Hydrogen Peroxide Effects on Natural-Sourced Polysacchrides: Free Radical Formation/Production, Degradation Process, and Reaction Mechanism—A Critical Synopsis

Numerous reactive oxygen species (ROS) entities exist, and hydrogen peroxide (H2O2) is very key among them as it is well known to possess a stable but poor reactivity capable of generating free radicals. Considered among reactive atoms, molecules, and compounds with electron-rich sites, free radicals emerging from metabolic reactions during cellular respirations can induce oxidative stress and cause cellular structure damage, resulting in diverse life-threatening diseases when produced in excess. Therefore, an antioxidant is needed to curb the overproduction of free radicals especially in biological systems (in vivo and in vitro). Despite the inherent properties limiting its bioactivities, polysaccharides from natural sources increasingly gain research attention given their position as a functional ingredient. Improving the functionality and bioactivity of polysaccharides have been established through degradation of their molecular integrity. In this critical synopsis; we articulate the effects of H2O2 on the degradation of polysaccharides from natural sources. Specifically, the synopsis focused on free radical formation/production, polysaccharide degradation processes with H2O2, the effects of polysaccharide degradation on the structural characteristics; physicochemical properties; and bioactivities; in addition to the antioxidant capability. The degradation mechanisms involving polysaccharide’s antioxidative property; with some examples and their respective sources are briefly summarised.


Introduction
Free radicals are a group of reactive atoms, molecules, and compounds with electronrich sites produced as a result of metabolic reactions during cellular respiration [1,2]. In biological/food systems, free radical reactions are predominantly present and play a key role in biochemical pathways [1]. Collectively, free radicals are classified in two groups, namely reactive oxygen species (ROS) and reactive nitrogen species (RNS). The ROS involves oxygen-based highly reactive free radicals whereas the RNS involves nitrogen moieties associated with oxygen. In other words, RNS is a subset of ROS. Since oxygen radicals were discovered as harmful molecules [3], the causative agent of oxidative stress [4], and among essential free radicals generated during cellular metabolism, research interest on ROS has increased impressively. The ROS remains in the group of reactive oxygen-containing molecules possessing one or more unpaired electrons capable of existing independently [5]. The ROS comprises radical species and non-radical species. The former involves chemically reactive oxygen radicals such as hydroxyl, superoxide, peroxy, and alkoxy radicals, whereas the latter involves non-radical derivatives of oxygen such as hydrogen peroxide (H 2 O 2 ,), hypochlorous acid (HOCl), singlet oxygen (O 2 ), ozone (O 3 ), and peroxynitrite (ONOOH) [6]. Moreover, non-radical species though not free radicals can still easily result in free radical reactions in living organisms, once conditions are favourable [7,8]. The ROS can be seen as a natural by-product of aerobic cellular metabolism with a double role, in physiological regulation and a key biological messenger in cell signalling cascades [9][10][11] such that when in low concentration, could cause oxidative stress, and when in excess or high concentration, could significantly damage the cell structures. Oxidative stress has always been hypothesized to be linked to excess production of ROS and/or impaired antioxidant defense capacity [12] or has also been perceived to occur due to an imbalance between the production of ROS and the available antioxidant or radical scavengers [13]. Over time, the accumulation of oxidative stress determined by the action of free radicals can cause lipid, protein, and DNA damage or alterations [7,14,15], as well as many life-threatening diseases including cancer, cardiovascular, Alzheimer disease, and related neurodegenerative diseases [11,16] diabetes mellitus, rheumatoid arthritis, cataracts, respiratory disease and aging process [7].
Numerous ROS entities exist, and H 2 O 2 is very key among them as it is well known to possess stable [7,17] but poor reactivity [1] capable of generating free radicals. In addition, H 2 O 2 is a metabolite of superoxide metabolic reactions [17] and a substrate for the production of hydroxyl radicals during cellular respiration in living organisms. Since its discovery, there has been a substantial increase in research interest regarding in vivo and in vitro formation mechanisms of H 2 O 2 -a ubiquitous endogenous molecule considered toxic to human tissues [18], and how it functions as a biomarker (target) in directing therapeutic delivery for oxidative stress-related diseases [19]. H 2 O 2 is generated via respiratory chain cascade, but also a by-product of cellular metabolism [17]. Chemically, in contrast to superoxide anion (O 2 •− ) and hydroxyl (OH•) radical, H 2 O 2 is not very reactive [1] especially in the absence of transition metal ions and can act as a mild reducing and oxidizing agent. Though H 2 O 2 does not readily oxidize proteins, lipids, and DNA [20], how it gets involved in such numerous physiological processes as cell signal transduction, cell differentiation (apoptosis), growth, and maintenance, as well as mediation of immune responses [17] demonstrates its dual role potential. Like other ROS molecules, and besides the dual functionality of H 2 O 2 , there are reported levels of H 2 O 2 being cytotoxic to cells when the concentration is greater than 50 µM and limited cell cytotoxicity when concentration is at or below about 20-50 µM [20]. The effect of H 2 O 2 in a living system is dependent on the type of cell, its concentration, its physiological state, and duration of exposure [17,[20][21][22]. Interestingly, H 2 O 2 can be part of redox signalling but can induce oxidative destruction of organs and tissues, associated with other inflammatory responses when overexpressed in cells [11,23]. Note worthily, H 2 O 2 can produce very reactive OH• free radicals either in the presence of redox metal ions, such as copper or iron (Fenton reaction) or in the presence of oxygen radical (Haber-Weiss reaction) [24]. Hence, the need for antioxidants to scavenge free radicals from ROS which are causative agents of oxidative stress cannot be over-emphasized.
Antioxidants are substances able to inhibit oxidation caused by free radicals and unstable molecules produced in a living organism. They are applied to foods to inhibit free radical formation due to lipid oxidation and reduce the in vivo free radical concentration after (food) digestion [25]. The oxidative processes of antioxidants are underpinned by free radicals, being propagated by chain reactions in foods. For the most part, lipid oxidation (LOx) is sure to occur via mechanisms that involve complex processes such as autoxidation, enzymatic/thermal oxidation, and photo-oxidation. These processes are already well explained elsewhere [26,27]. For emphasis, antioxidants have two major groups that include: natural and synthetic types. Natural antioxidants have two major groups, namely, enzymatic and non-enzymatic antioxidants. Enzymatic antioxidants have primary (catalase, glutathione peroxidase (GPx), and superoxide dismutase) and secondary (glucose-6-phosphate dehydrogenase and glutathione reductase) enzymes, whereas nonenzymatic antioxidants have carotenoids, cofactors, flavonoids, minerals, phenolic acids, and vitamins. On the other hand, synthetic antioxidants can include butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), tert-butylhydroquinone (TBHQ), and esters of gallic acid (e.g., propyl gallate) [27]. Nowadays, nutritional awareness has significantly influenced consumer demand and acceptance for safe, organic, and natural antioxidant sources. The preference for organic (food) products and/or additives such as antioxidants over synthetic ones by most consumers has increased interest in the use of antioxidants from natural sources. In addition, the components that make the antioxidant functions/mechanism work include its hydroperoxide destroyers, metal chelators, oxygen quenchers, radical scavengers, as well as (antioxidant) regenerators [27].
Polysaccharides are among the most abundant biomaterials which have attracted serious research interest in the field of pharmacology and biochemistry [25,28,29]. In recent years, polysaccharides obtained from natural (plants, animals, and microorganisms) sources have shown promising biological activity, such as antioxidation, antitumor, immunity, hypoglycaemic, and anti-aging properties, as well as cardiovascular, liver, and kidney protective effects [30][31][32][33][34], which is indicative of their high potential, especially in the field of medicine [25,35]. However, the high molecular viscosity/weight of many polysaccharides is considered to limit their pharmaceutical application. Modifying the chemical aspects of polysaccharides can therefore help achieve promising pharmacological/therapeutic targets [36,37]. The biological activities of polysaccharides may depend on several structural parameters, such as the branch structures, glycosidic linkages, and their types, molecular weight, and monosaccharide compositions. For instance, the β-(1→3) and β-(1→6) glycosidic linkages present within the repeating units of the polysaccharides have been deemed vital for anticancer activity [38]. Natural polysaccharides' bioactivities and application are significantly influenced by the molecular weight of the native polysaccharide. For instance, a polysaccharide with larger molecular weight, on the one hand, may not easily penetrate cell membrane barriers in organisms and this could result in impaired biological effects [34,39,40]. On the other hand, Wu et al. [41] have reported higher biological activity in low molecular weight natural polysaccharides. Thus, finding efficient methods for the degradation of natural polysaccharides is crucial, if the biological activity of polysaccharides were to be facilitated. The physical, chemical, enzymatic, and/or a combination of these modification methods could substantially enhance both characteristics and utilities of (natural) polysaccharides.

