of fresh onions have hardly been investigated in previous studies. Therefore, in the present study, to evaluate the nutritional and taste characteristics of fresh onions produced in Minamishimabara city, we investigated the antioxidant activities and taste qualities in SU and K13, and compared them with those of a general-use variety of fresh onion, ‘Shippowase No. 7’ (S7), which is cropped in famous onion production regions of Japan, such as Saga and Hyogo prefectures.
Sample preparation Fresh onions of cultivars S7, SU, and K13 were used in this study. They were produced on the same farm in Minamishimabara city, Nagasaki, Japan. In 2018, onions of S7 were harvested in May, and those of SU and K13 were harvested in March, because the optimal harvest periods of these onions differed. Five onions were used (as replicates) for each cultivar for analysis. All samples were homogenized separately using a blender (magic BULLET MB-1001; OAK LAWN MARKETING, INC., Nagoya, Japan) and Polytron™ PT2100 homogenizer (Kinematica AG, Luzern, Switzerland), and the individual extracts stored at −25 °C until further use.
Oxygen radical absorbance capacity (ORAC) assay 0.2 g of each sample extract was homogenized with 0.5 mL phosphate-buffered saline (PBS; pH 7.4). The extract was centrifuged at 15 000 × rpm at 4 °C for 10 min, and the supernatant was collected. The insoluble fraction (pellet) was homogenized with 0.3 mL of PBS (pH 7.4) and centrifuged at 15 000 × rpm at 4 °C for 10 min, and the supernatant was collected. This and the first supernatant were combined (as the water-soluble fraction). Subsequently, the pellet from the second extraction was homogenized with 0.8 mL of acetone, and vortexed at 1 500 × rpm at room temperature for 60 min. This extract was centrifuged at 15 000 × rpm at 4 °C for 10 min, and the supernatant was collected (as the acetone extract). ORAC values were measured separately in the water-soluble fraction (hydrophilic (H)-ORAC value) and the acetone extract (lipophilic (L)-ORAC value), using the OxiSelect ORAC Activity Assay Kit (Cell Biolabs, Inc., USA), and these ORAC values were summed (to provide the total (T)-ORAC value). Results were expressed as 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) equivalents (µmol Trolox eq per 100 g fresh weight).
1,1-Diphenyl-2-picrylhydrazyl (DPPH) free radical-scavenging activity DPPH free radical-scavenging activities were measured with the Prominence Ultra-Fast Liquid Chromatography (UFLC) system (Shimadzu Co., Ltd., Kyoto, Japan), according to the method described by Yamaguchi et al. (1998). A TSKgel Octyl-80Ts column (4.6 mm i.d. × 150 mm, 5-µm particle size) (GL Sciences Inc., Tokyo, Japan) was used for separation. 0.4 g of sample was homogenized in 20 mL of 100 mmol/L Tris-HCl buffer (pH 7.4). The extract was centrifuged at 3 000 × rpm at room temperature for 10 min, and the supernatant was collected. Subsequently, 200 µL of supernatant and 200 µL of 500 µmol/L DPPH were mixed and incubated at room temperature for 20 min, and filtered through a 0.45 µm nylon microfilter (Starlab Scientific Co., Ltd., Shaanxi, China). The UFLC conditions were as follows: mobile phase, 70% (v/v) methanol; flow rate, 1.0 mL/min; reaction temperature, 40 °C; injection volume, 10 µL; monitoring was carried out with a UV/VIS detector at 517 nm (0.064 AUFS) to measure the DPPH concentration remaining after reaction of the sample and DPPH to evaluate the antioxidant activity of each sample. Results were expressed as Trolox equivalents (µmol Trolox eq per 100 g fresh weight).
Total polyphenol Total polyphenol concentration was measured using the Folin-Ciocalteu method as previously described (Jaworska et al., 2015). 0.2 g of sample was homogenized in 1.0 mL of 80% (v/v) methanol (containing 0.5% (v/v) HCl), and incubated at room temperature for 24 h. The extract was centrifuged at 12 000 × rpm at 4 °C for 10 min, and the supernatant was collected. 10 µL of the supernatant were added to wells in a microtiter plate, and 15 µL of two-fold diluted Folin-Ciocalteu reagent and 60 µL of ultrapure water were added to each well and incubated at room temperature for 5 min, after which 75 µL of 2% (w/v) sodium carbonate were added, the mixture was incubated at room temperature for 15 min and the absorbance was measured at 765 nm. Gallic acid was used as the standard. Results were expressed as gallic acid equivalents (mg gallic acid eq per 100 g fresh weight).
