Consumers associate the color of food with its freshness and quality. More and more attention is being paid to natural colorants that bring additional health benefits to humans. Such natural substances are the carotenoids (yellow to orange), the anthocyanins (red to blue), and the betalains (red and yellow), which are very sensitive to exposure to light, air, high temperatures, and chemicals. Stability and diversity in terms of color can be optimized by using environmentally friendly and selective extraction processes that provide a balance between efficacy, safety, and stability of the resulting extracts. Green solvents like water, supercritical fluids, natural deep eutectic solvents, and ionic liquids are the most proper green solvents when combined with different extraction techniques like maceration, supercritical extraction, and ultrasound-assisted or microwave-assisted extraction. The choice of the right extracting agent is crucial for the selectivity of the extraction method and the stability of the prepared colorant. The present work reviews the green solvents used for the extraction of natural food colorants from plants and focuses on the issues related to the selectivity and stability of the products extracted.

Recently, there has been a growing interest in protein-rich foods and functional foods, as well as plants characterized by high levels of drought resistance. Vigna unguiculata L. Walp. from the Fabaceae family, also known as cowpea, was chosen as the subject of the present study. The antioxidant potentials of 15 genotypes cultivated in Bulgaria, Southern Europe, were determined. The research focused on the green mass used as feed and on the seeds used as an “ancient” traditional crop. The total phenolic content (TPC), total flavonoid content (TFC), radical scavenging capacity, determined via the DPPH method, and crude protein content were measured. The seed coat extracts showed higher contents of phenolics (291.0 ± 4.6 mgGAE/g) and flavonoids (83.5 ± 1.1 mgCE/g) and a stronger radical scavenging capacity (50.4 ± 0.7 µmolTE/g) than those obtained from the leaves (22.0 ± 0.5 mgGAE/g, 13.7 ± 0.5 mgCE/g, and 19.7 ± 0.6 µmolTE/g, respectively). The protein content in the seeds ranged from 20.5% to 27.0%. According to the obtained results, the cowpea genotypes with dark-colored seed coats showed greater antioxidant potentials but lower protein contents. Due to its high antioxidant content, strong radical scavenging capacity, and high protein content, V. unguiculata L. shows great potential as a functional food for humans and animals.

Malva sylvestris L. is a herbaceous plant, distributed worldwide, rich in biological active compounds, and known for its health benefits. In this study, extracts from different parts (leaves, flowers, and roots) of this plant were prepared using green classic (70% ethanol) and natural deep eutectic solvents (NADESs) based on choline chloride and acetic acid (NADES1) or glycerol (NADES2). Their antioxidant, antibacterial (against B. cereus, S, aureus, E. coli, and P. aeruginosa), and antifungal activity (against P. chrysogenum, F. oxysporum, A. parasiticus, A. flavus, A. niger A. carbonarius, and A. ochraceus) were compared. Ethanolic extracts were characterized with the highest total contents of phenols, flavonoids, and condensed tannins. Ethanolic and NADES flower extracts were the richest in the antioxidants tested. Alkaloids were extracted in low quantities. The experimentally determined antioxidant potential of the extracts proved the highest DPPH scavenging activity of ethanolic extracts and the lowest of NADES1 extracts. The ABTS scavenging capacity of NADES1 and ethanolic extracts displayed comparable results, while NADES2 extracts were characterized as having the highest FRAP activity. NADES1 extracts manifested pronounced antibacterial activity, partially due to the low pH of the pure solvent, as well as inconsistent antifungal activity—from moderate to a complete lack of activity. A strong positive correlation was reported between the DPPH radical scavenging capacity and phenolic compound content. Future detailed investigations on the mechanism of the antimicrobial activity of NADES1 extracts are necessary to clarify the observed phenomenon of the decreased antifungal potential of NADES1 extracts compared to the pure solvent NADES1.

