Plant Bioactive Phenolic Compounds: Comparison
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Please note this is a comparison between Version 2 by Peter Tang and Version 1 by Alice Vilela.

Polyphenols, as well as volatile compounds responsible for aromatic features, play a critical role in the quality of vegetables and medicinal, and aromatic plants (MAPs). The research conducted has shown that these plants contain biologically active compounds, mainly polyphenols, that relate to the prevention of inflammatory processes, neurodegenerative diseases, cancers, and cardiovascular disorders as well as to antimicrobial, antioxidant, and antiparasitic properties. 

  • aromatic plants
  • bioactive compounds
  • consumers
  • medicinal plants
  • phenolic compounds
  • plant breeders
  • volatile compounds
  • vegetables

1. Plant Bioactive Phenolic Compounds

Vegetables and medicinal and aromatic plants (MAPs) are important sources of bioactive phenolic compounds and have a key role in the development of compounds eliciting beneficial health effects [6][1]. Phenolic bioactive compounds of plant origin are those secondary metabolites possessing desired health benefit effects [7][2]. They might be produced from two distinct pathways: (i) shikimic acid (phenylpropanoids) and (ii) acetic acid (phenols) [8][3]. Due to their abundance in vegetables and MAPs, the study of phenolic compounds’ (simple phenolics, coumarins, lignans, flavonoids, isoflavonoids, anthocyanins, proanthocyanidins, and stilbenes) effects on health has increased in recent years, due to the growing evidence indicating that polyphenols are a major class of bioactive phytochemicals. Their consumption may play a role in the prevention of several chronic diseases as potent antioxidant properties, prevention of diseases induced by oxidative stress, and prevention of some specific cardiovascular (mainly high cholesterol levels, high blood pressure) and neurodegenerative diseases (such as Alzheimer’s or Parkinson’s, type II diabetes, cancers, urinary tract infections) [9,10,11,12,13][4][5][6][7][8]. However, the health effects of phenolic compounds are dependent on their type, quantity consumed, as well as on their bioavailability.
The amount of total phenolic compounds is greater in dark vegetables, such as red kidney beans, black beans (Phaseolus vulgaris), and black gram (Vigna mango). Bravo [9][4] determined (by regarding dry matter, mg/100 g) the amount of total phenolic compounds in several vegetables such as black gram (540–1200), chickpeas (78–230), cowpea (175–590), common beans (34–280), green gram (440–800), pigeon peas (380–1710), Brussel sprouts (6–15), cabbage (25), leek (20–40), onion (100–2025), parsley (55–180), and celery (94).
Phenolic acids have been recently widely studied because of their potential protective roles. Phenolic acids have a benzene ring, a carboxylic group, and one or more hydroxyl and/or methoxyl groups. They are usually divided into benzoic acid derivatives (i.e., hydroxybenzoic acids, C6-C1) (Figure 1a) and cinnamic acid derivatives (i.e., hydroxycinnamic acids, C6-C3) (Figure 1b), based on the constitutive carbon structures. The amount of hydroxybenzoic acid (C6-C1 derivatives) (e.g., gallic acid, salicylic acid, salicylaldehyde, and protocatechuic acid) is typically low in edible plants [14][9]. Phenolic acids may make up about one-third of the phenolic compounds in the human diet; these substances have a powerful antioxidant activity that may help protect the body from free radicals [9,15][4][10].
Figure 1.
Chemical structures of hydroxybenzoic acids (
a
) and hydroxycinnamic acids (
b
).
According to Khadem and Marles [16][11], gallic acid has antineoplastic and bacteriostatic activities, and salicylic acid exerts anti-inflammatory, analgesic, antipyretic, antifungal, and antiseptic properties. Protocatechuic acid has also been described as having several bioactivities such as anti-inflammatory, antifungal, and antioxidant ones [17][12]. For instance, p-hydroxybenzoic acid has been isolated from many sources including carrots (Daucus carota) [18][13] and protocatechuic acid from onion, garlic, and relatives (Allium spp.) [19][14].
The hydroxycinnamic acids (C6-C3 derivatives) are more abundant than the hydroxybenzoic acids. The four most common hydroxycinnamic acids are ferulic acid, caffeic acid, coumaric acid, and sinapic acid. These acids are frequently present in plants in the combined forms such as glycosylated derivatives or esters of tartaric acid, shikimic acid, and quinic acid rather than in the free form. Hydroxycinnamic acids are recognized as powerful antioxidants playing an essential role in protecting the body from free radicals. Several hydroxycinnamic acid derivatives, such as caffeic acid, chlorogenic acid, ferulic acid, p-coumaric acid, and sinapic acid, present strong antioxidant activities by inhibiting lipid oxidation and scavenging reactive oxygen species (ROS) [10][5]. Chlorogenic acid and caffeic acid inhibit the N-nitrosation reaction and prevent the formation of mutagenic and carcinogenic N-nitroso compounds [20][15].
