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.     

Solvent Free Microwave-Assisted Extraction of Antioxidants from Sea Buckthorn (Hippophae rhamnoides) Food By-Products

After processing, every extraction process generates huge amount of unintended wastes, especially from fruits and vegetables which represent a major disposal problem for the food industry. They are promising sources of bioactive compounds that could be used for their favourable nutritional properties. Sea buckthorn juice production results in generation of large amount of by-products, which are suggested to contain substantial amounts of valuable natural antioxidants. Extracts obtained by solvent-free microwave hydrodiffusion and gravity (MHG) technique and conventional solvent extraction (CSE) method were analysed with HPLC for quantification of flavonoids along with evaluating their phenolic contents by Folin-Ciocalteu method and reducing power by the reduction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical. MHG is a green extraction method which offers important advantages like shorter extraction time (15 min), cleaner feature (no solvent or water used) and extraction of valuable flavonoids (Isorhamnetin, isorhamnetin 3-O-glucoside, isorhamnetin 3-O-rutinoside and quercetin 3-O-glucoside) at optimised power (400 W). Along with extracting similar flavonols in enough concentratioin, MHG extract has shown much higher phenolic contents (1,147 milligram gallic acid equivalents (GAE) per gram) against CSE extract (741 mg GAE/g) with greater antioxidant activity determined by DPPH assay.     

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.     

Composition and antioxidant activity of raisin extracts obtained from various solvents

Experiments were conducted to determine if the contents of phenolics and browning reaction products and antioxidant activity of raisin extracts were closely dependent on the extraction solvent. Enhanced extraction yields were obtained from solvent containing higher water concentrations. However, total phenolic content (TPC) was highest for extracts obtained from solvent to water ratios of 60:40 (v/v), whereby the extract obtained from ethanol:water (60:40, v/v) had the highest TPC of 375 mg gallic acid equivalents (GAE)/100 g extract. HMF content was highest in extracts obtained from 60% solvent, regardless of solvent type. The extract obtained from 60% methanol had the highest HMF content at 199 μg/g extract. Although the 60% solvents provided extract with high antioxidant components, the antioxidant activity of raisin extracts obtained from 80:20 (v/v) solvent/water was significantly higher than other raisin extracts obtained from different solvent concentrations. Phenolic acids, HMF, and low-molecular-weight flavonoids were responsible for the antioxidant activity, but not the high-molecular-weight flavonoids.     

Subcritical water extraction of flavonol quercetin from onion skin

Subcritical water could be an excellent alternative to organic solvent as a medium for extracting flavonol quercetin, due to its temperature-dependent selectivity, safety, efficiency of recovery, and lower cost. This study investigated the application of subcritical water extraction (SWE) of quercetin from onion skin and evaluated the effect of key operation conditions by varying the temperature (100-190 °C), extraction time (5-30 min), and mixture ratio of onion skin and diatomaceous earth (DE) (0.5:3.5-2:2) under high pressure (90-131 bar). The maximum yield of quercetin (16.29 ± 0.75 mg/g onion skin) was obtained at extraction temperature of 165 °C, extraction time of 15 min, mixture ratio of 1.5:2.5 for onion skin and DE. The SWE was compared with three conventional extraction methods in terms of the efficiency. The quercetin yield by SWE was over eight-, six-, and fourfold greater than those obtained using the ethanol, methanol, and water-at-boiling-point extraction methods, respectively.     

Optimisation of microwave‐assisted extraction of phenolic compounds from broccoli and its antioxidant activity

The effect of independent variables of microwave‐assisted extraction (MAE), extraction temperature (50–90 °C), microwave power (100–200 W), solvent concentration (methanol/water, 50–90%, v/v) and extraction time (1–27 min) on total phenolics and flavonoids contents as well as antioxidant activity of broccoli extracts was investigated. Response surface methodology was successfully applied for optimisation and the high correlation of the mathematical model indicated that a quadratic polynomial model could be used for optimisation of investigated responses. Considering the maximum amount of extracted total phenolics, total flavonoids and antioxidant activity expressed as ORAC value, the optimal conditions for all three investigated responses were obtained (temperature ?71.11 °C; microwave power ?167.03 W; solvent concentration ?75.95%; and extraction time 16.34 min), which is in agreement with optimal conditions obtained for each investigated responses. The development of simultaneously MAE methods for total phenolics, total flavonoids and antioxidant activity of broccoli extracts might simplify assessment of broccoli as valuables source of antioxidants.     