Justification of Synopsis and Objective Statement
Ranked third among the macromolecules after nucleic acids and proteins that carry essential biological information, polysaccharides and their associated (protein) complexes from natural sources (such as bacteria, fungi, algae, and plants) continue to display effectiveness as non-toxic substances, demonstrated by diverse biological functions [42]. It is important to reiterate that among the active polysaccharides from natural sources, those that are sulfated are considered acceptable and safe [43]. Various polysaccharide extraction methods are possible due to novel techniques like enzyme-assisted extraction (EAE), microwave-assisted extraction (MAE), as well as ultrasonic-assisted extraction (UAE). Additionally, polysaccharide extraction has largely focused on both effects as well as the yield of a single variable, not forgetting its activity (that is, polysaccharides) [44]. However, considering the existence of ROS such as H 2 O 2 , the presence of free radicals that induce oxidative stress, and the need for an antioxidant to curb its (free radicals) overproduction within biological systems (in vivo and in vitro), it is important to understand how the aforementioned (physical, chemical, and/or enzymatic modification) methods can extend the characteristics and utilities of (natural) polysaccharides.
The chemical method of polysaccharide modification (degradation) via oxidation has been of increasing research interest, given the contributions to this body of knowledge specific to the degradation of polysaccharides via oxidation method largely by the H 2 O 2 method. Although studies focused on the degradation of polysaccharides by free radicals increasingly grows, there is a paucity of literature that has synthesized the polysaccharide degradation by free radicals from H 2 O 2 , to particularly demonstrate its potential use in the agro-food industry, together with it being a viable candidate for in vivo biological activity especially when incorporated within the food matrix/system. On this premise, this current critical synopsis articulates the H 2 O 2 effects on the degradation of polysaccharides from natural sources. Specifically, the structure of this synopsis will give emphasis to free radical formation/production, the effects of polysaccharide degradation with H 2 O 2 on the structural characteristics, physicochemical properties, and bioactivities, in addition to the antioxidant capability, and reaction mechanism.

Survey Methodology
To actualise this critical synopsis, the first step taken was to formulate the research questions, ensuring the contribution of this review is relevant to this field, and at the same time, considering the intended/target audience. In the next step, after having agreed with the research questions, we developed a search strategy. This allowed for the establishment of both inclusion and exclusion criteria, which determined the appropriate research articles, and considered relevant specific to degradation of polysaccharides by free radicals from H 2 O 2 . Based on this chosen topic, the search terms such as polysaccharides, natural antioxidants, free radicals, ROS, polysaccharide degradation, H 2 O 2 (formation/sources and reaction mechanism), and natural polysaccharides were identified and selected.
Published papers and their references were collated from databases like EMBASE, Google Scholar, Web of Science, Interscience Online Library, MEDLINE, ScienceDirect, and PubMed. By way of inclusion and exclusion criteria, the research articles not addressing the research question(s) were expunged, considering the year of publication and the language of the article (publications in languages other than English were excluded). Importantly, the data from the literature of selected studies were analysed and relevant information incorporated helped to expatiate the discourse of this conducted critical synopsis.

