Behaviour of Listeria monocytogenes in New Zealand fresh apple supply chains

Abstract

Listeria monocytogenes (L. monocytogenes) is a pathogenic bacterium. It can be a market access issue if found on fresh produce like apples, especially in countries with "zero tolerance" for bacteria like the USA. Although New Zealand has recorded no outbreak or recalls for apples, the potential exists for L. monocytogenes to be present on apples through the soil, water, and packhouse. The literature review found knowledge gaps in understanding the survival of L. monocytogenes under dynamic temperature conditions, which is a reality in the international or domestic supply chain. The thesis investigated: (i) The effect of dynamic temperatures on L. monocytogenes in the international supply chain, (ii) The effect of dynamic temperatures on L. monocytogenes in the domestic supply chain, (iii) The effect of storage temperatures on the survival of three lineages of L. monocytogenes on apples, (iv) The effect of commercial storage temperatures on the survival and attachment of three lineages of L. monocytogenes on two apple cultivars. In conclusion, the research has proved that dynamic temperatures in the international and domestic apple supply chains did not result in the growth of L. monocytogenes for various apple cultivars. The study also demonstrated that L. monocytogenes of lineage I and II has better survival and biofilmmaking capacity under commercially critical apple supply chain temperatures than lineages III. The comparative quantitative challenge study using MPN and qPCR methods explored options for a faster and more accurate way to quantify L. monocytogenes and found higher bacterial concentrations in the qPCR than MPN method, which could be due to nucleic acids originating from dead cells in qPCR interfering with the final results. This thesis provides helpful insights into the survival of L. monocytogenes on different apple cultivars that can be used to develop effective risk mitigation strategies for the fresh apple supply chain.