Temperature abuse of foods is one of the leading causes of foodborne illness outbreaks. Food can be considered unsafe any time it is left in the temperature range of 140 degrees Fahrenheit to 41 F for more than six hours. The most rapid bacterial growth occurs in the 60 F to 120 F range.
To be safe, cool foods from 140 F to 70 F in less than two hours and then to 41 F in less than four hours.
Even if the food has been heated to boiling, spores of bacterial pathogens such as Clostridium perfringens, Clostridium botulinum and Bacillus careus can survive. When temperatures get near 100 F, these spores can germinate, and the bacteria start reproducing very rapidly. For example, if there were just 10 Clostridium spores in a food and optimal conditions existed for four hours, the organism could multiply to levels well above 100 million. This is more than enough to cause sickness.
The following graph shows cooling curves of chili in different containers. Chili cooled in a pickle bucket in a 40 F cooler did not cool to 41 F even after 20 hours in the cooler. Chili cooled in a two-gallon plastic container cooled more rapidly but required about 10 hours to reach 41 F.
Acceptable cooling occurred when chili (at a depth of 2 inches) was placed in a shallow metal pan in a 40 F cooler. Chili (at a depth of 10 inches) in a two-gallon plastic bucket cooled within acceptable limits only when an ice bath was used and the mixture was stirred periodically.
Leftovers and pre-prepared foods that have been properly cooled should reach 165 F within two hours during the reheating process.