Supercritical fluid extraction and hydrodistillation for the recovery of bioactive compounds from Lavandula viridis L’Hér

The chemical profiles of bioactive essential oil and extracts obtained by hydrodistillation (HD) and supercritical fluid extraction (SFE), respectively, from Lavandula viridis were compared. The SFE was performed at 40 °C and at extraction pressures of 12 or 18 MPa in two different separators. Evaluation of the essential oil and SFE extracts by GC–FID and GC–IT–MS revealed that oxygen-containing monoterpenes were the major constituents in both cases, but there were important differences between the chemical profiles produced by the different extraction techniques. More compounds were isolated by HD but higher yields were achieved by SFE. Camphor was the main component identified in the essential oil (31.59 ± 1.32%), and in extracts from the first (1.61 ± 0.34%) and second SFE separators (22.48 ± 1.49%) at 12 MPa. In contrast, the first separator SFE extract at 18 MPa (heavy compounds) was dominated by myrtenol (5.38 ± 2.04%) and camphor (4.81 ± 1.93%), whereas the second separator SFE extract (volatiles) was dominated by verbenone (13.97 ± 5.27%). The essential oil and heavy compound extracts from the first separator possessed antioxidant and anti-cholinesterase activities. Our data show that phytochemicals from the aerial parts of L. viridis could be developed as natural antioxidant and anti-cholinesterase drugs, with particular applications in the symptomatic treatment of Alzheimer’s disease.     

Comparison of two isolation methods for essential oil from rosemary leaves: Hydrodistillation and microwave hydrodiffusion and gravity

Traditional hydrodistillation (HD) and innovative Microwave Hydrodiffusion and Gravity (MHG) methods have been compared and evaluated for their effectiveness in the isolation of essential oil from fresh Rosmarinus officinalis leaves. The microwave method offers important advantages over traditional alternatives, namely: shorter isolation times (15 min against 3 h for hydrodistillation), environmental impact (energy cost is fairly higher to perform HD than that required for rapid MHG isolation), cleaner features (as no residue generation and no water or solvent used), increases antimicrobial activities, increases antioxidant activity and provides a more valuable essential oil (with high amount of oxygenated compounds). It offers also the possibility for a better reproduction of natural aroma of the essential oil from rosemary leaves than the HD essential oil. Moreover, microwave procedure yielded essential oils that could be analysed or used directly without any clean-up, solvent exchange or centrifugation steps. Scanning electron microscopy shows important structural changes for MHG extraction in contrast to those obtained by HD. Electron micrographs show clearly that the cells are broken and damaged during microwave treatment. Finally, the mechanism of Microwave Hydrodiffusion and Gravity is proposed and discussed.     

Phenolic content and antioxidant activity of extracts from whole buckwheat (Fagopyrum esculentum Möench) with or without microwave irradiation

The purpose of this study was to extract phenolic compounds and antioxidant activity from buckwheat with water, 50% aqueous ethanol, or 100% ethanol using microwave irradiation or a water bath for 15 min at various temperatures (23–150 °C). Phenolic content of extracts increased with increasing temperature. In general, phenolic contents in microwave irradiated extracts were higher than those heated with a water bath. The highest phenolic content, 18.5 ± 0.2 mg/g buckwheat, was observed in the extract that was microwave irradiated in 50% aqueous ethanol at 150 °C. The highest antioxidant activities, 5.61 ± 0.04–5.73 ± 0.00 μmol Trolox equivalents/g buckwheat, were found in the 100% ethanol extracts obtained at 100 and 150 °C, independent of heat source. These results indicate that microwave irradiation can be used to obtain buckwheat extracts with higher phenolic content and similar antioxidant activity as extracts heated in a water bath.     

Enzymatic water extraction of taxifolin from wood sawdust of Larix gmelini (Rupr.) Rupr. and evaluation of its antioxidant activity

An enzyme incubation–water extraction (EI–WE) method was developed and optimised for the extraction of the natural antioxidant taxifolin and of the total flavonoids from wood sawdust of Larixgmelini (Rupr.) Rupr. A factorial design and a central composite design approach were used for method optimisation. Optimal conditions were 0.5 mg/ml cellulase and 0.5 mg/ml pectinase, a pH of 5.0, a temperature of 32 °C and 18 h incubation time. The flavonoids and taxifolin were extracted in hot water at 50 °C for 30 min, with a solid to liquid ratio of 1:20. Under optimised conditions, the yields of taxifolin and total flavonoids increased from 1.06 ± 0.08 to 1.35 ± 0.04 mg/g and 4.13 ± 0.17 to 4.96 ± 0.29 mg/g, respectively. DPPH and BHT assays revealed that the EI–WE samples had 1.8- and 1.68-fold higher antioxidant activities than the controls. SEM results revealed the structural disruption of wood sawdust with enzyme incubation.     

