Green and efficient extraction of rutin from tartary buckwheat hull by using natural deep eutectic solvents

A Systematic Review on Waste as Sustainable Feedstock for Bioactive Molecules—Extraction as Isolation Technology

In today’s linear economy, waste streams, environmental pollution, and social–economic differences are increasing with population growth. The need to develop towards a circular economy is obvious, especially since waste streams are composed of valuable compounds. Waste is a heterogeneous and complex matrix, the selective isolation of, for example, polyphenolic compounds, is challenging due to its energy efficiency and at least partially its selectivity. Extraction is handled as an emerging technology in biorefinery approaches. Conventional solid liquid extraction with organic solvents is hazardous and environmentally unfriendly. New extraction methods and green solvents open a wider scope of applications. This research focuses on the question of whether these methods and solvents are suitable to replace their organic counterparts and on the definition of parameters to optimize the processes. This review deals with the process development of agro-food industrial waste streams for biorefineries. It gives a short overview of the classification of waste streams and focuses on the extraction methods and important process parameters for the isolation of secondary metabolites.

Sustainable Development and Storage Stability of Orange By-Products Extract Using Natural Deep Eutectic Solvents

The citrus industry produces large amounts of waste rich in bioactive compounds that have important effects on human health. Their extraction was performed using organic solvents, and a greener alternative to those solvents are natural deep eutectic solvents (NADES). The present study aimed to obtain and optimize extracts rich in polyphenols and flavonoids from orange peels using NADES and monitor polyphenol stability in the extracts for 30 days. The software COSMOtherm (conductor-like screening model) was used to screen fourteen NADES. The most promising solvents were lactic acid:glucose (LA:Glu) with an extraction yield of 1932 ± 7.83 mgGAE/100 gdw for TPC (total polyphenol content) and 82.7 ± 3.0 mg/100 gdw for TFC (total flavonoid content) and in the case of L-proline:malic acid (LP:MA) was 2164 ± 5.17 mgGAE/100 gdw for TPC and 97.0 ± 1.65 mg/100 gdw for TFC. The extraction process using LA:Glu and LP:MA was optimized, and the results showed that the selected variables (%NADES, solid:liquid ratio, and extraction time) had a significant influence on the extraction of TPC and TFC. Results showed that NADES improve the stability of TPC. These findings revealed that NADES are efficient for the extraction of bioactive compounds from orange by-products, and these extracts can represent an alternative for the food industry to enrich food products with natural ingredients.

Antioxidant Capacity, Vitamin C and Polyphenol Profile Evaluation of a Capsicum chinense By-Product Extract Obtained by Ultrasound Using Eutectic Solvent

Habanero pepper leaves and stems (by-products) have been traditionally considered waste; however, bioactive compounds such as polyphenols, vitamin C and carotenoids have been identified that can be used for formulation of nutraceuticals or functional foods. Furthermore, the extraction of these bioactive compounds by using environmentally friendly methods and solvents is desirable. Thus, the aim of this study was to assess the antioxidant capacity, total polyphenol content (TPC), the phenolic profile and vitamin C content in extracts obtained from by-products (stems and leaves) of two varieties (Mayapan and Jaguar) of habanero pepper by ultrasound-assisted extraction (UAE) using natural deep eutectic solvents (NADES). The results showed that NADES leads to extracts with significantly higher TPC, higher concentrations of individual polyphenols (gallic acid, protocatechuic acid, chlorogenic acid, cinnamic acid, coumaric acid), vitamin C and, finally, higher antioxidant capacity (9.55 ± 0.02 eq mg Trolox/g DM) than UAE extraction performed with methanol as the solvent. The association of individual polyphenols with NADES was confirmed by principal component analysis (PCA). Overall, NADES is an innovative and promising "green" extraction technique that can be applied successfully for the extraction of phenolic compounds from habanero pepper by-products.

A Development Strategy of Tailor-made Natural Deep Eutectic Solvents for the Enhanced Extraction of Hydroxynaphthoquinones from Alkanna tinctoria Roots

Natural hydroxynaphthoquinone enantiomers (HNQs) are well-described pharmaceutical and cosmeceutical agents especially present in the roots of Alkanna tinctoria (L.) Tausch, a species native to the Mediterranean region. In this work, eco-friendly natural deep eutectic solvents (NaDESs) were developed for the selective extraction of these compounds. An extensive screening was performed using more than sixty tailor-made NaDESs. The impact of the intrinsic physicochemical properties on the HNQs extraction efficiency as well as the specificity towards the different enantiomeric pairs was thoroughly investigated. As a result of a multivariate analysis and of the one factor-a-time solvent optimization, the eutectic mixture composed of levulinic acid and glucose (LeG) using a molar ratio of 5 : 1 (mol_HBA : mol_HBD) and 20% of water (w/w) was found as the most appropriate mixture for the highest extraction efficiency of HNQs. Further optimization of the extraction process was attained by response surface methodology, using a temperature of 45 °C, a solid-to-liquid ratio of 30 mg/mL, and an extraction time of 50 min. A maximum extraction output of 41.72 ± 1.04 mg/g was reached for HNQs, comparable to that of the commonly used organic solvents. A solid-phase extraction step was also proposed for the recovery of HNQs and for NaDESs recycling. Our results revealed NaDESs as a highly customizable class of green solvents with remarkable capabilities for the extraction of HNQs.

