Summary

The concentration of nitrate and nitrite in a vegetable reflects its food safety, quality and even how efficiently fertiliser was used in the production of that vegetable. Vegetables provide the majority of nitrate found in the diet. Leafy vegetables tend to have a higher nitrate concentration than other vegetable types such as root or fruit vegetables. Further, leafy vegetables can accumulate nitrate beyond levels that are optimum for plant growth. Once eaten, nitrate can be reduced to nitrite in the body and can combine with amines to form carcinogenic nitrosamines that are associated with gastric cancer.

This project assessed the levels of nitrate and nitrite occurring in Australian produced leafy vegetables and the production factors that impact on these. A survey of leafy vegetables available on the market was conducted over a one-year period with vegetables obtained from different growing areas in New South Wales, Queensland and Victoria. Greenhouse experiments were then carried out at the NSW Department of Primary Industries Gosford Horticultural Institute investigating how light and nitrate fertiliser affect the concentration of nitrate in a range of leafy vegetables.

The survey clearly demonstrated that Australian leafy vegetables generally have a low concentration of nitrite but that 27% of samples had accumulated nitrate, leafy Asian vegetables in particular. It was demonstrated that nitrate supply is the key factor controlling the accumulation of nitrate in harvestable shoots. Light conditions in Australia, even in protected situations such as under shade, are not reduced enough to exacerbate nitrate accumulation. For Australia, management of nitrate accumulation in vegetables will be achieved through the efficient use of fertilisers in production. A limitation to this is that growers currently do not have the tools to easily manage nitrate in Asian vegetable crops.