The Effect Of High Pulsed Electric Field On The Viscoelastic Dynamics And Dielectric Properties Of Fruits And Vegetables

A high pulsed electric field(HPEF)technique show potentials to reduce the drying time and energy consumption of a vacuum freeze-drying technique,which can maintain the nutritional and sensory properties of the foods.HPEF is a non-thermal technology,and has been widely used to reduce the population of pathogenic microorganisms and increase the life of liquid foods and fresh products without greatly affecting their nutritional and sensory properties.It is important to understand effects of the HPEF treatment and the optimized operation parameters on changing the dynamic viscoelasticity and dielectric properties of fruits and vegetables for improvement of the vacuum freeze-drying process.In this study,two kinds of fruits and two kinds of vegetables-apple,pear,potato,radish-were chosen as raw material.This study including the following contents:(1)The effects of high pulsed electric field pretreatment on the compressive mechanical properties of fruits and vegetables have been studded.The compressive mechanical parameters such as maximum compressive load,ultimate strength,yield strength and elastic modulus were studied.The microstructure deformation,macroscopic mechanical properties,deformation and failure process of potato was gained by the system of micoscopic image and mechanical test.The fitting equation and the optimal parameters of yield strength and HPEF parameter were analyzed and further optimized by SAS software.The result could provide a theoretical basis for optimize the technologic paramater of freeze-drying and the design of processing equipment.(2)The viscoelastic properties of four kinds of fruits and vegetables after HPEF pretreatment were studied by estimating their relaxation and creep parameters from experimental stress relaxation and creep data.The Generalized Maxwell model and Burgers four-parameter model were fitted to the experimental stress relaxation and creep data respectively.All the models were valid for quantifying the relaxation behavior and creep behavior.Dielectric properties(i.e.dielectric constant and loss factor)have been measured for fruits and vegetables using open ended coaxial probe technique.The variables pulse strength,pulse width,and pulse number of HPEF were studied using a central composite rotatable design.The effects and significance of the models on the response variable and on the dielectric properties of fruits and vegetables were evaluated using a response surface curve and analysis of variance.(3)The effect of HPEF on the viscoelastic properties of have been studied.Immediately before measuring,the fruits and vegetables were washed and hand peeled,and then cut into 13-mm diameter and 5-mm thickness using a cork borer.High pulsed electric field generator was used for HPEF pretreatment the fruits and vegetables with the parameters set as:1)1 000 V/cm of pulse strength,60 μs of pulse length,15 of pulse number,and 2)1 250 V/cm of pulse strength,90 μs of pulse length,30 of pulse number,and 3)1 500 V/cm of pulse strength,120μs of pulse length,45 of pulse number.After HPEF pretreatment,the dynamic viscoelaticity behavior was performed with DMA testing machine using compression platen(diameter of 15 mm).The linear viscoelastic range(LVR)was determined through stress sweeps.A constant amplitude of 10 μm was then chosen(common to the LVR of all samples)to obtain the mechanical spectra as frequency sweeps from 1 to 100 Hz at 25℃ and temperature sweeps from 25 to 90 ℃ at 10Hz.The dynamic viscoelastic test of fruits and vegetables were analyzed by TA Universal Analysis software.Analysis of variance was applied to the data.In order to study the differences among samples,all the data were analyzed with the use of SAS software version 9.2.The microstructure analysis was performed by Scanning Electron Microscopy(SEM)on apple samples at 1)and 2)and 3)pretreatment parameters,and untreated samples.The storage modulus(G’)and loss modulus(G")and loss tangent(tan8)on different HPEF pre-treatment parameters and different oscillation frequency were plotted to characterize the viscoelastic behavior.The theoretical analysis about dynamic viscoelastic mechanics is presented.Results showed that all samples showed a slight linear increase in G’,G" and tanδ as frequency diminished.A statistically significant differences in G’,G" and tanδ between different varieties was confirmed.For apple samples,the HPEF pretreatment led to the decrease for G’and G" by 72.36%and 78.09%,respectively.It primarily attributed to the changes of the cell structure and the intracellular space of the cell as revealed by scanning electron microscopy(SEM).The same tendency of potatoes G’ and G" decreasing