Nitrate in raw and cooked vegetables: content, variation and influence of cooking methods

Nitrate concentrations in vegetables are pivotal in dietary nitrate benefit-risk assessments. This study examined nitrate and nitrite levels in vegetables commonly consumed in Jordan, highlighting the impact of different cooking methods (boiling, steaming, baking, stir-frying, and deep frying) on nitrate content. The study examined the nitrate and nitrite levels in 270 raw vegetables and vegetable-based dishes purchased from retail markets and restaurants in Amman, Jordan, during the winter months of 2022 and 2023. Furthermore, 91 experimentally -cooked vegetables were tested for their nitrate and nitrite contents before and after cooking to determine the effect of cooking methods on nitrate levels. The nitrate and nitrite concentrations were measured using an in-house validated molecular absorption spectrometric method. Nitrate levels varied considerably within and between vegetable groups and species. Celery had the highest mean nitrate content (1700 mg/kg), and radish (1300 mg/kg) and beetroot (900 mg/kg) were next. Approximately 32% of vegetables contained moderate amounts of nitrate (200–1000 mg/kg). Leafy greens such as spinach, tossa jute, and lettuce contain nitrates in averages exceeding 500 mg/kg. Among fruit vegetables, zucchini and eggplant have the highest mean nitrate levels (311.6 and 221.6 mg/kg, respectively). Flower, root (excluding radishes and beetroot), and legume (excluding green beans) vegetables have nitrate levels below 200 mg/kg. On average, the mean nitrite level in raw and cooked vegetables was less than 1 mg/kg. Different cooking methods influence vegetable nitrate content differently. Boiling reduced nitrate by 22-40%. Steaming had no significant impact. Although deep-frying, stir-frying, and baking increased nitrate by 130%, 66.7%, and 57.6%, respectively, the overall effect was not statistically significant for deep-frying and baking. These findings are essential for public health and food safety authorities in evaluating nitrate exposure and assessing the risk and benefit of dietary nitrate intake. The data may support the development of evidence-based guidelines for safe nitrate intake. Key words: Nitrate, nitrite, raw vegetables, cooked vegetables, cooking methods