The Study Of Enzymatic Glycosylation Modified Milk Protein Concentrates And Its Application In Microcapsule

Milk protein concentrates(MPC)have high protein content and high digestible amino acid scores,but compared with traditional milk protein ingredients(such as sodium caseinate and whey protein),the emulsification of MPC is poor.It was reported that the casein micelle structure affected the emulsification of MPC and limited the industrial application of MPC.In this work,MPC was modified by green chemical reaction,that is,transglutaminase(TG enzyme)and oligochitosan(COS)were used to modify MPC,and the effect of enzymatic glycosylation modification on the structure and performance of MPC was studied,especially the improvement of its properties;Then,using modified MPC as the wall material and conjugated linoleic acid as the core material,prepared emulsions and microcapsules,exploring the protective effect of modified MPC for the core material.The main results are as follows:(1)The MPC was modified by oligochitosan by TG enzyme.The results of SDS-PAGE showed that the reaction produced macromolecular polymer.Since the electrophoretic band showed that whey protein reacted at pH 7.0 and 37°C for 0~4 h,it almost did not participate in the reaction,the polymer may be the casein component of MPC(α-casein,β-casein,κ-casein)formed by cross-linking reactions within and between molecules.Infrared spectroscopy showed that with reaction time,the degree of glycosylation gradually increased.The absorption peak intensity of the modified MPC at 3300~3600 cm-1 and 1050~1150 cm-1 increased,indicating that the vibration of N-H and O-H groups were enhanced.The three-phase contact angle showed the difference in hydrophilicity and hydrophobicity of MPC with reaction time.The modified MPC first increased in hydrophilicity and then became more hydrophobic.Rheological analysis showed that as the degree of protein crosslinking increasing,the apparent viscosity of the protein solution also increased.The solubility of modified MPC at different pH(pH 2~11)was investigated.The results showed that the enzymatic glycosylation modification is conducive to the improvement of the solubility of MPC,in particular,it is improved by 26.3%~52.0% and 16.0%~24.3% at pH 2~4 and pH 10~11,respectively.Finally,although theoretically crosslinking will inhibit the digestibility of protein in vitro,preliminary investigations on the digestibility of modified MPC in vitro showed that the digestibility of the modified MPC increased by 7.3%,which may be due to the glycosylation of oligochitosan was conducive to improving the digestibility of MPC in vitro.The synergistic effect of protein cross-linking and oligochitosan glycation on the digestion performance of MPC in vitro showed that the digestibility of modified MPC was not inhibited,but relatively improved.(2)The effect of enzymatic glycosylation modification on the emulsification of MPC was investigated.The results showed that the emulsifying activity index of modified MPC at 2 h was increased by 45% compared with MPC.AFM,DLS and QCM-D were further used to characterize the protein particle size and molecular chain viscoelasticity during enzymatic glycosylation,and to explore the mechanism of improved emulsifying activity.The results showed that the particle size of MPC increased from 150 nm to 350 nm within the reaction time of 0~4 h,which was mainly caused by oligochitosan grafting.In addition,the cross-linking of protein molecules would also make the statistical average of the hydrated particle size of MPC(Dh)increase;QCM-D showed that the enzymatic glycosylation had an effect on the molecular chain rigidity and flexibility(molecular chain viscoelasticity)of the MPC itself,and the viscoelasticity of the MPC molecular chain increased within 0~2 h,which may be the grafted oligochitosan chain increased the flexibility of the protein,that is,the molecular chain viscoelasticity increased;However,with reaction time,the degree of protein crosslinking in the modified MPC also increased,so the rigidity of the modified MPC increased.The contribution of the glycosylation and protein crosslinking to the molecular weight rigidity/viscoelasticity competed with each other,so the viscoelasticity of the modified MPC molecule decreased slightly at 4 h.(3)Using modified MPC as the wall material and conjugated linoleic acid(polyunsaturated fatty acid)as the core material which is easily oxidized prepared emulsions.MPC,MPC+COS,WPI+SC(mass ratio of 1:4,the mixture has a similar composition as MPC,but does not contain casein micelle structure)were used to compare the wall material properties with the control group.The accelerated experiment at high temperature during the storage period of 24 h showed that the enzymatic glycosylation modification improved the oxidation resistance of MPC emulsions.Although the change in POV value of the modified MPC emulsion(increment of 20.23±0.47 meq/kg oil)was still slightly higher than the WPI+SC group(increment of 14.13±1.05 meq/kg oil),but had a significant improvement compared to the MPC group(increment of 77.79±9.68 meq/kg oil).The emulsion prepared by the modified MPC obtained protection effect on the core material close to the high-quality wall material WPI + SC group.The conjugated linoleic acid microcapsule powder was further prepared by spray drying.The efficiency of the modified MPC microcapsules reached 95.36±1.43%,which there was not significantly different with the WPI+SC group(microcapsule efficiency was 97.05±0.87%).Accelerated experiments during the storage showed that the CLA retention rate of modified MPC was improved by 33% compared with MPC.Moreover,the change of POV value,the secondary oxidation product malondialdehyde value and the core material retention rate of the modified MPC microcapsules within 14 days were not significantly different from those of the WPI+SC group.SEM and CLSM images showed that the pores of modified MPC powder were reduced,and the distribution of oil on the surface was similar to the WPI+SC wall material group.The above results indicated that when enzymatic glycosylated MPC was applied in microcapsule,it could obtain better core material protection effect comparable to WPI+SC,which was conducive to the industrial application of MPC as new milk protein ingredients.