高压脉冲电场预处理对果蔬动态黏弹特性的影响.

High pulsed electric field (HPEF) pretreatment is helpful to induce breakdown and displacement effect in cell of fruits and vegetables, which is useful to improve drying speed. We should study the effect of HPEF on the physical properties of fruits and vegetables, which puts the base for optimizing the parameters and mechanism analysis. In this study, 2 kinds of fruits and 2 kinds of vegetables, apple, pear, potato and radish were chosen as raw material. The moisture contents of apple, pear, potato and radish were approximately 86.89%, 89.46%, 78.35% and 93.61%, respectively. Immediately before measuring, the fruits and vegetables were washed and peeled by hand, and then cut into 13 mm diameter and 5 mm thickness using a cork borer. High pulsed electric field generator was used for the HPEF pretreatment of the fruits and vegetables, and the parameters were set as follows: 1) 1 000 V/cm pulse strength, 60 ?s pulse length, pulse number of 15, 2) 1 250 V/cm pulse strength, 90 ?s pulse length, pulse number of 30, and 3) 1 500 V/cm pulse strength, 120 ?s pulse length, pulse number of 45. After HPEF pretreatment, the dynamic viscoelasticity 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 10 Hz. The test and analysis of dynamic viscoelastic of fruits and vegetables were performed 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 (statistical analysis system) software (version of 9.2). The microstructure analysis was performed by scanning electron microscopy (SEM) on apple samples with the 3 sets of pretreatment parameters, and untreated samples. The storage modulus, loss modulus and loss tangent on different HPEF pretreatment parameters and different oscillation frequency were plotted to characterize the viscoelastic behavior. The theoretical analysis about dynamic viscoelastic mechanics was presented. Results showed that all samples showed a slight linear increase in storage modulus, loss modulus and loss tangent as frequency diminished. A statistically significant difference in storage modulus, loss modulus and loss tangent between different varieties was confirmed. For apple samples, the HPEF pretreatment led to the decrease of storage modulus and loss modulus by 72.36% and 78.09%, respectively. It was primarily attributed to the changes of the cell structure and the intracellular space of the cell as revealed by the SEM. The same decreasing tendency of storage modulus and loss modulus with the increasing of HPEF parameters was observed for potatoes. However, the radish showed a contrary tendency that the storage modulus and loss modulus of radish tissue increased with the increasing of 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 were 1 000 V/cm pulse strength, 60 ?s pulse length, and pulse number of 15, the storage modulus and loss modulus of pear tissue reached the maximum. The reason may be the changes in pear cell turgor pressure caused by low HPEF pretreatment, and then with the further increasing of the HPEF parameters, pear cell membrane could be damaged and the dynamic viscoelasticity of pear tissue decreased. With the temperature increasing, the storage modulus 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 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 the optimization of drying process parameters and the improvement of the industrial pretreatment technique in vacuum freeze-drying processing. [ABSTRACT FROM AUTHOR]