H 2 O 2 Formation and Factors Influencing its Production
The H 2 O 2 remains a key affiliate of the group of ROS next to superoxide anion (O 2 •− ) and OH• radicals, which is produced through the respiratory chain cascade and a by-product of cellular metabolism [17,45,46]. As a member of ROS, its reactivity targets proteins, lipids, and DNA alteration or damage, which brings about oxidative stress [47][48][49]. H 2 O 2 , as a crucial second messenger in normal cellular signalling [11], has had its overproduction associated with various life-threatening diseases [2,6,11,20] since it is considered cytotoxic [1,20]. Besides being the simplest form of peroxide [5] having two oxygen atoms covalently bonded (single bond), it is endogenously produced in diverse cellular aspects of tissues or organs (peroxisomes, endoplasmic reticulum, and mitochondria) at high levels of oxygen utilisation [7].
However, many studies have reported that H 2 O 2 is a metabolite of O 2 •− produced in the mitochondria during cellular respiration propelled by electron transport chain (ETC) [6,7,17], and a substrate for the production of OH• radical during cellular metabolism [7]. The O 2 •− (half-life of 10 −6 s), OH• radical (half-life of 10 −10 s), and H 2 O 2 (stable half-life) are key members of ROS [7]. H 2 O 2 as a non-radical is fairly reactive (in the absence of transition metal ions) probably due to its stable half-life, but its reactivity is enhanced when it reacts with transition metal ion to yield OH• radical (Fenton reaction), as well as when it reacts with O 2 •− to yield OH• radical (Haber-Weiss reaction) [24].
Fenton reaction : Haber-Weisss reaction : In the first instance (Equation (1)), the Fenton reaction begins with the production of superoxide radical via cellular metabolism. During cellular respiration, some electrons that escaped the electron transport chain can interact with oxygen (O 2 ) to yield (unstable) O 2 •− , a member of ROS. As a non-enzymatic product, the generation of superoxide anion (O 2 •− ) increases with an increase in metabolic rate, which causes cellular components destruction. Consequently, low amounts of superoxide anion (O 2 •− ) are maintained in the cells with the aid of a metalloprotein enzyme [2] called superoxide dismutase (SOD), which catabolizes superoxide anion (O 2 •− ) to H 2 O 2 and subsequently catabolized (detoxified) by the enzyme catalase (metalloenzyme) and glutathione peroxidase (GPx) [7] to water and oxygen.
As shown in Figure 1, there is the electron transport chain, which involves a series of electron transporters embedded within the inner mitochondrial membrane that shuttles the cascade of electrons from the NADH and FADH 2 to molecular oxygen. It is at this process that protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water. Specifically, O 2 •− levels may rise, at times, after the metabolism of alcohol by actions of alcohol dehydrogenase (ADH) enzyme, and NAD + generates a lot of nicotinamide adenine dinucleotide (NADH) [50]. As a result, more H 2 O 2 is produced which may not be either detoxified and or hydrolysed, by the limited amount of enzyme (catalase and glutathione peroxidase). Moreover, the H 2 O 2 would react with the transition metal ions, such as iron (Fe 2+ ) and copper (Cu + ), respectively found in the cells as ferritin and ceruloplasmin [51], and in that way, able to generate OH• radical, which remains a highly unstable molecule [52,53].
On the other hand, the Haber-Weiss reaction (Equation (2)) mechanism, which generates OH• radical from H 2 O 2 and O 2 •− catalysed by iron as described by Fritz Haber and his student Joseph Joshua Weiss [5] is considered as among the key foundation of free radical biochemistry [53]. Figure 2 shows the role of H 2 O 2 in cell dysfunction and transformation. Notably, both the Fenton and the Haber-Weiss reactions have proven the role of H 2 O 2 as an intermediate and substrate respectively, for the production of OH• radicals responsible for oxidative stress and cellular damages [5,53]. The OH• radical, as a neutral form of hydroxyl ion (OH − ), remains the most reactive member of the ROS because of its short-lived half-life, estimated to be about 10 −10 s. The destructive action of this unstable molecule (OH•) is implicated in series of cell dysfunction, transformation, and even death [5,54,55].
0, x FOR PEER REVIEW 6 of 33 Figure 1. The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to molecular oxygen. In the process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water (Source: Electron Transport Chain [50]).
On the other hand, the Haber-Weiss reaction (Equation (2)) mechanism, which generates OH• radical from H2O2 and O2 •− catalysed by iron as described by Fritz Haber and his student Joseph Joshua Weiss [5] is considered as among the key foundation of free radical biochemistry [53]. Figure 2 shows the role of H2O2 in cell dysfunction and transformation. Notably, both the Fenton and the Haber-Weiss reactions have proven the role of H2O2 as an intermediate and substrate respectively, for the production of OH• radicals responsible for oxidative stress and cellular damages [5,53]. The OH• radical, as a neutral form of hydroxyl ion (OH -), remains the most reactive member of the ROS because of its short-lived half-life, estimated to be about 10 −10 s. The destructive action of this unstable molecule (OH•) is implicated in series of cell dysfunction, transformation, and even death [5,54,55]. The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH 2 to molecular oxygen. In the process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water (Source: Electron Transport Chain [50]). The formation of H2O2, a member of ROS, is the outcome of an unavoidable aerobic metabolism or reaction [56]. Many studies have understood H2O2 as poorly reactive yet somewhat stable. However, the destructive activity and cytotoxic effects of H2O2 have been owed to the formation of OH• radical, when it reacts with either superoxide anion(s) or transition metals. The formation/production of H2O2 in vitro and in vivo [57] can, The formation of H 2 O 2 , a member of ROS, is the outcome of an unavoidable aerobic metabolism or reaction [56]. Many studies have understood H 2 O 2 as poorly reactive yet somewhat stable. However, the destructive activity and cytotoxic effects of H 2 O 2 have been owed to the formation of OH• radical, when it reacts with either superoxide anion(s) or transition metals. The formation/production of H 2 O 2 in vitro and in vivo [57] can, respectively, take place in certain beverages and humans. The production of H 2 O 2 by various food systems has been reported previously. For instance, studies have shown H 2 O 2 production from thiol-rich proteins during barley (malt) beer production [58] and in the association of L-cysteine with Cu(II)-catalysts during malting and beer brewing [59]. Also, H 2 O 2 can be generated in vitro in certain polyphenol-rich beverages such as green tea, black tea [60][61][62], red wine, and coffee [60,63]. Polyphenols are a large group of compounds naturally found in fruits, vegetables, and certain beverages, possessing antioxidant properties. The protective effect of polyphenols which is attributed to its antioxidative property is believed to be due to its ability to act as a free radical scavenger, quenching the activities of ROS such as O 2 •− radical, OH• radical, and H 2 O 2 [62,64]. In contrast, the production of H 2 O 2 by polyphenols understood from the (polyphenol) pro-oxidant action via autoxidation [60,65] has been associated with induced mutagenesis and promotion of cancer [62], as well as to promote oxidative damage to DNA, lipids, and proteins in the presence of transition metal ions under certain conditions in vitro [66].
The  [61] (+)-catechin, and gallic acid [67]. Further, Grzesik et al. [63] reported that (black and green) teas and coffee but not cocoa were shown to produce considerable concentrations of H 2 O 2 . In such beverages (green tea, black tea, and coffee), the H 2 O 2 generating property is poised to coincide with the content of phenolic compounds. This suggested that polyphenols are liable for H 2 O 2 production in beverages. Results from previous researches indicated that the formation of H 2 O 2 from the aforementioned phenolic compounds in an aqueous solution would depend on the content and composition of polyphenols, structure of the phenolic compound, oxygen, metal ions, temperature, pH, and incubation time [66]. Therefore, the combined effect of these conditions would result in the formation as well as diverse concentration(s) of in vitro H 2 O 2 production.

(b) In vivo production of H 2 O 2
The H 2 O 2 can be naturally produced in vivo for example, in humans, as a by-product of oxidative metabolism [5] by the dismutation of superoxide radical, both non-enzymatically and catalysed by superoxide dismutase (SOD) enzymes [68]. Besides the superoxide dismutase (SOD) enzyme, there are other oxidase enzymes such as glycollate and monoamine oxidases, directly linked to the production of H 2 O 2 [68,69]. Additionally, the glutathione peroxidase and thioredoxin-linked peroxidase are enzymes contained in the mitochondria. However, the effectiveness of H 2 O 2 removal by the enzymes remains unclear, given the rate at which it is generated in the mitochondria [70,71]. Although certain tissues may be exposed to higher H 2 O 2 concentrations with the mitochondria being at the helm of activity in human cells, most cells may also be exposed to certain H 2 O 2 levels.
The formation of H 2 O 2 in the oral cavity, oesophagus, stomach, respiratory system, kidney, urinary tract and bladder, vascular endothelial and circulating blood vessel as well as ocular tissues are well documented. Lambert et al. [72] reported the in vivo generation of H 2 O 2 in micromolar concentrations produced when green tea solution is held in the mouth. Halliwell et al. [68] reported that beverages like green tea, black tea, and instant coffee contain H 2 O 2 at concentrations above 100 µM, probably because of the joint action of autoxidation of antioxidants in the beverages and oral bacteria. Production of H 2 O 2 by oral bacteria has been previously reported, although its impact on oral tissues remains unknown.
Moreover, H 2 O 2 can be present in exhaled air of healthy humans although its concentration is higher in those with inflammatory lung diseases and in cigarette smokers [68]. This H 2 O 2 in exhaled air can possibly be produced by oral bacteria since cells lining the respiratory system are exposed to about 21% oxygen. Besides H 2 O 2 detected in freshly voided human urine [68], it could be at a higher concentration in the urine of coffee drinkers [63]. The presence of H 2 O 2 in human urine may be due to the dismutation of superoxide radicals which may come from the diet via autoxidation. Adahi et al. [73] detected some traces of superoxide dismutase (SOD) in the urine. Additionally, the exhalation of H 2 O 2 and its removal through urine could be part of the human excretory mechanism.
The in vivo generation of H 2 O 2 in blood plasma by the action of xanthine oxidase enzyme is degraded in part by traces of catalase. Like in blood plasma, ocular fluids have been reported to generate H 2 O 2 constantly which is usually rapidly removed [74,75]. Though the origin of ocular H 2 O 2 is unknown, oxidation of ascorbate may likely be a contributory factor [76]. Importantly, it is expected that H 2 O 2 would accumulate as a result of the joint action of in vivo dismutation of superoxide radical and in vitro autoxidation of phenolic compounds (antioxidants). However, there is no evidence that these beverages (teas and coffee) promote or induce mutagenesis and carcinogenesis in vivo. Possibly, any threat the in vitro generated H 2 O 2 from polyphenols is likely to pose could be inhibited by phenolic compounds and other constituents of the beverages [60], or even catalase and glutathione peroxidases in the body, and decomposed by the reaction with biomolecules [62]. In a research on dietary antioxidants as a source of H 2 O 2 , Grzesik et al. [63] reported that 27 out of 52 antioxidants generated H 2 O 2 , suggesting the availability of other constituents capable of exerting antioxidant property in the beverage.

