1. Introduction
Many antioxidant compounds can be found in fruits and vegetables including phenolics, carotenoids, anthocyanins, and tocopherols [
1]. Approximately 20% of known plants have been used in pharmaceutical studies, impacting the healthcare system in positive ways such as treating cancer and harmful diseases [
2]. Plants are able to produce a large number of diverse bioactive compounds. High concentrations of phytochemicals, which may protect against free radical damage, accumulate in fruits and vegetables [
3]. Plants containing beneficial phytochemicals may supplement the needs of the human body by acting as natural antioxidants [
4]. Various studies have shown that many plants are rich source of antioxidants. For instance, vitamins A, C, E, and phenolic compounds such as flavonoids, tannins, and lignins, found in plants, all act as antioxidants [
3]. The consumption of fruits and vegetables has been linked with several health benefits, a result of medicinal properties and high nutritional value [
5]. Antioxidants control and reduce the oxidative damage in foods by delaying or inhibiting oxidation caused by reactive oxygen species (ROS), ultimately increasing the shelf-life and quality of these foods [
6]. Beta carotene, ascorbic acid, and many phenolics play dynamic roles in delaying aging, reducing inflammation, and preventing certain cancers [
7]. Increasing the consumption of fruits and vegetables has been recommended by many agencies and health care systems throughout the world [
8].
The objective of this paper is to provide a review of phytochemical studies that have addressed extracting, measuring and identifying bioactive compounds of plants. This review includes an overview of the lipid oxidation process, details of plants known to be antioxidant and antimicrobial sources, phenolic compounds, antioxidants from vegetables and fruits, cancer prevention, extraction techniques for phenolic compounds, isolation and purification of bioactive molecules, and techniques for structural classification of bioactive molecules.
3. Lipid Oxidation
Lipid oxidation can occur during the processing, shipping, and storing of many foods. Lipids (such as triglycerides, sterols, and phospholipids) readily become oxidized with exposure to an oxidative environment [
41]. Lipid molecules, especially those carrying polyunsaturated double bonds (i.e., linolenic acids), readily undergo oxidation within foods. Oxidatively stable oil with high melting temperature is necessary for solid fat application, and thus, highly saturated seed oil (palmitic acid and stearic acid) would be suitable for this end use [
42]. Soybeans provide 56% of the world’s oilseed production. However, the percentage of saturated oil is very low in seed plants (about 10%), if compared to unsaturated oil (about 90%) [
43]. Palmitic acid improves the oxidative stability of soybean oil, and can also be used to produce
trans-fat-free shortening, margarine, and cosmetic products. However, this saturated short-chain fatty acid is undesirable for nutrition because its consumption results in an unfavorable lipoprotein profile in blood serum [
44]. Stearic acid does not exhibit these cholesterolemic effects on human health [
45]. Stearic acid is less likely to be incorporated into cholesterol esters and has a neutral effect on the concentration of blood serum LDL cholesterol [
46,
47]. Extensive research has been performed in order to increase stearic acid content oil production in the most widely consumed legume crop in the world, soybeans. By employing induced mutagenesis, seed stearic acid content was increased by up to 7 times [
48].
Lipid oxidation in food systems can be caused by oxygen free radicals or reactive oxygen species. Free radicals are molecules with one or more unpaired electrons that work independently to cause oxidation [
49]. Reactive oxygen species are a perfect example of oxygen free radicals. Reactive oxygen species do not solely contain free radical molecules, but also some non-free radicals that can influence lipid oxidation. Examples of non-free radical reactive oxygen species are hydrogen peroxide (H
2O
2), hydrochloric acid (HCl), ozone (O
3), and molecular oxygen (O
2) [
42]. Molecular oxygen can react with linoleic acid about 1450 times faster than triplet oxygen. One of the major causes of oil rancidity is molecular oxygen. Lipid oxidation caused by the chain reaction of free radicals can be illustrated in three stages: initiation, propagation, and termination [
42]:
- (1)
Initiation:
RH + initiator → R
ROOH + initiator → ROO•
- (2)
Propagation:
R + O2 → ROO
ROO + RH → ROOH + R•
- (3)
Termination:
R + R → R-R
ROO• + R → ROOH
The processes above occur in response to several physical or chemical factors including heating, radiation, temperature, metal ion catalysts, reactive oxygen species, and photosensitizers such as chlorophyll. The initiation step, shown in Equation (1), often happens at either an allytic methylene group of an unsaturated fatty acid (RH) or a lipid-hydroperoxide (ROOH). Next, the generated free radical (R•) reacts with oxygen to form a peroxy radical (ROO•). This product can directly react with another lipid molecule to produce a lipid hydroperoxide (ROOH), and thus a lipid free radical (R•). This causes continuously cascading chain reactions to occur until the free radicals are neutralized by other free radicals. This whole stage is shown in Equation (2). In the termination step, there are two radicals that have converted into non-free radical products, and thus will stop the cascade mode of the chain reaction according to Equation (3). Moreover, the reaction chain can also be terminated by some antioxidants or free radical scavengers. Metal ions, especially those of iron and copper, effectively catalyze these reactions [
50].