Rosemary (Rosmarinus officinalis L.) extracts have been used as diuretic, analgesic, expectorant, antirheumatic, and antimutagenic agents. Caffeic acid and its derivatives, such as rosmarinic acid (Figure 2) and chlorogenic acid, have been thought to be the most important ones responsible for the therapeutic properties of rosemary extracts, as they have antioxidant effects and contribute to the bioactive function of rosemary [21][16].
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Figure 2.
Chemical structures of rosmarinic acid and chlorogenic acid.
Among the phenolic compounds identified by Zheng and Wang [22][17], rosmarinic acid was the predominant phenolic compound in Salvia officinalis and Thymus vulgaris.
In previous years, the use of the active phenolic acid compounds (such as chlorogenic acid, ferulic acid, cinnamic acid, and rosmarinic acid) in food has increased. Thus, the study of plants’ phytochemicals is important and essential [23][18].
Coumarins are a large class of C6-C3 derivatives belonging to the benzo-α-pyrone group, which exist in the free or combined form as heterosides and glycosides in certain plants; most of them are isolated from chlorophyll-containing plant materials [24][19]. Species rich in coumarins included Aesculus hippocastanum (Horsechestnut), Passiflora incarnata (Passionflower), Lawsonia inermis (Henna), Hypericum perforatum (Saint John Wort), Tilia cordata (Lime Tree), and Uncaria tomentosa (Cat’s Claw) [25][20]. Coumarins can be categorised into four types. Simple coumarins are the hydroxylated, alkoxylated, and alkylated derivatives of the benzene ring of coumarin, and the corresponding glycosides. Furanocoumarins compounds consist of a five-member furan ring attached to the coumarin nucleus, divided into linear and angular types with a substituent at one or both remaining benzenoid positions. Pyrano coumarins are analogous to the furanocoumarins but contain a six-member ring. The last type is coumarins substituted in the pyrone ring [26][21]. Several products that contain a coumarin moiety show excellent biological activities such as antitumor, antibacterial, antifungal, anticoagulant, vasodilator, analgesic, and anti-inflammatory activities [24,27,28][19][22][23].
Lignans are a diverse group of bioactive phenolic compounds formed of two β-β-linked phenylpropane units; they are present in different parts of plant species in free form or combined form as glycoside derivatives. Lignans are found in vegetables such as in the brassica family where fresh edible weights (mg/100 g) between 0.185 to 2.32 of can be found, for instance, for broccoli (98.51), Brussels sprouts (50.36), cauliflower (9.48), green cabbage (0.03), red cabbage (18.1), white cabbage (21.51), and kale (63). They can also be found in green beans (22.67), tomato (2.15), cucumber (3.8), zucchini (7.02), green lettuce (1.17), and carrot (7.66). However, spinach, white potatoes, and mushrooms contain an amount below 0.1 mg/100 g (fresh edible weight) of lignin [29,30][24][25]. Lignan presents a great antioxidant activity and may be effective in the treatment of cardiovascular disease, coronary heart disease, and diabetes [31][26].
Flavonoids have the general structural C6-C3-C6, in which the two C6 units are phenolic and linked by a C3 group. They can be divided into flavones, flavonols, flavanones, and flavanols, according to the oxidation state of the central pyran ring, as well as in anthocyanins and isoflavonoids, with different antioxidant, antibacterial, antiviral, and anticancer activities [15,32][10][27].
Flavones usually occur as glycosides of apigenin and luteolin in plants (Figure 3). Flavones are found in celery (22–108 mg/kg fresh weight) and showed the proprieties of lowering the levels of total and low-density lipoprotein (LDL) cholesterol and also have anti-inflammatory and anticancer activities [33][28]. In other vegetables, the amounts are (mg/kg of luteolin and apigenin, respectively): 0.41 and 0.05 in water spinach; 0.09 and 0.03 in cucumber; 0.16 and 1.07 in purple cabbage; 1.18 and 0.31 in Chinese cabbage; 0.16 and 0.92 in white cabbage and 0.22 and 0.04 in onion [34][29].
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Figure 3.
Structures of the major flavones.