Total quercetin Total quercetin concentration was determined with the Prominence UFLC system (Shimadzu Co., Ltd.), according to the method described by Németh et al. (2003) and Watanabe et al. (2013). An Inertsil ODS-3 column (4.6 mm i.d. × 150 mm, 5-µm porticle size) (GL Sciences Inc.) was used for separation. 0.2 g of the sample was homogenized in 1.0 mL of 80% (v/v) methanol (containing 0.5% (v/v) HCl), and incubated at room temperature for 24 h. The extract was centrifuged at 12 000 rpm at 4 °C for 10 min, and supernatant was collected. The supernatant was filtered through a 0.45 µm PTFE microfilter (Starlab Scientific Co., Ltd.). The UFLC conditions were as follows: mobile phase A, 0.2% formic acid; mobile phase B, acetonitrile; gradient, 20%–48% mobile phase B (0–9 min), 48%–60% mobile phase B (9–18 min), 60%–20% mobile phase B (18–33 min); flow rate, 0.8 mL/min; reaction temperature, 35 °C; injection volume, 10 µL. Quercetin and its derivatives were monitored with a UV/VIS detector at 360 nm. Total quercetin concentration was the sum of quercetin, quercetin 3-glucoside, quercetin 3, 4′-diglucoside, and quercetin-4′-O-glucoside, which were detected separately. The individual quercetin species were identified by their respective retention times and characteristic spectra compared with those of quercetin and quercetin glycoside standard solutions. Total quercetin concentration results were expressed as mg quercetin per 100 g fresh weight.
Total vitamin C Total vitamin C concentration was determined with the Prominence UFLC system (Shimadzu Co., Ltd.), according to the method described by Yuasa et al. (2018). An Inertsil SIL-100A column (4.6 mm i.d. × 250 mm, 5-µm particle size) (GL Sciences Inc.) was used for separation. 3.0 g of sample was homogenized in 20 mL of 5% (w/v) metaphosphoric acid, and made up to 30 mL using 5% (w/v) metaphosphoric acid. The extract was centrifuged at 3 000 rpm at room temperature for 10 min, and the supernatant was collected. Subsequently, 0.5 mL of 5% (w/v) metaphosphoric acid, 2–3 drops of 0.2% (w/v) 2,6-dichloroindophenol sodium salt dehydrate, 1 mL of 2% (w/v) thiourea (in 5% (w/v) metaphosphoric acid), and 0.25 mL of 2% (w/v) 2,4-dinitrophenylhydrazine (in 4.5 mol/L H2SO4) were added to 1.0 mL of the supernatant. The reaction mixture was incubated at 50 °C for 90 min, and an aliquot of 1.0 mL of ethyl acetate was added to the sample and vortexed at 1,500 × rpm at room temperature for 30 min. An aliquot (0.5 mL) of the supernatant (ethyl acetate layer) was collected after centrifugation at 2 000 × rpm at room temperature for 10 min. The supernatant was dehydrated by using anhydrous sodium sulfate, and these were filtered through a 0.45 µm nylon microfilter (Starlab Scientific Co., Ltd.). The UFLC conditions were as follows: mobile phase, ethyl acetate: hexane: acetate (50:40:10 by vol.); flow rate, 1.5 mL/min; reaction temperature, 40 °C; injection volume, 10 µL, with detection being monitored with a UV/VIS detector at 495 nm. Total vitamin C was identified by its retention time and characteristic spectrum compared with L-ascorbic acid standard solutions. Results were expressed as mg per 100 g fresh weight.
