Friday, March 18, 2016

Reheating food: Hazardous or hoax?

These days, I get updated on current affairs through Facebook. I find news websites a tad depressing to read, and it seems that you can more or less get updated through FB, since people can be counted on to share about relevant issues. If there's an earthquake somewhere, or if there is some good sale going on, you'd be certain that someone on your friends list would post about it. I've realized that the filtering that Facebook does to your feed can be pretty useful, since it means that I tend to get updates from the people or pages whose posts I read more, and that prevents information overload. (FB's filtering isn't so great when it comes to small businesses though, but that is another issue altogether.)

I really do enjoy reading posts about varied topics, and I myself share interesting articles on the blog's FB page. However, there is another class of articles that I am not a fan of: the health-related ones that usually involve a certain amount of fear-mongering, with titles which include words like "toxic", "lethal", "can kill". My issue with these is how many of these articles do not seem to be reliable and are not substantiated with scientific research, yet many go on to share these articles as if the information was truth.

One example would be a post about seven foods that become poisonous upon reheating, which has been making its rounds recently. I spotted a few friends and pages that I follow sharing this post, and I know it got some people quite worried about freezing and reheating their food. This particular post bothered me a fair bit, which is why I am writing in an attempt to encourage everyone to read with discernment. That aside, I wanted to take the opportunity to research their points on food safety a little further, since it is a relevant issue for us, as we freeze and reheat food frequently.

Firstly, about reader discernment. These days, anyone can set up a website or blog sharing information about anything. Most of the information online is not policed or vetted, which means the best way to read anything on the web is with a huge pinch of salt.  I found this list of questions very helpful for evaluating online information, and it tackles aspects such as coverage, objectivity and accuracy. Even what is deemed as a reputable source may sometimes give erroneous information, for example, a news site may misinterpret the findings from a scientific paper. To a certain extent, scientific research, often deemed as reliable information source, has its limitations too, since the type of tests and sample sizes may not be ideal, and fraud can also be an issue.

All in all, we need to remember that not everything online is true, and we need to be wise in how we deal with information in cyberspace. We also need to teach our children the skills to evaluate information as they learn how to navigate the web (I can't emphasize to you how important this is, having marked so many project reports from kids who plagiarized chunks of information filched from random sources!).

Now back to the article on reheating food. Going by the list of questions to evaluate internet information, the article wasn't up to mark on many aspects. It had issues with regards to authority: all the information was copied from another article, and no author was stated. A quick check on Google showed that there were at least ten articles containing the same points floating around in cyberspace, all with no scientific evidence being cited to back up the points. Another indicator which set off alarm bells in my head were the reader comments that followed. However, the point of this post is not to slam the websites, or the writers (which is why I am not linking to the said posts), so let's move on to looking at what interests me most: do these stated foods really turn toxic when reheated? (In the process of researching this matter, I've tried to use reliable sites with accurate information, to the best of my ability.)

1. Spinach

Spinach was the first culprit in the dangerous food list, being said to "contain nitrates which will change into nitrites when reheated". A quick search online yielded many articles which stated that spinach becomes toxic when reheated, but most of these were written along the same lines of the original article, and these didn't cite any references or give a proper explanation. However, I found a short piece by European Food Information Council that confirmed the point about nitrates and nitrites:

"Nitrate itself is totally harmless, but it can be converted to nitrites, and then to nitrosamines, some of which are known to be carcinogenic. Enzymes present in bacteria convert nitrate to nitrite. This happens especially when spinach is heated, stored and then later reheated. Nitrite itself is a harmless compound, but it should be avoided by infants of up to 6 months. It can affect the ability of the blood to transport oxygen by transforming haemoglobin, the protein that carries oxygen in the blood, into methaemoglobin, a form of the protein which is unable to carry oxygen. This can be dangerous for babies and is commonly known as “Blue Baby Syndrome”." 

