Effect Of Ultrasonic Treatment On The Structure And Functional Properties Of Milk Protein

Low-frequency ultrasound?20¹00 kHz?is a safe and environmentally friendly m odification technology.It can change structure,spatial conformation,and aggregation of proteins through cavitation,mechanical,thermal and microfluidic effect,thus affecting t heir physicochemical and functional properties.In this study,defatted milk was used a s material,we used different ultrasonic treatment,three power including 200 W,400W,600 W power combined with 7 treatment time including 2,4,6,8,10,15 and 20min,to study structural and functional properties of normal temperature and preheate d skim milk?65°C for 30 min,90°C for 15 min?,whey protein and casein.The ai m is to obtain the best ultrasonic treatment conditions to improve the different charact eristics of dairy products,and provide theoretical basis for the application of ultrasoun d modification technology in milk processing.The results are as follows:1.Ultrasonic treatment had no significant effect on the p H value of skim milk,whey protein and casein?p<0.05?;200 W and 400 W ultrasonic treatment significantly decreased the average particle size and turbidity of sonicated samples with the increa se of treatment time?p<0.05?,the average particle size and turbidity were decreased f ollowed by increased after 600 W ultrasonic treatment;fluorescence spectral analysis s howed that different ultrasound power caused the maximum fluorescence intensity and emission wavelength(?_max)of skim milk,whey protein,casein changed in different tin me,which leading to change of tertiary structure.2.The emulsification activity index and emulsifying stability of ultrasonic treatment were significantly higher than those in the control.The best results of 200 W ultrasonic treated skim milk was at 10 min?1.242 m²/g?and 15 min?106.16%?;400 W was at 4 min?1.353 m²/g?and 8 min?109.15%?;600 W was at 2 min?1.278 m²/g,108.87%?.The above properties of whey protein were the best after the ultrasonic treatment of 200 W,400 W at 20min?1.808 m²/g,90.71%,1.926 m²/g,97.13%?,and 600 W at 4 min?1.781 m²/g,96.21%?.The emulsification activity and emulsification stability of casein 200 W at 20 min?1.711 m²/g,87.80%?,400 W at 20 min?1.742 m²/g?and 4 min?89.04%?,600 W at 10 min?1.738 m²/g,93.52%?were the largest.3.In the foaming and foaming stability,in addition to the foaming stability of 600 W treated skim milk were lower than the control,the other ultrasound conditions were improved the foaming and foaming stability.200 W sonicated skim milk was maximal at 8 min?149.87%,88.70%?,400 W at 4 min?156.80%?and 20 min?93.26%?,and 600 W at 2 min?152.67%?.Whey protein 200 W was maximal at 20 min?231.33%,90.20%?,400 W at 6 min?243.07%,92.32%?and 600 W at 2 min?230.53%,90.69%?.Casein 200 W,400 W was maximal at 20 min?166.13%,69.18%,168.67%,69.64%?and 600 W at 10 min?167.87%,69.26%?.4.Milk was pretreated at 65°C for 30 min combined with different time treatment of 400W showed that emulsification activity index,emulsification stability,foaming and foaming stability were the best at 15 min?1.150 m²/g?and 4 min?95.63%?,2 min?160.67%?,20 min?45.80%?,respectively;90°C 15 min preheated milk at 20 min?1.285 m²/g?,8 min?90.37%?,8 min?166.67%?,20 min?56.73%?were the best,respectively.The average particle size of preheated milk at 65°C for 30 min decreased first and then increased slightly with the increase of the ultrasound time.At 90°C for 15 min,the average particle size showed a decreasing trend.Fluorescence spectrum analysis showed that the fluorescence intensity of preheating treatment at 65°C for 30 min and 90°C for 15 min was changed,the hydrophobicity changed,and the tertiary structure changed.5.In terms of rheological properties,the ultrasonic treated skim milk showed obvious shear thinning effect.the viscoelasticity and thixotropy of the skimmed milk after sonication increased compared to the control;the gel point of the skim milk was delayed.After gel formation,the storage modulus G'is greater than the loss modulus G''.6.Scanning electron microscopy observed that small particles on the surface of non-lipid casein particles of skim milk was gradually increased after different time ultrasonic treatment of 400 W,but no significant difference were found in different treatment of skim milk in casein micelles.