Lipoxygenases (EC 1.13.11) can also act, causing oxidation to produce the peroxides in food materials that contain lipids. Hydrogen peroxide is one of the primary products of the oxidation, and it is very unstable and easily converts into secondary products. The final product of oxidation may include different chemical groups such as aldehydes, ketones, alcohols, acids, or hydrocarbons. These kinds of compounds can have a negative effect on the appearance, quality, and edibility of a food product by changing the texture, color, flavor, and safety of foods, or also by producing unacceptable off odors or off tastes, even negatively affecting the nutritional value [
50].
4. Plants as a Source of Antioxidants
Antioxidants can be defined as bioactive compounds that inhibit or delay the oxidation of molecules [
42]. Antioxidants are categorized as natural or synthetic antioxidants. Some synthetic antioxidants commonly used are: BHT, BHA, propyl gallate, and tertbutylhydroquinine. Many scientists have concerns about safety because synthetic antioxidants have recently been shown to cause health problems such as liver damage, due to their toxicity and carcinogenicity. Therefore, the development of safer antioxidants from natural sources has increased, and plants have been used as a good source of traditional medicines to treat different diseases. Many of these medicinal plants are indeed good sources of phytochemicals that possess antioxidant activities. Some typical examples of common ingredients that have been used in ethnic foods are tamarind, cardamom, lemon grass, and galangal basil. These spices or herbs have been shown to contain antioxidants [
51].
Deterioration of food due to either bacterial or fungal infection has always been a major concern, causing huge losses to food industries and societies throughout the world [
51]. Moreover, the spread of food pathogens has become a major public health concern. With an increasing awareness of the negative effects of synthetic preservatives, there has been increased demand for the use of nontoxic, natural preservatives, many of which are likely to have either antioxidant or antimicrobial activities [
52,
53]. Herbs have always been used for flavor and fragrance in the food industry, and some of them have been found to exhibit antimicrobial properties [
54]. Therefore, the call for screening and using plant materials for their antioxidant and antimicrobial properties has increased. Approximately 20% of all plant species have been tested in both pharmacological and biological applications to confirm their safety and advantages [
3]. A summary of the types of compounds, plant species, plant parts from which compounds were extracted, etc. can be found in
Table S1.
4.1. Presence of Antioxidant in Red Algae
Red algae are aquatic plant species considered one of the oldest groups of eukaryotic algae [
55]. The antioxidant activity of a red alga,
Palmaria palmate, has been studied. The results reported that 9.68 μg of ascorbic acid and 10.3 μg of total polyphenol can equally reduce activity in 1 mg of dulse extracts. The reducing activity was correlated with aqueous/alcohol soluble compounds due to the presence of functional groups such as hydroxyl, carbonyl, etc., which lead to reduced or inhibited oxidation [
56].
4.2. Antioxidants from Monocots
Ashawat et al. studied the antioxidant properties of ethanolic extracts of
Areca catechu and showed that
Areca catechu had the highest antioxidant activity when compared to other eudicots like
Centella asiatica, Punica granatum, and
Glycyrrhiza glabra [
57]. Londonkar and kamble studied
Pandanus odoratissimus L. in order to determine its antioxidant activity [
58]. Zahin et al. screened
Acorus calamus to estimate antioxidant activity and total phenolic contents [
59]. The observations confirmed that there was a significant correlation between the phenolic content and antioxidant activity. Another monocot,
O. sanctum, showed that the inhibition of lipid peroxidation in vivo and in vitro increased proportionally with an increase in the concentration of the extract.
4.3. Antioxidants from Vegetables
Consumption of vegetables has been linked to a reduction in the risk of many diseases, such as cancer and cardiovascular disease, when studied in epidemiological studies [
59]. Numerous studies have attempted to screen vegetables for antioxidant activity by using different oxidation systems. These vegetables include carrots, potatoes, sweet potatoes, red beets, cabbage, Brussels sprouts, broccoli, lettuce, spinach, onions, and tomatoes. In addition to the concise studies, which have used different methodologies to release bioactive compounds, it is becoming increasingly difficult to ignore advanced extraction methods, which have paved the way to extract bioactive compounds rapidly. Despite scientists’ successes in showing the activity of vegetables’ bioactive compounds, there is little known about the activity of the antioxidant components that have been isolated from these vegetables. Researchers have tended to focus on advanced methods to isolate and measure the activity of antioxidant compounds such as flavonoids, phenolic acids, tocopherols, carotenoids, and ascorbic acid [
60].
