A multi-scale analysis of the microbial response of Listeria to novel processing technologies, as affected by system (micro)structure and natural microflora

Minimal food processing methods, e.g. cold atmospheric plasma (CAP), ultrasound and the use of natural antimicrobials, are of interest to replace traditional decontamination techniques, as they are milder and thus maintain “fresh-like” food characteristics. Synergistically combining these techniques can increase the inactivation efficiency by acting as a hurdle for microbial growth. However, the efficacy and mechanism of action of combined treatments remains unclear, potentially leading instead to stress adaptation, antimicrobial resistance (AMR) development and/or post-treatment bacterial survival. Natural microflora present in foods could also present a challenge to microbial growth. Most studies on the inactivation of food-related pathogens by these novel processing methods are conducted in liquid broths, or in/on specific food products. However, many foods are solid(like) e.g. soft cheeses, meats, and studies in real foods are informative only for the product studied. Food structure can impact the non-thermal processing efficacy, and the diffusion/efficacy of natural antimicrobials. This thesis presents, for the first time, a fundamental study on microbial inactivation by natural antimicrobials, ultrasound and/or CAP in structured food model systems of controlled rheological composition and complexity. In this thesis, viscoelastic food models of various compositions and (micro)structural characteristics are developed and characterised. Significant structural effects are identified on a microscopic scale, i.e. the system viscosity and growth morphology (surface/immersed/planktonic) affect Listeria colony size and growth location/distribution. Selective surface growth on the protein phase of a complex biphasic protein-polysaccharide model system is observed for the first time. Furthermore, the efficacy of combined novel processing technologies is shown to depend on the microbial species, the system viscosity, the growth morphology (surface/immersed/planktonic), and the order in which treatments are applied. This work sheds light on the combined efficacy of novel processing techniques for the inactivation of Listeria in structured food models, highlighting the importance of accounting for structural effects when designing inactivation processes for the food industry