Taste responses The taste responses were determined using the Taste Sensing System TS-5000Z (Intelligent Sensor Technology, Inc., Kanagawa, Japan), which is equipped with ceramic reference electrodes and artificial lipid-based membrane sensors [CA0 (sourness), C00 (acidic bitterness), AE1 (astringency), AAE (umami) and CT0 (saltiness)]. CA0, C00, AE1, AAE and CT0 correspond to organic acids (e.g., citric acid, lactic acid, malic acid and acetic acids), astringency components (e.g., tannic acid, caffeic acid and epigallocatechin gallate), some bitterness components (e.g., bitterness amino acids and quinine), umami components (e.g., umami amino acids and 5′-nucleotides), and sodium chloride and organic acids (e.g., lactic acid and malic acid), respectively (Yuasa et al., 2018; Toyota et al., 2016; Kobayashi et al., 2010). 20 g of the sample was homogenized in 70 mL of ultrapure water, and the extract was filtered through a filter paper No. 5C (Advantech Co., Ltd., Osaka, Japan), with the filtrate being used for determination of taste responses. The taste response was measured by immersing the sensors in a standard solution (30 mmol/L of KCl and 0.3 mmol/L of tartaric acid) to obtain the membrane potential (Vr). Then, the taste sensors were immersed in the sample solution to obtain the potential (Vs). The difference in the potentials (Vs − Vr) is termed the initial taste responses (sourness, acidic bitterness-A, astringency-A, umami, and saltiness). Next, the sensors were rinsed lightly with the standard solution and were immersed in the standard solution again to obtain the potential (Vr'). The difference in potential (Vr' − Vr) is called the aftertaste responses (acidic bitterness-B, astringency-B, and richness). Taste responses were analyzed by the analysis application in the basic process mode. Results for the two test cultivars were expressed as fold-changes, relative to the corresponding result from the onion cultivar S7 (S7 = 0).
The taste responses for sourness and astringency in the initial taste responses were below −13 and 0 (the baseline of taste in humans), respectively, so were not analyzed because these taste response levels correspond to “tasteless”.
Sugars and Brix The sweetness sensor of the Taste Sensing System TS-5000Z is not adequately sensitive for the evaluation of the sweetness of foods. Therefore, the sweetness of fresh onions was evaluated using sugars and brix in the present study.
Sugars (D-glucose, sucrose, and fructose) were using the E-kit ENZYTEC D-Glucose Sucrose D-Fructose (J. K. International Inc., Tokyo, Japan), according to the manufacturer's instructions. Results were expressed as g per 100 g fresh weight.
The intensity of sweetness in D-glucose and fructose is different compared with that in sucrose. The intensities of sweetness in D-glucose and fructose are 0.65-fold and 1.25-fold greater, respectively, than the intensity of sweetness in sucrose (Miyagi et al., 2011). Therefore, we calculated the “degree of sweetness” for the evaluation of total sweetness using the mathematical formula stated below.
 |
Brix was measured using the Pocket Saccharimeter APAL-1 (AS ONE Corporation., Osaka, Japan). Results were expressed as %.
5′-Guanylate (GMP) 5′-GMP is umami component in vegetable foods, thus the concentration of 5′-GMP was measured using the Prominence UFLC system (Shimadzu Co., Ltd.), according to the method described by Adachi et al., (2002). A Shimpack WAX-1 column (4.0 mm i.d. × 5.0 mm, 3-µm particle size) (Shimadzu Co., Ltd.) was used for separation. 0.5 g of sample was homogenized in 10 mL of ultrapure water. The extract was centrifuged at 3 000 rpm at room temperature for 10 min, and the supernatant was collected and filtered through a 0.45 µm nylon microfilter (Starlab Scientific Co., Ltd.). The UFLC conditions were as follows: mobile phase, 50 mmol/L phosphate buffer (pH 3.1); flow rate, 1.0 mL/min; reaction temperature, 40 °C; injection volume, 10 µL, with detection being monitoring with a UV/VIS detector at 260 nm. The 5′-GMP was identified by its retention time and characteristic spectrum compared with a 5′-GMP standard solution. Results were expressed as mg per 100 g fresh weight.