Simultaneous optimization of the extraction process of Yangyin Yiqi Huoxue prescription with natural deep eutectic solvents for optimal extraction yield and antioxidant activity: A comparative study of two models

INTRODUCTION

Natural deep eutectic solvents (NaDESs) are green and effective solvents that are used to extract 3 flavonoids from Yangyin Yiqi Huoxue prescription, a traditional Chinese prescription.

METHODS

A total of 6 types of NaDESs were systematically screened and evaluated for the total extraction yield of puerarin, calycosin, and formononetin by high-performance liquid chromatography. Then, a 4-factor-three-level experimental scheme designed by the Box-Benhnken Design was applied on the basis of a single experiment to determine the extraction yield and the antioxidant property. Finally, the extraction process was optimized through response surface methodology (RSM) and the genetic neural network (GNN), respectively.

RESULTS

The use of betaine-lactic acid as an extractant displayed significant advantages in the screening process. The optimum extraction parameters provided by GNN were as follows: water content 25% (v/v), liquid to material ratio 190 mg/ml, extraction time 37 min, and extraction temperature 63 °C. Under this condition, the average experimental comprehensive evaluation values of the extraction yield and antioxidant properties were 3.12 mg/g and 86.27%, and the relative deviations to the predicted values were 0.30% and 1.44%, respectively. In addition, the experimental results of GNN were better than those of RSM (p < 0.01).

CONCLUSIONS

We found the application of GNN to be effective and credible for bi-objective optimization of extraction yields and antioxidant activity in this study. Moreover, our results provide a reference and a theoretical basis for experimental and future industrial extraction for multi-objective situations.

Advances on Cyclocarya paliurus polyphenols: Extraction, structures, bioactivities and future perspectives

Cyclocaryapaliurus (C. paliurus) is an edible and medicinal plant, distributed in southern China. As a kind of new food raw material, the leaves of C. paliurus are processed as tea products in daily life. C. paliurus is recognized as a good source to polyphenols, showing excellent bioactivities, which has attracted more and more attention. Polyphenols are important functional bioactive components in C. paliurus. C. paliurus polyphenols perform nutritional functions in anti-diabetes, anti-hyperlipidemic, anti-obesity, anti-oxidant, and other activities. In this review, we summarize the research progress of extraction technologies, structural characteristics, and bioactivities of C. paliurus polyphenols. Other potential functions of C. paliurus polyphenols are prospected. This review provides a reference for further research and applications of C. paliurus polyphenols in a field of functional food and medicines.

Effects of five extraction methods on total content, composition, and stability of flavonoids in jujube

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Five methods of extracting flavonoids from jujube were compared in different aspects.

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The extraction methods can significantly influence the flavonoid compositions.

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DES-UAE method had outstanding ability to maintain the stability of flavonoids.

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DES-UAE is an efficient and green method for extracting flavonoids from jujube.

The present study investigated the effects of different extraction methods including water-water bath (W-WB), ethanol-water bath (E-WB), deep eutectic solvent (DES) combined with ultrasound-assisted extraction (DES-UAE), microwave-assisted extraction (DES-MAE), and enzyme-assisted extraction (DES-EAE) on flavonoids (total flavonoid content, flavonoid composition, and stability) in jujube. The highest total flavonoid content of 8.03 mg/g was obtained by the DES-MAE extraction. Fifteen types of flavonoids were identified from jujube. The amount of rutin produced by the E-WB and DES-UAE methods was 66.88 ± 1.58 μg/g and 45.23 ± 3.22 μg/g, respectively. The retention of flavonoids in DES-UAE extracts were 98.15 ± 0.51%, 64.25 ± 2.21% after 2 h of high temperature treatment at 90 °C and 21 days of dark storage, respectively. The flavonoids extracted by different methods were suitable for dark storage under different light contrasts, where the retention of flavonoids extracted by DES-UAE method was 86.44 ± 2.45%. In conclusion, DES-UAE would be an efficient method for flavonoid extraction from jujube.

