The occurrence of nitrate and nitrite in Mediterranean fresh salad vegetables and its modulation by preharvest practices and postharvest conditions.

Winter and summer nitrate/nitrite concentrations in 11 salad vegetables were surveyed using a validated HPLC-DAD method. Nitrate was highest in rocket, both in winter (x̅ = 3974 mg kg-1 fw) and summer (x̅ = 3819 mg kg-1 fw). High nitrate accumulators included spinach, purslane, chards, dill, coriander and parsley. Wide intra-species variability and levels in excess of permitted maxima highlighted the importance of monitoring vegetable production methods to protect consumer health. Occurrence of detectible nitrite (14-352 mg kg-1 fw) was most frequent in winter head cabbage. Three additional experiments examined the seasonal effects of nitrogen (N) fertilization rate, application method, formulation and postharvest storage on nitrate and nitrite levels in lettuce, rocket and spinach. Violation of current nitrate limits is likely when total N exceeds 200 kg ha-1, particularly in rocket and spinach. Postharvest nitrate reduction requires exogenous microbial nitrate reductase activity, which is unlikely to be achieved without visible loss of quality.