I've heard that you need to let meat cool down after cooking before you store in in refrigeration, something about bacteria building up when you do that. Any idea if that's true?
In an ideal world, you would chill any hot food product rapidly before putting it in the refrigerator. However, modern refrigerators can handle the load from a mass of hot food, so it is better to put the food in the refrigerator than to hold it at room temperature to cool.
The goal is to minimize the amount of time in the danger zone (40-140 F, 4-60 C) during which pathogens can grow.
As an Environmental Health Officer who has investigated thousands of cases of food poisoning above, 70% of which were caused or exacerbated by poor temperature control, including leaving food out of a refrigerator too long, I have never discovered a case caused by putting hot food in a refrigerator. Good practice is to cool it as quickly as is reasonable, but never let meat be out of a refrigerator after cooking for more than 90 minutes.
My parents always told me to let things cool down outside the refrigerator before putting them in. There are two reasons for this; avoiding bacteria buildup in the food already in the fridge, and lowering power consumption.
When you add a warm bowl of just about anything to your fridge, the overall temperature in the fridge will rise until the fridge is able to compensate and lower the temperature. The theory (and I would stress that I have not tested this myself) this could increase the buildup of bacteria in the foods closest to the warm substance, as their temperature would conceivably be raised to above four degrees celsius (40 F).
The second point, power consumption, refers to the fact that the internal thermostat in the fridge will activate the engine and coolant pump in the fridge to compensate for the rise in temperature, which in turns draws additional power from the grid.
Always chill food fast as possible, no matter what!! If your fridge gets too warm, turn it down beforehand. Never leave food out at room temp. in order to let it cool...that really creates risk. .you want it cool, as fast as possible, not worrying about your fridge. . If there is other food in there that gets compromised, even at the lowest setting, then you need a new fridge.
There is actually many reasons why you would want to let food cool down before putting in the fridge, as you can see.
Yes, ideally you want the food to cool down as fast as possible. And yes you want to consider things such as bacteria build up, on the food cooling down and any food around it that may warm up.
But one good point that was not made is that, you don't want the excess moisture build up in the container of the food cooling down. When food is cooling down, it releases a lot of energy in the form of moisture which can spoil the food much faster. So it is good practice to cool off food before covering and (unless you have a walk in fridge) before putting in the fridge.