Buckwheat Hull-Enriched Pasta: Physicochemical and Sensory Properties

This work aimed to evaluate the effect of partial replacement of semolina with 0, 1, 5, 10, 15, and 20% of ground buckwheat hull (BH) on the chemical composition, antioxidant properties, color, cooking characteristics, and sensory properties of wheat pasta. Pasta samples were prepared by dough lamination (tagliatelle shape) and dried at 55 °C until the moisture content was 11–12% (wet basis). Analyses of samples showed that the addition of BH caused an increase in fiber content in pasta from 4.31% (control pasta) to 14.15% (pasta with 20% of BH). Moreover, the brightness and yellowness of BH-enriched products were significantly decreased compared to the control sample, and the total color difference ranged from 23.84 (pasta with 1% of BH) to 32.56 (pasta with 15% BH). In addition, a decrease in optimal cooking time, as well as an increased weight index and cooking loss, was observed in BH-enriched pasta samples. Furthermore, BH-enriched cooked pasta had significantly higher total phenolic content and antioxidant activity but an unpleasant smell and taste, especially if the level of BH was higher than 10%.

Phenolics in buckwheat hull extracts and their antioxidant activities on bulk oil and emulsions

Buckwheat hulls are discarded as waste, although they have more phenolic compounds than buckwheat groats. The antioxidant activities of buckwheat hull extracts prepared with water, 50% ethanol, and 100% ethanol were investigated in bulk oil, oil-in-water (O/W), and water-in-oil (W/O) emulsions. The relationship between the phenolic compositions of the extracts and their antioxidant activities in the three different lipid systems was also evaluated. Fifty percent ethanol extract had the highest total phenolic content (327 mg gallic acid equivalent [GAE]/g extract) followed by water and 100% ethanol extracts (211 and 163 mg GAE/g extract, respectively). The total oxidation rate (k) was not significantly different among the bulk oils added with the buckwheat hull extracts. However, in the O/W emulsion, the k was more reduced by the 50% and 100% ethanol extracts than by the water extract at the concentration of 100 µg GAE/g (2.9, 2.8, and 3.7 Totox/day, respectively). The k of the W/O emulsion was more reduced by the 100% ethanol extract than by the water and 50% ethanol extract at the concentration of 100 µg GAE/g (3.8, 4.7, and 4.5 Totox/day, respectively). Multivariate statistical analysis revealed that the contents of phenolic acids and their derivatives were the highest in the water extract among the extracts, while the contents of flavonoid glycosides and methylated polyphenols were the highest in the 50% and 100% ethanol extracts, respectively. The results suggest that flavonoid glycosides and methylated polyphenols could be potential candidates for retarding the oxidation of the emulsion system. PRACTICAL APPLICATION: Buckwheat hull extracts could retard lipid oxidation. Flavonoid glycosides and methylated polyphenols in buckwheat hull extracts may have an antioxidative effect on lipids. Thus, buckwheat hulls could be used as an antioxidant in lipid systems, as flavonoid glycosides and methylated polyphenols are properly extracted from buckwheat hulls.

Fractionated extraction of polyphenols from mate tea leaves using a combination of hydrophobic/ hydrophilic NADES

A new methodology for the selective extraction of antioxidants from mate tea leaves (and decaffeinated mate tea leaves), using different natural deep eutectic systems (NADES), is reported in this paper. A fractionated extraction was carried out and the optimization of the extraction conditions such as solid/liquid ratio, temperature, time, stirring and the use of ultrasound assisted extraction (UAE) technology was performed. The results demonstrate that a sequential extraction using, in a first step, an hydrophobic system Men:Lau (2:1) and, in a second step, an hydrophilic lactic acid-based NADES, leads to two distinct extracts: the first one rich in pigments and the second one rich in polyphenols. NADES systems were able to extract 30% more of the polyphenolic components of the mate tea leaves matrices, when compared with traditional solvents/techniques. Moreover, it has been shown that the incorporation of the extract in the NADES, compared to the same extract in aqueous medium was beneficial for the stabilization of the antioxidants. It maintains their functionality at least for three months, reaching 41% more versus the extracts obtained by traditional solvents/techniques. The absence of caffeine in the extracts did not shown to have any effects on the stability results.

Deep eutectic solvents microbial toxicity: Current state of art and critical evaluation of testing methods

Deep eutectic solvents (DESs) were described at the beginning of 21st century and they consist of a mixture of two or more solid components, which gives rise to a lower melting point compared to the starting materials. Over the years, DESs have proved to be a promising alternative to traditional organic solvents and ionic liquids (ILs) due to their low volatility, low inflammability, easy preparation, and usually low cost of compounds used in their preparation. All these properties encouraged researchers to use them in diverse fields and applications e.g., as extractants for biomolecules and solvents in pharmaceutical and cosmetic industries. Nevertheless, despite undeniable potential of DESs, there is still controversy about their toxicity. Besides the low number of studies on this topic, there are also some contradicting reports on biocompatibility of these solvents. Such misleading reports could be mainly attributed to the lack of well design standard protocol for DESs toxicity determination or the use of out-off-purpose methodology. Thus, to better apply DESs in green and sustainable chemistry, more studies on their impact on organisms at different trophic levels and the use of proper techniques are required. This review focuses on DESs toxicity towards microorganisms and is divided into three parts: The first part provides a brief general introduction to DESs, the second part discusses the methodologies used for assessment of DESs microbial toxicity and the obtained results, and finally in the third part the critical evaluation of the methods is provided, as well as suggestions and guidelines for future research.