Free amino acids Free amino acids have umami (e.g., L-aspartate and L-glutamate), bitterness (e.g., L-histidine, L-arginine, L-tyrosine, L-valine, L-methionine, L-isoleucine, L-leucine, and L-phenylalanine), or sweetness (e.g., L-serine, glycine, L-alanine, and L-threonine). Therefore, free amino acids profile were determined with the Prominence UFLC system (Shimadzu Co., Ltd.), according to the method described by Glevarec et al. (2004). An Inertsil ODS-3 column (4.6 mm i.d. × 150 mm, 3-µm particle size) (GL Sciences Inc.) with an Inertsil ODS cartridge guard column (4.0 mm i.d. × 10 mm, 3-µm particle size) (GL Sciences Inc.) were used for separation. 0.2 g of sample was homogenized in 0.4 mL of 0.1 mol/L HCl. The extract was centrifuged at 15 000 rpm at 4 °C for 10 min, and the supernatant was collected. An aliquot (10 µL) of the supernatant was transferred into an Eppendorf tube, in which the sample was dried at 45 °C with an aspirator. After addition of 20 µL of ethanol: ultrapure water: triethylamine (2:2:1), the tube was vortexed. The sample was dried at 45 °C with an aspirator, 50 µL of ethanol: ultrapure water: triethylamine: phenyl isothiocyanate (7:1:1:1) was added and the tube was vortexed. The suspension was incubated at room temperature for 20 min, and then dried at 45 °C with an aspirator. The dried material was dissolved in 500 µL of the mobile phase A, and passed through a 0.45 µm nylon microfilter (Starlab Scientific Co., Ltd.). The UFLC conditions were as follows: mobile phase A, 60 mmol/L acetate buffer solution (pH 6.6): acetonitrile (94:6 v/v); mobile phase B, 60 mmol/L acetate buffer solution (pH 6.6): acetonitrile (40: 60 v/v); gradient, 0–55% solvent B (0–20 min), 55–100% solvent B (20–25 min), 100–0% solvent B (25–40 min); flow rate, 0.6 mL/min; reaction temperature, 40 °C; injection volume, 6 µL, with monitoring being carried out with a UV/VIS detector at 250 nm. Free amino acids were identified by their retention times and characteristic spectra compared with an amino acid mixture standard solution, type H (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan). Results were expressed as mg per 100 g fresh weight.
Organic acids The organic acids have soreness taste, thus L-malic acid, pyruvic acid, acetic acid, citric acid, and fumaric acid were determined by separation with the Prominence UFLC system (Shimadzu Co., Ltd.). An Inertsil ODS-3 column (4.6 mm i.d. × 150 mm, 3-µm particle size) (GL Sciences Inc.) with an Inertsil ODS cartridge guard column (4.0 mm i.d. × 10 mm, 3-µm particle size) (GL Sciences Inc.) were used for separation. 0.5 g of the sample was homogenized in 10 mL of ultrapure water, and the supernatant was collected after centrifugation at 3 000 × rpm at room temperature for 10 min. The UFLC conditions were as follows: mobile phase, 0.1% (w/v) phosphoric acid: acetonitrile (97.5:2.5 v/v); flow rate, 0.3 mL/min; reaction temperature, 30 °C; injection volume, 6 µL, with monitoring with a UV/VIS detector at 210 nm. Individual organic acids were identified by their retention times and characteristic spectra compared with standard solutions of individual organic acids. Organic acid concentrations were expressed as mg per 100 g fresh weight.
Statistical analysis Data are shown as the means ± SD or means. Statistical analysis was performed using Excel 2016 (Microsoft Japan Co., Ltd., Tokyo, Japan) and IBM SPSS Statistics version 25 (IBM Japan, Tokyo, Japan). Samples were compared using the Scheffé's F test. A p value < 0.05 was considered to be significant.
