Onion high-pressure processing: Flavonol content and antioxidant activity

Onion flavonol content and antioxidant activity have been related to human health promoting effects. Quercetin and quercetin glucosides (quercetin-4′-glucoside and quercetin-3,4′-diglucoside) have been reported the main onion flavonols in recent literature. Impact of combined treatments of high-pressure processing (HPP) and temperature on onion nutritional attributes has been scarcely studied.Our study aimed to investigate the impact of HPP technology combined with temperature on onion (Allium cepa L. var. cepa, ‘Grano de Oro’) total phenol content, flavonol content, and antioxidant capacity. The experimental design comprised a response surface methodology according to a central composite face-centred design. The variable ranges were 100-400 MPa (pressure) and 5-50 °C (temperature), time was set up constant to 5 min.Response surfaces of onion total quercetin, quercetin-4′-glucoside, and quercetin-3,4′-diglucoside content showed a similar pattern. The application of low temperature (5 °C) combined with pressures of 100 and 400 MPa triggered to a better extraction of these flavonols among the treatments analysed. Response surface of the EC₅₀ antioxidant parameter as a function of pressure and temperature showed a clear trend towards an increase in onion antioxidant activity when applying pressures from 100 to 400 MPa. Four hundred megapascals/5 °C-processed onion showed an approximately 33% higher quercetin-4′-glucoside content compared with the untreated onion, and maintained the antioxidant activity of the untreated onion.     

Distribution of quercetin-3,4′-O-diglucoside,quercetin-4′-O-monoglucoside,and quercetin in different parts of the onion bulb (Allium cepa L.) influenced by genotype

Flavonoids have gained much attention because of their proposed positive effects for human health. Onions are a rich source of flavonoids, consisting mainly of the major flavonols quercetin-3,4′-O-diglucoside (QDG) and quercetin-4′-O-monoglucoside (QMG) in the bulb and the aglycone quercetin in the outer scales. In this study, distribution of these three flavonoids was determined in 16 onion cultivars (Allium cepa) using HPLC–DAD. Three different parts of the onion bulb, the inner layers, the middle layers and the outer scales were analysed. The analysis showed varietal differences in the selected onion cultivars. The cultivars with the highest total flavonoid content were the red skinned ‘Red Baron’ and the yellow skinned cultivars ‘Ailsa Craig’ and ‘Prilep’. The distribution of the total flavonoid content in the different parts of the onion bulb showed the following order: middle layers > outer scales > inner layers. In the inner layers QDG was the major flavonoid, while in the middle layers QDG and QMG were in equal amounts. In the outer scales quercetin was the major flavonoid prior to QMG.     

Flavonol glucosides in Allium species: A comparative study by means of HPLC–DAD–ESI-MS–MS

Cultivars and consumption typologies of some Allium species can significantly vary from a chemical point of view and even small differences can be important for their characterization and differentiation. Bulbs of three varieties and four consumption typologies of onion (Allium cepa L.) and two varieties of shallot (Allium ascalonicum Hort.) were subjected to HPLC–DAD–ESI-MS–MS analysis. Seven flavonol glucosides were identified in all the samples, two of which, quercetin 3,4′-diglucoside and quercetin 4′-glucoside, represent about the 90% of the overall contents. Cultivars and consumption typologies of the Allium species under study show significant differences in flavonol contents, from the very low quantity of antioxidant compounds in white onion, about 7 mg/kg against 600–700 mg/kg that were found in red and gold varieties, to the enormous content of flavonols that are present in onions of prompt consumption, where quercetin 4′-glucoside exceeds 1 g/kg and quercetin 3-glucoside is present in a ratio higher then 10:1 with respect to its value in the other onion typologies. Shallots are very rich in the two major flavonols.     