Biodegradable Solvents: A Promising Tool to Recover Proteins from Microalgae

The world will face a significant protein demand in the next few decades, and due to the environmental concerns linked to animal protein, new sustainable protein sources must be found. In this regard, microalgae stand as an outstanding high-quality protein source. However, different steps are needed to separate the proteins from the microalgae biomass and other biocompounds. The protein recovery from the disrupted biomass is usually the bottleneck of the process, and it typically employs organic solvents or harsh conditions, which are both detrimental to protein stability and planet health. Different techniques and methods are applied for protein recovery from various matrices, such as precipitation, filtration, chromatography, electrophoresis, and solvent extraction. Those methods will be reviewed in this work, discussing their advantages, drawbacks, and applicability to the microalgae biorefinery process. Special attention will be paid to solvent extraction performed with ionic liquids (ILs) and deep eutectic solvents (DESs), which stand as promising solvents to perform efficient protein separations with reduced environmental costs compared to classical alternatives. Finally, several solvent recovery options will be analyzed to reuse the solvent employed and isolate the proteins from the solvent phase.

Ultrasound-Assisted Natural Deep Eutectic Solvent Extraction and Bioactivities of Flavonoids in Ampelopsis grossedentata Leaves

We performed ultrasound-assisted extraction coupled with natural deep eutectic solvents (NADES) to achieve the green and efficient preparation of flavonoid extract from Ampelopsis grossedentata leaves. We then evaluated its antioxidant and antiproliferative activities. A NADES consisting of choline chloride and glucose at a molar ratio of 4:1 with 20% water was determined to be the most suitable solvent. The optimal extraction conditions were: a liquid-to-solid ratio of 30 mL/g, an ultrasonication power of 490 W, and an ultrasonication time of 6.5 min. The actual flavonoid yield was 83.93%, which was close to the predicted yield. Further, 86.75% of the flavonoids were recovered by adding the same volume of phosphate buffer saline (100 mM, pH of 7.0) to the extract solution. Although the chemical antioxidant activities of the flavonoid extract were slightly inferior to those of dihydromyricetin, the flavonoid extract could still effectively inhibit the proliferation of human breast MDA-MB-231 cells by inducing cell apoptosis, retarding the cell cycle, changing the mitochondrial membrane potential and scavenging intracellular reactive oxygen species (ROS). The obtained results can provide a reference in the development of plant-derived functional foods.

L-Proline-Based Natural Deep Eutectic Solvents as Efficient Solvents and Catalysts for the Ultrasound-Assisted Synthesis of Aurones via Knoevenagel Condensation

Aurones are minor flavonoids that possess a wide variety of bioactivity, including antioxidant, anticancer, and enzyme inhibitory activity. L-proline-based natural deep eutectic solvents (NaDES) were synthesized and applied as solvents and catalysts for the Knoevenagel condensation reaction between benzofuranone and substituted benzaldehydes to produce aurones in high yields and purity. The reaction between benzofuranone and vanillin served as the model reaction. After screening three NaDESs, and testing microwave, as well as ultrasound as energy sources, we concluded that the optimum results are obtained using L-proline/glycerol 1:2 as catalyst and solvent and ultrasound irradiation. The scope of the reaction was evaluated using a variety of benzaldehydes, and the corresponding aurones were obtained in moderate to satisfactory yields (57–89%) and high purity. An important additional feature of the described methodology is the recyclability and reusability of the NaDES, which was recycled and effectively reused after 6 cycles.

