Effect Of Microwave Vacuum Freeze-Drying On Gel Properties Of Egg White Protein

Egg white powder is easy to store and transport,good preservation performance,retain the flavor and nutrition of egg white;Egg white powder is widely used in the food industry,However,the processing process will affect the quality and functional characteristics of protein powder,which limits the application of protein powder.Compared with vacuum freeze drying,microwave vacuum freeze drying has the advantages of high dehydration efficiency,low energy consumption and good product quality.However,this technique has not been applied to the study of egg white drying,and the effect on the gelation characteristics of egg white powder is not clear.Therefore,aiming at the influence of microwave vacuum freeze drying on the gelation characteristics of egg white powder,this study explored the influence of different microwave vacuum freeze drying conditions on the gelation characteristics of protein powder(hardness,cohesiveness,chewiness,resilience,water loss rate)and protein structure characteristics.The moisture migration,protein gel properties and structure changes of egg white powder during microwave vacuum freeze drying were studied.The effects of different drying methods on the gelation characteristics and protein structure of egg white powder were compared and studied.The main research results are as follows:(1)Effects of microwave vacuum freeze drying conditions on the characteristics and structure of egg white protein gel.The effects of microwave vacuum freezing drying conditions on the gelation characteristics(gel hardness,cohesiveness,chewiness and resilience)and protein molecular structure of egg white powder were investigated.The results showed that the gel hardness was the highest when the microwave power was 500 W.With the increase of microwave power,the cohesiveness and chewiness increased first and then decreased.At 500 W,the minimum ratio of a-helix was 20.29%.And at 700 W,the maximum proportion of irregular crimp was 30.10%,and the lysozyme protein molecules aggregated.The enthalpy value of egg white powder protein was inversely related to the gel hardness and reached 245.70 J/g at 500 W.With the increase of vacuum degree,the hardness of egg white powder gel increased first and then decreased.At 150 Pa,the maximum gel hardness is 387.90 g.Elasticity and water loss were inversely proportional to gel hardness.At 150 Pa,the proportion of the a-helix was 19.86% at the minimum and the enthalpy was 227.99 J/g at the minimum.The lysozyme accumulation occurred at 90 Pa.With the increase of loading,the hardness,cohesiveness and chewiness of the gel increased first and then decreased.At 200 g,the hardness reached a maximum of 387.449 g.The a-helix first decreased and then increased(P<0.05).In addition,the peak temperature of the protein gradually decreased.At 300 g,the upper protein was heated for a long time leading to molecular aggregation.(2)Optimization of microwave vacuum freeze drying process of egg white powder.The influence of microwave power,vacuum degree and loading amount on the gel hardness was investigated by single factor experiment.The results showed that the gel hardness increased first and then decreased with the increase of microwave power,vacuum degree and loading capacity,and reached the optimum at the microwave power of 500 W,vacuum degree of 140 Pa and loading capacity of200 g,respectively.By means of Box-Behnken rotation test,the gel hardness was taken as the response value,and the gel hardness of the egg white powder obtained under the drying conditions of microwave power 503 w,vacuum degree 150 Pa and load capacity 195.2 g was obtained to be 393.159±5.23 g.(3)Changes of water migration and gel properties during microwave vacuum freeze drying.Under the optimal conditions,the egg whites were dried.The water migration pattern in the egg whites was determined by low-field nuclear magnetic resonance(NMR),and the gel characteristics and protein structure were analyzed.The results showed that the free water in the early stage of drying was the first to be removed.The water content in egg white decreased exponentially.In the later stage of drying,weak bound water and bound water account for more,which leads to the decrease of drying rate.The gel hardness of egg white powder increased with the extension of drying time,and reached the maximum of 399.11±4.11 g at 5 h.The gel network structure was also denser and smoother.The cohesiveness and chewiness increased first and then decreased with drying time.At 5 h,the total and surface sulfhydryl groups of egg white reached the maximum,52.31% and 3.07%,respectively.In the early stage of drying,the protein molecules were expanded under the influence of drying conditions,and the content of the protein molecules was continuously decreased as the hydrogen bond of the a-helix was broken.The peptide chain structure of the protein was not affected by the drying treatment,and some protein molecules aggregated at 6 h.(4)Effect of drying method on gelation characteristics and structure of egg white powder.Microwave vacuum freeze-drying(MFD)was compared with spray-drying(SD)and vacuum freeze-drying(FD)to explore the effects of different drying methods on the gelation characteristics and protein structure of egg white powder.The results showed that the gel hardness of MFD egg white powder was16.24% higher than that of FD,but 19.82% lower than that of SD egg white powder.The Gel cohesiveness,resilience and chewiness of MFD egg white powder were greater than SD.The a* value(redness)and b* value(yellowness)of MFD egg white gel were significantly lower than that of SD egg white gel.It indicates that MFD is whiter than SD egg white powder gel,which is easy to be accepted by consumers.The structure analysis showed that the content of free sulfhydryl group was proportional to the hardness of the gel.The MFD and SD a-helix were 51.84%and 40.67% lower than that of FD egg white powder,respectively.The order of fluorescence intensity at 340 nm was: FD>MFD>SD.Both MFD and SD have denser gel structures than FD egg white powder.In summary,both MFD and SD can improve the gel hardness of egg whites.Although the spray dried egg white gel has a higher hardness,the microwave vacuum frozen egg white gel has advantages in Cohesiveness and resilience.The gel is whiter and more acceptable to consumers.MFD gels are whiter in color and more acceptable to consumers.MFD egg white powder is more promising.