Mechanistic understanding of internalization of bacteria into leafy greens during a vacuum cooling process

Abstract. Vacuum cooling is a common unit operation in the leafy greens industry and is considered as a very efficient approach to extend the shelf-life of fresh produce. However, during this popular process, bacteria can internalize into the produce. This emphasizes the need for better understanding of the internalization mechanisms and identification of the risk factors contributing to food-borne outbreaks. We developed a coupled multiphase transport model to simulate internalization of pathogenic bacteria into fresh leafy greens during the vacuum cooling process. This mechanistic model includes transport of water and gas into/out of a leaf section, bacterial transport, and heat transfer in the leaf. It covers different pathways that bacteria are able to internalize into a leaf during this process. The model was solved and qualitatively validated by using some literature data for leaf temperature, weight loss and the total amount of internalized bacteria into the leaf during the vacuum cooling process. Our results indicate that vacuum cooling helps internalization of bacteria by changing leaf microstructures and providing high pressure gradients when vacuum is being released. The results are further used to provide practical recommendations to reduce the risk of bacterial internalization into leafy greens during the vacuum cooling process.