Analysis of the Major Flavonol Glycosides Present in Four Varieties of Onion (Allium cepa) and Changes in Composition Resulting from Autolysis

The major flavonoids of mature onion bulb were confirmed as the 3,4′-O-diglucoside (Qdg) and 4′-O-monoglucoside (Qmg) of quercetin using a combination of chromatographic comparisons, mass spectrometry and nuclear magnetic resonance spectroscopy. These two components account for over 85% of the total flavonoids in three varieties of onion with Qdg as the main component. Quercetin is detected in these long stored onions but only at low levels of less than 2% of the total. The remaining flavonoid fraction (approx 15%) comprises upto 17 different components of which quercetin-3-O-glucoside and isorhamnetin glucoside are prominent members although each contribute less than 1% of the total flavonoid fraction. There are significant differences in the levels of Qdg and Qmg between the four different onion varieties analysed; Qdg varying from 50–1300 mg kg^-1 fresh onion tissue and Qmg from 36–394 mg kg^-1. Maceration of the tissue for the three varieties tested led to a loss of Qdg and the appearance of Qmg and free quercetin. In the variety Rijnsburger 50% of the Qdg was degraded in 5 h and had completely disappeared after 24 h. These changes in Qdg can be quantitatively explained by increases in Qmg and free quercetin. The possible significance of quercetin glycosides in the diet is discussed. © 1997 SCI     

Physicochemical characteristics of rapidly dried onion powder and its anti-atherogenic effect on rats fed high-fat diet

Rapidly dried onion (Allium cepa L. cv. Momiji No. 3) powder (OP) prepared from the outer layers (from second to fourth scale leaves from the surface) of onion bulbs was analysed for its quercetin and polyuronide contents, the effects of enzymatic treatment and the anti-atherogenic effect on rats fed a high-fat diet. Quercetin 4′-glucoside (50%), free quercetin (30%) and quercetin 3,4′-diglucoside (20%) were identified as quercetin derivatives, and boiling-water extraction was effective in extracting these compounds. OP contained 12.9% of polyuronides, the basic skeleton of pectin. Enzymatic degradation (cellulase and pectinase, 50 °C for 12 h, pH 6.0) of OP was effective in obtaining a slurry of smaller particle sizes. The free quercetin increased and the glucosides decreased with enzyme treatment. In Wistar rats fed an OP-added high-fat diet, the total cholesterol, HDL-cholesterol and triglyceride concentrations were not significantly different from the rats fed a high-fat diet without OP. However, the atherogenic index (AI) of Wistar rats fed an OP-added high-fat diet was lower (AI = 3.3) than rats fed the diet without OP (AI = 4.1). The incremental elastic modulus (IEM) of the aorta from rats fed the OP-added diet was also significantly lower than that of the rats fed the diet without OP. The AI and IEM values of the rats fed the OP-added diet were quite similar to the values of rats fed the diet without OP but were allowed spontaneous exercise. These results suggest that OP intake is effective for decreasing the risk of arteriosclerosis.     

Identification and quantification of major phenolic compounds from mango (Mangifera indica, cv. Ataulfo) fruit by HPLC–DAD–MS/MS-ESI and their individual contribution to the antioxidant activity during ripening

Mango (Mangifera indica L.) is an economically important fruit throughout the world. ‘Ataulfo’ mango, a leading cultivar in Mexico, has the highest content of phenolic compounds among several commercial varieties of mango. However, the individual identification and antioxidant contribution of these phenols during ripening of mango fruit is unknown. Qualitative and quantitative analysis of the major phenolic compounds found in ‘Ataulfo’ mango fruit pulp was conducted in four stages of ripeness, using high-performance liquid chromatography coupled to mass spectrometry. The antioxidant contribution of each of the major phenolic compounds was calculated. The major compounds identified were chlorogenic acid (28–301 mg/100 g DW), gallic acid (94.6–98.7 mg/100 g DW), vanillic acid (16.9–24.4 mg/100 g DW), and protocatechuic acid (0.48–1.1 mg/100 g DW). The antioxidant contribution of the four phenolic acids increased during ripening. Gallic acid accounted for the highest contribution (39% maximum value), followed by chlorogenic acid (21% maximum value). This could indicate that these phenolic compounds may have an important role in the antioxidant metabolism in ‘Ataulfo’ mango fruit during ripening, and promoting health benefits to consumers.     

