| Whey is a by-product of cheese production.It has abundant branched chain amino acid,reasonable amino acid ratio and ideal function.However,high intensity heat treatment may reduce the solubility of whey protein,thereby affecting its functionality.Therefore,in order to improve the functional properties of whey protein and expand its application in food industry,the study of this paper uses non-heat treatment which is milder and has less side effects,so the ultrasonic treatment is used to modify whey protein.The whey protein structure,gel properties,digestive characteristics,and antioxidant activity and mechanism of the digesta in cell model were also studied in this paper.(1)The solubility was used as an indicator to investigate the effect of different frequencies and time of ultrasonic pretreatment on whey protein.It was found that the solubility of whey protein pretreated by ultrasound increased by 8.22%.Then the protein structure content was used as indicators to investigate the effect on the structure of whey protein.Ultraviolet spectroscopy,endogenous fluorescence spectroscopy and circular dichroism spectroscopy were used for analysis,it was found that the secondary and tertiary structure of whey protein changed significantly(p < 0.05)compared with the control sample.Single-frequency ultrasonic pretreatment had the least effect on the secondary structure.Compared with the control sample,the content of α-helix in the secondary structure of whey protein decreased by 14.36%(p < 0.05),the content of β-sheet increased by 13.33%(p < 0.05),the content of β-turn decreased by 4.87%(p < 0.05),and the content of irregular curl increased by 6.4%(p < 0.05).Single-frequency and triple-frequency ultrasonic pretreatment had the most significant effect on the endogenous fluorescence intensity of whey protein,and the peak fluorescence intensity increased by 14.0% under the condition of greater influence.The single-frequency ultrasonic pretreatment had the most significant effect on the surface hydrophobicity of whey protein.The content of free sulfhydryl group of whey protein pretreated by dual-frequency ultrasound and triple-frequency ultrasound was the most significant.Under the condition of large influence,the surface hydrophobicity and free sulfhydryl group increased by 93.08% and 73.74%,respectively.Dual-frequency ultrasonic pretreatment had the most significant effect on carbonyl content of whey protein,and the carbonyl content increased by 41.22%.(2)Take solubility and texture and rheological properties of D-(+)-Gluconic acid δ-lactone(GDL)-induced cold set gel as indicators,the effects of multimode ultrasonic pretreatment(different ultrasound frequencies and processing time)on functional properties of whey protein were investigated.The results showed that ultrasonic pretreatment could improve the solubility,apparent viscosity,hardness and viscoelasticity.Dual-frequency ultrasonic pretreatment had the most significant effect on whey protein solubility,and the solubility increased by 10.47%.The optimum processing parameters of single-frequency,dual-frequency and triple-frequency ultrasonic working modes were selected as 20 kHz 30 min,20/35 kHz 15 min,20/35/50 kHz 30 min.Then the apparent viscosity and gel properties were used as indicators to investigate the effects of ultrasonic pretreatment on apparent viscosity,gel texture,gel rheological properties and gel bonding force of whey protein and its GDL-induced cold set gel.It was found that ultrasonic pretreatment decreased the apparent viscosity of whey protein solution,and the gel hardness was significantly(p < 0.05)increased by 50.99%.(3)The degree of hydrolysis(DH),free amino groups and molecular weight change were used as indicators to investigate the effect of ultrasonic pretreatment on digestibility of whey protein.The digestive characteristics of whey protein pretreated with 20/35 kHz 15 min changed most significantly,the DH in gastrointestinal digestion and free amino acid groups content were significantly(p < 0.05)increased by 60.87% and 21.76,respectively.It also increased the digestibility of α-lactalbumin in gastric digestion and β-lactoglobulin in gastrointestinal digestion.(4)The antioxidant activity was used as an indicator,evaluate the protective effect of ultrasonic pretreatment on oxidative damage induced by hydrogen peroxide of whey protein gastrointestinal digesta by the methods of antioxidant activity in vitro(reducing power,DPPH radical scavenging capacity,hydroxyl radical scavenging activity,chelating ability of ferrous ions),contents of reactive oxygen species(ROS),glutathione,degree of lipid and protein oxidation,and antioxidant enzymes in Caco-2 cell model.It was found that,ultrasonic pretreatment of whey protein gastrointestinal digesta antioxidant activity in vitro were significantly(p < 0.05)improved.Under the optimal ultrasonic pretreatment condition(20/35 kHz 15 min),the ROS content in cells was significantly(p < 0.05)decreased by 30.18%.The activities of glutathione peroxidase,catalase and superoxide dismutase increased by 23.52%,57.07% and 11.35%(p < 0.05)respectively.And the level of lipid and protein oxidation decreased.(5)Transcriptome sequencing was used to analyze differentially expressed genes,GO enrichment and KEGG pathway analysis,it was found that the whey protein digesta pretreated by ultrasound could regulate the expression of some genes in Caco-2 cells,so as to improve the ability of Caco-2 cells to resist oxidative damage.In summary,ultrasonic pretreatment can not only improve the functional properties of whey protein,but also change the digestive characteristics of whey protein.Ultrasonic pretreatment also enhanced the antioxidant activity of whey protein gastrointestinal digesta in Caco-2 cell models. |
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