Abstract
Plastic is a persistent pollutant. When plastic “breaks down”, its structure fragments until the plastic particles are eventually no longer visible to the naked eye. Plastic particles < 5 mm are known as microplastics. When microplastics are in the environment, they are weathered by mechanical and chemical forces, allowing the plastics to sorb to pollutants easily. Limited research exists on the interactions between crop plants and microplastics. There is concern that the smallest microplastics (< 10 µm) may adhere to the surfaces of leafy vegetables such as lettuce (Lactuca satvia). If plastics adhere to leafy vegetables and are retained after washing, then they are likely consumed by humans. This consumption is concerning as both clean and contaminated microplastics have the potential to cause damage to human cells and bioaccumulate. Published papers have identified the presence of microplastics in lettuce and observed a cytotoxic effect when microplastics were exposed to human (Caco-2) intestinal models. However, no research has investigated the impact of microplastic contaminated plant-based food on human intestinal models.
This research initially aimed to determine whether microplastics adhere to the surface of lettuce leaves and if washing was effective at removing microplastics. We found that microplastics adhere to lettuce leaves and that washing, while effective in reducing the number of microplastics retained by the lettuce leaf, was unsuccessful in removing all microplastics from the leaf. Leaf macro surface morphology and whole leaf contour were identified as factors that influenced the adhesion and retention of microplastics.
The second aim of this research was to identify a level where microplastics had a cytotoxic effect on a human intestinal model (Caco-2). A level of 50,000 microplastics per mL of medium was identified. As 50,000 microplastics per mL of medium is 100-fold the level of microplastics found in the environment, these results are not concerning. This aim was then extended to identify if level at which microplastics had a cytotoxic effect was affected by the presence of a lettuce digest. The presence of lettuce was found to be a bioprotective factor. Red lettuces conferred particularly high levels of bioprotection, with no cytotoxic effects on the human intestinal model identified at any microplastic concentration. This finding has significant implications for regulations surrounding food safety.