In-situ Raman Spectroscopic Study Of High Pressure Shift Freezing And Application Of High Pressure Shift Freezing On Wolfberry

As an emerging freezing technology,the principle of high-pressure shift freezing(HPSF)is to reduce the freezing point of water by elevating pressure,so that the water in foodstuff can keep in a liquid state at low temperature.With this fact,a large super-cooling degree can be obtained by cooling the sample under pressure then release the pressure suddenly.With the current and conventional technologies,the pressure chamber used in high-pressure freezing equipment must be made by the mental material with high density for ensuring a stable running of device.Thus,it is difficult to realize an in-situ observation on the state change of food under high-pressure,as well as the amount of ice crystals formed in food during HPSF.For this,we set up an in-situ observation platform for high-pressure freezing,and analyzed the Raman spectra of pure water and 15%sucrose solution at high-pressure and low-temperature with this platform.We improved the S_D phase marker method and applied it on determining the phase transition temperature of aqueous solution under high-pressure.According to the phase transition temperature obtained by using this method,different technical parameters of HPSF were set and the experiments were carried out on 15%sucrose solution,then the crystallization during HPSF with different super-cooling degree were analyzed.Finally,in order to verify and evaluate the real applicational effect of HPSF in food,the fresh wolfberry which with sugar content of about 15%was selected as the experimental object,and the applicational effect of HPSF was evaluated from quality change and color change of frozen wolfberry.The main research contents and conclusion of this paper are as follows:(1)Study on Raman spectroscopy of pure water and 15%sucrose solution at high-pressure and low-temperature.The results showed that when the temperature dropped from 10?to-40?,the maximum“red shift”number for pure water was 50 cm^-1 at 200 MPa(generally is150 cm^-1 under 0.1 MPa),and the maximum“red shift”number for sucrose solution was only30.3 cm^-1 at 150 MPa,where the minimum number was at 200 MPa.This indicated that both high-pressure and the addition of sucrose solute could improve the hydrogen bonding combining capacity of water,and the highest combining capacity was at 200 MPa.For pure water and sucrose solution,as the temperature decreased,the area ratio of fitting peaks in the low frequency region increased,but decreased in the high frequency region.For sucrose solution,the variation of area ratio with temperature was least at 200 MPa,and the linear correlation between the area ratio and temperature was the weakest as well.This indicated that the hydrogen bonding energy of the system was the highest at 200 MPa,and the hydrogen bonding structure was least affected by temperature.In conclusion,both high pressure and the addition of sucrose solute can stabilize the hydrogen bonding network structure of water,and the most stable system was at 200 MPa,which was least affected by temperature as well.(2)Study on the phase transition of pure water and 15%sucrose solution under high-pressure by using S_D phase maker method.The system keeps in a liquid state when S_D>1,and by decreasing of temperature,S_D value decreases gradually which means the system changes from the liquid to solid state.When S_D value is decreased to 1,it means the beginning of phase transition,where the inflection point of the plot S_D and temperature represents the phase transition point.The results showed that the phase transition temperature of pure water at 200MPa was-22?,and that of 15%sucrose solution at 150,200 and 250 MPa were-19,-31 and-25?,respectively.This indicated that high-pressure and sucrose solute stabilized the hydrogen bonding network structure of water,then induced the drop of 22 and 9?on phase transition temperature of pure water.The determination of phase transition temperature could provide data basis for the selection on technical parameters of HPSF.For pure water,the selection range of supercooling degree is 0?22?,while for food systems with sugar content of 15%,the selection range are 0?19?,0?31?and 0?25?,respectively.(3)According to the obtained phase transition temperature of sucrose solution under different pressure levels,the technical parameters for HPSF were set and the experiments were carried out on 15%sucrose solution.Then the crystallization of solution during HPSF was studied by Raman spectroscopy.The results showed that the“red shift”of“ice peak”appeared after pressure relief,and the higher super-cooling degree,the lower Raman shift number of“ice peak”.This indicated that the structure of ice crystal formed during HPSF was more stable than the“ice-like”structure formed under high-pressure and low-temperature,and higher degree of super-cooling was conductive to the formation of ice crystals with larger density and more stable structure.The amount of ice crystals formed in sucrose solution within 1 min after pressure relief was calculated.It was found that when super-cooling degree was increased from10 to 30?,the amount of ice crystals increased by about 27%,but only increased 3.2%during20?30?,which indicated that with elevating the super-cooling degree,crystallization rate increased significantly,but began to slow down when higher than 20?.(4)The fresh wolfberries were treated with 150 MPa/-10?,200 MPa/-20?and 250MPa/-15?,respectively,then the quality attribute changes of frozen wolfberries were studied.The results showed that the fullness,cell membrane integrity,hardness,soluble sugar content and drip loss of wolfberries were most affected by super-cooling degree.The higher super-cooling degree,the finer ice crystals formed inside food,and the lower cell damage to wolfberry.It was also found that higher super-cooling induced less water migration after freezing.From a comprehensive consideration,the HPSF parameters of 200 MPa and-20?are more conductive to maintaining the quality of wolfberry.(5)Study on the color change of wolfberries after different freezing conditions,the browning index(BI)and chromaticity(C)value were selected as the major evaluation indicators.The correlation analysis results showed that the BI of wolfberries was mainly related to PPO activity and the contents of Erucic acid,Chlorogenic acid and Rutin.Combined with test results,the pressure from 150 to 200 MPa activated PPO,then resulted in the contents reduction on Erucic acid,Chlorogenic acid and Rutin.The change of C value was mainly related to the content of carotenoids.Combined with test results,the higher super-cooling degree of HPSF(200?250 MPa treatments)caused lower degradation on carotenoids.From a comprehensive consideration,the HPSF parameters of 250 MPa and-15?are more conductive to maintaining the color of wolfberry.