Nitrate and nitrite levels in commonly consumed vegetables in Hong Kong

Levels of nitrate and nitrite in 73 different vegetables, a total of 708 individual samples grouped into leafy, legumes, root and tuber, and fruiting vegetables, which are traded mainly in Hong Kong, were measured. Where available, five samples of each vegetable type were purchased from different commercial outlets during the winter of 2008 and summer of 2009. Levels of nitrate and nitrite were determined by ion chromatography and flow injection analysis, respectively. Nitrate and nitrite levels of all samples ranged 4-6300 and 0.8-9.0 mg kg-1, respectively. Nitrate concentrations for the different groups, in descending order, were leafy root and tuber fruiting and legume vegetables. More than 80% of vegetables had mean nitrate concentrations less than 2000 mg kg-1, but mean nitrate concentrations of three types of leafy vegetables, namely Chinese spinach, Shanghai cabbage and Chinese white cabbage, were 3500 mg kg-1. On the other hand, nitrite concentrations were generally low - 1 mg kg-1 on average. Nitrate in vegetables (i.e. Chinese flowering cabbage, Chinese spinach and celery) can be reduced significantly (12-31%) after blanching for 1-3 min, but not after soaking.