Date of Award

13-11-2024

Document Type

Thesis

School

School of Electrical & Electroncis Engineering

Programme

Ph.D.-Doctoral of Philosophy

First Advisor

Dr.M.Balasubramanian

Keywords

Pulsed Electric Field, Test Chamber, Loading Effect, Response Surface Methodology, Box Behnken, Log Reduction

Abstract

Pulsed Electric Field (PEF) treatment is one of the efficient non-thermal food processing techniques which is being preferred as a replacement for thermal pasteurization methods. The effectiveness of PEF treatment was measured in terms of reduction in the microbial load in the food, extension of shelf life of the food and retention of nutritional properties of the food. The successful implementation of the PEF treatment depends upon various aspects such as design of pulse generator, parameters of pulse generator, shape and size of the treatment chamber and most importantly the characteristics of each food items. Design and fabrication of a suitable pulse generator for PEF treatment is mainly governed by the type and volume of food to be treated. For the processing of food items using PEF technique, different shapes of pulse voltages such as square pulse, exponential pulse and oscillatory pulse voltages were preferred.

However, each type of food offers different loading effect to the pulse generator due to its varying impedance based on the type of food and volume of food to be treated. Identifying suitable type of pulse shape and pulse generation circuit to get optimum treatment for a given food type is an important task. In this work, the main focus was to analyze the impact of different volumes of juice samples on the performance of pulse generator circuit. The performance of pulse generator was investigated by analyzing the loading effect of juice samples on pulse generator as a simulation study, followed by numerical computation to optimize the key process parameter and validation of results using a hardware circuit.

In order to analyze the loading effect, different shapes of pulse voltages such as square pulse, exponential pulse and oscillatory pulse voltages were generated using power electronic converter circuits and the same was applied to different volumes of juice samples. The juice samples were considered as an R-C equivalent circuit and the equivalent circuit was connected as a load to pulse. Two distinctly dimensioned test chambers with parallel plate electrodes were fabricated for different volumes ranging from 1 ml to 500 ml.

The generated pulse voltages were applied to the juice samples and response of the pulse generator were measured in-terms of voltage, electric field intensity, pulse power consumption and specific energy dissipation. The loading effect were analyzed for both the test chambers (named, Test Chamber 1 & Test Chamber 2) dimensions. From the analysis, it was observed that the change in pulse output voltage magnitude and the change in pulse shape with respect to change in juice volumes is evident for test chamber 1. In addition, the electric field intensity decreases with the increase in volume.

In order to get the appreciable field intensity between the electrodes of test chamber, the dimensions of the first test chamber were modified and the new R-C parallel branch model was considered as a load across the PEF generator circuits. It was evident from the results of test chamber 2 that the field intensity obtained for all the volumes were higher when compared to test chamber 1. Particularly, the results of bipolar square pulse generator were higher compared to other generator circuits. Though the electric field intensity applied to the sample was appreciable, there is a reduction in the voltage magnitude with increase in sample volume.

Among all the process parameters, specific energy dissipation was the key process parameter which results in maximum log reduction. To optimize the effect of treatment chamber design and its volumes on the key process parameter [specific energy dissipation], Response surface methodology with the Box-Behnken design was applied. For this analysis, the features were selected with the typical range of distance, voltage, conductivity, pulse duration and volume. The corresponding response of specific energy dissipation was obtained.

From the results, it was clear that most significant effects were observed from the gap distance between electrodes when compared to other parameters. Most importantly, there was no significant effect on response due to the volume with other parameters. It was found from the results that to achieve effective microbial inactivation, the gap spacing needs to be maintained less and the same should remain unchanged with the increase in the volume of the treatment chamber.

For the validation of loading effect analysis and for the treatment of fruit juice, a high voltage bipolar square pulse generator was fabricated and the generated 1 kV pulse was applied across the different volumes from 1 ml, 10 ml and 100 ml of apple juice sample. From the treatment results, the microbial load in the apple juice gets reduced and the inactivation efficiency of 71.5% was achieved with a log reduction of 4.8.

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