Factors That Influence the Production of H 2 O 2
As was earlier mentioned, the in vivo and in vitro formation of H 2 O 2 is dependent on some factors that could inhibit or enhance its production, and they include the composition and structure of phenolic compounds, oxygen, metal ions, pH, enzyme, and incubation condition (temperature and time).
Phenolic compounds with OH-group, substituted at the 2 and 4 positions of phenol, would generate H 2 O 2 [80]. Such production would significantly increase by the insertion of a third OH-group. This, therefore, corroborates the relationship between the H 2 O 2 generating property of phenolic compounds and their structures [61] (Figure 3). Akagawa et al. [62] reiterated this, in that the insertion of a third adjacent hydroxyl group, that is, the presence of gallate substituent (pyrogallol moiety) in the catechins, would increase the formation of H 2 O 2 . In practice, a significant amount of H 2 O 2 can be produced by gallic acid. The H 2 O 2 generating property in these beverages was found to coincide with the content of phenolic compounds, indicating that polyphenols are responsible for the production of H 2 O 2 in beverages.

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Factors That Influence the Production of H2O2
As was earlier mentioned, the in vivo and in vitro formation of H2O2 is dependent on some factors that could inhibit or enhance its production, and they include the composition and structure of phenolic compounds, oxygen, metal ions, pH, enzyme, and incubation condition (temperature and time).
Phenolic compounds with OH-group, substituted at the 2 and 4 positions of phenol, would generate H2O2 [80]. Such production would significantly increase by the insertion of a third OH-group. This, therefore, corroborates the relationship between the H2O2 generating property of phenolic compounds and their structures [61] (Figure 3). Akagawa et al. [62] reiterated this, in that the insertion of a third adjacent hydroxyl group, that is, the presence of gallate substituent (pyrogallol moiety) in the catechins, would increase the formation of H2O2. In practice, a significant amount of H2O2 can be produced by gallic acid. The H2O2 generating property in these beverages was found to coincide with the content of phenolic compounds, indicating that polyphenols are responsible for the production of H2O2 in beverages.

(b) Oxygen and metal ions
Oxygen is essential for the viability and maintenance of cell metabolism but has been found to play a key role as a potential promoter of in vitro and in vivo free radical

(b) Oxygen and metal ions
Oxygen is essential for the viability and maintenance of cell metabolism but has been found to play a key role as a potential promoter of in vitro and in vivo free radical reactions [81,82]. Electronically it is an highly excited molecule that exists in two excited states upon activation. In the mitochondria, the formation of H 2 O 2 is initiated by oxygen during the mitochondrial respiratory chain, where about 85% of oxygen is metabolized. The transition metal ions mainly considered by researchers are iron, manganese, copper, and sometimes zinc because these species are major players in free radical or electron transfer reactions [1]. The joint action of transition metal ions and oxygen in the production of H 2 O 2 has been reported in many studies. According to Mochizuki et al. [83], tea polyphenols are easily oxidized to the corresponding quinones by oxygen (O 2 ) in the presence of transition metal ions, incidentally resulting in the formation of O 2 •− and H 2 O 2 through autoxidation ( Figure 4). In addition, higher H 2 O 2 formation has been detected in tea infused in tap water containing transition metal ions than a tea infused with deionised water [63], thus suggesting that the autoxidation of polyphenols would produce H 2 O 2 , catalysed by transition metal ions. The autoxidation of polyphenols by oxygen to yield semiquinone radical and O 2 •− in the absence of metal ions was reported by Akagwa et al. [62] though with a slow reaction rate due to the low redox potential of the O 2 /O 2 -. On the other hand, metal ion (Cu 2+ ) catalysed autoxidation of polyphenols to yield semiquinone radical and Cu + have also been reported. Subsequently, O 2 •− and quinone is produced when the generated semiquinone radical is rapidly oxidized by oxygen [83]. Hence, the generated O 2 •− helps to make the H 2 O 2 spontaneously via dismutation [62], and also to H 2 O 2 and semiquinone radical via oxidation of polyphenols.

(c) pH
Multiple studies have reported the correlation between pH and H2O2 formation from tea polyphenols. Mochizuki et al. [83] reported that the autoxidation of polyphenols depends on the pH of the medium (tea) and is accelerated by alkaline pH. Similarly, Grzesik et al. [63] reported that the rate of autoxidation of compounds increased with pH, which corroborated with findings of Rinaldi et al. [84] and Akagawa et al. [62] wherein the strong electron-donating ability of acid dissociated the phenolic compound, as polyphenols underwent autoxidation at a higher rate with increased pH. Nakayama et al. [61] detected H2O2 in polyphenol-rich beverages (tea and coffee) when the pH value is above 5.8 and its concentration increased with pH up to 8.0. A maximum formation of

(c) pH
Multiple studies have reported the correlation between pH and H 2 O 2 formation from tea polyphenols. Mochizuki et al. [83] reported that the autoxidation of polyphenols depends on the pH of the medium (tea) and is accelerated by alkaline pH. Similarly, Grzesik et al. [63] reported that the rate of autoxidation of compounds increased with pH, which corroborated with findings of Rinaldi et al. [84] and Akagawa et al. [62] wherein the strong electron-donating ability of acid dissociated the phenolic compound, as polyphenols underwent autoxidation at a higher rate with increased pH. Nakayama et al. [61] detected H 2 O 2 in polyphenol-rich beverages (tea and coffee) when the pH value is above 5.8 and its concentration increased with pH up to 8.0. A maximum formation of H 2 O 2 at pH 7.0-9.0 for tea polyphenols has also been reported [62]. However, the addition of milk and lemon respectively decreased and prevented H 2 O 2 formation in tea [60,63]. This was shown by the lower pH ranges in milk (pH 6.5-6.7) and lemon (pH 2-3), which suggested H 2 O 2 formation as pH-dependent, where a decrease in pH reduces or prevents its (H 2 O 2 ) formation and vice versa.