Antioxidant activity of fresh onions Antioxidant activities and antioxidant concentrations of the different onion cultivars are shown in Table 1. In K13, H-ORAC and T-ORAC values were significantly higher than in S7 and SU (p < 0.05), with L-ORAC values in K13 being significantly higher than in S7 (p < 0.05). In K13, DPPH free radical-scavenging activity was higher than in S7 and SU (p < 0.05). In SU, DPPH free radical-scavenging activity was higher than in S7 (p < 0.05). With respect to antioxidant concentrations, total polyphenol and total vitamin C concentrations in K13 were significantly higher than in S7 and SU (p < 0.05), whereas total quercetin concentration in SU and K13 was significantly higher than in S7 (p < 0.05). These results suggested that fresh onions of K13 and SU had higher antioxidant activities than the general-use cultivar S7, in part because of their higher quercetin concentrations. Furthermore, a high level of total vitamin C concentration in K13 contributed to a high level of antioxidant activity in this cultivar. Therefore, in SU and K13, high antioxidant activity is one of their strengths, useful for achieving new regional brand food status in Minamishimabara city, Nagasaki. Whereas the late-season variety onion have a 8 mg/100 g vitamin C concentration in Japan (i), those of the early-season cultivars SU and K13 were 1.7 and 2.2 times higher than the late-season variety onion concentration. This characteristic of being vitamin C-rich is another of the benefits of SU and K13. In addition, such seasonal effects could have influenced the total polyphenol and total quercetin concentrations in fresh onions in the present study. On the other hand, antioxidant activities in K13 were higher than that in SU, which were consistent with observations made in our previous study (Yuasa et al., 2018). The results of the present and previous study suggest that the new product, K13, has higher antioxidant activity compared to the conventional product, SU.
| Shichihowase No. 7 (S7) |
Super-up (SU) |
Kazusa No. 13 (K13) |
|
|---|---|---|---|
| H-ORAC value (µmol Trolox eq/100g) |
304.0±32.6a | 303.0±67.8a | 407.4±49.0b |
| L-ORAC value (µmol Trolox eq/100g) |
50.3±22.9a | 91.7±20.3ab | 122.1±31.7b |
| T-ORAC value (µmol Trolox eq/100g) |
354.3±30.9a | 394.7±82.9a | 529.5±55.4b |
| DPPH free radical-scavenging activity (µmol Trolox eq/100g) |
119.6±35.4a | 226.7±52.9b | 372.9±76.0c |
| Total polyphenol (mg gallic acid eq/100g) |
15.85±1.68a | 17.56±4.46a | 22.94±4.25b |
| Total quercetin (mg/100 g) |
5.59±1.69a | 17.14±5.01b | 19.49±5.20b |
| Total vitamin C (mg/100 g) |
10.88±0.69a | 13.85±1.64a | 17.89±5.83b |
Values are compared among the three breed varieties. H-ORAC, Hydrophilic oxygen radical absorbance capacity. L-ORAC, Lipophilic oxygen radical absorbance capacity. T-ORAC, Total oxygen radical absorbance capacity. DPPH, 1,1-diphenyl-2-picrylhydrazyl. Values are the means ± S. (n=5).
A high intake of flavonoids is associated with a low cardiovascular disease risk and mortality (McCullough et al., 2012), and A high intake of total polyphenols and flavonoids (specifically flavanones and dihydroflavonols) could reduce risk of diabetes in elderly persons at high risk of cardiovascular disease (Tresserra-Rimbau et al., 2016). Consequently, polyphenol intake, particularly flavonoid intake, could minimize the risk of occurrence and mortality associated with some lifestyle diseases, and it is suggested that the effects of flavonoids on such lifestyle diseases vary based on the types of flavonoids consumed. Quercetin is a flavonoid that is present in a number of vegetable foods such as onion, asparagus, red leaf lettuce, romaine lettuce, cherry tomato, tomato, apple, and green tea infusion, and the onion is a major dietary source of quercetin (Nishimuro et al., 2015). Quercetin has a number of functions, such as antioxidant activity (Boots et al., 2008), anti-obesity and fat reduction effects (Dong et al., 2014), and systolic blood pressure reduction effect (Egert et al., 2009). Therefore, foods containing high quercetin concentrations are being developed for the purpose of providing a high quercetin dietary intake for humans. For example, some varieties of onion have been bred to contain high levels of quercetin, such as the red onion “Quer-rich” (Muro et al., 2010) and the onion “Quergold” (Muro et al., 2015) bred in Hokkaido. Some varieties of green tea have been bred to provide potentially rich sources of high-quercetin drinks (Monobe et al., 2015). Therefore, these quercetin-rich foods are receiving considerable attention from health-conscious consumers. In onions, more than 85% of the flavonoids consist of quercetin glucoside (Tsushida and Suzuki, 1995), which was also observed based on the total polyphenol and total quercetin concentrations in the SU and K13 cultivars in the present study. Therefore, most of the polyphenols in SU and K13 potentially consist of quercetin. In addition to the above quercetin-rich foods (e.g., Quer-rich and/or Quergold), SU, and K13 could be good sources of dietary quercetin.