Flavonols have been extensively studied and are extensively distributed in plants [35,36,37,38,39,40,41,42,43,44][30][31][32][33][34][35][36][37][38][39]. They are frequently the conjugated form of glycosides such as kaempferol, quercetin, and myricetin (Figure 4). Quercetin levels in the edible parts of most vegetables are generally (of fresh weight, mg/kg) below 10, except for onions (284–486), kale (110), broccoli (30), French beans (32–45), and slicing beans (28–30) [35][30]. Kaempferol could only be detected (fresh edible weight, mg/kg) in kale (211), endive (15–91), leek (11–56), and turnip tops (31–64) [35][30]. A rich source of flavonols are onion leaves that contain (fresh weight, mg/kg) 1.497 of quercetin and 832 of kaempferol [37][32], and also sweet potato leaves (purple) showed 156 mg/kg of myricetin and 267 mg/kg of quercetin [34][29]. According to Erlund [45][40], quercetin is an antioxidant protecting against reactive oxygen species and shows also antiatherosclerosis, anticancer, anti-inflammatory, and cholesterol-lowering properties. Flavanones are colourless compounds characterised by the absence of a double bond in the 2, 3-position of the pyrone ring, and are isomeric with chalcones. Low concentrations of flavanones, namely naringenin, are found in tomatoes [46][41].
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Figure 4.
Structures of the major flavonol aglycones.
Monomeric flavan-3-ols include catechin, epicatechin, gallocatechin, catechin gallate, epicatechin gallate, epigallocatechin, epigallocatechin-3-gallate, and gallocatechin gallate (Figure 5). Catechin and epicatechin are the most abundant flavanols found in fruits, while in the seeds of some leguminous, the most abundant flavanols are gallocatechin, epigallocatechin, and epigallocatechin gallate [47][42].
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Figure 5.
Structures of monomeric flavan-3-ols.
Isoflavonoids are flavonoids that have their B ring fused with the C3 position of ring C, which are phenolics with phytoestrogenic activity (Figure 6). The concentrations of isoflavones in soybean products ranged from 580 to 3800 mg/kg of fresh weight [54][43].
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Figure 6.
Chemical structures of isoflavonoids.
The basic structures of anthocyanins are anthocyanidins, in which the two aromatic rings A and B are linked by a heterocyclic ring C that possesses oxygen. More than 23 different anthocyanidins have been found with pelargonidin, cyanidin, peonidin, delphinidin, petunidin, and malvidin being the most common (Figure 7). Anthocyanins in plants mainly exist in conjugated form as glycosides. Monomeric anthocyanin changed the hydroxylation and methoxylation patterns on the B ring; the nature, position, and the number of conjugated sugar units; the nature and number of conjugated aliphatic or aromatic acid groups; the existence or lack of an acyl aromatic group in the molecule [55][44]. They are usually present in any pink to purple vegetables such as black beans (Phaseolus vulgaris) (delphinidin (11.98); malvidin (6.45); petunidin (9.57) in mg/100 g, edible portion); kidney red beans (Phaseolus vulgaris) (pelargonidin (2.42); cyanidin (1.19) in mg/100 g, edible portion) [56][45]; common raw beans (Phaseolus vulgaris var. Zolfino) (delphinidin (2.50); malvidin (0.10); petunidin (0.14) in mg/100 g, edible portion) [38][33]; redraw cabbage (Brassica oleracea) (cyanidin (72.86), delphinidin (0.01); pelargonidin (0.02) in mg/100 g, edible portion) [42,56][37][45] and in cowpeas (blackeyes, crowder, southern) (Vigna unguiculata) (cyanidin (94.72); delphinidin (94.60); malvidin (34.28); peonidin (11.07); petunidin (27.82) in mg/100 g, edible portion) [57][46]. The protective effects of anthocyanins include antiedema, antioxidant, anti-inflammatory, and anticarcinogenic activities [58][47].
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Figure 7.
Chemical structures of anthocyanidins.
Condensed tannins, also recognised as proanthocyanidins, mainly comprise a flavan-3-ol unit to form dimers, oligomers, and polymers of up to 50 monomer units (Figure 8). Proanthocyanidins have complex structures depending on the number of the flavan-3-ol units, the location and type of interflavan linkage in the molecule, and the nature and position of substituents on the flavan-3-ol unit. Proanthocyanidins can be classified into procyanidins and prodelphinidins based on their hydroxylation patterns of A and B rings [33][28]. The proanthocyanidin contents in spinach (Spinacea oleracea) and radish leaves (Raphanus sativus) are 88.46 and 13.57 proanthocyanidins in mg/100 g fresh weight, respectively [59][48]. Proanthocyanidins have antioxidant activity responsible for cardioprotection, cancer chemoprevention, and lowering cholesterol amounts [33][28].
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Figure 8.
Chemical structure of procyanidins.
