Table 1.

Measurement methods and changes in antioxidant activity of polyphenol–protein complex.

Measurement Methods	Proteins	Polyphenols	Reaction Conditions	Influence on Antioxidant Activity of Polyphenol–Protein Complexes	Mechanism Explanations	Ref.
DPPH method ABTS method	β-lactoglobulin	Ferulic acid	pH 9.0 at 20 °C for 24 h	The free radical scavenging ability of the complexes was significantly improved	The introduction of hydroxyl groups was the reason for the enhanced antioxidant activity of the complexes	[51]
	Zein	Chlorogenic acid, gallic acid, and caffeic acid	pH 12.0 at 25 °C under atmospheric air for 24 h	The antioxidant activity changes of control zein and zein–polyphenol complexes	The synergistic antioxidant effect of polyphenols	[42]
	Zein	Chlorogenic acid and gallic acid	pH 6.5 cross-linked by EDC and NHS	The antioxidant activity of zein was enhanced by covalent grafting of polyphenols	The hydroxyl groups in polyphenols were still available after conjugation and terminate the radical chain reaction	[48]
	Whey protein	Gallic acid, ferulic acid, and tannic acid	pH 9.0 at 25 °C under atmospheric air for 24 h	The phenolic compounds contributed to the increase in the antioxidative activity of the modified whey protein	The antioxidative activity of the complex is dependent on the type and concentration of phenolic compounds	[52]
	β-lactoglobulin	Chlorogenic acid	25 °C for 24 h	The addition of chlorogenic acid improved the free radical scavenging ability of β-lactoglobulin	The protein combined with polyphenols showed better antioxidant capacity	[30]
	β-lactoglobulin	Cyanidin-3-O-glucoside(C3G)	pH 9.0 at 20 °C for 2 h	The free radical scavenging capacities of complexes were improved	C3G conjugation imparted excellent antioxidant properties to the complexes	[53]
	Wheat gluten hydrolysate	Chlorogenic acid	H2O2/ascorbic acid, under atmospheric air at 25 °C for 2 h	The radical-scavenging activity of conjugates increased significantly	The structure changed, and consequently, some antioxidant amino acids exposed after forming conjugates	[46]
	Camel whey	Quercetin	H2O2/ascorbic acid, under atmospheric air at 25 °C for 24 h	The radical scavenging activity of the conjugates produced was enhanced	The quercetin contributed OH groups after conjugation	[47]
	Protein isolate from large yellow croaker roe	EGCG	H2O2/ascorbic acid, under atmospheric air at 25 °C for 24 h	the conjugation significantly increases the antioxidant capacity of native protein	The better antioxidant capacity of the conjugate may be a result of the addition of EGCG to increase the hydroxyl content	[54]
ORAC method	Mung bean globulin	Mung bean polyphenol	At 25, 70, 85 or 100 °C for 2 h	With increased addition of polyphenols, the antioxidant capacity of the system showed a trend of first increasing and then decreasing slightly	The combination of polyphenols with globulins led to the introduction of phenolic hydroxyl groups that can scavenge free radicals. When the interaction ratio was larger, the sites that could supply hydrogen and electrons to free radicals were masked	[55]
	Whey protein	Caffeic acid and EGCG	pH 3.5 or 7.0 at 25 °C for 60 min in the dark	The complexation suppressed the antioxidant capacity compared to the isolated compounds	The suppression may be due to hydrophobic interaction and H-bonding between these compounds	[56]
	Ovalbumin	Catechin	H2O2/ascorbic acid, under atmospheric air at 25 °C for 24 h	The antioxidation ability of ovalbumin was improved via its conjugation with catechins	The conjugation introduced a large amount of phenolic hydroxyl groups	[43]
	Whey protein	EGCG	H2O2/ascorbic acid, under atmospheric air at 25 °C for 24 h	The conjugate exhibited stronger antioxidation ability than whey protein	The presence of EGCG resulted in the increase of hydroxyl groups in whey protein	[57]
HRSA method	β-lactoglobulin	Caffeic acid	pH 2.5, 6.0, or 8.5, cross-linked by EDC and NHS	The activity of the complex significantly higher than the un-derivatized β-lactoglobulin	-	[49]
	β-lactoglobulin	Curcumin	pH 6.0 or 7.0 at 25 °C	In the presence of β-lactoglobulin, the antioxidant capability of complexes is remarkably higher than curcumin alone	The high activity of complexes may be contributed by both curcumin and β-lactoglobulin	[58]
	Myofibrillar protein	Hydrophilic and hydrophobic	pH 9.0 at 25 °C	The incorporation of polyphenol enhanced the antioxidation activities	The enhanced antioxidation activities were related to the hydroxyl groups substituents in a polyphenol ring	[59]
FRAP method	β-casein	Chlorogenic acid	pH 7.0, at 25 or 65 °C for 30 min, 100 or 121°C for 15min	Complexes showed a synergetic effect on FRAP activity	The reducing groups originally buried in β-casein are exposed and enhance the FRAP of the complexes	[60]
	β-lactoglobulin	Chlorogenic acid	pH 7.0, at 25, 65 or 85 °C for 30 min, 100 for 15min, or 121°C for 10 min	The addition of β- lactoglobulin could enhance chlorogenic acid′s ability to resist thermal oxidation	The complexes protected the chlorogenic acid from oxidation reaction	[61]
	β-lactoglobulin	EGCG	pH 7.0, at 25, 65 or 85 °C for 30 min, 100 for 15min, or 121°C for 10 min	The addition of β- lactoglobulin inhibits the antioxidation ability of EGCG	The formation of complex leads to the occupation of active hydroxyl group in EGCG	[62]