I wanted to read up more about the whole process of the conversion of nitrates to nitrites, and I found this very informative article from the Centre for Food Safety in Hong Kong. To summarize, spinach does contain a high amount of nitrates, and this is converted to nitrites by enzymes within the plant cells, and by bacterial action on the vegetables before cooking, after cooking, and after ingestion. It is probably not the action of reheating the spinach that is the issue, but that the cooked spinach has been left at room temperature before being refrigerated, since this period of time allows for an accumulation of nitrites.

2. & 3. Celery, carrots and beets
These three were also mentioned as having high levels of nitrates. However, I unearthed this factsheet on nitrates and nitrites, and it stated that the vegetables that contain the most nitrates include lettuce, spinach, beetroot, celery and radishes. Going by the table of nitrate levels of various vegetables, it seemed like carrots do not contain high levels of nitrates.

If the levels of nitrate/nitrite are a cause of concern, then following these steps would help to reduce their levels:
:: Storing the fresh vegetables in the fridge, which reduces enzyme and bacterial activity.
:: Peeling root vegetables, as the nitrates are concentrated in the skin and just below the skin.
:: Discarding the stems of leafy vegetables such as lettuce and spinach might help to reduce the nitrate levels.
:: Washing leafy vegetables before cooking, and blanching them in boiling water and discarding the cooking water, which helps to wash away some of the nitrates.
:: Cooking the vegetables immediately after chopping or mashing, as chopping releases the enzymes from the cells and speeds up the formation of nitrites.
:: Freezing any foods containing these vegetables, if not to be consumed immediately, so as to stop bacterial action which causes nitrite accumulation. [A side note: this means it should be fine to heat up those frozen frittatas containing spinach, if you froze them quickly after cooking!]
:: Not storing these foods in the fridge for more than 12 hours.
:: Ensure food for infants containing such vegetables is prepared for immediate use, and only introduce these foods when they are older.

Like what was mentioned in this articlethe nitrates and nitrites in vegetables should not pose a problem to us, and reheating the vegetables should not be an issue if the food has been stored properly prior to reheating. However, the consumption of these vegetables would be an issue in young infants due to the risk of methaemoglobin.

Finally, for those who are concerned that the presence of nitrites in vegetables might lead to the formation of nitrosamines (some which are carcinogenic), some studies have shown that the Vitamin C found in the same vegetables have a protective effect against various cancers. However, processed meat (such as sausages) can be a source of nitrosamine exposure. These meats contain nitrites and nitrates which are used as preservatives, as well as amines, produced during the breakdown of protein. The nitrites in processed meat can combine with amines to give nitrosamines. So if you're worried, eat your vegetables (but ensure you follow food preparation and storage guidelines), but ditch the processed meats!

4. & 7. Potatoes and mushrooms
The article stated that potatoes are harmful when reheated because "their dietary and health benefits are lost". I tried researching further, but I couldn't find any article discussing this in detail. Going by the nutritional content of potatoes, key nutrients include carbohydrates (mainly in the form of starch), as well as Vitamin B complex and Vitamin C. Since both of these vitamins are heat sensitive, the process of cooking the potatoes would decrease the levels of vitamins. Subsequent storage and reheating of the meal might further decrease the vitamin content. However, even with this loss of vitamins, I do not think we can conclude that reheating and consuming potatoes would be harmful.

In the case of mushrooms, the article mentioned that reheating might cause "heart and digestive problems". However, again no scientific evidence was found that reheating mushrooms (assuming these are the edible ones!) might cause health issues, so long they were properly refrigerated after cooking.

That being said, it is important that the cooked food is stored properly in the fridge if it is meant to be reheated and consumed later, since leaving the potatoes or mushrooms at room temperature may result in the growth of the bacterium Clostridium botulinum. This bacterium occurs naturally in the soil, hence it is sometimes present on potatoes or mushrooms. It  produces spores, and these are heat resistant to a certain degree, so the process of cooking/baking potatoes may not destroy the spores. The spores can then be activated as the potatoes are cooled, especially in low oxygen conditions (such as when the potatoes are foil-wrapped). While the bacterium and spores do not cause disease, they produce botulinum toxin, which causes respiratory and muscular paralysis, an illness known as botulism. Potatoes seem to be one of the common foods associated with botulinum, while mushrooms are less often mentioned. However, a study has shown that the toxin can accumulate in fresh mushrooms that are stored at room temperature. As the toxin is heat-labile, heating foods to temperatures of above 80 ºC for at least ten minutes would greatly reduce the possibility of illness.