4.4. Antioxidants from Fruits
Fruit consumption has also been linked to a reduction in the risk of many diseases [
61]. Peaches (
Prunus persica L.) are an economically important fruit in many countries. Studies have shown that phenolic compounds found within various peach genotypes are a major source of potential antioxidants [
60]. Interestingly, peaches have shown a great inhibition of low density lipoprotein (LDL) oxidation with a percentage of antioxidant activity of 56–87%. This antioxidant activity can be attributed to its essential compound content including hydroxycinnamic acids, chlorogenic, and neochlorogenic acids, but not to carotenoids such as b-carotene and b-cryptoxanthin. Moreover, low antioxidant activity was found in peach peel. In contrast, Plumb et al. pointed out that hydroxycinnamic acids do not contribute to the inhibition of lipid peroxidation of the liver using plums and peaches because hydroxycinnamic acids had weak ability to scavenge hydroxyl radicals [
62].
Grape (
Vitis vinifera L.) is a fruit crop grown throughout the world. Grapes and its juices have been recently studied by [
62]. Phenolic compounds were high in both fresh grapes and commercial grape juices. The percentage of inhibition LDL oxidation was about 22% to 60% for fresh grapes, while it was approximately 68% to 75% for commercial grape juices, when standardized at 10 mg gallic acid equivalents (GAE). According to [
63], both grapes and its juices exhibited high oxygen radical absorbance capacity (ORAC), and the anthocyanin pigment malvidin-3,5-diglucoside was a major compound isolated in grapes. Anthocyanins with malvidin nucleus malvidin 3-
O-(6-
O-p-coumaroylglucosido)-5-glucoside and phenolics were common compounds isolated from wild grapes (
Vitis coignetiae). Wangensteen et al. tested the activity of many bioactive compounds by releasing them from grape pomace, and demonstrated that bioactive compounds have the ability to significantly inhibit LDL oxidation in the human body [
64]. Grape seeds are an amazing source of polyphenol compounds including monomerics such as catechin, epicatechin, and gallic acid, and polymerics such as procyanidins [
65].
Both polyphenols and carotenoids are the major phenolic compounds of apples (
Malus domestica L.) including caffeic, quinic, and p-coumaric acids. These polyphenols can act as antioxidants. Flavanol monomers and oligomers, as well as quercetin, contribute to the beneficial health aspects of fruits and vegetables [
65]. Apple pomace has mainly been used as a major source of polyphenols such as chlorogenic acid [
66,
67]. In addition phenolics like caffeic, p-coumaroyl quinic, arbutin, p-coumaric acids, and especially flavonol procyanidins have been mentioned as constituents of apple pomace [
68]. The ability of procyanidins to work as oxygen radical scavengers, superoxides, and hydroxyl radicals was estimated. Despite the low content in total phenols in apples obtained by using acetone 70%, it has shown strong antioxidant activities towards oxidation of linoleic acid. In this case, the major bioactive compounds obtained were chlorogenic acid and phloretin glycosides; however, Vitamin C was a minor fraction in apple juice [
69].
Antioxidant and antibacterial activities of various solvent (ethyl acetate, acetone, methanol, and water) extracts of Punica granatum peel were examined by applying the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging method. The results obtained showed a significantly higher decreasing power in the methanol extracts and a significantly higher antibacterial activity in the acetone extracts.
Soong and Barlow investigated the antioxidant activity and phenolic content of various fruit seeds [
70]. Petroleum ether was used to get rid of the excess fat from the seeds and extraction has been carried out with methanol. The 2,2-azino-bis-3-ethylbenzthiazoline-6-sulfonic acid (ABTS), DPPH, and the ferric reducing ability of plasma (FRAP) methods were used to investigate the antioxidant activity. Abdille et al. examined the antioxidant activity of
Dillenia indica fruit using different kinds of solvents using DPPH, phospho-molybdenum, and β carotene bleaching methods [
71]. The methanol extracts showed the highest antioxidant activity, followed by the ethyl acetate and water extracts. Antioxidant activity of
Syzygium cumini fruit in vitro has been investigated [
71]. Antioxidant activity was measured by DPPH, superoxide, lipid peroxidation, and hydroxyl radical scavenging activity methods. The results brought to light a significant correlation between the concentration of the extract and the percentage of inhibition of free radicals. The antioxidant property of the fruit might be from the presence of antioxidant vitamins, anthocyanins, phenolics, and tannins. It has been reported that blackberry (
Rubus fruticosus L.) fruit extracts produced in varying climatic regions showed that antioxidant activity depended on the genotype, rather than the climate or season [
10]. Juntachote and Berghofer measured the stability of the antioxidant activity of ethanolic extracts for Holy basil and galangal using DPPH, superoxide, β carotene bleaching, reducing power, and iron chelation methods [
72]. They found higher antioxidant activity at neutral pH compared to an acidic pH. Holy basil and galangal extracts provided strong iron chelation activity, superoxide anion scavenging activity, and reducing power proportional to the concentration of the extracts. Liyana-Pathirana et al. investigated the antioxidant activity of cherry laurel fruit (
Laurocerasus officinalis Roem) and its concentrated juice (Pekmez) using in vitro methods such as superoxide, DPPH scavenging activity, and inhibition of LDL oxidation [
73]. The results confirmed the presence of a significantly higher antioxidant activity in pekmez compared to the cherry laurel fruit. Employing in vitro methods such as DPPH and superoxide scavenging activity, Orhan et al. measured the antioxidant activity of
Arnebia densiflora Ledeb and observed that polar extracts had a higher antioxidant activity compared to non-polar extracts [
74]. Rathee et al. studied the antioxidant activity of
Mammea longifolia buds extracted in both methanol and aqueous ethanol. The results found a significant antioxidant activity, and the activity of aqueous ethanol was higher than methanol. The antioxidant activity of leaf extracts of
Annona species in vitro reveals that
Annona muricata possessed a higher antioxidant activity compared to
Annona squomosa [
75].
