In higher temperatures bacteria reproduce faster, but once temperature hits about 65 °C (150 °F) they start dying because it’s too much heat for them. The inside of a car doesn’t get quite as hot as that, but gets sufficiently warm for bacteria to reproduce very fast. Food in steamers and under lamps does get hot enough to kill off bacteria.
According to US food safety standards, there is the “Rule of 4”:
#Between 40 and 140 degrees Fahrenheit is safe for 4 hours.*
This applies to cars and heat lamps. Keeping food outside of that temp range is safe, as bacteria have difficulty operating/surviving.
*it’s technically between 41 and 135 degrees, but that’s harder to remember.
Food is supposed to be kept at 140F in order to kill bacteria. Cars can get hot, but it’s rare for them to get THAT hot. Warm areas over 90, but below 130, are ideal conditions for bacteria growth. So a car will most likely encourage bacteria growth rather than kill it.
So if the heat source heats the food to 140 then bacteria will die and it can be left out pretty much all day. If not then the food can only be kept out for a couple of hours before it is deemed unsafe.
Certain temperature control. At very high temperatures, bacteria is killed (about 140°F +), and at cold temperatures (40°F or below), it doesn’t reproduce.
Anything in-between allows for rapid growth of harmful bacteria. I suppose if you had a thermometer in the car to ensure it was consistently above 140°F or below 40°F, it would be OK, but we don’t generally have that.
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