Taste qualities of fresh onions The taste responses to the three fresh onion cultivars are shown in Fig. 1. In SU and K13, umami (p < 0.05) and saltiness (p > 0.05) were lower, but astringency was higher (p < 0.05) than in the general-use cultivar, S7. Sugar profiles and Brix values are shown as Table 2. In SU and K13, D-glucose and fructose concentrations were lower than in S7 (p < 0.05); however, the sucrose concentrations did not differ significantly among the three onion cultivars. In SU, the total sugar concentration and the degree of sweetness were lower than in S7 (p < 0.05), and lower Brix scores were observed in SU than in S7 and K13 (p < 0.05). Thus, the umami of SU and K13 are lower than S7, and the sweetness of SU was low compared with S7 and K13. In vegetable foods and sake, saltiness of taste response is an index for rich taste of food-likeness (Yuasa et al., 2018; Toyota et al., 2016), because saltiness sensor do not respond sodium chloride but also some organic acids. In SU and K13, umami and saltiness were lower than in S7; therefore, the taste of these fresh onions may be refreshing-taste, and they could be appropriate for vegetable salads compared with S7. Conversely, in cases when differences in the relative scores of taste responses that are lower than 1.0, this differences among tastes of foods cannot be recognized in human. Therefore, umami and astringency in such fresh onions may not be different. In future studies, sensory evaluation of tastes and preferences between the onions evaluated in the present study should be carried out.
Taste responses in fresh onions. Values are expressed as fold change relative to S7 each taste quality (S7=0). S7, Shippowase No. 7. SU, Super-Up. K13, Kazusa No. 13. #Initial taste, †Aftertaste. Values are means (n=5). a–b p < 0.05 (Scheffé's F test).
| Shichihowase No. 7 (S7) |
Super-up (SU) |
Kazusa No. 13 (K13) |
||
|---|---|---|---|---|
| Sugars (g/100g) |
D-glucose | 2.35±0.07a | 1.87±0.07b | 1.95±0.14b |
| Sucrose | 0.43±0.18 | 0.65±0.47 | 0.66±0.20 | |
| Fructose | 2.07±0.07a | 1.64±0.20b | 1.72±0.16b | |
| Total sugar | 4.84±0.09a | 4.16±0.51b | 4.34±0.23ab | |
| Degree of sweetness | 4.54±0.09a | 3.92±0.52b | 4.07±0.21ab | |
| Brix (%) |
- | 7.80±0.17a | 6.70±0.12b | 7.65±0.56a |
| 5′-Guanylate (mg/100 g) |
- | 0.18±0.03a | 0.35±0.11b | 0.22±0.03a |
| Free amino acids (mg/100 g) |
L-aspartate | 37.36±3.62a | 29.82±2.16b | 25.94±4.44b |
| L-glutamate | 29.36±2.16a | 31.74±1.95a | 39.36±4.36b | |
| L-serine | 20.86±2.66a | 9.00±2.63b | 11.70±4.50b | |
| Glycine | 1.20±0.19a | 2.12±0.15b | 2.16±0.32b | |
| L-histidine | 1.84±0.17a | 0.94±0.09b | 1.12±0.08b | |
| L-arginine | 70.54±6.95a | 30.66±16.94b | 26.20±12.98b | |
| L-threonine | 14.54±2.20a | 4.38±0.18b | 8.44±1.51c | |
| L-alanine | 8.62±0.72a | 4.36±0.72b | 7.70±1.75a | |
| L-proline | 3.10±0.60a | 0.66±0.05b | 0.92±0.31b | |
| L-tyrosine | 5.00±0.76a | 0.84±0.30b | 2.82±1.43c | |
| L-valine | 7.18±1.43a | 2.24±0.33b | 4.14±1.00c | |
| L-methionine | 0.24±0.05a | 0.22±0.88a | 0.44±0.13b | |
| L-cysteine | 2.42±0.35a | 0.52±0.13b | 0.96±0.36b | |
| L-isoleucine | 7.14±1.72a | 0.64±0.36b | 2.94±1.01c | |
| L-leucine | 9.70±1.35a | 1.04±0.50b | 2.88±1.45b | |
| L-phenylalanine | 9.56±1.09a | 1.82±0.48b | 4.70±1.38c | |
| L-lysine | 9.04±0.80a | 3.50±0.67b | 4.94±0.73c | |
| Total | 237.76±10.62a | 124.54±19.22b | 147.48±19.59b | |
| Organic acids (mg/100 g) |
L-malic acid | 268.52±83.47 | 288.59±50.83 | 380.27±152.59 |
| Pyruvic acid | 14.59±1.46 | 14.47±0.91 | 16.41±3.63 | |
| Acetic acid | 4.14±0.61 | 4.27±0.81 | 5.10±1.43 | |
| Citric acid | 142.19±4.47 | 164.90±21.54 | 173.20±15.97 | |
| Fumaric acid | 0.96±0.90 | 0.90±0.28 | 0.92±0.19 | |
| Total | 430.39±87.60 | 473.13±55.45 | 575.91±155.65 |
Values are compared among the three breed varieties. Values are the means ± SD (n=5).
