Effect Of Ultrasound-Assisted Freezing On Processing Quality Of Dough And Its Mechanism

The changes of the gluten network and molecular structure are the main reasons,which result in change of dough quality.The cavitation,mechanical effect and homogenization of ultrasonic waves can induce nucleation,and enhance the mass transfer and heat transfer in the medium,which increase the freezing rate of dough.It can increase the freezing rate,thus reducing the proportion of large ice crystals and improving the quality of the frozen dough.The main results were as follows:（1）In this chapter,we investigated the effects of different ultrasonic powers of dough freezing time,texture characteristics and moisture migration.The results showed that the freezing time of dough was shortened by 9.6 min when ultrasonic-assisted in whole immersion freezing process（UWF）at the power density of 80 W/L.And the freezing time was shortened by 2.0 min,when ultrasonic-assisted in maximum ice crystal generation zone（UMF）at the power density of 60 W/L.Ultrasound-assisted freezing（UAF）treatment significantly increased the elastic modulus（G’）and viscous modulus（G’’）of freezing dough,but the hardness decreased.When the power density was 60 W/L,the transverse relaxation time of the dough was shortened,the bound water content was increased,and the non-freezing water content of UWF and UMF treatment were increased by 1.96%and 3.06%,respectively.So the UAF treatment improves the water holding capacity of dough.（2）The effects of UWF and UMF on the molecular structure of gluten were studied by fluorescence spectrum and infrared spectroscopy.The results showed that the sulfhydryl content（SH）and surface hydrophobicity（H₀）of gluten increased first and then decreased with the increase of power density.UAF increased the relative percentage of gluten?-helix and the secondary structure tended to be ordered.The UAF treatment increased the absorbance of ultraviolet spectrum and fluorescence spectra of the gluten,which changed the spatial conformation of samples.Pearson Correlation Analysis and Principal Component Analysis revealed that UAF treatment improved the quality of frozen dough by changing SH content and gluten secondary structure.（3）The molecular structure of glutenin and gliadin was studied at the power density of60 W/L.It was found that compared with the traditional immersion freezing（IF）,UAF treatment significantly reduced the SH and H₀ contents of glutenin and gliadin,made them closer to the fresh samples.The relative percentage of?-helix of glutenin and gliadin increased by 20.59%and 6.89%respectively,which made the protein secondary structure tended to be ordered.Electron scanning microscopy（SEM）and atomic force microscopy（AFM）intuitively reflected that UAF treatment improved the microstructure of protein samples.（4）Moisture distribution and protein structure changed during dough freezing were in situ monitored by minir Raman spectroscopy and low field nuclear magnetic resonance（L-NMR）.The results showed that through the transverse relaxation time and magnetic resonance imaging（MRI）of the dough during freezing,most of the water was converted from liquid to solid during the freezing of 30-35 min.UAF treatment could alleviated the weakened of hydrogen bond in water molecules during freezing by analyzed the vibration range of water molecules in Raman spectroscopy.The UAF treatment slowed down the gluten secondary structure,SS conformation and amino acid side chain changed in the freezing process,through the analysis of the Raman spectral fingerprint interval.Therefore,UAF treatment improved the quality of frozen dough.