4.5. Cooking Herbs as an Important Source of Antioxidants
The antioxidant activity of 32 herbs belonging to 21 different families has been screened [
76]. The finding confirmed that there was a positive correlation between the total antioxidant activity and total phenolic content. Lu and Yeap Foo studied
Salvia officinalis (L.) for its antioxidant activity and polyphenol content and reported that rosmarinic acid and various catechols were responsible for the radical scavenging activity and caffeic acid was responsible for the xanthine oxidase (EC 1.17.3.2) inhibition [
77]. Zhao et al. investigated the antioxidant activity of
Salvia miltiorrhiza and
Panax notogensing [
78]. The results showed that
Salvia miltiorrhiza had a higher reducing power and scavenging activities against free radicals, including superoxide and hydroxyl radicals, although it showed weak hydrogen peroxide scavenging.
Furthermore, Javanmardi et al. tested the Iranian
Ocimum sp. accessions to determine the antioxidant activities and total phenolic contents and demonstrated that the antioxidant activity increased in parallel with the total phenolic content [
51].
Evaluation of the pomegranate peel extracts to discover its antioxidant and antimutagenic activities using different solvents such as ethyl acetate, acetone, methanol and water has been carried out [
51]. Dried extracts were examined by using the Ames test and the phosphorus-molybdenum method to test both anti-mutagenic and antioxidant activities. The results showed the highest anti-mutagenic and the lowest antioxidant activity in the water extract.
Moreover, the phenolic content and antioxidant activity of parsley (
Petroselinum crispum) and cilantro (
Coriandrum sativum) have been tested [
79]. The total phenolic content was observed to be different between parsley and cilantro leaves and stems, as well as methanol and water extracts. The methanol leaf extracts exhibited significant antioxidant activity towards both lipid- and water-soluble radicals. The works also investigated the antioxidant activity of aqueous plant extracts using in vitro methods such as DPPH scavenging activity and FRAP. The results revealed a strong correlation between total antioxidant activity and phenolic content and a weak correlation between cupric ion chelators and polyphenols. The antioxidant activity and lipid peroxidation inhibition of
Satureja montana L. subsp.
Kitaibelii extracts were tested using hydroxyl radical scavenging. The results obtained showed that there was a significant correlation with total phenolic content [
9].
4.6. Antioxidant from Legumes
Antioxidant property of methanol extracts of
Mucuna pruriens L. (
Fabaceae) seed extracts has been investigated in vitro using the DPPH radical scavenging method. The results obtained showed a positive correlation between the antioxidant activity and the total phenolic compounds [
80]. Siddhuraju and Manian studied horsegram (
Macrotyloma uniflorum Lam.) seeds to measure the antioxidant and free radical scavenging activity [
81]. Acetone extracts had a higher activity of about 70% [
81]. Samak et al. studied
Wagatea sp. to measure its scavenging activities of superoxide and hydroxyl radicals and showed a high oxidation inhibition because it was rich in both phenolic and flavonoid contents. The authors also reported that bark and leaf extracts of
Wagatea sp. exhibited high scavenging action against super radicals [
82].
4.7. Antioxidants from Trees
Antioxidants from trees have been also measured. Phenolics from almond hulls (
Prunus amygdalus L.) and pine sawdust (
Pinus pinaster L.) have been extracted employing various methods in order to determine the gram fresh yield of polyphenol compounds and antioxidant activity [
83]. The antioxidant activity was measured by the DPPH radical scavenging method. The results showed that ethanol was most appropriate either for phenolics or any bioactive compounds, while methanol was more selective for extracting polyphenolics. The antioxidant activity of juniper (
Juniperus communes) fruit extracts has been investigated in vitro [
84]. The results confirmed that both water and ethanol extract showed strong antioxidant activity. The concentration of 60 μg/mL of water and ethanol extracts exhibited 84% and 92% inhibition, respectively, on the peroxidation of linoleic acid. Ibrahim et al. studied the antioxidant activity of
Cupressus sempervirens L., and set up goals to isolate quercetin, rutin, cupress flavone, caffeic acid, and para-coumaric acid. The results showed higher antioxidant activity related to quench DPPH and identified these active compounds successively [
85].