European elderberry (Sambucus nigra L.) rich in sugars,organic acids,anthocyanins and selected polyphenols

Sugars and organic acids in the fruit of two cultivars and three selections of black elderberry (Sambucus nigra L.): ‘Haschberg’, ‘Rubini’, ‘Selection 13’, ‘Selection 14’ and ‘Selection 25’ were quantified. The anthocyanin as well as quercetin profiles of this plant material were also established by the use of HPLC/MS. Significant differences in the concentration of sugars and organic acids were detected between the widely spread cultivar ‘Haschberg’ and all other cultivars/selections; ‘Haschberg’ was the richest in organic acids (6.38 g kg⁻¹ FW), and it contained the least sugar (68.5 g kg⁻¹ FW). The following major cyanidin based anthocyanins were identified in the fruit of black elderberry: cyanidin 3-sambubioside-5-glucoside, cyanidin 3,5-diglucoside, cyanidin 3-sambubioside, cyanidin 3-glucoside and cyanidin 3-rutinoside. The most abundant anthocyanin in elderberry fruit was cyanidin 3-sambubioside, which accounted for more than half of all anthocyanins identified in the berries. The ‘Rubini’ cultivar had the highest amount of the anthocyanins identified (1265 mg/100 g FW) and the lowest amount was measured in berries of the ‘Selection 14’ (603 mg/100 g FW). The ‘Haschberg’ cultivar contained a relatively low amount of anthocyanins in ripe berries (737 mg/100 g FW). From the quercetin group, quercetin, quercetin 3-rutinoside and quercetin 3-glucoside were identified; the latter prevailing in black elderberry fruit. The cultivar with the highest amount of total quercetins was ‘Selection 25’ (73.4 mg/100 g FW), while the ‘Haschberg’ cultivar contained average amounts of quercetins (61.3 mg/100 g FW). The chemical composition of the ‘Haschberg’ cultivar, the most commonly planted, conforms to the standards for sugars, anthocyanins and quercetins and exceeds them in the content levels of organic acids, the most important parameter in fruit processing.     

Antioxidative phenolic constituents of skins of onion varieties and their activities

The antioxidant activity of phenolic constituent of skin and selected flesh of different coloured onions (Pearl, Red, Yellow and White) were determined. The green shoot obtained after sprouting of the red onion was also analyzed to study the changes in the phenolic constituents during germination. For the first time, all tests were carried out separately for the free, esterified and insoluble-bound phenolic constituents of onion samples. The content of phenolics extracted from onion skins was approximately six times higher than that of their flesh counterparts. Among onion varieties, pearl onion skin showed the highest phenolic content (26.4 mg quercetin eq/g freeze dried sample). A similar trend was observed for free radical scavenging activity of the tested samples. Extracts from edible part of onion showed lower activity in all antioxidant tests carried out. The HPLC–MS analysis showed that quercetin 3,4′-diglucoside, quercetin, and kaempferol were the predominant phenolics in all onion extracts tested.     

Comparison of the antioxidant activity of two Spanish onion varieties

Phenol content and antioxidant activity of two Spanish onion varieties, namely white onion and Calçot de Valls, have been studied. White onions contained higher phenol content than Calçot onions, with values which ranged from 2.57 ± 0.51 to 6.53 ± 0.16 mg gallic acid equivalents/g dry weight (GAE/g DW) and 0.51 ± 0.22 to 2.58 ± 0.16 mg GAE/g DW, respectively, depending on the solvent used. Higher phenol content was associated with higher antioxidant capacity. White onion extracts had the highest antioxidant activity at 86.6 ± 2.97 and 29.9 ± 2.49 μmol Trolox/g DW for TEAC and FRAP assays, respectively, while the values for the Calçot variety were 17.5 ± 0.46 and 16.1 ± 0.10 μmol Trolox/g DW.     

