Time and Temperature Make A Difference
Temperature and time influence the growth of bacteria. This includes those that cause food poisoning. Bacteria need food, warmth, moisture and time to grow and multiply.
The U.S. Department of Agriculture recommends keeping hot foods hot (above 140 degrees Fahrenheit) and cold foods cold (below 40 degrees Fahrenheit). The "danger zone" is between these temperatures. Food may not be safe if held in this zone for more than two to three hours. Bacteria multiply rapidly in this temperature range.
Food can reach the two-to three-hour time limit cumulatively. For example, a cooked roast is left for one hour at room temperature. Its refrigerated, then left out for another hour (without proper reheating). This is comparable to leaving it out for two consecutive hours. Refrigerating or freezing cooked meat inhibits bacteria. However, any bacteria already on the meat remains alive. These bacteria will multiply when you put the meat back into the danger temperature zone.
At danger zone temperatures, bacteria can double their numbers every 20 to 30 minutes. In summary; keep foods out of the danger zone temperatures of 40 degrees to 140 degrees F. Remember to cumulatively figure time periods that food is held at room temperature. This means cooked food should be rapidly cooled to refrigerator temperatures. Reheat quickly to temperatures above 165 degrees Fahrenheit.
For more information, contact the Extension office in your area.
Source: University of Minnesota Extension Service, InfoU Script # 637