Higher values of antioxidant activity have been obtained by using a methanolic solvent to extract the bioactive compounds from
Anacardium occidentale, while other solvents like ethyl acetate gave lower values of antioxidant activity [
85]. Kaur et al. studied the Chickrassy
Chukrasia tabularis A. Juss leaves to confirm its ability to inhibit lipid peroxidation and showed that there was a large inhibition considering its high content of phenolic compounds [
86]. Finally,
Acacia nilotica L. antioxidant activity has been measured using ethyl acetate as a solvent to extract phenolic compounds [
86]. The results exhibited the highest antioxidant activity when the concentration of extracts was relatively high.
4.8. Antioxidant from Shrubs
Many shrubs have been shown to contain antioxidant activity. Singh et al. tested several plants to measure the antioxidant activity from different extracts. The antioxidant activity was determined by using peroxide value, thiobarbituric acid, DPPH radical scavenging activity, and reducing power. The results showed that the antioxidant activity of
Coriandrum sativum L. and
Sarcolobus globosus L. exhibited high activity by using acetone solvent, and its activity was similar to synthetic antioxidants [
87].
Eleven Algerian medicinal plants have been measured for phenolic compound content and antioxidant activity using the ABTS method. The tested plants showed antioxidant activity. Artemisia campestris L. had better antioxidant activity than caffeic acid and tocopherol. Moreover, HPLC analyses exhibited a good correlation between the antioxidant activity and hydroxycinnamic derivative content.
Evaluation of
Vitex negundo Linn seed antioxidant activity using different methods such as superoxide, hydroxyl, and DPPH scavenging activity has been carried out [
87]. The highest antioxidant activity was in both raw and dry heated seed extracts, while lower antioxidant activity was observed in the hydrothermally processed samples.
4.9. Characterization of Antioxidants from Other Eudicots
The nitric oxide and superoxide scavenging activity of green tea have been studied by Nakagawa and Yokozawa [
88], who concluded that certain tannins had the ability to exhibit excellent antioxidant activity. Zin et al. estimated the antioxidant activity of the extracts from various parts of Mengkudu (
Morinda citrifolia L.), including the leaves, fruits, and roots, using different solvents such as methanol and ethyl acetate [
89]. Ferric thiocyanate and thiobarbituric acid were used as models to observe and evaluate the antioxidant activity. The results exhibited a higher antioxidant activity in the methanol extract of Mengkudu root, although it was not significantly different from tocopherol and BHT extracts. The methanol extracts of the fruits and leaves showed unassuming activity. According to these scientists, the antioxidant activity in the roots resulted from polar and non-polar compounds, but the antioxidant activity in leaves and fruits was only due to non-polar compounds.
Increase of the antioxidant activity of fennel (
Foeniculum vulgare) seed extracts in vitro has been shown to be proportional to the increase in the concentration of extract [
89]. Nine other extracts of Bolivian plants have been measured for radical scavenging and antioxidant activity using the DPPH and β carotene bleaching methods [
90]. It was found that the ethyl acetate fractions had higher radical scavenging and antioxidant activity compared to the other extracts. It has been reported that the bioactive compounds of
Rhodiola rosea extracted in methanol showed a significant yield of phenolics, about (153 ± 2 mg/g) [
91]. Wangensteen et al. investigated the antioxidant activity of
Ss globosus using DPPH scavenging and inhibition of lipoxygenase [
64]. Coriander had a high capacity to inhibit oxidation. There was also a positive correlation between total phenolics and antioxidant activity. Moreover, it was observed that the leaves of the coriander had higher antioxidant activity than the seeds [
91].
Antioxidant activity of
Phyllanthus niruri was estimated using methanol and water as a solvent. The extracts of leaves and fruits exhibited high antioxidant activity by using the inhibition of lipid peroxidation and DPPH scavenging [
64]. The results also noticed a higher superoxide scavenging activity in the aqueous extract compared to the methanol extract. Moreover, the antioxidant and free radical scavenging activity of
Phyllantus species from India in an aqueous extract has been also evaluated [
92]. The antioxidant activity was estimated using DPPH, β carotene, superoxide, nitric oxide scavenging, and reducing power methods. The extract of
Coleus aromaticus exhibited a moderate inhibition on DPPH and nitric oxide scavenging activity.