Pressurised hot water extraction with on-line particle formation by supercritical fluid technology

In this work, an on-line process for pressurised hot water extraction (PHWE) of antioxidants from plants as well as drying of the extract in one step by particle formation based on the use of supercritical carbon dioxide (SC-CO₂) has been developed. This process has been called WEPO®, water extraction and particle formation on-line. With this process, dried extracts from onion with the same composition of quercetin derivatives as non-dried extracts have been obtained as a fine powder with spherical particles from 250 nm to 4 μm in diameter. The major compounds present in the extract were quercetin-3,4′-diglucoside, quercetin-4′-glucoside and quercetin. An auxiliary inert gas (hot N₂) was used to enhance the drying process. Parameters such as temperature (120 °C), SC-CO₂ and N₂ pressures (80 and 12.5 bar, respectively) and flow rate of SC-CO₂ (10 ml/min), have been settled by trial-and-error in order to achieve a fine and constant spray formation. Water content, size and morphology, antioxidant capacity and quercetin content of the particles were studied to evaluate the efficiency of the WEPO process. Results were compared with the ones from extracts obtained by continuous flow PHWE followed by freeze-drying. Results showed that both processes gave similar results in terms of antioxidant capacity, concentration of quercetin derivatives and water content, while only WEPO was able to produce defined spherical particles smaller than 4 μm.     

Variation of total phenolics, anthocyanins, ellagic acid and radical scavenging capacity in various raspberry (Rubus spp.) cultivars

The aim of this study was to assess the level of some phytochemicals in 19 raspberry cultivars grown in Lithuania. The content of total ellagic acid measured after 20 h acidic hydrolysis of investigated raspberry cultivars, varied from 119.8 (cv. ‘Pokusa’) to 323.5 mg/100 g (cv. ‘Bristol’). The content of total phenolics ranged from 278.6 (cv. ‘Pokusa’) to 714.7 mg/100 g (cv. ‘Bristol’). The total anthocyanins content varied from 2.1 (yellow cv. ‘Beglianka’) to 325.5 mg/100 g (black cv. ‘Bristol’). The radical scavenging capacity of the tested raspberry cultivars highly correlating with their total phenolics and total ellagic acid content (r = 0.90 and 0.92, respectively). The results of this study expand the knowledge about variation in the content of valuable bioactive compounds in raspberries and may help for the selection and validation of the most productive cultivars.     

Identification and quantification of flavonol aglycons in cactus pear (Opuntia ficus indica) fruit using a commercial pectinase and cellulase preparation

The applicability of pectinases and cellulases as soft hydrolysing agents on flavonol glycosides was investigated for identification and quantification of flavonol aglycons in cactus pear fruit. Freeze-dried samples of cactus pear fruit’s peel (cactus pear peels) and onions were treated with commercial pectinase and cellulase preparations at 50 °C for different time periods (up to 16 h). Additionally isorhamnetin-3-O-rutinoside and quercetin-3,4′-O-diglucoside were used as model compounds. In parallel, samples of cactus pear peels and onions were treated by the traditional acidic hydrolysis using HCl. Following hydrolysis, flavonols were characterised using HPLC–DAD. Enzymatically, all model compounds and plant material were highly hydrolysable. Flavonol glycosides were completely hydrolysed after 16 h (cactus pear) and 4 h (onion), respectively. While the acidic hydrolysis caused degradation of the flavonols and produced protocatechuic acid as a degradation product, the enzymatic hydrolysis gave gentle effects and did not produce any protocatechuic acid at all.     

A remarkable influence of microwave extraction: Enhancement of antioxidant activity of extracted onion varieties

Four (red, yellow, white and grelot onion) varieties of Allium cepa, a rich source of quercetin (flavonol) glycosides, were studied for their total content of reducing compounds (TCRC), flavonol content and antioxidant activity evaluation. Extracts obtained by solvent free microwave hydrodiffusion and gravity (MHG) technique and conventional solvent extraction (CSE) were analysed with high performance liquid chromatography (HPLC) for quantification of flavonoids. Three different methods were selected for evaluating the antioxidant capacity of the different onion varieties (after the determination of their phenolic content by the Folin–Ciocalteu method): the reduction of the stable DPPH (2,2-diphenyl-1-picrylhydrazyl) radical, the ORAC (oxygen radical absorbance capacity) method, and the inhibition of the AAPH-induced peroxidation of linoleic acid in SDS micelles. The highest antioxidant capacity was observed for red onion, followed by yellow, white and grelot onion. In spite of the low recovery of extractable flavonoids (quercetin 3,4′-diglucoside, 4′-glucoside and 3-glucoside), MHG remained the preferred extraction method in comparison to the conventional method, as all the samples obtained under microwave-assisted extraction (MAE) exhibited the highest antioxidant activities in all the tests. Also the microscopic observations of extracted tissues showed that at cellular level, microwaves induced disruptions of vacuoles and cell walls thus promoting the effectiveness of this method.     