Quinones are phenolic compounds with conjugated cyclic dione structures, such as that of benzoquinones, derived from aroma compounds by the conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double bonds. The most common skeletal structures of quinones found in plants are p-quinone, o-quinone, anthraquinone, naphthoquinone, and naphtodianthrone (Figure 9).
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Figure 9.
Quinone structures.
Stilbenes are a group of phenolic compounds that share a similar chemical structure to flavonoids, in which the two aromatic rings (A and B) are linked by a methylene bridge. One of the most aroma compounds stilbenes is present mostly in glycosylated forms is trans-resveratrol (Figure 10). Resveratrol is a phytoalexin that has been particularly studied as it shows several biological activities, reduces the formation of atherosclerotic plaque, present neuroprotective, antidiabetic, anti-inflammatory, antioxidant, anticarcinogenic effects, and antiviral activity [60,61][49][50]. It was also shown in several studies that trans-piceid a 3-β-glucosylated form of trans-resveratrol could inhibit platelet aggregation [62,63][51][52] and oxidation of human low-density lipoprotein (LDL).
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Figure 10.
Chemical structure resveratrol.
Flavonoids are largely distributed in vegetables and they have been studied mainly because of their potential health benefits as antioxidants and chemopreventive agents [48][53]. However, until now no recommended daily intake of these compounds has been established mainly because the composition data are incomplete, the biological activities are not well determined, and especially because the bioavailability and pharmacokinetic data are inconclusive. Emerging science from some studies suggests that flavonoid-rich diets may lower the risk of some diet-related chronic degenerative diseases [66,67,68][54][55][56] but a few clinical and laboratory reports indicate that very high doses of certain flavonoids may have adverse effects [69,70][57][58]. Therefore, it is important to accurately assess flavonoid intakes from the perspectives of both disease prevention and safety [71,72][59][60]. The specific action of each phenolic compound from vegetables and medicinal and aromatic plants is not easy to measure since only a small part of it is truly absorbed and, also, it may potentially transform [73][61]. Numerous dietary phenolic compounds are antioxidants able to quench ROS and toxic free radicals formed from the peroxidation of lipids and, consequently, have anti-inflammatory and antioxidant properties. Flavonoids are recognised as preventing the production of free radicals by chelating iron and copper ions to directly scavenge ROS and toxic free radicals and inhibit lipid peroxidation, which may damage DNA, lipids, and proteins, linked to ageing, atherosclerosis, cancer, inflammation, and neurodegenerative diseases [74][62].
Many of these reported biological functions have been attributed to free radical scavenging activity and there has been intensive research on the natural antioxidants derived from plants [32,75,76,77][27][63][64][65]. Hundreds of epidemiological studies have correlated the antioxidant, anticancer, antibacterial, cardioprotective, anti-inflammation, and immune system promoting roles of plants enhanced by phenolic content. Table 1 and Table 2 summarise important bioactivities related to the presence of phenolic identified in vegetables and MAPs widely consumed in the world. For example, Salem et al. [78][66] found that extracts of artichokes rich in polyphenols were capable of inhibiting the production of histamine, bradykinin, and chemokines. These authors discovered that polyphenols present in extracts were capable of acting synergistically, enhancing their anti-inflammatory potential. Additionally, Sharma et al. [79][67] observed that extracts of onion were capable of inhibiting the bacterial growth of Staphylococcus sp. and Escherichia coli, due to the presence of quercetin aglycone, quercetin-4′-O-monoglucoside, and quercetin-3,4′-O-diglucoside. However, the intensity of the antagonistic effect was dependent on the concentration of each compound in each onion variety assessed. In 2018, Dzotam et al. [80][68], using extracts of nutmeg rich in 7-trihydroxyflavone, observed an antibacterial activity of such extracts against the multidrug resistant Gram-negative bacteria Providencia stuartii and Escherichia coli. A recent study showed that Thymus extract rich in rosmarinic acid and 3,4-dihydroxybenzoic acid was capable of exhibiting antiradical and antioxidant properties and enhanced gastrointestinal digestion [81][69].
Table 1.
Phenolic compounds present in some vegetables consumed worldwide and the main bioactivities pointed.