Valorisation of grape pomace by the extraction of phenolic antioxidants: Application of high voltage electrical discharges

The aim of this study was to optimise the electrically assisted extraction in order to obtain grape pomace extracts with high polyphenols content, which would be potentially interesting for applications as natural antioxidants. High voltage electrical discharges (HVED) were applied for intensification of the extraction. The effects of the energy input, the electrodes distance gap and the liquid-to-solid ratio were studied. Diffusion was then carried out in different mixtures of water and ethanol for one hour at 20, 30, 40 and 60 °C. The most efficient extraction was (i) an HVED pre-treatment at 80 kJ/kg with an electrodes distance of 5 mm in a liquid-to-solid ratio of 5 followed by (ii) a diffusion with 30% ethanol in water at 60 °C for 30 min. The highest total polyphenols content reached 2.8 ± 0.4 gGAE/100gDM with a corresponding antioxidant activity of 66.8 ± 3.1 gTEAC/kgDM. The polyphenols extraction rate increased with temperature in accordance to an Arrhenius type of relationship: activation energy of 21.5 ± 1.1 kJ/mol for HVED treated systems against 0.5 ± 0.2 kJ/mol for untreated ones.     

Enhanced extraction of flavanones hesperidin and narirutin from Citrus unshiu peel using subcritical water

Flavanones including hesperidin and narirutin constitute the majority of the flavonoids that occur naturally in citrus fruits. The main purpose of this study was to extract valuable natural flavanones from agricultural by-products such as citrus peels using subcritical water extraction (SWE). Thus, the application of SWE to extraction of flavanones hesperidin and narirutin from Citrus unshiu peel was evaluated, and the effect of key operating conditions was determined by varying the extraction temperature (110–200 °C) and time (5–20 min) under high pressure (100 ± 10 atm). The maximum yields of hesperidin (72 ± 5 mg/g C. unshiu peel) and narirutin (11.7 ± 0.8 mg/g C. unshiu peel) were obtained at an extraction temperature of 160 °C for an extraction time of only 10 min. These yields accounted for approximately 99% of the total amount of these flavanones in the original material. The SWE was compared with three conventional extraction methods in terms of the extraction time and recovery yields for hesperidin and narirutin. The hesperidin yield by SWE was more than 1.9-, 3.2-, and 34.2-fold higher than those obtained by extraction methods using ethanol, methanol, or hot water, respectively, and the narirutin yield was more than 1.2-, 1.5-, and 3.7-fold higher.     

Optimisation of accelerated solvent extraction of antioxidant compounds from rosemary (Rosmarinus officinalis L.), marjoram (Origanum majorana L.) and oregano (Origanum vulgare L.) using response surface methodology

The present study optimised the accelerated solvent extraction (ASE) conditions (Dionex ASE® 200, USA) to maximise the antioxidant capacity of the extracts from three spices of Lamiaceae family; rosemary, oregano and marjoram. Optimised conditions with regard to extraction temperature (66–129 °C) and solvent concentration (32–88% methanol) were identified using response surface methodology (RSM). For all three spices results showed that 129 °C was the optimum temperature in order to obtain extracts with high antioxidant activity. Optimal methanol concentrations with respect to the antioxidant activity of rosemary and marjoram extracts were 56% and 57% respectively. Oregano showed a different response to the effect of methanol concentration and was optimally extracted at 33%. The antioxidant activity yields of the optimal ASE extracts were significantly (p < 0.05) higher than solid/liquid extracts. The predicted models were highly significant (p < 0.05) for both total phenol (TP) and ferric reducing antioxidant property (FRAP) values in all the spices with high regression coefficients (R²) ranging from 0.952 to 0.999.     