Effects of Novel Extraction Strategies on the Recovery of Phenolic Compounds and Associated Antioxidant Properties from Buckwheat Hull (Fagopyrum esculentum)

This study investigated the effects of novel extraction technologies, including ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), pulsed electric field (PEF), high-pressure processing (HPP), enzyme-assisted extraction (EAE), and conventional extraction, on the recovery of phenolic compounds and associated antioxidant properties from buckwheat hull (Fagopyrum esculentum). Initially, twenty-four extraction strategies were investigated. Based on the results of the total phenolic content and antioxidant properties (DPPH and FRAP), twelve strategies (i.e., US (n = 2), PEF (n = 1), MW (n = 4), HPP (n = 4), and a control method) were selected for phenolic profiling carried out using liquid chromatography-mass spectrometry (LC-MS). Forty-one phenolic compounds were identified in the extracts, and a scanning electron microscope (SEM) analysis was also carried out on the treated residues to analyze the surface damage post-treatments. The results showed that samples treated with US (16.14 ± 0.06), PEF (9.94 ± 0.02), MW (12.63 ± 0.13), and HPP (21.76 ± 0.78) contained the highest total phenolic content (mg GAE/100 mg of DW). In the case of the antioxidant activities, the highest DPPH activities were obtained using HPP, MAE, and UAE, while no clear pattern was recorded in the case of FRAP activities. The highest DPPH and FRAP activities observed were 80.91 ± 0.22% and 23.98 ± 0.2 mg Trolox equivalents/100 mg, respectively. Additionally, the LC-MS results identified eleven different groups of phenolic compounds in buckwheat hull extracts, including anthocyanin, flavanol, flavanones, flavones, flavonol, phenolic acids, isoflavones, lignans, and quinones.

Natural Deep Eutectic Solvents as a Key Metal Extractant for Fractionation in Speciation Analysis

The research aimed to use natural deep eutectic solvents (NADES) as an extractant for fractionation of compounds of selected elements from young barley and to compare it with the fractionation of elements from certified element materials. The use of such a comparison made it possible to prove the possibility of extracting the same forms of elements (species) from different materials, which confirmed the option of using NADES as extractants in speciation analysis. The research was conducted with hyphenated techniques—separation by high-performance chromatography coupled to an isotope-specific detector, mass spectrometry (MS) with ionization in inductively coupled plasma (ICP)—which are widely used in speciation analysis. Natural deep eutectic solvents also help introduce Green Analytical Chemistry principles (GAC). According to the results of our studies, the use of different NADES permit the extraction of various metals from a single sample. Moreover, using other natural solvents of eutectic properties helps extract different species of a given metal.

A Green Method of Extracting and Recovering Flavonoids from Acanthopanax senticosus Using Deep Eutectic Solvents

In recent years, green extraction of bioactive compounds from herbal medicines has generated widespread interest. Deep eutectic solvents (DES) have widely replaced traditional organic solvents in the extraction process. In this study, the efficiencies of eight DESs in extracting flavonoids from Acanthopanax senticosus (AS) were compared. Response surface methodology (RSM) was employed to optimize the independent variable including ultrasonic power, water content, solid-liquid ratio, extraction temperature, and extraction time. DES composed of glycerol and levulinic acid (1:1) was chosen as the most suitable extraction medium. Optimal conditions were ultrasonic power of 500 W, water content of 28%, solid-liquid ratio of 1:18 g·mL⁻¹, extraction temperature of 55 °C, and extraction time of 73 min. The extraction yield of total flavonoids reached 23.928 ± 0.071 mg·g⁻¹, which was 40.7% higher compared with ultrasonic-assisted ethanol extraction. Macroporous resin (D-101, HPD-600, S-8 and AB-8) was used to recover flavonoids from extracts. The AB-8 resin showed higher adsorption/desorption performance, with a recovery rate of total flavonoids of up to 71.56 ± 0.256%. In addition, DES solvent could efficiently be reused twice. In summary, ultrasonic-assisted DES combined with the macroporous resin enrichment method is exceptionally effective in recovering flavonoids from AS, and provides a promising environmentally friendly and recyclable strategy for flavonoid extraction from natural plant sources.

Green extraction of apigenin and luteolin from celery seed using deep eutectic solvent

In recent years, deep eutectic solvent (DES) has attracted a great deal of attention as an environmentally friendly solvent and could be used as an alternative to conventional solvents. In this study, 34 choline and betaine-based deep eutectic solvents were prepared and investigated the ability to extract apigenin and luteolin from the celery seed. The results showed that DES composed of betaine hydrochloride and propylene glycol had the best extraction efficiency. Using one factor at a time combined with response surface methodology, the optimal conditions of extraction were determined as follows: time of extraction: 19 min, extraction temperature: 75 °C, the water content in solvent: 40% (w/w). Antisolvent (water) combined with distillation was proposed as an efficient method to recover apigenin and luteolin from the DES extract. The relationship between components of DES and solubility of apigenin, luteolin was also established which was the starting point for the prediction of extractability of DES. Molecular dynamics revealed that betaine hydrochloride and propylene glycol could interact with each other by the formation of two types of hydrogen bond while water molecules might play an important role in the ability to dissolve apigenin and luteolin of DES.

Magnetic deep eutectic solvents: formation and properties

Deep eutectic solvents (DESs) have been well-known as novel solvents due to their unique properties, which are dispensable for the development of green chemistry in future. The CoCl2·6H2O and NiCl2·6H2O-based...

