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My boss was arguing that this is true and that it's a well known fact amongst culinary professionals that you should never put hot food in the refrigerator (he used to be a baker and had restaurants and such.)

A quick Google search seems to suggest this is a widely held belief that I've never heard of before:

Snopes Thread:

I remember my grandmother would let food cool down on the counter before storing it in the refrigerator. Someone told me that you should never put hot food in the refrigerator as germs/bacteria would multiply. Is this true? Is this an urban myth?

Yahoo Answers

If you put food that's still hot in the fridge, it will raise the temperature of the fridge. Unless you have a huge commercial refrigerator with a strong motor, it will take awhile for the temperature to go back down to the safe level. This means that every single thing that is is the fridge will be at risk for bacterial growth.

Metafilter:

My girlfriend and I constantly disagree about this. She was always told, growing up, that you have to wait until recently-cooked food has cooled to room temperature before you can put it in the fridge. If you don't, she says, bacteria will grow on it much more easily and you will get sick.

Has this been investigated properly?

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This claim is from a time when refrigerators weren't nearly as efficient as today. By waiting for food to cool down on the counter, it will stay much longer inside the dangerous threshold (5-60ºC, 40-140ºF). If you put it in the fridge immediately, it will cool down to <5º much faster.

It won't raise the temperature in your fridge unless it's a 50x50cm 20cm high lasagna straight from the oven - just don't put it over other food containers.

This page by the USDA confirms it:

Hot food can be placed directly in the refrigerator or it can be rapidly chilled in an ice or cold water bath before refrigerating

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    Forgot: one logical reason to wait a little is to avoid condensation. Apr 6, 2012 at 22:45
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    In our class for our food handler's card at our local health department, they recommend putting hot food in smaller or shallow containers, so that they cool off more quickly in the fridge. A large container that takes more than 2 hours to cool, even in the fridge, is still in that danger zone. Apr 9, 2012 at 0:50
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    The USDA link is down, but here's a link from foodsafety.gov that lists "letting food cool before putting it in the fridge" as a dangerous mistake. Feb 25, 2014 at 16:55
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    @Solomonoff'sSecret, that isn't how science works though. Though this kind of question comes down to probability. Certainly you're free to do what you will, but on the other hand it is supported by biological sciences and physics. Suggestions are there to minimize the unneeded risk. It may be overly cautious, but that isn't what is being asked. If you have evidence that leaving food out is less risky than placement in a fridge, feel free to post that as an answer. May 22, 2019 at 19:53
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    @Solomonoff'sSecret: If I was starving, I'd eat a lasagna that fell on the ground. It wouldn't kill me. That doesn't mean eating lasagna off the ground is equally safe as eating lasagna that never touched the ground. You can't just say "well it made no difference for me and therefore it can never make a difference for anyone ever".
    – Flater
    May 23, 2019 at 12:35

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