Chemical and economic evaluation of natural antioxidant extracts obtained by ultrasound-assisted and agitated bed extraction from jussara pulp (Euterpe edulis)

This work aimed to evaluate the influence of ultrasonic and agitated bed extractions on the chemical composition and manufacturing costs of extracts obtained from jussara (Euterpe edulis) pulp. The effects of extraction time (5–180 min), temperature (25–55 °C), ethanol concentration (0–90% in acidified water) and solvent/pulp ratio (5–30 mL/g) on the extraction yield, phenolic content, anthocyanin content, antioxidant capacity and manufacturing costs were assessed. The yields provided by the ultrasound-assisted and agitated bed extractions were not significantly different. The anthocyanins and phenolic compound yields were significantly affected by the extraction time, the ethanol concentration in water and the solvent/feed ratio, but not by the temperature. In general, the antioxidant capacity of the extracts displayed tendencies similar to the anthocyanin and phenolic compound yields. The production of crude extracts obtained by ultrasound and agitated bed extraction incurred greater manufacturing costs compared to the market prices of assai extracts.     

Antioxidant activity of Spirulina platensis extracts by supercritical carbon dioxide extraction

Supercritical CO₂ extraction of antioxidants from Spirulina platensis was optimized using response surface methodology. About 10.26 g/kg of extracts from S. platensis could be obtained under the optimum conditions of 48 °C at 20 MPa over a 4 h period. The antioxidant activity of the extracts prepared under the optimized condition, determined by linoleic acid peroxidation inhibition method, was lower compared with BHT and Trolox, but significantly higher than α-tocopherol in 300 min and became similar to α-tocopherol after that. The components of the extracts were further analyzed, and the results showed that the extracts contained 85.1 g/kg of flavonoids, 77.8 g/kg of β-carotene, 113.2 g/kg of vitamin A and 3.4 g/kg of α-tocopherol, which may contribute greatly to their high antioxidant activity. The main fatty acids in the extracts were palmitic acid (35.32%), linolenic acid (21.66%) and linoleic acid (20.58%).     

Partial characterization of a new kind of Chilean Murtilla-like berries

The aim of this research was to characterize a new kind of Chilean Murtilla-like berries (Myrteola nummularia (Poiret) Berg. Myrtaceae, called by locals as Daudapo) vs.well known Murtilla, blueberries, raspberries and black chokeberries. Polyphenols, flavonoids, flavanols and tannins and the level of antioxidant activity by ABTS, FRAP and CUPRAC radical scavenging assays of methanol extract of studied berry samples were determined and compared. It was found that the contents of the polyphenol compounds and the level of antioxidant activity in extracts of berries differ significantly (P < 0.05). The significantly highest contents of polyphenol compounds were in methanol extract of non-ripe Murtilla (121.31 ± 5.9 mg GAE/g for polyphenols; 14.43 ± 0.7, 31.79 ± 1.5, and 9.93 ± 0.3 mg CE/g for flavonoids, tannins and flavanols, respectively). Also the antioxidant activity according to ABTS, FRAP and CUPRAC was significantly highest in methanol extract for non-ripe Murtilla (878.18 ± 41.2, 486.92 ± 23.3 and 1012.42 ± 43.2 ??M TE/g, respectively). The amount of polyphenol compounds and their antioxidant activities of Murtilla berries are significantly higher than in other studied berries and are comparable with blueberries and raspberries, however, these indices in the Murtilla-like non-ripe berries were the following: 31.55 ± 1.4 mg GAE/g for polyphenols; 5.22 ± 0.3, 12.16 ± 0.6 and 2.24 ± 0.1 mg CE/g for flavonoids, tannins and flavanols; ABTS, FRAP and CUPRAC: 244.22 ± 12.1, 81.32 ± 3.9 and 203.83 ± 9.3 ??M TE/g, respectively. The correlation between the polyphenol compounds and the antioxidant activities were relatively high. DPPH kinetic measurements were used to compare, distinguish and discriminate the antiradical activity among berry methanolic extracts by multivariate analysis. 3-D fluorescence was used as an additional tool for the characterization of the polyphenol extracts during various stages of ripening and different berries cultivars. The interaction between methanol polyphenol extracts of Murtilla-like and bovine serum albumin (BSA) showed that the new kind of berries has a strong ability, as other studied berries, to quench the intrinsic fluorescence of BSA by forming complexes.In conclusion, for the first time these berries were analyzed and compared with widely consumed cultivars, using their polyphenols' composition, antioxidant activities and fluorescence properties. The ability of Murtilla-like berries to quench the intrinsic fluorescence of BSA and relatively high content of polyphenol compounds can be used as a new source of antioxidants.     

