Inactivation, Cell Injury and Growth of Microorganisms Exposed to Pulsed Electric Fields Using a Continuous Process

Doktorsavhandling, 2002

Pulsed electric fields (PEF) is a proven technology to inactivate microorganisms under non-thermal conditions. The PEF technology has a potential as a "quality friendly" alternative to traditional methods of food preservation. The PEF method involves the application of short pulses of high electric field strength to a food product (or another medium) which is placed between two electrodes. Microbial cells which are exposed to electric fields for a period of time respond by structural changes of the cell membrane, leading to cell inactivation. To obtain high quality food products, also safe for consumption, an understanding of the mechanism of microbial inactivation is necessary. This study provides evidence that permeabilization is involved in the inactivation of vegetative cells of microorganisms. Increased membrane permeabilization corresponds to a higher number of cells being inactivated. Apparently, PEF does not cause sublethal injury to bacterial cells. The critical factors determining microbial inactivation by PEF can essentially be classified as process parameters, microbial characteristics, and factors stemming from the cell environment. In this study, the PEF effect was found to be highly species dependent, with yeast cells being more sensitive than bacterial cells. Among the bacterial species, the Gram positive genera were the least affected. Greater microbial inactivation could be achieved by an increase in any one of the process parameters evaluated in this study, such as electric field strength, pulse duration, number of pulses and process temperature. A non-linear model, which includes electric field strength, pulse duration and number of pulses as independent variables, was shown to describe the kinetics of PEF inactivation well. The microbial inactivation is also significantly affected by environmental factors in the surrounding media, such as pH and aw. The PEF effect is significantly enhanced at lower pH values. The effect of changing the aw was less pronounced; however, it seems to have a slightly protective effect, giving a better survival at lower aw values. To predict the shelf life of a PEF processed product, it is important to understand subsequent recovery and growth of survivors during product storage and distribution. In this study, the growth following exposure to PEF was found to be linked to environmental factors as well as process factors, with a higher growth rate resulting from more severe treatment conditions. The number of surviving cells was found to be the most important factor determining time-to-detection.