Construction And Application Of Ultrasound-assisted PH-shifting Treatment Of Different Milk Protein Self-assembly Delivery Carriers

Many bioactive substances have shown the poor water solubility and tend to degradate during digestion and applacation.Food-grade delivery systems contribute to improve the water solubility and protect these active ingredents from external environmental damage.The effects of ultrasound-assisted pH-shifting treatment on milk protein concentrate(MPC),micellar casein concentrate(MCC)and whey protein isolate(WPI)were discussed in this research,including the structural,physicochemical and functional properties for carrier construction.Astaxanthin(ASTA)is prone to chemical degradation during processing,storage and digestion due to its highly unsaturated conjugated structure,resulting in the loss of its antioxidant activity.Moreover,ASTA has poor water solubility and poor oral bioavailability,so it is difficult for ASTA to be applcation in functional foods.In this study,ASTA was used as a model substance for nutrient delivery,and the optimal combined treatment conditions were discussed.Moreover,the encapsulation rate,antioxidant activity,bioaccessibility,and stability of ASTA during storage after embedding by milk protein were anlysized.The study of the delivery of astaxanthin by the addition of DHA small molecule ligands to milk proteins was also investigated.The results showed that protein solutions treated with different degrees of ultrasound-assisted alkaline pH-shifting significantly reduced the particle size of MPC(from 224.87 nm to 100.58 nm)and MCC(from 257.83 nm to 82.28 nm),and the solubility of the three protein powders,especially MCC(from 59.02% to 99.50%)had a significant increase.In addition,the turbidity of the three protein solutions had a significant decline after treatment(P<0.05).The exposure of the hydrophobic groups of the proteins by the treatment led to an increase in the surface hydrophobicity of the proteins and an improvement in the foaming properties.The treatment at pH 11 resulted in the highest surface hydrophobicity and foaming properties.The secondary structure of the treated milk proteins at pH 11 was also examined for the reduction ofα-helix and β-sheet structures and the increase of β-turn and random coils.The SDSPAGE analysis revealed that the molecular weight of the casein protein had no change in the combined treatment,but the molecular weight of the whey protein was found,with the appearance of high molecular weight polymers after the treatment.In conclusion,the combined treatment of ultrasound and pH-shifting was found to be effective in improving the physicochemical properties and functions of milk proteinbased powders,especially those containing casein,and pH 11 was the optimal treatment condition for subsequent ASTA delivery studies.The results showed that the combined-treated milk proteins was beneficial to the encapsulation rate of ASTA compared with the untreated milk proteins,and the encapsulation rates of ASTA was 90.26%,92.74% and 96.71% for MPC,MCC and WPI,respectively.The zeta potential of ASTA-milk protein was lower and the structure was more stable after the combined treatment.And the fluorescence intensity of the protein had a drop after ASTA bounding to the milk protein,which caused a fluorescence burst of the milk protein.At the same time,ASTA binds to the hydrophobic region of the protein,thus reducing the surface hydrophobicity of the protein.The DPPH radical scavenging activity,ABTS radical scavenging activity and·OH radical scavenging activity of ASTA-milk protein complexes were significantly increased compared to free astaxanthin(P<0.05).During digestion,astaxanthin in the milk protein delivery system had higher bioaccessibility compared to free astaxanthin,and they were 44.44%,43.96%,and 55.96% for ASTA-MPC,ASTA-MCC,and ASTAWPI complexes,respectively,which showed more effective in improveing the digestion and absorption of astaxanthin for the organisms.The retention of astaxanthin in ASTA-milk protein complexes formed by the combined treatment was higher during storage.Therefore,the ultrasound-assisted pH-shifting treatment of the milk protein delivery vehicle can effectively improve the water solubility,bioavailability and stability of astaxanthin during storage,making it a good vehicle for bioactive substance delivery.Using DHA as a protein ligand to improve the protein’s affinity for biologically active substances,a DHA-mediated ultrasound-assisted pH-shifting-treated milk protein delivery study of astaxanthin was carried out.DHA resulted in a significant increase in the particle size of MPC,MCC,and WPI(P<0.05)and the better dispersion.The zeta potential was lowest at DHA of 0.36 μmol/m L,the potential was the lower and the sample was the more stable,so the optimal addition of DHA was at 0.36μmol/m L.The particle size of ASTA-milk protein-DHA complex formed after the addition of DHA was significantly larger than that of ASTA-milk protein,but its zeta potential was lower.Meanwhile,the addition of DHA significantly increased the encapsulation rate of ASTA by milk protein(P<0.05).ASTA-MPC-DHA and ASTAMCC-DHA complexes had an increase in the bioaccessibility of ASTA by 11.60% and11.48%,respectively.However,interestingly,the bioaccessibility of ASTA-WPI-DHA complexes was greatly reduced.The retention of ASTA during storage was higher in the complexes formed by the addition of DHA compared to the complexes without the addition of DHA ligands.Therefore,DHA can be a good ligand for milk protein delivery carriers to improve the efficiency of milk protein delivery of bioactive substances.