Anti-melanogenesis properties of quercetin- and its derivative-rich extract from Allium cepa

In an effort to find a new whitening agent, we have found that the methanol extract of the dried skin of Allium cepa showed inhibition of melanin formation. Bioassay-guided fractionation led to the isolation of quercetin (1) and quercetin 4’-O-β-glucoside (3) from A. cepa as the inhibitors of melanin formation in B16 melanoma cells with IC₅₀ values of 26.5 and 131 μM, respectively. In addition, we evaluated the effect of some quercetin derivatives, such as isoquercitrin (2), quercetin 3,4’-O-diglucoside (4), rutin (5) and hyperin (6) on B16 melanoma cells. These quercetin derivatives did not show any inhibition of melanin formation. Furthermore, the ORAC values of compounds 1–6 were 7.64, 8.65, 4.82, 4.32, 8.17 and 9.34 μmol trolox equivalents/μmol, respectively. Dried skin of red onion showed inhibitory activity against melanin formation in B16 melanoma cells, as well as antioxidant properties.     

Effects of binary solvent extraction system,extraction time and extraction temperature on phenolic antioxidants and antioxidant capacity from mengkudu (Morinda citrifolia)

An investigation into the effects of ethanol concentration (0–100%, v/v), extraction time (20–120 min) and extraction temperature (25–65 °C) on the extraction of phenolic antioxidants from mengkudu (Morinda citrifolia) was performed using a single-factor experiment. Total phenolic content (TPC) and total flavonoid content (TFC) assays were used for determination of phenolic compounds. Antioxidant capacity was evaluated by measuring the scavenging effect on 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and 2,2′-diphenyl-1-picrylhydrazyl (DPPH) radicals. Experimental results showed that extraction conditions had significant effect on extraction of phenolic compounds and antioxidant capacities. The optimised conditions were 40% ethanol for 80 min at 65 °C, with values of 919.95 mg GAE/100 g DW for TPC, 472.73 mg CE/100 g DW for TFC, 791.71 μmol TEAC/100 g DW for ABTS and 1928.5 μmol TEAC/100 g DW for DPPH. TPC was significantly correlated with DPPH under the effects of ethanol concentration (r = 0.932) and extraction time (r = −0.938).     

Phenolic compound profile of selected vegetables frequently consumed by African Americans in the southeast United States

The phenolic composition of vegetables commonly consumed by African Americans in the southeast United States was analyzed with HPLC–MS. The vegetable samples included collard greens, mustard greens, kale, okra, sweet potato greens, green onion, butter beans, butter peas, purple hull peas, rutabagas, eggplant, and purslane. Five compounds out of total 29 peaks detected from the 12 samples – caffeic acid, ferulic acid, quercetin, kaempferol, and isorhamnetin – were identified. No gallic acid, p-coumaric acid, myricetin, luteolin, apigenin, hesperetin, naringenin, or flavanols was detected. The major flavonoids were isorhamnetin, quercetin and kaempferol. Isorhamnetin was found in kale, mustard greens, and purslane. The content ranged from 2.8 to 23.6 mg/100 g fresh edible part. Quercetin was found in collard greens, mustard greens, kale, okra, sweet potato greens, purple hull peas, and purslane. The content ranged from 1.3 to 31.8 mg/100 g with the highest content in kale and lowest content in purslane. Kaempferol was found in collard greens, mustard greens, kale, sweet potato greens, green onion, and purslane. The content ranged from 1.1 to 90.5 mg/100 g. Caffeic acid was only found in sweet potato greens. Ferulic acid was found in collard greens, mustard greens, kale, okra, purple hull peas, and purslane. Although some peaks were found in eggplant, butter beans, butter peas and rutabagas, these peaks were not identified due to lack of reference compound and no flavonoid or phenolic acid was quantified in these samples. The results suggest that these indigenous vegetables among African Americans are good sources of the phenolic compounds, which can be useful for the prevention of cardiovascular and other chronic diseases.     