Vegetables

Main Phenolics

MAP Extracts

Main Phenolics Identified

Bioactivities Pointed

Ref.

Bioactivities Pointed

Ref.

Artichoke

(Cynara scolymus L.)

Hydroxytyrosol, verbascoside, apigenin-7-glucoside, oleuropein, quercetin, pinoresinol, and apigenin

Fern

(Asplenium nidus L.)

7-O-hexoside and quercetin-7-O-rutinoside

Anti-inflammatory activities of C. scolymus were found due to the synergistic effect of phenolic compounds. Inhibitory action of artichoke extracts in the inflammatory process such as histamine, bradykinin, and chemokine mediators’ processes was related to the phenolic content.

Antimicrobial activity against Proteus mirabilis Hauser,

Proteus vulgaris Hauser, and Pseudomonas aeruginosa (Schroeter). Migula was shown when fern extracts were applied at different concentrations.

[78][66]

[

89

][77]

Broccoli florets

(Brassica oleracea L. var. italica)

Ginkgo leaves

(

Hydroxybenzoic acid, hydroxycinnamic acid, flavone, polymethoxylated flavone, kaempferol glycosylated and kaempferol derivatives, quercetin-3-O-glucoside and derivatives, isorhamnetin-3-O-rutinoside, isorhamnetin glucoside, and related compounds

Ginkgo biloba

L.)

Hydroalcoholic extracts were capable of directly reacting with and quenching DPPH and Oxygen (ORAC) radicals. Flavonoids and derivatives showed significant positive correlations to DPPH, and ORAC.

Quercitin-3-O-glucoside

[

Ginkgo leaf extracts were capable of decreasing sunburn symptoms in UVB-induced skin in vivo models.

82][70]

[

90

][78]

Celery

(Apium graveolens L.)

High content of apiin, apigenin, and rutin, 3,7-dihydroxyflavone, cyanidin and diosmetin, and terpenes (α-ionone)

Antioxidant activity was highly correlated with the presence of apiin, apigenin, and rutin, mainly due to the lower BDE of O–H bonds in their B rings, which enhanced their H atom donating ability.

[83][71]

Green tea

(Camellia fangchengensis Liang and Zhong)

Procyanidin B1, B2, B3, procyanidin trimer, fangchengbisflavan A and B, catechin 7-O-β-glucopyranoside, epicatechin, (−)-epicatechin gallate, epigallocatechin, and epicatechin 3-(3-O-methyl) gallate

Antiradical and antioxidant activity against in vitro studies was shown.

[91][79]

Garlic

(Allium sativum L.)

Haskap berry

(Lonicera caerulea

The high content of total phenolic content,

vanillic acid, caffeic acid,

p-coumaric acid, ferulic acid, sinapic acid, cyanidin-3-(6′-malonyl)-glucoside)

L.)