Antioxidant activities of onion (Allium cepa L.) peel extracts produced by ethanol,hot water,and subcritical water extraction

Onion (Allium cepa L.) peels were extracted by ethanol, hot water and subcritical water (SW) extraction and their antioxidant activities were evaluated. Extraction yields of SW extraction were 4-fold higher than ethanol extraction. However, the ethanol extraction increased the total phenolics contents (327.5 mg GAE/g extract) and flavonoids contents (183.95 mg QE/g extract) in the onion peel extract. The onion peel extracts by ethanol extraction showed greater DPPH radical scavenging activities and greater antioxidant activities determined by ferric thiocyanate assay than those by hot water extraction and SW extraction at 165°C. Antioxidant activity of onion peel extract by SW extraction at 110°C was similar to that of ethanol extraction. HPLC profiles revealed that SW extraction at lower temperature (110°C) increased the concentration of quercetin. These results demonstrated that the onion peel extracts produced by SW extraction technique have great potential as a source for useful antioxidant.     

Phenolic and antioxidant potential of olive oil mill wastes

Olive oil mill waste was subjected to conventional liquid solvent extraction and supercritical fluid extraction using different solvents and carbon dioxide, respectively. The optimum solvent extraction conditions of phenols were 180 min using ethanol, at a solvent to sample ratio 5:1 v/w, and at pH 2. Solvent and SFE extracts were tested for their antioxidant activity by the DPPH radical scavenging method and by determination of peroxide value on virgin olive oil and sunflower oil. The ethanol extract exhibited the highest antiradical activity, and no correlation was found between antiradical activity and phenol content. The SFE extract exerted good antioxidant capacity although its phenolic yield was not quite high. Moreover, the ethanol extract appeared to be a stronger antioxidant than BHT, ascorbyl palmitate and vitamin E by the Rancimat method on sunflower oil. HPLC analysis of the extracts showed that the predominant phenolic compound was hydroxytyrosol. Various phenolic acids and flavonoids were also identified.     

Optimisation of total phenolic acids extraction from mandarin peels using microwave energy: The importance of the Maillard reaction

Microwave processing has been used for the extraction of antioxidants from mandarin peels using deionised water as the solvent. Optimum extraction conditions under constant heat load per unit mass were as follows: microwave power, 400 W; extraction time, 3 min; solid to solvent ratio, 1:2; and extraction temperature below 135 °C. These conditions did not change the colour of the extract significantly under neutral (pH 4.5-4.9) and acidic conditions (pH 1.5-2.5). The amounts of antioxidants, which were extracted when the heat load per unit mass was held constant, were not significantly affected by the solid to solvent ratio used, so the maximum solid to solvent ratio of 1:2 used here is recommended to minimise the amount of solvent used. The colour of the extract was more greatly affected by the extraction temperature than by the solid to solvent ratio. The total phenolic contents of mandarin peels were evaluated using the Folin-Ciocalteu assay (FCR) and the trends were compared with the results of the Oxygen Absorbance Radical Capacity (ORAC) assays. The presence of Maillard rections and Maillard by-products were also investigated at higher extraction temperatures and analysed by the High Performance Liquid Chromatography (HPLC) method. The results demonstrated that microwave extraction (MWE) is a fast and reliable method for phenolic compound extraction from citrus marc with the minimum use of solvent.     

Microwave assisted extraction of anthocyanins from grape skins

A new method for the analysis of anthocyanins in grapes based on a systematic study of the extractability of eleven anthocyanins from grapes has been developed. Microwave assisted extraction was applied as a prior stage to the chromatographic determination of anthocyanins in the extracts. The stability of anthocyanins under the extraction conditions was checked using a standardised extract from grape skins. Temperatures from 50 °C up to 150 °C were evaluated. A fractional factorial experimental design was developed to analyse the influence on the extraction process of six different extraction variables: solvent (mixtures of methanol and water), stirring, extraction temperature, extraction time, microwave power and extraction volume. The extraction solvent was the most important variable for the recovery of most anthocyanins from grapes. Finally, the influence of the extraction time was also studied. With this new method, anthocyanins can be extracted from grapes in 5 min, using 100 °C as extraction temperature and 40% methanol in water as the extraction solvent. Repeatability and reproducibility were also checked, the resulting RSDs (n = 9) were lower than 7% for glucosides, the main components, and lower than 9% for the acyl derivatives, the compounds found in the lowest concentrations.