Inactivation mechanisms of pathogenic bacteria in fruit juices by conventional and non-thermal treatments

Abstract

In response to growing consumer interest in minimally processed foods, the food industry seeks preservation technologies that ensure microbiological safety without compromising nutritional and sensory quality. While thermal processing is widely used in fruit juice and nectar production, it can lead to nutrient loss and undesirable changes in product quality. Alternative methods, such as high-pressure processing (HPP) and pulsed electric fields (PEF), offer promising non-thermal alternatives. To validate these technologies safely, surrogate microorganisms - non-pathogenic strains that mimic pathogen behavior - are essential. This cumulative thesis aimed to select suitable surrogate microorganisms for foodborne pathogens in strawberry and cherry nectar. The influence of the selected inactivation technology and the food matrix in which the bacteria are located on the determined D- and z-values was also examined. Additionally, the transcriptomic response of Escherichia coli to thermal and non-thermal treatments was investigated using RNA-Seq. Based on thermal resistance analysis, E. coli ATCC 11229 was identify as a suitable surrogate for tested Salmonella enterica strains, and E. coli ATCC 8739 for Shiga-toxin producing E. coli O157:H7 and O113:21. Additionally, a strong influence of the selected inactivation method and the dry matter content on the D-values was demonstrated. Transcriptomic analysis revealed that E. coli ATCC 8739 shows a similar response to thermal and high-pressure stress. Interestingly, the application of pulsed electric field, despite the lack of cell inactivation, strongly induced upregulation of genes, especially those related to metabolism. This work contributes to applied microbiology by identifying two suitable surrogate strains for validating microbial inactivation in fruit juice processing. Furthermore, it examines important aspects related to the thermal resistance of microorganisms and compares how innovative preservation technologies affect gene expression in E. coli.