Panax exhibited strong iron chelating and weak superoxide scavenging. Ajila et al. carried out bioactive compounds and antioxidant activity of mango peel extract [
93]. The results showed a higher concentration of anthocyanins and carotenoids in the ripe peel compared to the raw peel, while the raw peel exhibited higher polyphenol content. The range of IC50 values of lipid peroxidation and DPPH were 1.39–5.24 μg of gallic acid equivalent. Chen and Yen investigated the antioxidant activity and free radical scavenging capacity of dried guava leaves and fruit [
94]. The results confirmed that guava leaf and guava tea extracts had the ability to inhibit oxidation by 94–96% at a concentration of 100 μg/mL. Fruit extracts exhibited less activity compared to leaf extracts, while the scavenging effect increased with an increase in the concentration. Also, there was a correlation between antioxidant activity and phenolic compounds. Dastmalchi et al. investigated the chemical composition and antioxidant activity of water-soluble Moldavian balm (
Dracocephalum moldavica) in vitro by using DPPH, ABTS, and superoxide activity [
95]. The finding confirmed that polar compounds such as caffeic acid and rosmaric acid were responsible for the antioxidant activity observed.
Mulberry leaves were investigated to determine the antioxidant activity using different solvents [
95]. The procedure used DPPH and inhibition of lipid peroxidation methods to evaluate its activity. The results showed that the methanolic extract exhibited the highest yield of total phenolics, and it was the most essential antioxidant in all the methods used. The antioxidant activity of kale (
Brassica obraceae L.) has been screened after removing a fat fraction from the samples [
96]. The extraction process used methanol to investigate its antioxidant activity while using DPPH scavenging activity as tested method. The works successfully isolated nine phenolic acids using HPLC and MS, and confirmed that the total phenolic content was correlated with DPPH scavenging activity.
In another study, ethanol has been used to estimate the antioxidant activity of sun-dried cashew nuts (
Anacardium occidentale L.) skin [
97]. First, bioactive compounds were extracted with a protocol including lipid peroxidation, ABTS, and DPPH methods to measure the capability to inhibit oxidation. The results found that epicatechin was the major polyphenol in the extract, which was responsible for antioxidant activity.
Kaviarasan et al. measured the antioxidant and antiradical activity of fenugreek (
Trigonella foenum ssp.
graecum) seeds in vitro; the results showed that there was a positive relationship between the antiradical activity and phenolic compound content in the extract [
98]. Hexane and methanol were used to extract the bioactive compounds and measured the antioxidant activity of
Pueraria tuberosa by using ABTS, lipid peroxidation, and superoxide and hydroxyl scavenging activity. An independent study has shown an inhibition of the lipid peroxidation [
99].
The rhizome of the lotus (
Nelumbo nucifera Gaertn.) has been measured for its antioxidant activity in various solvent extracts using β Carotene bleaching and DPPH methods [
99]. Methanol extraction had a higher DPPH scavenging activity than acetone.
Helichrysum pedunculatum has been tested to determine the antioxidant activity, and total phenolic and flavonoid content [
100]. The results demonstrated that whenever the amount of phenolic content and flavonoid content was increased, higher antioxidant activity was obtained. Meot-Duros and Magn screened the leaves of
Crithmum maritmum to show if there was any correlation between the antioxidant activity and phenolic content and found a significant correlation between antioxidant activity and phenolic content when methanol was used as the solvent [
101].
Another dicot,
Tricholepis glaberrima L. (
Asteraceae), has been investigated for antioxidant activity using different kinds of extracts [
101]. Higher antioxidant activity was found by methanol, and a lower antioxidant activity in both chloroform and aqueous extracts. Sakat et al. investigated
Oxalis corniculata L. in order to measure the antioxidant and anti-inflammatory activity employing methanol as a solvent. The IC 50 values of DPPH and nitric acid were about 93 and 73.07 μg/mL, respectively [
102].
Jain et al. studied
Tabernaemontana divaricata L. to determine the phytochemical and free radical scavenging activities in vitro. The results indicated that the antioxidant activity was the same in both ethanol and water extracts, but less in petroleum ether [
103].
It has been reported that
Ascleipiadaceae and
Periplocoideae presented high antioxidant activity, with the presence of a strong correlation between antioxidant activity and phenolic content [
103]. Laitonjam and Kongbrailatpam studied the chemical composition and antioxidant activities of
Smilax lanceafolia by isolating the flavonol glycoside and steroidal saponin, which showed high antioxidant activity [
104]. Spinach (
Spinacea olerace L.) is among the most popular vegetables in the world. It was domesticated and first cultivated in West Asia. According to analytical chemistry, spinach is a source of violaxanthin and neoxanthin antioxidants that cannot be commercially produced [
105]. Although they may be present, pigments such as carotenoids can be masked by chlorophyll in greenish vegetables such as spinach [
106]. B-carotene, lutein, violaxanthin, and neoxanthin are the major carotenoids in raw spinach [
107]. Pumpkins belong to the family
Cucurbitaceae. This family is classified depending on the texture and shape of stems, such as in
Cucurbita pepo,
Cucurbita moschata,
Cucurbita maxima, and
Cucurbita mixta. Nowadays, the market offers a wide variety of vegetables, with pumpkin being one of them because of its many applications for nutrition or decoration [
108].