(d) Incubation condition
The incubation condition of tea polyphenols plays a significant role in H 2 O 2 formation. The action of temperature-time incubation conditions cannot be over emphasised. Studies have shown that an increase in incubation temperature and time increases the rate of H 2 O 2 formation. In the same vein, Nakayama et al. [61] reported an increase in H 2 O 2 formation when the incubation temperature of tea was increased from 37 • C to 77 • C. However, Nakayama et al. [66] reported that the concentration of H 2 O 2 in tea increased concavely with temperature when increased from 32 • C to 82 • C. Also, the amount of H 2 O 2 formation in tea has been reported to increase almost linearly with incubation time [66], suggesting that autoxidation of polyphenols is also time-dependent.

(e) Enzymes
Enzymes have been found to participate in the production of H 2 O 2 . Some H 2 O 2producing enzymes include but are not limited to superoxide dismutase (SOD), and other oxidase enzymes such as glycollate and monoamine oxidase [68,69], glucose oxidase, galactose oxidase, and cholesterol oxidase [85]. Superoxide dismutase catabolizes O 2 •− to form H 2 O 2 [7], while glucose oxidase catalyses the oxidation of β-D-glucose by oxygen to form H 2 O 2 and gluconolactone ( Figure 5) [85]. Also, cholesterol oxidase catalyses the oxidation of cholesterol by oxygen to yield H 2 O 2 ( Figure 6) while galactose oxidase catalyses the oxidation of primary alcohol and polysaccharides (for example, D-galactose) and primary alcohols by oxygen, resulting to corresponding aldehydes and H 2 O 2 as shown in the equation below:   Notably, some key situations associated with the formation of H 2 O 2 which can combine together via series of chain reactions under both in vitro and in vivo conditions are briefly presented in Table 1. In the mitochondria (in vivo), oxygen combines with an escaped electron in the electron transport chain to form O 2 •− , which is subsequently oxidised to H 2 O 2 by superoxide dismutase (SOD). This implies that the formation of H 2 O 2 in the mitochondria is dependent on the O 2 •− , which may increase in concentration with an increasing amount of NADH. On the other hand, in beverages (in vitro) such as tea and coffee, autoxidation of polyphenols by oxygen, catalysed by transition metal ions to yield corresponding quinones (depending on the composition and structure of phenolic compound) subsequently generates H 2 O 2 , whose concentration is dependent on the beverage incubation condition (temperature and time).  Notably, some key situations associated with the formation of H2O2 which can combine together via series of chain reactions under both in vitro and in vivo conditions are briefly presented in Table 1. In the mitochondria (in vivo), oxygen combines with an escaped electron in the electron transport chain to form O2 •− , which is subsequently oxidised to H2O2 by superoxide dismutase (SOD). This implies that the formation of H2O2 in the mitochondria is dependent on the O2 •− , which may increase in concentration with an increasing amount of NADH. On the other hand, in beverages (in vitro) such as tea and coffee, autoxidation of polyphenols by oxygen, catalysed by transition metal ions to yield corresponding quinones (depending on the composition and structure of phenolic compound) subsequently generates H2O2, whose concentration is dependent on the beverage incubation condition (temperature and time).  Polysaccharides are the most common biopolymers and copious organic material in existence [25,92,93]. For instance, the carbohydrate molecules of carbon and oxygen atoms differ in size and range from one to thousands of units. Simple sugars (monosaccharides) are the smallest unit of a polysaccharide molecule. Examples include glucose, fructose, mannose, and galactose. Polysaccharides are broadly classified as homo-polysaccharides, which can include, one type of monosaccharide units like glycogen, cellulose, and starch, and the hetero-polysaccharides, which can include two or more types of monosaccharide units like hyaluronic acid, heparin, etc. Furthermore, cellulose is a structural polysaccharide found in plants that forms the main structural component of plant cell wall [25] whereas glycogen and starch are storage polysaccharides found in animals (muscles and liver), and plants respectively. Besides the well-known animal/plant polysaccharides, there are other less popular polysaccharides that arise from sources like bacteria, fungi, and algae [25].
Ever since polysaccharides were discovered to possess some biological activity, there has been increasing research interest on this specific aspect in the field of biochemistry, life science, pharmacology, and medicine, together with its possible applications [25,94]. Garcia-Gonzalez et al. [95] highlighted the use of polysaccharides as key functional ingredients in the production of bio-based materials in food, cosmetics, medical devices, and pharmaceutics. However, polysaccharides obtained from plants, animals, and microorganisms have been shown to possess diverse properties not limited to antioxidative, anti-inflammatory, antimutagenic, antitumor, health-promoting, and therapeutic properties [40,94,[96][97][98]. With the many bioactivities/properties exerted by polysaccharides, our focus in this current review was on the resurgent antioxidative effect of polysaccharides, which can be explored as a potential novel functional food additive in food formulation.

Polysaccharide Degradation
There is evidence that, despite exerting antioxidative properties, not all polysaccharides have bioactivity, which is the situation, for example, of untreated pachymaran [99]. Additionally, some polysaccharides might not exhibit any bioactivities given their structural and physicochemical properties [94]. Many studies have shown that the functional role of polysaccharides is dependent on their degree of polymerization and molecular weight [46]. This was reiterated by Li and Shah [100] wherein the bioactivities of most polysaccharides were influenced by molecular weight and structural characteristics such as monosaccharide compositions, linkage patterns, anomeric carbon configuration, branching features, degree of polymerization, and sequence of sugar units. Despite these factors, several studies have shown that polysaccharides' bioactivities are significantly influenced by the molecular weight of the native polysaccharide because polysaccharides with too high (large) molecular weight may not easily penetrate cell membrane barriers in organisms due to their high viscosity and low solubility, and will therefore result to impaired biological effect [34,39,40,94,101]. Hence, the physicochemical properties and bioactivities of polysaccharides are significantly influenced by their structure; so the structure of polysaccharides must be modified via degradation to the desired molecular weights that will enhance the bioactivities of polysaccharides.

Methods of Polysaccharide Degradation
The growing realisation of the potential novel products with diverse applications which may be obtained from degradation (depolymerisation) of the structural integrity of polysaccharides gave rise to the development of different degradation methods [102]. Generally, the degradation of polysaccharides can be achieved via physical, chemical, and enzymatic methods [40,102,103], as well as biological means [94], to obtain a diversity of structural derivatives. Additionally, some other methods of polysaccharide degradation such as thermal, ultrasonic, mechanical, lyophilisation-mediated, metal-catalysed, free radical-catalysed [102], and irradiation [104] degradation have been previously described. Besides the advantages of primary degradation methods such as being simple to operate or easy to handle (chemical method), environmentally friendly (physical method), low cost, good safety, and mild reaction condition (biological method), each method also has its limitations. Multiple studies have reported environmental pollution and serious safety concerns (chemical method), utilization of specialized equipment which limits their industrial application (physical method), and difficulty in the screening of enzymes and microbes (biological method), as some of the challenges attributed to these treatment methods for polysaccharide degradation [105][106][107][108]. With the aforementioned methods of polysaccharide degradation, the chemical degradation through the oxidation method (either singly or combined) continues to attract researchers' interest. For example, Zhao. [34] showed that the free radical-catalysed H 2 O 2 method is frequently employed when they investigated polysaccharides from Tremella fuciformis. Furthermore, the fragmentation (degradation) of polysaccharides by ROS has been increasingly evidenced, as the bioactivities and overall functionality (of polysaccharides) gets altered [46]. Noteworthily, treating polysaccharides with H 2 O 2 (only) does not generally degrade its high molecular weight, instead, the activation of H 2 O 2 is necessary to produce highly reactive OH• radicals that can enhance its degradation property [106].
H 2 O 2 , as a member of ROS, during polysaccharide degradation operates under mild reaction conditions, with high efficiency, despite its inability to compromise the integrity of the main chain (polysaccharide) structure [40,98,109]. However, the various degradation methods employed are mediated by different reaction mechanisms, to yield potentially distinct products. Recently, desirable modified polysaccharides have been obtained with higher bioactivities by combining different degradation methods [94]. According to Wu et al. [104], the synergistic effect of combining different polysaccharide degradation methods significantly affects the depolymerisation kinetics of the process. Based [34,36,40,104,[110][111][112], as well as a compound treatment of H 2 O 2 -vitamin C-microwave treatment [107]. In other words, H 2 O 2 serves as an important source of reactive oxygen species (ROS) for these combined treatments, to assure adequate production of OH• radicals for significant degradation of polysaccharides.