The concentrations of taste and flavor components were then determined because of reported differences between the three onion cultivars with respect to some taste responses (Table 2). The concentration of 5′-guanylate in SU was higher than that in S7 and K13 (p < 0.05). L-glutamate and L-methionine concentrations in K13 were higher than S7 and SU (p < 0.05). In SU and K13, the glycine concentration was significantly higher (p < 0.05), and the concentrations of other and total free amino acids were significantly lower than in S7 (p < 0.05). Organic acid concentrations did not differ significantly among the three varieties. In oysters, although the concentration of umami-tasting amino acids (the sum of L-glutamate and L-aspartate) did not differ, high levels of taste responses to umami and saltiness were observed because of effect of including high bitterness amino acids (Yuasa et al., 2018). It has been shown that taste responses to umami in green tea are attributable to increases in total free amino acid concentrations (Kubo et al., 2014). Therefore, the present and previous studies suggest that differences in total free amino acid concentration or in the free amino acid profile affect the umami response among the onion varieties.
The saltiness sensor responded to differences in organic acid concentration in sake (Toyota et al., 2016). Noda and Makuta (2015) showed that taste responses to saltiness reflected a nitrate ion concentration-dependent increase in spinach (Spinacia oleracea) and spinach mustard (Brassica rapa var. perviridis). In the present study, the concentration of organic acids and nitrate ions (data not shown) were not significantly different among the onion varieties. Thus, it is suggested that the saltiness of SU and K13 was lower than that of S7 not so much because of a difference in organic acid and nitrate ion concentrations but because of differences in free amino acid concentration. In the present study, no relationship was observed between umami taste response and 5′-guanylate concentration. Sakamoto et al. (2005) showed that umami taste response in 1 mmol/L monosodium glutamate (MSG) and 0.01 mmol/L 5′-guanylate solution was indistinguishable from that of the same MSG concentration but 0.1 mmol/L 5′-guanylate solution. In the present study, the threshold 5′-guanylate concentration in onions required to detect a taste response were in the range 0.001–0.002 mmol/L. Therefore, it is suggested that the 5′-guanylate concentration in onions was insufficient for detection of the umami taste response, because no relationship was observed between 5′-guanylate concentration and umami taste response.
In the present study, fresh onions were harvested at different periods because the optimal harvest periods of the onions varied. Therefore, some antioxidants and taste and flavor characteristics of S7, SU, and K13 should be compared in similar harvest periods in future studies. Conversely, the evaluation of the taste characteristics of S7, SU, and K13 may be inadequate because the measurement parameters were only taste responses and some taste and flavor components; therefore, differences in taste and preferences for the onions should be investigated based on sensory evaluations in future studies.
In the present study, antioxidant activities and taste qualities of fresh onions grown in Minamishimabara city, Nagasaki, Japan were investigated. In the result, the advantages of SU and K13 were in terms of high levels of antioxidant activity and quercetin concentration, rather than in terms of taste or flavor. Thus, these onion varieties are expected to be marketed as new regional brand foods of Nagasaki, targeting health-conscious consumers.
Acknowledgments This study was supported by a grant from University of Nagasaki and Akao seed Co., Ltd.