A comprehensive review on current and emerging technologies toward the valorization of bio-based wastes and by products from foods

Industries in the agro-food sector are the largest generators of waste in the world. Agro-food wastes and by products originate from the natural process of senescence, pretreatment, handling, and manufacturing processes of food and beverage products. Notably, most of the wastes are produced with the transformation of raw materials (such as fruits, vegetables, plants, tubers, cereals, and dairy products) into different processed foods (e.g., jams, sauces, and canned fruits/vegetables), dairy derivatives (e.g., cheese and yogurt), and alcoholic (e.g., wine and beer) and nonalcoholic beverages (e.g., juices and soft drinks). Current research is committed not only to the usage of agro-food wastes and by products as a potential source of high-value bioactive compounds (e.g., phenolic compounds, anthocyanins, and organic acids) but also to the implementation of emerging and innovative technologies that can compete with conventional extraction methods for the efficient extraction of such biomolecules from the residues. Herein, specific valorization technologies, such as membrane-based processes, microwave, ultrasound, pulsed electric-assisted extraction, supercritical/subcritical fluids, and pressurized liquids, have emerged as advanced techniques in extracting various added-value biomolecules, showing multiple advantages (improved extraction yields, reduced process time, and protection to the bioactive properties of the compounds). Hence, this comprehensive review aims to analyze the ongoing research on applying such techniques in valorization protocols. A last-five-year review, together with a featured analysis of the relevant findings in the field, is provided.

Application of Extraction and Determination Based on Deep Eutectic Solvents in Different Types of Environmental Samples

Water sources are an indispensable resource for human survival. Monitoring the pollution status of the surrounding environment is necessary to protect water sources. Research on the environmental matrix of deep eutectic solvents (DESs) has expanded rapidly because of their high extraction efficiency for various target analytes, controllable synthesis, and versatile structure. Following the synthesis of hydrophobic deep eutectic solvents (HDESs), their application in aqueous matrices broadened greatly. The present review conducted a survey on the pollutant extraction methods based DESs in environmental matrices from two aspects, application methods and matrix types; discussed the potential risk of DESs to the environment and future development trends; and provided some references for researchers to choose DES-based extraction methods for environmental research.

Physicochemical Properties of Choline Chloride-Based Natural Deep Eutectic Solvents (NaDES) and Their Applicability for Extracting Oil Palm Flavonoids

Oil palm (Elaeis guineensis Jacq.) leaf (OPL) is abundantly generated from oil palm plantations as biomass that is rich in bioactive metabolites, primarily flavonoids. Six natural deep eutectic solvents (NaDES) were synthesized using a direct heating technique from different combinations of choline chloride with 1,2-propanediol (PD), 1,4-butanediol (BD), glycerol (GLY), glucose (GLU), maltose (MAL) and lactic acid (LA). The synthesized NaDES were subjected to physicochemical and biological evaluations comprising physical appearance, density, water activity, viscosity, polarity, thermal behaviors, spectroscopic analysis, cytotoxicity, radical scavenging activities and solubility tests. Compared to aqueous methanol, the synthesized NaDES, which appeared as a slightly to moderately viscous transparent liquid, showed favorable physicochemical properties as extraction solvents with a low cytotoxicity profile on cultured fibroblast cells. Further, the NaDES obtained from the choline chloride:lactic acid (LA) combination showed high free radical scavenging characteristics. Hydrogen bonding interactions were shown to play a significant role in the formation of the NaDES. Further, ultra-high-performance liquid chromatography ultraviolet/photodiode array (UHPLC-UV/PDA) analysis revealed that the NaDES from the choline chloride:glycerol (GLY) combination had comparable efficiencies with aqueous methanol regarding extracting flavonoids (luteolin and apigenin derivatives) from OPLs. The results of the present study suggested that the tailor-made NaDES were not only easy-to-use, stable and safe solvents but also suitable for extracting bioactive phytochemical compounds. The study highlighted their potential as an alternative green technology for applications in oil palm biomass utilization programs.

Deep Eutectic Solvents Application in Food Analysis

Current trends in Analytical Chemistry are focused on the development of more sustainable and environmentally friendly procedures. However, and despite technological advances at the instrumental level having played a very important role in the greenness of the new methods, there is still work to be done regarding the sample preparation stage. In this sense, the implementation of new materials and solvents has been a great step towards the development of "greener" analytical methodologies. In particular, the application of deep eutectic solvents (DESs) has aroused great interest in recent years in this regard, as a consequence of their excellent physicochemical properties, general low toxicity, and high biodegradability if they are compared with classical organic solvents. Furthermore, the inclusion of DESs based on natural products (natural DESs, NADESs) has led to a notable increase in the popularity of this new generation of solvents in extraction techniques. This review article focuses on providing an overview of the applications and limitations of DESs in solvent-based extraction techniques for food analysis, paying especial attention to their hydrophobic or hydrophilic nature, which is one of the main factors affecting the extraction procedure, becoming even more important when such complex matrices are studied.

