Changes in the nitrate and nitrite contents of fresh vegetables during cultivation and post-harvest storage

Nitrate and nitrite levels in both germinating seeds and growing vegetables were determined for the following vegetables: short-petioled cabbage (Brassica chinensis), long-footed cabbage (Brassica chinensis var. communis), field mustard (Brassica campestris), broad-leaf mustard (Brassica juncea var. rugosa) and water convolvulus (I pomoea aquatica Forsk). The seeds contained 16–32 ppm nitrogen in the form of nitrate and 0·3–0·8 ppm nitrogen in the form of nitrite. The nitrate nitrogen concentrations of the growing vegetables varied between 300 and 1200 ppm depending on species and their nitrite nitrogen contents were about 0·3–3 ppm. The nitrogen fertilizer urea seemed to induce remarkable nitrate accumulation in the growing vegetables. The influence of storage at different temperatures on the nitrate and nitrite contents of fresh and homogenized vegetables was studied. During the first few days storage temperatures of −10 or 2°C little change in the nitrate and nitrite concentrations was found. However during the first few days of storage at 26 or 32°C the nitrite concentration increased considerably while the nitrate concentration decreased. Nitrate reductase and nitrite reductase activities in the vegetables were estimated at various stages of growth but no significant changes in activity were observed.In view of the effect of dietary nitrate on salivary nitrite formation and on the endogenous formation of carcinogenic N-nitroso compounds. we suggest that more effort should be put into developing methods of cultivating low-nitrate vegetables.