	Whey protein isolation	EGCG, quercetin, apigenin, and naringenin	H2O2/ascorbic acid, under atmospheric air at 25 °C for 24 h	The complexes showed higher antioxidant activity, especially whey protein isolation-EGCG complex	A large number of phenolic hydroxyl groups were introduced into whey protein isolation	[13]
	Lentil protein	Quercetin, rutin and ellagic acid	pH 9.0, at 25 °C under atmospheric air for 24 h	The combination of polyphenols and proteins synergistically improves their antioxidant capacity	Coupling of polyphenols to lentil protein imparted protein reduction ability	[18]
	α-lactalbumin	Hydroxy safflower yellow A, neohesperidin dihydrochalcone and naringin dihydrochalcone	pH 7.0, at 25 °C	FRAP of the complex is significantly lower than that of its corresponding phenolic acid alone	The hydrogen bond between α-lactalbumin and chalcone is formed through hydroxyl, thus occupying hydroxyl	[11]
Metal-Chelating method	Porcine plasma protein hydrolysates	Tannic acid and oxidized chlorogenic acid	pH 9.0, at 25 °C under atmospheric air for 24 h	Improved metal chelating activity by trapping transition metals	The incorporation of phenolic compounds improves the antioxidant activity	[63]
	Silk sericin	Hydroquinone and pyrogallol	pH 9.0, at 25 °C under atmospheric air for 24 h	The metal chelating activity of the conjugates was improved	The hydroxyl groups of phenolic compounds can quench oxidants by providing hydrogen atoms	[64]
·O2− scavenging activityLDL oxidation	Gelatin	Catechin	Laccase, pH 7.0, at 20 °C for 24 h, under atmospheric air	Conferred the SOD-like antioxidant activity on gelatin, improved antioxidant activity of inhibiting oxidation of LDL	-	[65]
CAA method	Zein and bovine serum albumin	Resveratrol and caffeic acid	H2O2/ascorbic acid, under atmospheric air at 25 °C for 24 h	Resveratrol-loaded complexes exhibited higher antioxidant ability than free resveratrol	The complex nanoparticles improved chemical stability of the delivery system	[17]
	wheat protein, chickpea protein and soy protein isolate	Blueberry polyphenol	Protein-rich substrates were added to blueberry extract, and then spray drying or freeze-drying	Improved cellular antioxidant activity	-	[66]
	Wheat gluten hydrolysate	Chlorogenic acid	H2O2/ascorbic acid, under atmospheric air at 25 °C for 2 h	The conjugates showed significantly synergistically increased effect cellular antioxidant activity	The covalent binding enhanced the ability to promote the entry of chlorogenic acid into cells	[46]
	Wheat gluten hydrolysate	Chlorogenic acid	Interaction during in vitro digestion	The CAA of the mixture was higher than that of chlorogenic acid or hydrolysate alone	The interaction enhanced the cell entry and the stability of chlorogenic acid	[67]
POV methodTBARS method	Anchovy protein hydrolysate	Catechin, gallic acid and tannic acid	pH 9.0, at 25 °C under atmospheric air for 24 h	The POV level exhibited a remarkable reduction with the addition of conjugates	The conjugates could serve as the electron or hydrogen atom donors, leading to the break of free radical chain and reacting with certain peroxide precursors to prevent the formation of peroxides	[68]
	Ovalbumin	Procyanidin	pH 7.4, at 25 °C for 1 h	The oxidation degree of ovalbumin– procyanidin emulsion was lower than that of ovalbumin emulsion	The interaction altered the sensitivity of oxidation on ovalbumin and improved the ability to scavenge free radicals	[26]
	Soy protein isolate	EGCG	pH 9.0, at 4 °C under atmospheric air for 24 h	the Emulsions stabilized by the complexes exhibited better antioxidant capacity	EGCG delayed oil oxidation by donating a hydrogen from the hydroxyl groups and reduce the reactivity of the transition metal ions and oil by metal chelation	[69]
	Soy protein isolate	EGCG	pH 9.0, at 4 °C under atmospheric air for 24 h	The complexes provided superior oxidation resistance compared to pure protein	The proteins adsorbed at the oil–water interface is more sensitive to oxidation than unabsorbed proteins	[70]
	Oleosin	EGCG	pH 9.0, at 25 °C under atmospheric air for 12 h	The emulsion with the complexes shows high oxidative stability	the complexes had the ability to scavenge free radicals and chelate metal ions	[71]
CD method	Whey protein isolates	lotus seedpod proanthocyanin	H2O2/ascorbic acid, under atmospheric air at 25 °C for 24 h	The conjugate exhibited stronger antioxidant effects than then WPI alone	—	[72]
	Pea protein	Tannic acid	pH 7.0, at 25 °C under atmospheric air for 30 min	The lipid oxidation rate decreased with increasing tannic acid concentration in the emulsions	Tannic acid endowed the complexes with antioxidant activity and led to the formation of a thicker and denser coating around the oil droplets	[73]

DPPH: 2,2-diphenyl-1-picrylhydrazyl radical; ABTS: 2,2′-azinobis (3-ethylbenzthiazoline-6-sulfonic acid) radical; ORAC: Oxygen radical absorbance capacity; HRSA: Hydroxyl radical scavenging activity; FRAP: Ferric ion reducing antioxidant power; ·O²⁻: Superoxide anion radical; LDL: Low-density lipoprotein; CAA: Cellular antioxidant activity; POV: Peroxide value; TBARS: Thiobarbituric acid reactive substances; CD: Conjugated dienes; EGCG: (−)-Epigallocatechin-3-gallate; EDC: 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride; NHS: N-hydroxysuccinimide.