A positive and significant correlation between the content of total phenolic content and antimicrobial and antioxidant activity was found. The highest total phenolics content was significantly correlated with the lowest EC50 values for all the tested antioxidant activity assays.

Cyanidin-3-O-glucoside, cyanidin-3-O-rutinoside, chlorogenic acid, quercitin-3-O-rutinoside, quercitin-3-O-glucoside, and catechin

[

Extracts exhibited comparable anti-inflammatory effects to diclofenac which is a COX inhibitory medicine.

84,85][72][73]

[

92

][80]

Ginseng leaves

(Panax ginseng C. A. Mey.)

Nutmeg

Gallic acid and galangin

(

Myristica fragrans Houtt)

30,40,7-trihydroxyflavone

The antioxidant capacity in the lipophilic fraction was higher than those in hydrophilic fractions and positive correlations between antioxidant capacity and total phenolic content, gallic acid, and galangin were found.

Antibacterial activity of nutmeg extracts against the multidrug resistant Gram-negative bacteria Providencia stuartii Ewing and Escherichia coli was observed.

[86][74]

[

80

][68]

Leek

(Allium porrum L.)

Rosmarinic acid, quercetin, and apigenin glycosylated forms and respective derivatives

Extracts showed a favourable antimicrobial activity against

Lavandula

(Lavandula pedunculata Mill.)

O-glucuronide, and rosmarinic acid

Staphylococcus aureus, Bacillus subtilis, and Aspergillus niger. Extracts inhibit Hep2c, L2OB, and RD tumor cells in a dose-dependent manner after 48 h treatment period.

[87][

Caffeic acid, luteolin-7-75

Exhibited highest anti-inflammatory activity in rat RAW 264.7 macrophages by inhibiting nitric oxide production.]

[

93][81]

Onion

(Allium cepa L.)

Rosemary

(Rosmarinus officinalis L.)

Quercetin aglycone, quercetin-4′-O-monoglucoside, and quercetin-3,4′-O-diglucoside

Isorhamnetin-3-O-hexoside, carnosic acid, carnosol, rosmanol, epirosmanol, rosmaridiphenol, rosmarinic acid, and their methoxy derivatives

The antioxidant activity of onions was dependent on variation in the contents of quercetin compounds in all onion varieties assessed. Antibacterial activity against Staphylococcus sp. and Escherichia coli was dependent on variation in both phenolic profile and content.

Antioxidant and antiradical activities were observed. Exerted a direct cytocidal effect via upregulation of nitric oxide (NO) in cancer cells, which in turn acts in a proapoptotic manner and induces cell apoptosis.

[79][67]

[

94][82]

Watercress

(Nasturtium officinale L.)

Coumaric acid, sinapic acid, caftaric acid, quercetin, and quercetin derivatives were the major phenolic compounds identified

The radical scavenging activity (RSA) of root, stem, and leaves of watercress methanolic extracts were highly correlated with the variation of phenolics. Watercress leaves had similar antioxidant potential to that of tocopherol.

Oregano

[

88][76]
Table 2.
The key role of some important phenolics identified in some medicinal and aromatic plant (MAP) species extracts and respective bioactivities.

(

Origanum vulgare

L.)

Rosmarinic acid, 3,4-dihydroxybenzoic acid

The hydroalcoholic extract shows antioxidant activity in vitro and in vivo models. The oral formulation of oregano preserves antioxidant activity from gastrointestinal digestion.

[81][69]

Thymus

(Thymus algeriensis Boiss. and Reut)

Rosmarinic acid, caffeoyl rosmarinic acid, eriodictyol hexoside, kaempferol-O-hexoside, kaempferol-O-hexuronide, luteolin-O-hexuronide, apigenin-C-di-hexoside, and apigenin-O-hexuronide

Methanolic extracts were found to possess substantial antioxidant and antiacetylcholinesterase activities which were correlated to their phenolic contents; however, significant variations were observed between populations.

[95][83]

Sage

(Salvia officinalis L.)

Apigenin, carnosic acid, carnosol, rosmanol, epirosmanol, rosmarinic acid, and their methoxy derivatives

Antioxidant and antiradical activities were observed. Sage extracts were capable of exerting a direct cytocidal effect via upregulation of nitric oxide (NO) in cancer cells, in a proapoptotic manner which induced cell apoptosis.

