(Please note: I am not a food expert or toxicologist. Please do your own research or consult an expert before making a decision about reheating spinach. Nitrates caused by reheating spinach should be avoided by infants of up to 6 months!)
When my parents were visiting us here in Illinois last week, we had made some spinach-artichoke dip and were about to put the leftovers in the fridge when my mom told me that you can never ever keep spinach leftovers as reheating spinach is dangerous and causes cancer. I vaguely remembered something about this from my Dutch youth, but my American wife and her parents had never heard of such a thing. To be safe we threw the spinach away, but it did make me curious.
It turns out that it is a myth and reheating spinach is perfectly fine. It is true that reheating it many times can cause the formation of nitrites. These should be avoided by infants of up to 6 months, but the risks are negligible for adults.
What is funny, though, is that I could not find any English websites about the danger, but there are countless Dutch sites about it! Apparently, the life-threatening danger of spinach is only known to Dutch people.
Actually, the urban legend seems to be localized in the Netherlands, Indonesia (not surprisingly, as it is an old Dutch colony) as well as Germany and Switzerland. I’ve been reading some funny stories on several blogs from non-Dutch people being told by their Dutch friends that they shouldn’t have reheated the spinach.
On an episode of the British TV show Ready Steady Cook on the BBC, a Dutch viewer sent in a question stating that “as everyone knows one should never reheat spinach”, but she asked whether spinach soup was okay. The three British chefs were surprised by the statement about not reheating spinach. They had never heard of such nonsense.
I wonder how this myth got started in the Netherlands and why people are still believing it, even though most of the rest of the world doesn’t. The website of Iglo, a major spinach brand in the Netherlands, says that reheating spinach is no problem, but they still maintain that you shouldn’t keep it in your fridge for more than 2 days:
Vroeger was het opnieuw opwarmen van spinazie taboe. Zo zou er – net als bij andijvie en spitskool – te veel nitriet vrijkomen, wat schadelijk kan zijn voor de gezondheid. Uit onderzoek blijkt dat de toename van nitriet bij het opnieuw opwarmen van deze groenten meevalt. Hierbij is wel belangrijk dat de groenten snel worden afgekoeld en niet te lang in de koelkast worden bewaard (maximaal twee dagen). De spinazie van Iglo voldoet aan alle wettelijke normen voor nitraat. Doordat Iglo de spinazie direct na de oogst blancheert en invriest, wordt de vorming van enzymen stilgelegd en de vorming van nitriet tegengegaan. Daarom kunt u de spinazie van Iglo zonder problemen opwarmen. (Source: Iglo)