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

Abstract

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 long-term, 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 0.5 °C 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 each occasion, an extra set was analysed after a subsequent 8 days at 20 °C. When both the sea-freight 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.