[94][82]

2. Polyphenols as Prebiotics

As mentioned previously, polyphenols are natural compounds present in many vegetables and MAPs. In the human body, the majority of polyphenols have poor absorptions and they are retained in the intestine for more time where they can promote beneficial effects, specifically by affecting the gut microbiota [96,97,98][84][85][86]. This leads to a mutual reaction between polyphenolic compounds and gut microbiota. The polyphenols are biotransformed into low-molecular-weight phenolic metabolites by gut microbiota resulting in an increase in polyphenol’s bioavailability, responsible for the health effects derived from the consumption of polyphenol-rich plants, which may differ from the native compound found in the plants [97,98,99,100,101,102][85][86][87][88][89][90]. The properties of polyphenols are dependent on the bioactive metabolites produced when they are metabolised by the microbiota [103][91]. At the same time, specific polyphenols can modulate the gut microbial composition frequently by the inhibition of pathogenic bacteria and increase the growth of beneficial bacteria resulting in changes of gut microbial composition [104,105,106,107][92][93][94][95]. Finally, they may act as prebiotic metabolites and enhance the beneficial bacteria. It was demonstrated in animal studies that the consumption of polyphenols, especially catechin, anthocyanins, and proanthocyanidins, increases the abundance of Lactobacillus, Bifidobacterium, Akkermansia, Roseburia, and Faecalibacterium spp. [108][96]. Prebiotics were defined in 1995 as “nondigestible food constituents that beneficially act in the host by selectively stimulating the growth and/or activity of one or a limited number of bacterial species, already resident in the colon” [109][97]. Later, in 2010, prebiotics was defined as “a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora, benefits upon host well-being and health” [110][98]. Bioavailability of polyphenols is influenced by their structural characteristics, mainly by their degree of polymerisation [111,112][99][100]—for example, proanthocyanidins are not absorbed by the intestinal mucosa [112][100], only aglycones and some glucosides can be absorbed [113][101]. Additionally, the prebiotic effect of each polyphenol can be influenced by the plant source and the characteristic of the chemical structure of the compound, along with the individual differences in gut microbiota compositions [114][102].

3. Advances in Phenolic Compounds and Future Research Perspectives

As plant bioactive phenolic compounds have received increasing attention in recent years [115[103][104],116], the research concerning their biosynthesis, biological activities, extraction, purification processes, and chemical characterisations are of the utmost interest. New analytical strategies, such as Nuclear magnetic resonance (NMR) and Mass spectrometry (MS), have demonstrated their use in the identification of new molecular structures and characterisation of plant phenolic profiles [117][105]. Recently, Jacobo-Velázquez et al. [118][106] focused on most recent advances in plant phenolic research such as the functional characterisation of enzymes involved in the biosynthesis of flavonoids; the evaluation of pre- and postharvest treatments to increase the phenolic concentrations of different plants and the chemical characterisation of the phenolic profiles from different plants, and the evaluation of their bioactivities. Therefore, the development of analytical methods for exploring qualitative or quantitative approaches to analyse these bioactive phenolic compounds, in different plants, is essential. Sample preparation and optimisation of the extraction process (solid–liquid extraction, ultrasound-assisted extractions, microwave-assisted extractions, supercritical fluid extraction) are essential for achieving higher accuracy of results [119,120,121][107][108][109]. According to Swallah et al. [122][110] it is difficult to choose a universal method for the preparation and extraction of phenolic compounds from different plants, as they have different polarities, molecular structures, concentrations, hydroxyl groups, and several aromatic rings involved. Their analysis can be carried out by using different methods such as spectrophotometry, gas chromatography, liquid chromatography, thin-layer chromatography, capillary electrophoresis, and near-infrared spectroscopy, which are required to develop rapid, sensitive, and reliable methods [123,124][111][112]. Another challenge is the analysis of polymeric phenolic compounds, as their polydispersity results in poor resolution and detection, an example of which is proanthocyanidins, which have polydisperse structures for which method development is needed; consequently, characterising the unknown phenolic is one of the main challenges in the research on plant polyphenols [117][105].

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