6. Plants as an Antimicrobial Source
The antibacterial activity of
Punica granatum extracts has been investigated by using various solvents [
119]. The water extract had the ability to inhibit
Bacillus subtilis and
Staphylococcus aureus, but the organic solvents have the ability to inhibit the growth of all the organisms tested. Shariff et al. estimated the antibacterial activity of
Rauvolfia tetraphylla and
Physalis minima leaves. The chloroform extract was a more powerful inhibitor of pathogenic bacteria [
120].
Indian medicinal plants have been shown to have antimicrobial activity [
120]. About 77 extracts belonging to these plants have been tested for their antimicrobial ability against eight species of bacteria and four species of pathogenic fungi. The findings showed that water extracts of
Lantana camara L.,
Saraca asoca L.,
Acacia nilotica L., and
Justicia zeylanica L. caused the highest growth inhibition of all tested bacteria. The antimicrobial activity was the highest, ranging between 9.375 and 37.5 μg/mL and 75.0 to 300.0 μg/mL against both bacterial and fungal pathogens.
Devi et al. investigated
Achyranthes bidentata Blume to determine its phytochemical content and antibacterial activity [
121]. The antibacterial ability of the ethanol extract effectively inhibited
Bacillus subtilis,
Salmonella typhi, and
Klebsiella pneumoniae, but was less effective against
Pseudomonas species and
Staphylococcus aureus [
122]. Ethanolic extracts of
Gymnema montanum L. have been studied to measure its antimicrobial properties against
Salmonella typhi,
Pseudomonas aeruginosa, and
Candida albicans [
121]. The results indicated the highest presence of antimicrobial properties in the leaf extract of
G. montanum, correlating to its phenolic compound content. The antimicrobial activity of
Piper ribesoides L. from methanolic root extract against
Staphylococcus aureus has been reported [
123]. Interestingly, a small amount of 3.125 mg/mL was enough to inhibit harmful bacteria. Leaf extracts of
Caesalpinnia pulcherrimma (L.) showed higher antioxidant activity in water and ethanol extracts and lower antioxidant activity in petroleum ether extracts [
124].
Torilis japonica L. fruit has been observed to reduce the amount of spores, and the concentration of the vegetative cell was lower than the detection level. Ghosh et al. studied
Stevia rebaudiana Bertoni to measure its antimicrobial properties against 10 pathogens [
125]. The findings confirmed that
Staphylococcus aureus was more susceptible than others [
24]. Mahesh and Satish screened some important medicinal plants to show the antibacterial activity on human pathogenic bacteria [
126]. Water and methanol were used as solvents to extract the phenolic compounds. The finding confirmed that the methanol extract had a higher antimicrobial activity than the aqueous extract [
125].
Moreover, leaf extracts of
Acacia nilotica L.,
Sida cordifolia L.,
Tinospora cordifolia L.,
Withania somnifera L., and
Ziziphus mauritiana L. have been studied to determine the antibacterial activity against
Bacillus subtlis,
Escherichia coli,
Staphylococus aureus, and
Pseudomonas fluorescens, as well as studying the antifungal activity against
Aspergillus flavus,
Dreschlera turcica, and
Fusarium verticilloides [
126]. The highest antibacterial activity was noticed in
Acacia nilotica and
Sida cordifoliain leaves, and the highest antifungal activity was noticed in
Acacia nilotica bark. Water and methanol extracts of
Samanea saman (Jacq.) exhibited a significant effect against
Xanthomonas spp. and human pathogenic bacteria.
Pseudarthria viscida root has been studied to measure its antimicrobial activity using ethanol as a solvent. The results showed high antimicrobial activity when compared to standard drugs like ciprofloxacin and griseofulvin.
Ehsan et al. reported a high antimicrobial activity against
Staphylococcus aureus using methanol and ethanol extracts for
Hopea pariviflora Beddome [
127]. Ethanolic extracts of
Bryonopsis laciniosa have been investigated for their antimicrobial activity against different Gram-positive and Gram-negative bacteria. The growth of
Staphylococcus aureus,
Micrococus luteus, and
Bacillus cereus was inhibited, as shown by a decrease in the growth zone.