Alkanna tinctoria L. Tausch (“alkanet” or “alkanna”) is a medicinal plant. Extracts from alkanet roots have applications as natural food coloring agents. In addition, they have proven antioxidant effects. Three classical solvents (ethanol and acidified ethanol/water) and four natural eutectic solvents (NADES)—choline chloride/urea; choline chloride/citric acid; choline chloride/lactic acid; and sodium acetate/formic acid—were compared for their effectiveness as “green” solvents for the extraction of the alkanet pigment. Notably, this study is the first to apply choline chloride-based NADESs for alkanet extraction, providing a comprehensive profile of key bioactive compounds and their contributions to antioxidant activity using UV/Vis and FT-IR spectrometry, GC-MS, and HPLC-PDA-MS. Among the classical solvents, 50% acidified ethanol showed the highest extraction capacity, as indicated by its total flavonoid (708 ± 32 mgCE/L) and total phenolic (1318 ± 63 mgGAE/L) content. However, this extract exhibited the highest total alkaloid content (256 ± 15 µg/L) compared to the other classical extraction solvents. Consequently, absolute ethanol was identified as a more suitable alternative. Among the NADES, the sodium acetate/formic acid (1:2 mol/mol, NADES4) extract was the only one to show the presence of alkannins. This extract also contained high levels of phenols (355 ± 21 mg GAE/L) and tannins (163 ± 10 mg CE/L), exhibited strong antioxidant potential (DPPH: 131 µmol TE/g dw, FRAP: 7.49 mg Fe(II)/mg dw), and contained significantly lower alkaloid levels (7.0 ± 0.5 µg/L). Comparative analyses indicated that the sodium acetate/formic acid extract outperformed those prepared with ethanolic solvents and other NADES.

Due to its abundant content of biologically active compounds, Althaea officinalis L. (marshmallow) has been extensively researched and applied in both the food and pharmaceutical industries. This study aimed to evaluate hydroethanolic and natural deep eutectic solvent (NADES) extracts from leaves, flowers, and roots in terms of their chemical composition and biological activities. Extracts were characterized using chromatographic and spectrophotometric methods. Phytochemical profiling by HPLC-MS revealed a diverse composition, with 35 secondary metabolites identified or tentatively assigned, mainly hydroxycinnamic acid derivatives and flavonoid glycosides. GC-MS analysis of the ethanol extracts identified ten free amino acids, seven organic acids, several mono- and disaccharides, and one oligosaccharide. Their concentrations varied across different parts of the plant depending on the specific metabolism of the respective organ. Ethanolic extracts showed the highest total phenolic content (up to 176 mg GAE/L), while flower extracts exhibited the strongest antioxidant activity (DPPH up to 89 µmol TE/L). The antimicrobial potential of the extracts was assessed by the agar well diffusion method. NADES1 extracts demonstrated significant antibacterial activity, with inhibition zones reaching up to ~34 mm, whereas NADES2 extracts were largely inactive. In contrast, antifungal activity was negligible or absent across all samples.

The medicinal plant Malva neglecta Wallr. is known for its high concentration of beneficial bioactive compounds. This study investigated extracts prepared from the plant’s flowers, leaves, and roots. Different green solvents were used: 70% ethanol, and for the first time in relation to this plant species, natural deep eutectic solvents (NADES)—one based on choline chloride and citric acid (NADES1) and another using choline chloride and glycerol (NADES2). Key bioactive compounds were identified and quantified using spectrophotometric assays and HPLC-PDA-MS profiling to determine their role in the plant’s antioxidant activity. The analysis revealed that M. neglecta contains a wide range of flavonoid glycosides and phenolic acids, with the flowers and leaves exhibiting the highest diversity and concentrations of these compounds with a predominance of quercetin and kaempferol glycosides. Among the solvents tested, the ethanolic extracts showed the highest total contents of phenols, flavonoids, and condensed tannins. The flower extracts—regardless of the solvent used—exhibited the strongest antioxidant activity, as demonstrated by the DPPH, FRAP, and ABTS assays. Alkaloids were detected in all organs tested only in low quantities. The antibacterial (against Bacillus cereus, Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa) and antifungal activity (against Penicillium chrysogenum, Fusarium oxysporum, Aspergillus parasiticus, A. carbonarius, A. niger, A. flavus, and A. ochraceus) of the extracts was evaluated and compared. As a whole, the NADES1 extracts exhibited higher antibacterial potential than the ethanolic extracts. Such a clear trend regarding the antifungal activity was not observed. The highest antifungal activity was exhibited by NADES1 root extracts. NADES2 extracts showed a complete lack of antimicrobial effects.