| Casein exists in the form of casein micelles in milk.Micellar casein concentrates obtained by microfiltration can retain the natural structure of casein micelles.The process of rennet-induced micellar casein gels is very important to produce cheese and other casein-based gel products,which directly affects the structure,texture and taste of gel products.As a protein ingredient,micellar casein concentrates have a broad application prospect in food processing.However,its poor solubility restricts its popularization and application in dairy and other food industries.As an emerging and physical modification technology,ultrasound processing has been used in dairy industry.It has good influences on processing characteristics and does not affect the nutritional value of dairy products,such as improving the solubility and emulsifying characteristics of micellar casein concentrates.However,there are rare studies on the molecular mechanism of the change of gel properties by ultrasound treatment.Polysaccharides can be used as good stabilizers in yogurt and cheese due to its hydrophilicity,high molecular weight and gel properties.The binary composite gel of protein and polysaccharide has been more widely used in food,which has attracted more and more attention.However,there are few studies on the interaction between micellar casein concentrates and polysaccharides composite gel.In particular,there are few studies on the effect of polysaccharides on the formation of rennet-induced micellar casein gels.The objective of this paper was to study the effect of ultrasound treatment on the structure of micellar casein concentrates and explore the correlation mechanism between the structure of micellar casein concentrates and gel properties.We also aimed to obtain better gels and explore the interaction mechanism between modified micellar casein concentrates and polysaccharides.Multi-dimensional analysees(p H,particle size,zeta potential,surface hydrophobicity,protein structure,microstructure of gels,free sulfhydryl content,and fluorescence intensity)were performed to determine the structural changes and gel properties of micellar casein concentrate at different protein concentrations(8%,12%,16%)treated with ultrasound(40 k Hz,500 W,0-60 min).The shear effect produced by ultrasound treatment reduced the particle size of casein micelles.The particle size of 8%casein micelle solution decreased significantly from 337 nm to 282 nm after ultrasound treatment for 60 min.After ultrasound treatment for 60 min,the Zeta potential and surface hydrophobicity of 8% solution increased significantly from-20.7 m V and 137.74 to-24.7 m V and 161.95,respectively.Ultrasound treatment had no effect on the molecular weight of protein.It influenced the secondary structure of micellar casein concentrate.The content of random coil increased,which exposed hydrophobic amino acids and acidic amino acids.A denser gel network with smaller pore size was formed.The gel strength and water holding capacity were improved with the increase in ultrasound treatment time and protein concentration.After ultrasound treatment for 60 min,the water holding capacity and gel strength of 8%rennet-induced micellar casein gels increased significantly from 32.8% and 338 g to 36.2% and 628 g respectively.The content of free amino groups,the number of peptides,the ionic strength of peptides,and the distribution of peptides in the casein micelle solution and rennet-induced gel during gastric digestion were determined.A total of 1061 peptides were identified in the casein micelle solution after ultrasound,and996 peptides were identified in the untreated casein micelle solution.After rennet-induced gel formation,486 peptides were identified in the untreated group and 646 peptides were identified in the ultrasound group.The number of total peptides identified in the ultrasound casein micelle solution more than the untreated group.Large and medium peptides are digested into small peptides or amino acids.With the increase of digestion time,the content of free amino groups showed an increasing trend,and ultrasound treatment for 60 min improved the digestibility of casein micelle solution and rennet-induced gel.In casein micelle solution,ultrasound treatment significantly improved the gastric digestion stage at 90-120 min.In rennet-induced gels,ultrasound treatment affected the digestion rate throughout the digestion process.The casein micelle-κ-carrageenan composite gel was successfully prepared by rennet.The particle size,Zeta potential and Fourier transform infrared spectroscopy of the composite solution were determined to explore the interaction between protein and polysaccharide.The rheological properties of the gel formation process,the texture properties,water holding capacity and microstructure of the composite gel were determined.When the concentration of κ-carrageenan increased from 0 to 0.1%,the Zeta potential of the mixed solution changed significantly from-20.20 m V to-23.73 m V and the particle size increased significantly from 332 nm to 487 nm.The characteristic bond of κ-carrageenan moved.Zeta potential results and infrared spectroscopy results showed that the electrostatic adsorption interaction between κ-carrageenan and casein increased.The concentration of polysaccharide and the temperature conditions affected the rennet-induced gelation process and gel properties.Under room temperature reaction conditions,the addition of κ-carrageenan reduced the gel strength of the composite gel and improved the water holding capacity.When the concentration of κ-carrageenan increased from 0 to 0.1%,the water holding capacity increased significantly from 60% to 71%,and the gel strength decreased significantly from 56.89 g to 44.07 g.The addition of κ-carrageenan reduced the gel strength and water holding capacity at 4℃.When the concentration of κ-carrageenan increased from 0 to 0.04%,the water holding capacity decreased significantly from 60% to 32%,and the gel strength decreased significantly from 51.80 g to 11.36 g.The microstructure results showed that the composite gels form a more uniform and dense gel network structure with smaller pore size at room temperature.Microphase separation occurs after reaction at low temperature,resulting in a more porous structure and larger pore size.The rheological results showed that the rennet coagulation time was reduced with the addition of κ-carrageenan.The particle size,Zeta potential and Fourier transform infrared spectroscopy of the composite solution was measured to explore the interaction between protein and polysaccharide.The rheological properties of the gel formation process were determined to explore the effect of agar concentration on the gel formation process and the gel properties.When the concentration of agar increased from 0 to 0.5%,the Zeta potential of the mixed solution changed from-19 m V to-18.6 m V,there was no significant change.The particle size increased significantly from 357 nm to 467 nm and the characteristic bond of agar moved.The results of Zeta potential and infrared spectroscopy indicated that the interaction in the solution was through hydrogen bonding.The texture,water holding capacity and microstructure of the composite gel were investigated.When the addition of agar increased to 0.5%,the water holding capacity increased significantly from 63.30% to 92.44%,and the gel strength decreased significantly from 73.41 g to 60.57 g.The addition of agar transformed the casein micellar gel from a network-like structure to a sheet-like aggregate structure.Agar and protein both existed as a continuous phase.After heating at 80 °C for 30 min,the gel strength and water-holding capacity of the composite gel decreased compared with those before heating,and the microstructure was destroyed but the original network structure was still maintained.The water holding capacity of the composite gel after heat treatment was significantly higher than that of the casein micelle gel without adding agar.The results indicated the addition of agar significantly improved the water holding capacity and heating stability of the gel. |
PDF Full Text Request