Integration of Choline Chloride-Based Natural Deep Eutectic Solvents and Macroporous Resin for Green Production of Enriched Oil Palm Flavonoids as Natural Wound Healing Agents

Huge quantities of oil palm (Elaeis guineensis Jacq.) leaves (OPL) are generated as agricultural biomass from oil palm plantations. OPL are known to contain significant amounts of flavonoids. For maximal exploitation of these valuable antioxidant compounds, an innovative and sustainable extraction method employing natural deep eutectic solvents (NaDES) combined with ultrasonic assisted extraction was developed. Various NaDES composed of choline chloride as the hydrogen bond donor (HBD) and 1,2 propanediol (PD), 1,4 butanediol (BD), glycerol (GLY), glucose (GLU), maltose (MAL), and lactic acid (LA) as the hydrogen bond acceptor (HBA) were synthesized. The influence of these compositions, the methods of their synthesis, molar ratios, and water contents on their capacity to extract flavonoids from OPL was evaluated. Based on the results, it was found that methods which incorporate a heating step produced NaDES with the best capacity to extract OPL flavonoids. These thermal methods combined with molar ratios of 1:3 or 1:4 and water contents of 17 to 50% were found to be the optimal conditions for preparing NaDES, specifically when applied to the PD, BD, and GLY NaDES. Subsequently, UHPLC-UV/PDA-MS/MS analysis revealed NaDES extracts recovered by macroporous adsorption resin XAD7HP were able to optimally extract at least twelve luteolin and apigenin derivatives in OPL NaDES extracts prepared from glycerol and 1,4-butanediol demonstrated better and comparable efficiency as aqueous methanol in extracting flavonoids from OPL. The in vitro studies of antioxidant and wound healing properties supported these findings by exhibiting good free radical scavenging, cell proliferation, and migration activities. Additionally, the NaDES extracts also showed non-cytotoxicity effects at 1000 µg/mL and below on 3T3 fibroblast cells. Results of the study showed that NaDES could be a promising eco-friendly green solvent to extract bioactive OPL flavonoids that have great potential for applications as wound healing agents.

Current Progress in Natural Deep Eutectic Solvents for the Extraction of Active Components from Plants

In the last decade, natural deep eutectic solvents (NADESs) have gained more and more attention due to their green, convenient preparation, low toxicity and biodegradability. It is widely used in various fields, especially in the extraction of active components from plants, formed by the combination of hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) at a certain condition. In this article, six preparation methods of NADESs were summarized and the interactions that occur in the eutectic behavior of NADES including hydrogen bonding, electrostatic interaction and van der Waals force were also reviewed. What is more, its significant extraction capacity on flavonoids, phenols, alkaloids and plant pigments endows its extensive applications in the extraction of active components from medicinal plants. Extraction factors including solvents properties (viscosity, carbon chain length, number of hydroxyl groups), extraction condition (water content, extraction temperature, extraction time, solid-liquid ratio), extraction method and recycling method were discussed. In addition, NADESs can also be combined with other technologies, like molecular imprinting, monolithic column, to achieve efficient and specific extraction of active ingredients. Further systematic studies on the biodegradability and biotoxicity are put forward to be urgent.

The evaluation of phenolic content, in vitro antioxidant and antibacterial activity of Mentha piperita extracts obtained by natural deep eutectic solvents

The focus of this study was to evaluate whether six choline chloride-based natural deep eutectic solvents (NADES) could serve as solvents for the extraction of bioactives from the leaves of Mentha piperita. NADES extracted significantly higher amounts of phenols from peppermint than 70% ethanol and may be useful in the extraction of targeted major compounds from peppermint, like rosmarinic acid, at a similar level as 70% ethanol. The microdilution method for in vitro antibacterial activity showed that all NADES exhibit bacterial growth inhibition at a lower concentration than 70% ethanol, especially NADESs containing organic acids. The majority of NADES extracts neutralize DPPH radical at a lower concentration than conventional solvent and showed similar ability to reduce Fe³⁺ to Fe²⁺ ions in FRAP assay. NADES can be useful in the isolation of phenolic compounds from plant sources and should be considered as novel, sustainable, and low-cost solvents with a variety of applications.

