Characterization of Bioactive Compounds and Antioxidant Activity of Plants—Editorial Comments and Summary

The biological potential of plant objects depends on the active compounds they contain. These compounds have benefits for the pharmaceutical, nutraceutical, and functional food industries. Bioactive plant compounds have a pronounced pharmacological effect that is often superior to that of synthetic drugs, with low toxicity and minimal side effects. The antioxidant activity of bioactive compounds is an important aspect of their biological activity. The total antioxidant effect of such compounds is characterized by the presence of various forms of nafiveural substances and their combined action, manifested in the formation of effective redox systems and synergistic cycles. Given the special interest in the investigation of bioactive compounds and their antioxidant activity, the identification of promising plant objects is a potentially rewarding task for researchers.

This Special Issue aimed to cover all research aspects related to medicinal, aromatic, and edible plant objects; characterize the investigated plant extracts, including methods for their extraction, purification, comprehensive profiling characterization; and perform quantification and elucidation of their mechanisms of action with a focus on antioxidant activity. A total of five papers (all research papers) are presented in this Special Issue. The contributors provide novel and comprehensive insights into the chemical composition of medicinal plants, as well as those with potential antioxidant properties.

In their study, Lin et al. [1] evaluated the utility of using extracts of different parts of Chenopodium formosanum Koidz, an indigenous cereal plant of Taiwan, i.e., the leaves, stems, and unhulled and hulled seeds, since most existing studies focused on the extract of C. formosanum seeds. To select objects for further study, the authors prepared water, ethanolic, methanolic, and ethyl acetate extracts of the four parts of C. formosanum. After evaluating their biological activities, unfermented and fermented C. formosanum leaf extracts were selected for further analysis. The authors’ data revealed that the combination of extraction and fermentation considerably improved the antioxidative, anti-inflammatory, antimicrobial, skin-whitening, antiaging, and moisturizing activities of C. formosanum leaves. The C. formosanum leaf extract demonstrated better efficiency and a significantly lower cytotoxicity than the seed extract. The authors also realized a molecular docking analysis of C. formosanum leaf extract, which showed that quercetin expressed a skin-whitening effect while epicatechin possessed the highest antiaging activity. Thus, the work performed demonstrated that fermented C. formosanum leaf methanolic extract can be useful for use in functional products, botanical drugs, and cosmetics.

An original study by Alzubaidi et al. [2] investigated the biosynthesis of silver nanoparticles using ethanolic flaxseed (Linum usitatissimum L.) extract as an efficient reducing factor. The authors proved the production of silver nanoparticles through color-shifting observation of the mixture of silver nitrate turning from yellow to a reddish suspension after the addition of the extract and evaluating it with UV–visible control. Moreover, the FTIR spectrum was used to determine functional groups. Additionally, the authors evaluated the morphology and structure of silver nanoparticles using a scanning electron microscopy and X-ray diffraction examinations, which revealed spherical silver nanoparticles with a diameter of 46.98 nm and a crystalline structure. The antibacterial activity of silver nanoparticles was investigated against Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, and Streptococcus pyogenes, while the antioxidant effect was investigated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) technique. According to the obtained results, flaxseed extract demonstrated promising antioxidant activity as well as antibacterial activity against both Gram-positive and Gram-negative bacteria.

Another important study was conducted by Razuvaeva et al. [3] on the neuroprotective, energy-protective, and antioxidant effects of Klasea centauroides (L.) Cass. (Serratula centauroides L.) extract in cholinergic deficiency caused by long-term scopolamine administration. The authors stated that K. centauroides extract accelerated passive-avoidance-conditioned reflex development and ensured preservation over a longer time period under cholinergic deficiency conditions. K. centauroides extract increased the resistance of brain tissues to the toxic effects of scopolamine, reducing the number of neuron regressive forms in the cerebral cortex and hippocampus. Moreover, the authors found that the use of K. centauroides extract reduced the malonic dialdehyde content in brain structures and increased the catalase and antioxidant system glutathione unit activities. These findings are important because they support existing recommendations for the prevention of cognitive impairments with herbal remedies.

The antioxidant potential of Ficus umbellata Vahl water and methanolic extracts was assayed by Silihe et al. [4] using DPPH and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid assays, total antioxidant capacity, and ferrous reducing power. The authors revealed that at tested concentrations, F. umbellata extracts demonstrated strong radical scavenging activity that was dose- and time-dependent, as well as total antioxidant capacity and ferrous ions reducing power. Furthermore, the anti-inflammatory protective effects of the same extracts were determined in Wistar rats with carrageenan-induced paw oedema, and it was found that methanolic extract of F. umbellata at doses of 50 and 200 mg/kg had antiedematous properties with oedema inhibition percentages of 71.16% and 72.98%, respectively. F. umbellata extracts also manifested protective effects against the hemolysis of murine erythrocyte membranes induced by heat and denaturation of proteins at concentrations ranging from 0.125 to 2 mg/mL.

Finally, a study on the effect of pre-treating Arabica coffee beans with cold atmospheric plasma, microwave radiation, and slow and fast freezing on the antioxidant activity of aqueous coffee extract was conducted by Tarasov et al. [5]. The authors measured the moisture content of coffee beans and antioxidant activity of aqueous extracts. It was stated that green coffee showed a decrease in moisture content after microwave treatment, and roasted coffee showed an increase in moisture content after freezing. Moreover, the antioxidant activity of all extract samples increased by 4.1–17.2% after slow freezing and fast freezing at −19 °C for 24 h. The authors noted that microwave radiation treatment at 800 W for 60 s increased the antioxidant activity of green coffee extracts by 5.7%, while the changes in the antioxidant activity of roasted coffee extracts were insignificant. As a result of cold atmospheric plasma treatment with dielectric barrier discharge parameters of 1 μs, 15 kV and 200 Hz for 5 and 15 min, green coffee showed a decrease in the antioxidant activity of the extract by 3.8% and 9.7%, respectively, while the changes in the antioxidant activity of roasted coffee extracts were insignificant. The authors propose using the results obtained for slow and fast freezing, microwave radiation, and combined treatments at the pre-extraction stage of coffee bean preparation to increase the yield of antioxidant extractives.

Although submissions for this Special Issue have been closed, more in-depth research into the antioxidant activity of plants continues to address the challenge of identifying plants for the development of new medicines.