| Pulsed electric field (PEF) technology has the advantage of low processing temperature, short processing time and low energy consumption, which can maximize the preservation of food nutrients, characteristic flavor and freshness. In this paper, we will take the study of Polyphenol oxidase in apple as an example, to explore the passivated effect and passivated mechanism on Polyphenol oxidase caused by PEE. And study the quality of apple products (fresh fruit and juice) based on the passivation. Specific contents and results are as follows:(1) Effect of PEF on inactivating PPO of apple. Based on the factorial experiments (voltage, residence time, frequency, and pulse width), the response surface analysis was used to optimize the conditions to inactivate PPO activity. The optimal conditions for PPO inactivation were:25 kV of voltage,530 Hz of frequency,8 p.s of pulse width, and 45 s of residence time, the PPO activity inhibition rate reached 51.86%.(2) The influence of PEF on fresh apple quality. Texture profile analysis (TPA) was applied to study hardness, springiness, cohesiveness and other related physical and sensory quality indicators. The results showed that, compared with the control group, in the range of selected PEF parameters, hardness, adhesiveness, gumminess, chewiness were decreased by 20.57,13.26, 19.36, and 14.27%. But cohesiveness and springiness increased by 4.89 and 14.08%. With the conclusion got from central composite design to make the minimum changes in textual properties of apple, the electric field parameters were:22 kV of voltage,600 Hz of frequency,30 s of residence time,8.0 μs of pulse width.(3) The influence of PEF on apple’s micro-morphological features. Scanning electron microscopy (SEM) analysis showed that PEF treatment had an impact on apple’s pulp cells. With the increase of the intensity of PEF treatment, pulp cells arranged more disorderly and structured loosely. Cell walls appeared curly and rupture. The effect of materials and patterns of containers on PEF to fruits was in the following order:metal> glass (not covered)> plastic (not covered)> glass (covered)> plastic (covered). The research on change rate of electrical conductivity proved that PEF had an effect on apple’s pulp. A metal packaging of apples would have a sharp increase in the conductivity, while a glass and plastic packaging of apple pieces by PEF treatment the conductivity changed relatively stable.(4) PEF impact on the quality of the fresh apple juice. The results showed that:PEF treatment could inhibit the enzymatic browning of fruit juice. Compared to the control, the freshly prepared apple juice from the pulps treated at the maximal condition by PEF had significantly different color value and browning index (BI) (P<0.01). The total soluble solids and pH did not change significantly (P>0.05), and the ascorbic acid in reduced form did not decrease significantly (P>0.05); A fuzzy mathematical evaluation was applied for the sensory analysis of three kinds of apple juice and score their sensory quality. The comprehensive quality sorting order:Control group>PEF-superior group>heat treatment scoring 85.25 points,82.00 points, 78.75 points.(5) Preliminary exploration of the mechanism of enzymatic inhibition. The nature of the enzyme is protein. Take protein as test, the Ultraviolet spectroscopy (UV), Fourier transform infrared spectroscopy (FT-IR), and SDS-PAGE technique were applied to study the change of protein structure under the PEF treatment. The results showed that with the increase of pulse intensity or the extension of time, the surface of protein free sulfhydryl content increased, while the total amount of sulfhydryl decreases; IR analysis test and subsequent deconvolution (FSD), second derivative spectrum and Gaussian curve fitting processing, calculating the relative percentage protein secondary structure in the amide I and discovering a-helix content decrease and β-fold is on the rise; SDS-PAGE analysis showed that the electrophoretic bands of protein subunits did not change before and after the pulse processing, PEF has no effect on protein subunits... |
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