with increasing HPEF parameters was observed.However,the radish showed a contrary tendency that the G’ and G" of radish tissue increased with increasing HPEF parameters.High cell turgor pressure may be the main reason for the increase in radish dynamic viscoelasticity after HPEF pretreatment.To pears,as HPEF pretreatment parameters is 1 000 V/cm of pulse strength,60 μs of pulse length,and 15 of pulse number,the G’ and G" of pear tissue is reached maximum.The reason may be due to the changes in pear cell turgor pressure which caused by low HPEF pretreatment,then with the further increasing of the HPEF parameters,pear cell membrance could be damaged and the dynamic viscoelasticity of pear tissue decreased.With the temperature increasing,the storage modulus(G’)of apple,pear,potato,and radish decreased by 34.44%±3.47%,49.04%±10.64%,28.12%±10%,and 55.25%±5.61%,respectively.The loss modulus(G")of them decreased by 66.49%±8.05%,82.12%±15.12%,44.66%±7.79%,and 67.84%±2.54%,respectively.These results can provide useful information for optimization of drying process parameters and improve the industrial pretreatment technique in vacuum freeze-drying processing.(4)The effect of the HPEF pretreatment on the rheological properties of fruits and vegetables juices have been studded.The rheological properties of fruits and vegetables juices were tested,including steady shear viscosity,dynamic shear rheology,and thixotropy.The shear viscosity curve was fitted by the power law model,and the correlation coefficient was 0.87.Because of the flow characteristic index n<1,liquid fruits and vegetables belong to pseudoplastic fluid.The effects of high voltage pulsed electric field pretreatment on the rheological properties of fruit and vegetable juices and the effect of temperature on the rheological properties of fruit and vegetable juices were studied.The results can provide reference for the processing technology of fruit and vegetable juice,the research of the processing technology,the design of the processing equipment,the quality control,the product control and so on.(5)In order to find out the influence of the operational conditions of the HPEF process on the dielectric constant and loss factor of fruits and vegetables,a factorial design has been used.The results were analyzed using the response surface method and analysis of residuals.Ten models were obtained for each fruits and vegetables,including five models between dielectric constant and HPEF at 0.5,2.5,4.5,6.5,8.5GHz and five models between loss factor and HPEF.The highest R2 value of the models for apple is 0.8343 at frequency of 8.5 GHz by ε’,for pear is 0.8610 at frequency of 2,5 GHz by ε",for potato is 0.9124 at frequency of 4.5 GHz by ε",and for radish is 0.8873 at frequency of 4.5 GHz by ε".The effects of the variables and their respective errors were analyzed statistically(calculating 95%confidence limits).Analysis of residuals was also performed,which consisted of the following:a significance test of model fit based on analysis of variance(ANOVA)in order to determine the goodness of fit of the model;calculation of the coefficient of determination(R2),which provides a measure of the proportion of variation(through a regression equation),and the F-test which is an analysis of the variance of a regression.The results obtained were significant at the 95%confidence limit,since the calculated F values(7.27 for apple,8.27 for pear,15.04 for potato,11.38 for radish)were higher than the Tableulated F value(0.0008 for apple,0.0025 for pear,<0.0001 for potato and radish).The coefficient of determination(R2)obtained for this models were 0.8014,0.8610,0.9124 and 0.8873 indicating that 80.14%of the variation in dielectric properties of apple can be explained by the fitted model,86.10%of pear,91.24%of potato,88.73%of radish,i.e.,all the regressions are significant.(6)In order to study the relationship of the dielectric properties and moisture content of fruits and vegetables.Two experimental techniques based on coaxial probe and network analyzers were used to measure the dielectric properties of them in the frequency range 500 MHz to 8.5 GHz.Mathematical models of the relationship between relative permittivity(dielectric constant),dissipation factor and moisture content of fruits and vegetables were esTablelished.It was found that the relative dielectric constant of apples,radishes and potatoes decreased with the increase of frequency from 500 MHz to 8.5 GHz,but the dielectric loss factor of them decreased at first with increasing frequency,and then increased with further increasing frequency.The dielectric properties were significantly different among the three varieties.Moreover,a significant correlation was observed between dielectric properties and drying characteristics.