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General guidelines about optimum nutrition and cooking usually say that one should avoid too much heating and air/light exposure of food to minimize the loss of vitamins and nutrients.

However, there are some products in the market that act like a thermos food flask that one can use to pack hot meals in the early morning and then take to work, to eat around lunch time with the food still very hot (after 6-7 hours). But doesn't keeping the food in tight hot jar/container at such high temperature for 6 or 7 hours actually destroy the nutrients in the food, even if it is tightly sealed?

In other words, if one has the options of packing the food cold and microwaving it later at work versus packing it hot in a thermos and keeping it hot all the way until lunch, which option is healthier?

2 Answers 2

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You asked:

if one has the options of packing the food cold and microwaving it later at work versus packing it hot in a thermos and keeping it hot all the way until lunch, which option is healthier?

I'm not able to answer your question in regard to nutrients and vitamins, but I might have something to share about what is healthier in another aspect. It seems that it is important to keep food either in very low or very high temperatures to prevent bacterias from growing. See this quote from "Temperature of Foods Sent by Parents of Preschool-Aged Children" (2011):

Biological hazards, especially bacteria, are found everywhere, including most foods, and multiply rapidly when exposed to warm moist conditions. Control of the temperature of food is an important way to prevent bacteria from growing and possibly causing foodborne illness. Bacteria are preserved in a state of suspended animation when refrigerated or frozen and most are killed when food is heated to an internal temperature >74°C (165.2°F). Keeping foods >60°C (140°F) or <4°C (39.2°F) is critical in the prevention of foodborne illness.

Foods left in the temperature zone of 4°C (39.2°F) to 60°C (140°F) for >2 hours are unsafe to consume and must be discarded because of the production of heat-resistant toxins by bacteria that can cause foodborne illness.

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The main short-term detrimental effect of heating on nutrients occurs during cooking. Keeping the food warm for several hours would further destroy some nutrients but to a lower extent.

The only essential nutrients that can be partially destroyed (30-50%) by heating are vitamins (mainly B vitamins and vitamin C) (nutritiondata.self.com). Minerals are not destroyed, but some of them can leak out of foods during cooking in water. Carbohydrates, proteins and fats can change during heating but are not "destroyed" in the nutritional sense.

On the linked website, there is a table that shows how food processing (freezing, drying, cooking, draining and reheating) affects vitamins. Keeping food in a thermo is similar to reheating, during which additional 30-50% of vitamins B and C can be destroyed (at temperatures expected in your flask probably less than 30%).

In summary:

  • Cooked and cooled food would retain more vitamins than food kept warm for several hours.
  • A more important issue is an increased risk of bacterial overgrowth and food poisoning from a food that was kept warm for several hours.

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