Effect of Polysaccharide Degradation with H 2 O 2
H 2 O 2 continues to be a promising candidate for polysaccharide degradation. Polysaccharide degradation (depolymerisation) with H 2 O 2 or in combination with another technique, by changing its functional role, would initiate its (polysaccharide) modification. Largely, the H 2 O 2 is used in polysaccharides degradation given that it is easily available, easy to handle, and environmentally friendly since it decomposes to water and oxygen [106,113] as well as its ability to form reactive OH• radical through Fenton reaction [104]. In free radical-catalysed depolymerisation, the hydroxyl radicals via bond (glycosidic linkage) cleavage on the polysaccharides chain [102], would bring about molecular modification. Consequently, the modification of polysaccharides would lead to changes in physicochemical (relative molecular mass, viscosity, and solubility) and structural properties, as well as improved bioactivities. Li et al. [94] obtained enhanced biological properties in degraded/modified polysaccharides. Thus, both physicochemical and structural properties will be tersely discussed below: (a) Physicochemical Properties Polysaccharide degradation can cause numerous changes in the physicochemical properties, from a decrease in its crystallinity as a result of structural rearrangements [114] to the improved tensile strength of its chitosan polysaccharide [94,115]. To understand the effect of polysaccharide degradation on the physical and chemical property, we attempt to examine aspects like its molecular weight, intrinsic viscosity, and water solubility, which are discussed briefly below.
(i) Molecular weight The molecular weight of polysaccharides is the basis for their bioactivities. Polysaccharides with high molecular weight are associated with low bioactivity, due to the compactness of their structures, low surface area, and stronger intramolecular hydrogen bonds [30]. Interestingly, the intrinsic viscosity and water solubility of polysaccharides depends significantly on their molecular weight, which could indicate a clear correlation between these properties. High molecular weight polysaccharides could lead to increased viscosity and decreased water solubility. On this premise, the degradation of high molecular weight polysaccharides could result in a modified (polysaccharide) type, able to easily permeate the cell membrane barriers of organisms, which could, in turn, exert improved bioactivity. Suggestively, the bioactivity of polysaccharides significantly would depend on their molecular weight.
The molecular weight of degraded or modified polysaccharides would decrease significantly, compared to the native (unmodified) polysaccharide. For example, Zhang et al. [34] reported a successful degradation of high molecular weight polysaccharides from 599,580 to 26,895 Da. In addition, the rate of polysaccharide degradation into lower molecular weight (degraded) polysaccharide significantly depends on the concentration of degradation reagent (H 2 O 2 ). A high molecular weight Astragalus polysaccharide (11,033 Da) treated with different concentrations of H 2 O 2 (4%, 6%, and 14%) was significantly modified (degraded) to lower molecular weight polysaccharides of 8376, 4716, and 2600 Da, respectively [30]. Therefore, whilst degradation of polysaccharides enhances their bioactivities, the rate of degradation into lower molecular weights is determined by the concentration of H 2 O 2 which was utilized during the degradation. Notwithstanding that low molecular weight polysaccharides can exert added bioactivities, degraded polysaccharides with very low molecular weight, on the other hand, might bring about a reduction or loss in polysaccharide bioactivity as the inactive polymeric structure develops [101]. That is to say, the destruction of the polysaccharide ring structure by the H 2 O 2 degradation system in very low molecular weight polysaccharide also breaks its intramolecular hydrogen bond, making the carbonyl group be in an open chain, thereby resulting in reduced bioactivity.

(ii) Intrinsic viscosity
Multiple studies have shown the correlation between molecular weight and intrinsic viscosity. Due to the high molecular weight of polysaccharides, the high intrinsic viscosity of polysaccharides limits their absorption and utilization in vivo [40,94]. Notably, the intrinsic viscosity of degraded or modified polysaccharides decreases significantly with a decrease in molecular weight compared to the native polysaccharide. Xu et al. [40] observed a decrease in intrinsic viscosity of degraded polysaccharides from 2.26 to 1.43 mPa.s. Similarly, Wu et al. [104] observed a reduction in intrinsic viscosity of chitosan polysaccharides by 82.19% and 70.04% when the chitosan polysaccharide was treated with SPP irradiation-H 2 O 2 and SPP irradiation degradation method, respectively. This implies that the rate of reduction of intrinsic viscosity of polysaccharides is higher when two or more polysaccharide degradation methods are combined. Zhang et al. [34] observed a successful degradation with a 77.07% decrease in the intrinsic viscosity of polysaccharides, from 285.24 to 65.39 mL/g using Fe 2+ -H 2 O 2 -Vc degradation method. Similarly, Li et al. [94] observed a decrease in intrinsic viscosity of modified polysaccharide solution from 34.67 to 22. 92 mL/g which was believed to be attributed to a reduction of the hydrogen bonds. It is evident that the molecular weight of polysaccharides is concordant to their viscosity. This was consistent with Zhang et al. [36] who reported that the decrease in viscosity of polysaccharides is attributed to the reduced interaction between macromolecules, due to a slight decrease in their molecular weight.
(iii) Water solubility Most bioactive polysaccharides have not found useful applications because of their poor water solubility, a result of their high molecular weight [40,94]. On the other hand, many studies have recorded a significant increase in water solubility of polysaccharides after degradation or depolymerisation,. Xu et al. [40] found that the solubility of de-graded polysaccharides increased from 18.24 to 24.40 mg/mL with a decrease in molecular weight. Free radical-catalysed degradation offered by H 2 O 2 has been found to increase polysaccharide solubility because of the inauguration or incorporation of carboxyl groups, ionic groups, or other hydrophilic groups into the polymer structure [94]. In addition to increased water solubility, degraded polysaccharides had a greater surface area which translates to higher reaction rates [34,98]. Indeed, the increase in water solubility of degraded polysaccharides would bring about a significant transformation of polysaccharides especially to their application, to exert improved bioactivities.

