Ultrasound-assisted free radical modification on the structural and functional properties of ovalbumin-epigallocatechin gallate (EGCG) conjugates

Ultrasound-assisted preparation of protein–polyphenol conjugates and their structural and functional characteristics

“…Compared with SPI, all peak intensities and positions of the SPI–EGCG conjugates are changed significantly, especially for the SPI–EGCG conjugates prepared via the ultrasound-assisted treatment. This indicates that the dual effects of polyphenol grafting and ultrasound induced protein structure rearrangements and conformational changes [18] . For example, compared with SPI, the amide A band of the complex exhibits a significant blue shift by approximately 5 cm −1 , which may be attributed to the reaction between the hydroxyl groups in EGCG and protein amino groups [9] .…”

“…Compared with SPI alone, the contents of sulfhydryl and amino groups in all SPI–EGCG conjugates are significantly reduced, indicating that polyphenols were grafted onto the amino and sulfhydryl groups in the protein structure through C–N or C–S bonds [14] . Furthermore, compared with the non-ultrasonicated SPI–EGCG conjugates, the amino content in the ultrasonicated SPI–EGCG conjugates is significantly reduced, and SPE2 exhibits the lowest amino content, indicating that the free radicals generated during ultrasonication promoted the reaction between polyphenols and protein amino groups [18] . Meanwhile, compared with the non-ultrasonicated SPI–EGCG sample, the ultrasound-assisted composite exhibits a higher sulfhydryl content, which is likely caused by the opening of the protein structure and dissociating SPI aggregates, as well as more exposure of its sulfhydryl groups after ultrasound treatment [16] , [19] .…”

“…The ultrasound-assisted alkali and free radical treatments increased the grafted amount of polyphenols, and the alkali treatment technique produced the highest polyphenol content for the following two reasons. (1) Ultrasonication and the alkali treatment opened the protein structure, exposing more polyphenol-binding sites (more sulfhydryl and amino groups are exposed) [22] , and (2) the free radicals generated by ultrasound synergistically catalyzed the oxidation of polyphenols to quinones under the alkaline conditions, thereby reacting with the side-chain amino acids of the protein [18] . It is noteworthy that the ultrasound-assisted enzyme treatment reduced the grafting amount of polyphenols, which could be caused by the decrease in the activity of polyphenol oxidase owing to the high levels of pressure, temperature, shear force, and free radicals generated by the acoustic hydrolysis of water during ultrasonication [23] .…”

“…In addition, after ultrasonication, the SPI–EGCG conjugates prepared by the ultrasound-assisted traditional methods exhibited a higher peak intensity and larger blue shift at the maximum wavelength. This phenomenon may be attributed to the (1) interactions between the introduced hydrophilic hydroxyl groups and protein hydrophobic groups and (2) exposure of aromatic amino acids during the ultrasound treatment, which made the microenvironment more hydrophilic [18] .…”

Covalent polyphenols-proteins interactions in food processing: formation mechanisms, quantification methods, bioactive effects, and applications

“…In addition, the cavitation effect breaks up the droplets and generates lots of OH radicals in solution, while simultaneously, there is mechanical unfolding of protein structure coupled with reaction of exposed SH and amino groups with OH radicals to produce activated protein derivatives, amid covalent interaction with polyphenolic compounds ( Figures 2C , D ). It was shown that ovalbumin-EGCG (OVA-EGCG) conjugates produced via ultrasound-assisted free radical treatment could be used as potential emulsifiers and antioxidants, thus expanding the application of OVA as a dual functional component ( 33 ). This phenomenon could potentially be attributed to the increased molecular flexibility of OVA resulting from the treatment, which in turn, might contribute to the improved emulsification properties.…”