Behaviour of Listeria monocytogenes in New Zealand fresh apple supply chains

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

Decline of Listeria monocytogenes on fresh apples during long-term, low temperature simulated international sea-freight.

Listeria monocytogenes has caused outbreaks of foodborne illness from apples in the USA, and is also a major issue for regulatory compliance worldwide. Due to apple’s significance as an important export product from New Zealand, we aimed to determine the effect of longterm, low-temperature sea-freight from New Zealand to the USA (July) and Europe (March- April), two key New Zealand markets, on the survival and/or growth of L. monocytogenes on fresh apples. Temperature and humidity values were recorded during a shipment to each market (USA and Europe), then the observed variations around the 0.5C target temperature were simulated in laboratory trials using open (‘Scired’) and closed (‘Royal Gala’ for the USA and ‘Cripps Pink’ for Europe) calyx cultivars of apples inoculated with a cocktail of 107-108 cells of seven strains of L. monocytogenes. Samples were analysed for L. monocytogenes quantification at various intervals during the simulation and on eachoccasion, an extra set was analysed after a subsequent 8 days at 20°C. When both the seafreight simulations concluded, L. monocytogenes showed 5 log reductions on the equatorial surface of skin of apples, but only about 2.5 log reduction for USA and about 3.3 log reduction for Europe in the calyx. Cultivar type had no significant effect on the survival of L. monocytogenes for both sea-freight simulations, either in the calyx or on the skin (P > 0.05). Most of the reduction in the culturable cells on the skin occurred during the initial 2 weeks of the long-term storage simulations. There was also no significant difference in the reduction of L. monocytogenes at 0.5 or 20°C. No correlation was observed between firmness or total soluble solids and survival of L. monocytogenes. Because the inoculated bacterial log reduction was lower in the calyx than on the skin, it is speculated that the risk of causing illness is higher if contaminated apple cores are eaten. The result suggested that the international sea-freight transportation does not result in the growth of L. monocytogenes irrespective of time and temperature. The results of this study provide useful insights into the survival of L. monocytogenes on different apple cultivars that can be used to develop effective risk mitigation strategies for fresh apples during long-term, low-temperature international sea-freight transportation

Semi-commercial hot water treatments for control of bull’s eye rot of apples

Bull’s eye rot (BER) of apples is caused by a postharvest fungal pathogen (Phlyctema vagabunda syn. Neofabraea alba). Previous laboratory experiments found hot water treatments (HWT) resulted in a significant reduction of BER incidence for artificially inoculated fruit so the feasibility of HWT to control naturally infected fruit in a semi-commercial trial was tested. One bin (1934 fruit) of naturally infected ‘Scired’ apples was harvested from a Hawke’s Bay orchard with a known high incidence of BER, then placed in a coolstore for 1 week until treated. All fruit were passed through a high-pressure water blaster then air dried. Approximately half the contents of the bin (1034 fruit) were packed into Friday trays in apple boxes with a plastic polyliner. The other half (900 fruit) were treated for 2 min with hot water at 51°C in a semi-commercial hot water bath before packing. All fruit were then coolstored for 20 weeks before assessment for BER. This HWT resulted in a 6-fold reduction of BER incidence so was an effective treatment for BER in a semi-commercial test.</jats:p