Many cooks thing it is best to cool leftover foods on the counter before putting them in the refrigerator. They (mistakenly) believe that allowing the food to cool more slowly will reduce the likelihood that the food will spoil or that the hot food will cause the fridge to lose energy. Both of these notions are false and dangerous, according to the FDA.
The FDA is on record as saying that leftover food (particularly meat) should be refrigerated immediately after serving and should certainly not be left out for more than 2 hours. Food bacteria can double every 30—40 minutes, says the FDA. In fact, the agency points to several documented outbreaks of food poisoning that were linked to meat left to cool for too long at room temperature.
Because the kinds of bacteria that contaminate food thrive at temperatures between 40 and 140 degrees Fahrenheit, leftovers should always be stored in a fridge set at 40 degrees or lower. Another tip is to divide large quantities of food into several storage containers for quicker cooling. Finally, the FDA reminds cooks never to reheat leftovers more than once.