The effect of solar radiation on the flavonol content in broccoli inflorescence

The effect of solar radiation on the quercetin and kaempferol contents in the inflorescence of three broccoli cultivars (‘Lord’, ‘Marathon’ and ‘Fiesta’) was investigated from 1999 to 2001. Great differences in the contents of both flavonols, dependent on growing time and cultivar, were found. Quercetin and kaempferol contents varied from 14.3 to 81.0 mg kg⁻¹ f.w. and from 35.9 to 213 mg kg⁻¹ f.w., respectively. Inflorescences of the cultivar ‘Lord’ were characterised by the highest mean content of quercetin and those of cultivar ‘Fiesta’ of kaempferol. The contents of both flavonols were highly positively correlated with total solar radiation in the period from planting to the harvest of broccoli inflorescences.     

Flavonoid pattern of sage (Salvia officinalis L.) unifloral honey

The aim of the present paper was to determine the flavonoids in monofloral sage (Salvia officinalis L.) honey which is characteristic and specific for the area of Croatian coast and islands. For that purpose 38 sage honey samples from two production seasons were analysed. After specific pollen content determination, and analyses of selected physicochemical parameters which confirmed that samples are in compliance with national and international regulations and can be regarded as unifloral sage honeys, flavonoid fraction was isolated and analysed using RP-HPLC/DAD method. The HPLC analysis showed that all examined sage honey samples contain quercetin (3,3′,4′,5,7-pentahydroxyflavone), luteolin (3′,4′,5,7-tetrahydroxyflavone), kaempferol (3,4′,5,7-tetrahydroxyflavone), apigenin (4′,5,7-trihydroxyflavone), chrysin (5,7-dihydroxyflavone) and galangin (3,5,7-trihydroxyflavone), as well as p-coumaric (trans-4-hydroxycinnamic acid) and caffeic acid (3,4-dihydroxycinnamic acid). Total amount of identified flavonoids varied from 109.4 μg/100 g of honey to 589.9 μg/100 g of honey, with the average of 288.5 μg/100 g of honey. All analysed honey samples showed common and specific flavonoid profile which could be the basis for differentiating sage from other monofloral honeys.     

The microbial degradation of onion flavonol glucosides and their roasting products by the human gut bacteria Eubacterium ramulus and Flavonifractor plautii

Flavonoids are important constituents of the human diet. One source for flavonols, a major subclass of the flavonoids, is onion. It contains high amounts of quercetin glycosides, primarily quercetin-3,4′-di-O-glucoside (QDG) and quercetin-4′-O-monoglucoside (Q-4′-MG). Due to their high reactivity flavonols are susceptible to thermal degradation as used in food processing. Especially boiling and roasting influence the flavonoid content of food products. Quercetin and several of its glycosides may serve as substrates for human gut bacteria. For example, Eubacterium ramulus and Flavonifractor plautii are capable of cleaving the aglycone quercetin to form 3,4-dihydroxyphenylacetic acid (DHPAA) and phloroglucinol which to some extent can be degraded further. The aim of this study was to find out whether E. ramulus and F. plautii are also capable of degrading Q-4′-MG and QDG by and to investigate the influence of a thermal treatment (roasting) of the onion glucosides on the subsequent microbial degradation. In this study, E. ramulus was capable of degrading Q-4′-MG and QDG, while F. plautii was not. Roasting of QDG at 180 °C for 5 min led to the formation of quercetin with Q-4′-MG and quercetin-3-O-monoglucoside (Q-3-MG) as intermediates. Roasting accelerated the microbial degradation of Q-4′-MG and QDG. In the case of F. plautii, microbial degradation was induced by quercetin which was formed during roasting and is a preferred substrate of this organism.