Plumbago zeylanica L. has been screened to measure the antibacterial activity in chloroform extracts to show antimicrobial activity against
Escherichia coli,
S. typhi, and
Staphylococcus aureus [
127]. However,
Bacillus subtilis and
Klebsiella were resistant. Khond et al. studied 55 medicinal plants to measure the antimicrobial activity [
128]. The higher antibacterial activities were in the extracts of
Madhuca longifolia L.,
Parkia biglandulosa L., and
Pterospermum acerifolium L. compared to the other plants screened. Pavithra et al. screened
Evolvulus nummularius L. for its antibacterial activity, finding that
Escherichia coli and
B. subtilis were the most inhibited by an ethanolic extract [
129].
Hygrophila spinosa Andres leaves showed significant antibacterial activity when collected between September to October, with less activity seen during other months [
129].
Artemisia pallens L. has been studied for its antimicrobial activity against seven species of bacteria [
130]. The results found that
Bacillus cereus was more sensitive to
A. pallens extracts. Also, a methanolic extract exhibited higher antibacterial activity than the other solvents used. Akroum indicated the antimicrobial activity of some Algerian plants [
131]. The results expressed higher antibacterial activity in methanolic extracts of
Linum capitatum,
Camellia sinensis,
Allium schoenoprasum,
Vicia faba,
Citrus paradise,
Lippia citriodora,
Vaccinium macrocarpon, and
Punica granatum. Bajpai et al. screened the antibacterial activity of
Pongamia pinnata leaves by using methanol and ethyl acetate extracts to confirm its ability against certain pathogenic bacteria [
132]. The results exhibited significant inhibition compared to streptomycin. It has been demonstrated that
Memecylon edule has higher antibacterial activity in chloroform extracts compared to other extracts [
132]. Gram-negative bacteria were more susceptible to the crude extracts compared to Gram-positive bacteria. Bansal et al. studied plants found in arid zones in order to determine the antibacterial efficiency [
133]. An ethanolic extract of
Tinospora cordifolia L. inhibited
Bacillus cereus and
Staphylococcus aureus. Kumar et al. reported
Andrographis serpyllifolia L. to have significant antimicrobial activity against tested organisms in methanol extracts of both aerial parts and root [
134].
Memecylon malabaricum,
Cochlospermum religiosum, and
Andrographis serpyllifolia have been rested for their possible antimicrobial activity [
135,
136]. Moderate activity against both Gram-positive and Gram-negative bacteria was observed. The antimicrobial activity of an ethanolic extract of
Anethum graveolens was better than the aqueous extract. Khanahmadi et al. [
137] found a higher antibacterial activity against Gram-positive bacteria compared to Gram-negative bacteria when an ethanolic extract of
Smyrnium cordifolium Boiss was used [
136,
137]. Koperuncholan et al. studied some medicinal plants of the south eastern slopes of the Western Ghats [
138]. Gram-positive bacteria were more sensitive than Gram-negative bacteria to the plant extracts. Niranjan et al. screened
Schrebera swietenioides Roxb to measure the effectiveness against human pathogenic bacteria [
139]. Water and methanol extracts were most effective to prohibit growth of all the harmful bacteria tested.
Different studies have isolated tannins and saponins from some Indian medicinal plants, testing the antibacterial activity against
Klebsiella pneumoniae [
139,
140]. Ethanol extracts of
Tinospora cordifolia strongly inhibited
Bacillus cereus and
Staphylococcus aureus. Also, significant antibacterial activity from ethanolic extracts of
Coleus aromaticus L. has been found. The most effective range of inhibition was at concentrations of 25–39 μg/mL. Vinothkumar et al. evaluated a
Andrographis paniculata L. leaf extract’s ability to inhibit the growth of Gram-positive and Gram-negative bacteria. The results found that aqueous extracts inhibited harmful microbes [
134].
A positive effect of pumpkin has been observed by investigating its antimicrobial activity against
Staphylococcus aureus,
Bacillus subtillus,
Escherichia coli, and
Pseudomonas.
aeruginosa. Three different solvents were used to prepare the extracts: water, chloroform, and alcohol. The results showed that the alcohol extract was more powerful than both water and chloroform extracts.
Staphylococcus aureus was sensitive to all extracts. Recently, the novel antimicrobial activity of ultrasonicated spinach leaf extracts using random amplification of polymorphic DNA (RAPD) markers and electron microscopy against both Gram-positive and Gram-negative bacteria has been revealed [
134]. RAPD is an emerging technique used for diagnostic mutation detection within a genome. The range of the minimum inhibitory concentrations (MICs) of the extracted leaf spinach antimicrobial substances against
Escherichia coli and
Staphylococcus aureus was observed between 60 and 100 mg/mL. The optimal extraction conditions were at 45 °C, ultrasound power of 44%, and an extraction time of 23 minutes. The study showed that the treated bacterial cells appeared to be damaged by a reduction in cell number. In fact, it was inferred that spinach leaf extracts exert bactericidal activity by inducing mutations in DNA and causing cell wall disruptions.