A side note: Infants can also contract botulism, when they consume food containing Clostridium spores (such as honey), which then grow in their gut and produce botulinum toxin. This usually doesn't happen for children above 6 months and adults, as the immune system prevents the growth of the spores. This is why it is not recommended to feed honey to babies below one year of age.

(For more information, click here for an interesting read about the history of botulism, and here for more information about the bacterium and preventing food-borne botulism.)

5. & 6. Eggs and chicken
The article cautioned that eggs were "lethal" if consumed after reheating, and that the "protein in chicken undergoes a change in structure when eaten the following day". Again, I could not find any evidence to support this, and if you think about it, cooking the chicken the first time already alters proteins since the heat denatures them.

Like potatoes and mushrooms, the problem with reheating eggs and chicken would be the risk of food-borne illness. However, this bacterium in question this time is Salmonella, which can cause diarrhoea, fever and abdominal cramps. Usually the issue is when the eggs and chicken were not cooked adequately enough the first time (in terms of temperature and time duration), such that the bacteria present in the food is not killed. If the cooked food is left at room temperature and not refrigerated quickly, the bacteria multiplies and causes illness.

To prevent food-borne illness caused by bacteria such as Clostridium and Salmonella, ensure the following:

:: Store food (including eggs) below 4 ºC.
:: Oil infused with garlic or herbs may contain Clostridium, and should be refrigerated.
:: Do not use food in cans that have bulging or damaged lids, or any signs of leakage. Discard any expired canned food, and any eggs that have cracked.
:: Store raw meat separately from other produce and food in the refrigerator.
:: Refrigerate cooked food quickly and use within three days. If the portion to be stored is large, divide it to allow it to cool faster.


:: Wash hands, kitchen counters and utensils with soap before food preparation, after handling raw meat and eggs, and after visiting the bathroom.
:: Some recommend that poultry should not be washed, as the juices containing bacteria may contaminate kitchen surfaces or other food.
:: As home-canned foods may contain Clostridium, always heat these foods to 80 ºC for at least ten minutes.
:: Cook eggs until the whites and yolks are firm.
:: For dishes that require eggs to be raw or undercooked, use pasteurized eggs.
:: Poultry needs to be cooked to a minimum internal temperature of 74 ºC.
:: Reheat leftovers until piping hot before consumption.

:: Hold hot food above 57 ºC (poultry should be kept above 74 ºC), and keep cold dishes cold.
:: Do not leave cooked food out for more than two hours, and try to eat food immediately after cooking.
:: For lunchboxes and picnics, pack eggs or egg products with a frozen gel pack, or a frozen juice box.

(These tips were summarized from here, here, here and here. You can visit these sites for more information.)

In conclusion, reheating these seven foods is not dangerous, provided that the rules for proper food preparation and storage are followed. It is not reheating the food, but the presence of bacterial contamination and the improper handling of food that causes illness. Of course, it is ideal that you eat freshly prepared food, but freezing and reheating food should not pose a health issue.

PS: I hope this article has been helpful in debunking the myth that reheating certain foods can kill you. If you've found it useful, please share this!


  1. Thank you for doing the research. I used to live in Hong Kong where as you know domestic helpers are affordable. A friend once said to me, "Why did you eat leftover while you have a helper to cook for you? Leftover food is not healthy." So I've been wanting to find out if there's some truth in it. So thank you for sharing. Happy homeschooling! Cindy, Oregon, US.

  2. Good investigative work! Thank you!

  3. Brilliant article, thank you so much for writing this 😊



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