Vegetables (and most other natural foods) contain nitrate. High concentrations can be found in leafy vegetables. The concentration of nitrate depends on the variety, season, soil and water used for the cultivation. The quantity of nitrate present in a certain vegetable, thus, is completely unpredictable.

Nitrate is totally harmless; however, it can be converted to nitrite and nitrite to nitrosamines, some of which are known to be carcinogenic.

The conversion of nitrate to nitrite will occur in most products, but at a very low rate. Heating will increase the conversion rate. The longer the heat treatment, the more nitrite will be formed. Hence the recommendation not to heat leafy vegetables twice.

Nitrite itself is also a harmless compound. However, in view of the fact that acidic conditions favour the formation of nitrosamines from nitrite, coupled with the fact that nearly all foods contain some protein and have to pass the acid stomach, nitrosamine production cannot completely be prevented.

Lemon juice will reduce the formation of nitrosamines. Considering the fact that acidic conditions augment the formation of nitrosamines, lemon juice, which is very acid should increase the formation. However, it contains vitamin C, which also reacts with nitrite, thereby preventing the nitrosamine formation. The concentration of vitamin C determines how much nitrate is left and consequently how much is still available for nitrosamine formation.