(b) Structural Properties
The structure of polysaccharides influences their biological activity [30]. Polysaccharide degradation can produce different types of structures with varying bioactivities which lays a solid foundation for the functionality of the degraded or modified polysaccharide [94]. Free radical-catalysed degradation of polysaccharides offered by H 2 O 2 reduces the association of amylose molecules by introducing small amounts of carbonyl and carboxylate groups [116] into the polymer structure which can improve the hydrophilic properties of the modified or degraded polysaccharides [94,117]. However, the surface area of the fragmented structure of degraded or modified polysaccharides could decrease significantly compared to its native polysaccharides, specifically due to the destruction of the aggregates or original interconnection in the polysaccharides by free radical-catalysed treatment offered by H 2 O 2 [40,44]. Generally, by using only H 2 O 2 or in combination with other degradation techniques, the main chain structure in the degraded or modified polysaccharides may not alter, but the side chain structure can change [30]. The degradation of chitosan structure, according to Tian et al. [118], only alters the side groups, but no significant change took place within the main chain structure. Some enhancement can take place at the side groups on the main chain polysaccharide structure, such as hydroxyl (-OH) and carboxyl (-COOH) groups, especially after the polysaccharide degradation. This can happen owing to the exposure of its effective active sites (groups), which thereby increases its content. Moreover, Wang et al. [30] detected a slight increase in degraded polysaccharide after the polysaccharide chain was broken by the OH• radical produced by the H 2 O 2 degradation system, thus exposing the -COOH groups, whereas Zhao et al. [119] detected high levels of -COOH group in degraded polysaccharides. Similarly, Chang et al. [120] showed that a polysaccharide with a molecular weight of 129 KDa, degraded to lower molecular weights of 60 and 52 KDa, recorded an increase in their uronic acid content from 70.8% before degradation to 79.5% and 86.2% in the degraded polysaccharide, respectively. This was also consistent with the findings of Xu et al. [40] and Wu et al. [104]. Additionally, the helical structure of the main chain and hydrophilic group (hydroxyl group) influences the polysaccharides' bioactivities. Also, polysaccharides with compact structures tend to possess stronger intramolecular hydrogen bonds, leading to less accessibility or exposure of its effective active sites (groups). To shed more understanding on this, we tersely discussed polysaccharides' bioactivities and their workings.

(c) Bioactivities
There is increasing evidence that the functionality of polysaccharides such as their bioactivities can be enhanced only when their solubility in water increases, and this can be achieved via degradation or modification of polysaccharides [121]. In particular, the increase in hydrophilic properties of degraded polysaccharides, which has been attributed to their decreased molecular weight, lowered intrinsic viscosity, increased surface area, and structural changes (incorporation of carboxyl groups into the polymer structure), makes these degraded polysaccharides soluble in water, and as a result, exerts better bioactivities. Despite the many bioactivities, which include but are not limited to, antioxidant activity, antitumor activity, anti-inflammatory activity, anti-HIV activity, anticoagulant activity, antibacterial activity, antiviral activity, and immunomodulatory activity [94,121], our focus in this current literature synthesis is on the antioxidant activity of polysaccharides. Some researchers, however, have demonstrated that improved antioxidant activity of polysac-charides would associate with its degree of water-solubility [34,37]. To understand the workings of bioactivities, we shed some light on polysaccharides' antioxidant activity in terms of their radical scavenging capacity, reducing power, and metal ion chelating ability.
(i) Antioxidant activity Antioxidants are substances or compounds that inhibit oxidation caused by free radicals and unstable molecules produced in food or living organism. Antioxidants are specific in their action by performing their primary function of breaking chain reactions through free radical scavenging and also by performing their secondary function (preventive role) through chelation of metal ions that catalyse a free radical reaction. The antioxidant activity of substances or compounds is displayed through series of reaction mechanisms which include transition metals chelating ability, hydrogen electron transfer, and single electron transfer [81]. However, much research has shown that various polysaccharides obtained from plants, animals, fungi, bacteria, and algae possess antioxidant properties [94], especially when such a polysaccharide is the degraded or modified type. Specifically, polysaccharides are compounds with large molecular weights and as a result would have lesser non-reducing end contents, hence, weaker antioxidant activity [30]. Based on this fact, the degradation of polysaccharides can effectively better their antioxidant activity. Furthermore, series of studies on free radical-catalysed polysaccharide degradation have shown that the antioxidant activity of degraded polysaccharides was ascertained by determining their radical (OH•, O 2 •− , DPPH • , and ABTS) scavenging property, reducing power and metal ion chelating ability.

•
Radical scavenging capacity The scavenging capacity of free radicals (OH•, O 2 •− , DPPH • , and ABTS) by degraded polysaccharides has been documented in many studies. The OH• radical is one of the most unstable (half-life of 10 −10 s) and strongest oxidants among the ROS, randomly able to attack biomolecules, and cause oxidative damage to certain proteins, lipids, and nucleic acids [109,122]. Degraded polysaccharides have been found to show higher OH• radical scavenging capacity compared to the native (undegraded) polysaccharides. The OH• radical scavenging rate would be dependent on the concentration of degraded polysaccharides [109,122]. Wang et al. [30] reported an increase in radical scavenging rate when the concentration of degraded polysaccharide was increased from 0.15 mg/mL to 3 mg/mL. Similarly, Xu et al. [40] found that degraded polysaccharides of varying molecular weights exhibited different degrees of scavenging effects on the hydroxyl group, and this effect increased with decreasing molecular weight of polysaccharides. In addition, a polysaccharide with a molecular weight of 3.26 × 10 4 KDa, degraded to 1.30 × 10 4 and 9.62 × 10 3 KDa was found to show an increased OH• radical scavenging rates from 66.89% before H 2 O 2 degradation to 92.58% and 93.21% in the degraded polysaccharide respectively [40].
The O 2 •− radical (half-life 10 −6 s) is another key member of ROS, produced continuously in biological systems, and could either cause harm to cell components or serve as a precursor to other unstable reactive free radical species [40,100]. Just like in OH• radical, degraded polysaccharide showed higher O 2 •− radical scavenging capacity than the native polysaccharide, and the scavenging capacity was also enhanced with increasing concentration of degraded polysaccharide. The O 2 •− radical scavenging capacity of native polysaccharide (21%) and degraded polysaccharide (39%) at a concentration of 0.2 mg/mL increased significantly to 53.41% (native polysaccharide) and 62.12% (degraded polysaccharide) when the concentration was raised to 1.2 mg/mL [40]. Similarly, Wu et al. [104] also reported a significant increase in O 2 •− radical scavenging capacity (rate), when the concentration was 0.2 mg/mL with a scavenging rate of 12% and 32% for native polysaccharide and degraded polysaccharide was increased to 1.2 mg/mL, resulting in an increased scavenging capacity of 20% and 66% for native polysaccharide and degraded polysaccharide, respectively. Furthermore, DPPH • and ABTS radicals are (stable) free radicals frequently employed in radical scavenging assays for evaluating antioxidant capacity (of relevant compound(s)).
Similarly, studies have shown that degraded polysaccharides showed significant scavenging capacity on DPPH • and ABTS radicals, over the native polysaccharide [25,40,104,123] and the degree of the radical scavenging capacity of degraded polysaccharide on DPPH • and ABTS radical were concentration-dependent. This implies that the degraded polysaccharide can easily react with free radicals to terminate their radical chain reaction or donate electrons from the degraded polysaccharide's free hydroxyl group to reduce the radical to a more stable form [124]. Studies have also shown that degraded polysaccharides have more hydroxyl groups than their native polysaccharide which is an indication of a more available reductive hydroxyl group, which could accept and eliminate the free radicals [40,125]. Importantly, an increase in concentration and lower molecular weight of degraded polysaccharides play a significant role in the radical scavenging capacity [104], thus higher antioxidant capacity.

