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Can you heat spinach twice?

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.

Until recently it was assumed that re-heating of spinach and other leafy vegetables was dangerous, due to the formation of these nitrosamines. New research has shown that the concentrations of possible toxic nitrosamines is not as high as assumed. It is thus not dangerous to re-heat leafy vegetables. The only drawback is the (additional) loss of vitamins and other nutrients during re-heating, but this is not a food safety concern.