Pulsed Electric Field Food Treatment and Low Frequency Bioelectromagnetics
Sammanfattning: This thesis is divided in two parts, both of which address the interaction between biological cells (especially cell membranes) and electromagnetic fields. The first part deals with the application of intense pulsed electric fields (PEF) in foods. It is intended to be a non-thermal method to inactivate microorganisms. However, a moderate increase in temperature is present in this process due to ohmic heating where the pulsed electric field energy input is transformed into heat. We address the problem of minimizing this heating while at the same time ensuring microbiological safety. In Paper I, an overview of the electric field in a broad selection of designs for PEF chambers is presented. In Paper II, we applied two optimization criteria to increase the homogeneity of the electric field strength in a PEF chamber for flow-through food processing. Paper III widens the scope of this analysis to include the temperature increase brought about by ohmic heating in a nonhomogeneous flow profile liquid food. We performed simulations on four different chamber designs, two of which are tested experimentally. In Paper IV, the influence of process parameters on four different microorganisms was studied. The inactivation was studied as a function of electric field strength, pulse duration, and number of pulses in a flow-through chamber. Paper V applies a number of proposed empirical models to experimental data for three chamber designs. We performed regression analysis and found a suitable model to describe the microbial survival. Recommendations for comparison of empirical PEF data were formulated.The second part of this thesis deals with the problem of how to properly investigate biological effects of weak, extremely low frequency (ELF), electromagnetic fields. Research on biological effects of electromagnetic fields is conducted over a wide range of exposure parameters and biological systems and many demonstrated effects have been shown to be difficult to repeat. In Paper VI we reviewed the literature of independently replicated ELF cellular studies, aiming at identifying reasons for problems with replication. In general, the higher the degree of overlap in experimental design, the better the chance of obtaining a positive replication outcome. In particular, the origin of cells and serum batch were found to be important parameters to keep the same as in the original study. Large systematic variation between sets of experiments in biological assays can often be countered by normalizing the results of an "exposed" (E) experiment to that of a simultaneously performed "control" (C). In Paper VII we demonstrate that the commonly reported arithmetic mean of a number of E/C ratios always overestimates the true E/C ratio of a study. We implement a correction method that takes the overestimation into account when evaluating a simple t-test. Finally, we have investigated the magnetic flux densities in the streets of a downtown region of Göteborg, Sweden. Often, only residential and/or occupational magnetic field exposures are regarded in epidemiological studies. It was shown in Paper VIII that the average magnetic flux density in the streets of Göteborg is above 0.2 µT, a level commonly referred to as high exposures in epidemiological studies.
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