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Reducing power The reducing power of a compound is a potential indicator of its antioxidant activity [40,126]. Degraded polysaccharides with low molecular weight have strong reducing power. The reducing power of a blackcurrant polysaccharide with an initial molecular weight of 3.26 × 10 4 KDa degraded to a molecular weight of 9.62 × 10 3 KDa was found to increase from 0.183 before degradation to 0.414 in the degraded polysaccharide [40]. Additionally, a positive correlation exists between the concentration of polysaccharides and reducing power. The reducing power of native polysaccharide (0.02) and degraded polysaccharide (0.3) at a concentration of 0.2 mg/mL increased strongly to 0.11 in native polysaccharide and 0.87 in degraded polysaccharide when the concentration was increased to 1.2 mg/mL. This implies that reducing power increases with a decrease in molecular weight and increasing concentration of polysaccharides. The reducing power of degraded polysaccharides comes from their ability to donate a hydrogen atom to destroy free radical chain reactions, thus exerting its antioxidant activity [40,104]. The increased reducing power in low molecular weight degraded polysaccharides is attributed to the higher number of reducing and non-reducing ends, compared to their native polysaccharide [109].

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Metal ion chelating ability Transition metals such as iron and copper are well known to enhance the production of free radicals which are precursors of oxidative stress [40]. They are regarded as stimulants/facilitators of lipid peroxidation (Fenton reaction) consequently yielding more reactive alkoxyl and peroxyl radicals [5]. Amongst the transition metals, iron is the most important pro-oxidant due to its high reactivity [5,127]. Besides being an essential mineral for normal body physiology, the overproduction (excess) of iron can result in cellular damage from oxidative stress induced by free radicals. As a result, reduction in the concentration of the catalyzing transition metal-mediated autoxidation through metal ion chelation is commonly considered as an antioxidant mechanism [40,128]. Polysaccharides can inhibit free radical production by chelating metal ions instead of scavenging them directly [123]. Further, Chun-hui et al. [129] isolated two polysaccharide fractions (GAPS-1 and SAPS-1) from A. barbadensis Miller that showed good metal ion chelating ability against ferrous ion. Similarly, a polysaccharide from the fern Lygodium japonicum was reported to also show strong metal ion chelating potential [130,131]. Polysaccharide's metal ion chelating ability has been shown to serve as a secondary (preventive) antioxidant because of their redox potential reduction ability which stabilizes the oxidized form of the metal ions [132].
Furthermore, studies have shown that degraded polysaccharides exhibited significantly higher metal ion chelating ability on Fe 2+ over the native polysaccharide [40], as the metal-chelating effects of the degraded polysaccharides depend on concentration. Polysaccharide with a molecular weight of 3.26 × 10 4 KDa degraded to 1.30 × 10 4 and 9.62 × 10 3 KDa was found to show an increased metal chelating ability from 5.0% before H 2 O 2 degradation to 22.28% and 23.32 in the degraded polysaccharide respectively when concentration was increased from 0.2-1.2 mg/mL [40]. Specifically, the efficiency of polysaccharides in chelating metal ions is dependent on its structural configuration especially for functional groups of -OH, -SH, -COOH, -PO 3 H 2 , -C=O, -NR 2 , -S-, and -O- [123,133]. This was corroborated by Fan et al. [134] who confirmed that the chelating effect of degraded polysaccharides from leaves of Ilex latifolia Thunb. was partly due to the presence of functional groups such as the carboxyl group in the polysaccharide structure. Similarly, the presence of hydroxyl groups and the number of hydroxyl group on a polysaccharide structure have been found to influence the chelating affinity for the ferrous ion [123,135]. In line with polysaccharide's ability to act as a metal ion chelator, its chelation mechanism is owed to complexes formed between the metal ion and aforementioned functional groups on the polysaccharide structure [136]. Polysaccharides having exclusively hydroxyl groups in equatorial position (for example, glucose molecule) do not seem to form complexes with metal ions, rather a solely favorable arrangement or configuration of two or three hydroxyl groups allows for the formation of metal ion complexes [137]. This implies that structural configuration and the arrangement of functional group (OH group) on a polysaccharide influences the formation of metal ion complex, which is the stable form of the oxidized metal ion.

The Reaction Mechanism Involving Polysaccharides' Antioxidant Capacity on Free Radicals: A Primer
Polysaccharide has two mechanisms of degrading the OH• radical: (a) Removal of already generated OH• radicals through hydrogen electron transfer or single electron transfer [30,81]; and (b) inhibition of OH• radical formation [30,138]. Figure 7 shows the proposed reaction mechanism of OH• on a polysaccharide chain. Firstly, OH• radicals attack polysaccharides at different carbon positions and only a few of the polysaccharide backbones are influenced by these attacks, while oxo groups are simultaneously introduced into the polysaccharide chain, without altering the main chain. An OH• radical can abstract a hydrogen atom of the C-H chain of polysaccharides to form water, while on the other hand, a carbon-centred radical (carbon-free radical) is formed. In aerobic conditions, the carbonfree radical is oxidized to peroxy radicals and subsequently, reacts with hydroperoxyl radical (HO 2 •, often referred to as superoxide anion radical, O 2 •− in its ionized state) to produce ketone, lactone, or aldehyde [139,140].

Concluding Remarks and Future Prospects
Polysaccharide degradation by the free radical-catalysed system offered by H 2 O 2 is indeed effective in improving the bioactivity, especially the antioxidant activity. The current review has highlighted how H 2 O 2 singly or in combination with other techniques can degrade natural polysaccharides. How the effects of polysaccharide degradation would influence the structural characteristics, physicochemical properties, and bioactivities (antioxidant capacity) of degraded polysaccharides has also been enumerated in this review. The free radical-catalysed polysaccharide degradation method offered by H 2 O 2 is frequently employed because of its mild reaction conditions, high efficiency, inability to alter the integrity of the main chain structure of polysaccharides, and ease of OH• radical formation. Overall, the antioxidant capacity of polysaccharides is not dependent on a single factor but rather a combination of several related factors. Compared with the native polysaccharide, degraded polysaccharides exerts their antioxidant activities due to their low molecular weight, increased water solubility, decreased intrinsic viscosity, and modified structural characteristics.
Polysaccharide degradation comes with free radical scavenging capacity on OH• and O 2 •− radicals, transition metal chelating ability, and reducing power through their hydrogen atom donating ability. However, there is a need to further understand what happens before the degradation, specific to the level of purity of polysaccharide with antioxidant properties, and this should be the direction of future literature syntheses/experimental programs to supplement existing information, given that other antioxidant compounds such as polyphenols may form part of the coextractives retained in the polysaccharide. Degraded polysaccharides may find wide application in the food and beverage industries as novel functional ingredients. The direction of future studies should focus on how polysaccharides and other food components interact within the food or beverage matrix, as this may either change, enhance or inhibit the antioxidant potential of the polysaccharide. The information this overview provides should prove useful, not only to biotechnologists, food scientists, and technologists, but also processors, as well as product developers of novel and functional food formulations.