Deep Eutectic Solvents for the Extraction of Bioactive Compounds from Natural Sources and Agricultural By-Products

In this work, a review about the applicability of eutectic solvents, mainly deep eutectic solvents (DES) and natural deep eutectic solvents (NADES), for the extraction of bioactive compounds from natural products has been carried out. These alternative solvents have shown not only to have high extraction yields but also to be environmentally friendly, exhibiting very low or almost no toxicity, compared to conventional organic solvents. The last trends and main extraction methods that have been most widely used in studies using these emerging solvents have been reviewed, as well as the varied natural sources in which they have been used, including agro-food by-products. Besides the toxicity, biodegradability of these solvents is reviewed. Likewise, different reported bioactivity tests have been included, in which extracts obtained with these ecological solvents have been tested from antioxidant activity analysis to in vivo studies with rats, through in vitro cytotoxicity tests.

Theoretical insights into the cineole-based deep eutectic solvents

Deep eutectic solvents based on cineole as hydrogen bond acceptors and organic acids (succinic, malic, and lactic) as hydrogen bond donors are studied using a theoretical approach. The nature, strength, and extension of hydrogen bonding are analyzed, thus quantifying this prevailing interaction and its role in the fluid properties. Density functional theory was used to study small molecular clusters, and the topological characterization of the intermolecular forces was carried out using atoms in a molecule theory. Classical molecular dynamics simulations were considered to study nanoscopic bulk liquid properties and their relationship with relevant macroscopic properties such as density or thermal expansion. The reported results provide the characterization of environmentally friendly deep eutectic solvents and show the suitability of cineole for developing these sustainable materials.

A review of plant metabolites with metal interaction capacity: a green approach for industrial applications

Rapid industrial development is responsible for severe problems related to environmental pollution. Many human and industrial activities require different metals and, as a result, great amounts of metals/heavy metals are discharged into the water and soil making them dangerous for both human and ecosystems and this is being aggravated by intensive demand and utilization. In addition, compounds with metal binding capacities are needed to be used for several purposes including in activities related to the removal and/or recovery of metals from effluents and soils, as metals' corrosion inhibitors, in the synthesis of metallic nanoparticles and as metal related pharmaceuticals, preferably a with minimum risks associated to the environment. Plants are able to synthesize an uncountable number of compounds with numerous functions, including compounds with metal binding capabilities. In fact, some of the plants' secondary metabolites can bind to various metals through different mechanisms, as such they are excellent sources of such compounds due to their high availability and vast diversity. In addition, the use of plant-based compounds is desirable from an environmental and economical point of view, thus being potential candidates for utilization in different industrial activities, replacing conventional physiochemical methods. This review focuses on the ability of some classes of compounds that can be found in relatively high concentrations in plants, having good metal binding capacities and thus with potential utilization in metal based industrial activities and that can be involved in the progressive development of new environmentally friendly strategies.

Green and efficient removal of heavy metals from Porphyra haitanensis using natural deep eutectic solvents

BACKGROUND

Porphyra haitanensis now faces serious heavy metal pollution problems. Natural deep eutectic solvents (NADESs) have been recognized as a novel class of sustainable solvents, which can be used for heavy metal removal. In this study, 28 kinds of NADESs were prepared and investigated as eluent in the removal of lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), and copper (Cu) from P. haitanensis for the first time, and the adsorption mechanism of NADESs was also studied.

RESULTS

The removals were greatly improved by NADESs compared with control where the removal rates of Pb, Cd, Cr, As and Cu were 17.4-87.54%, 57.54-100%, 9.8-48.59%, 21.32-78.24% and 11.68-79.73%, respectively. The optimal condition was 10% water content and solid-liquid ratio of 1:20. Moreover, the addition of 20% natural surfactant arabic gum can further increase the heavy metals removal rates of NADESs. The adsorption mechanism experiments showed that the pseudo second-order model and the Freundlich adsorption model can better explain the adsorption mechanism of NADESs on heavy metals removal.

CONCLUSION

Taken together, a green and efficient method for removing heavy metals from P. haitanensis was established. © 2020 Society of Chemical Industry.

The Role of Hydrogen Bond Donor on the Extraction of Phenolic Compounds from Natural Matrices Using Deep Eutectic Systems

Recently, deep eutectic systems (DESs) as extraction techniques for bioactive compounds have surfaced as a greener alternative to common organic solvents. In order to study the effect of these systems on the extraction of phenolic compounds from different natural sources, a comprehensive review of the state of the art was carried out. In a first approach, the addition of water to these systems and its effect on DES physicochemical properties such as polarity, viscosity, and acidity was investigated. This review studied the effect of the hydrogen bond donor (HBD) on the nature of the extracted phenolics. The effects of the nature of the HBD, namely carbon chain length as well as the number of hydroxyl, methyl, and carbonyl groups, have shown to play a critical role in the extraction of different phenolic compounds. This review highlights the differences between DES systems and systematizes the results published in the literature, so that a more comprehensive evaluation of the systems can be carried out before any experimental trial.