One of the greatest kitchen challenges, for frugal home cooks and professional chefs alike, is what to do with leftovers. In fact, using up leftover foods was so important in the commercial kitchen that a whole speciality -- called "garde manger," literally "protecting the food" -- grew out of it. The bar is set higher for modern cooks, because it's not enough to make leftovers taste good. Now, they also need to be food safe. That's especially important if foods made from leftovers are also reheated.
Cool leftover foods immediately after a meal. Use shallow containers and plastic bags, as appropriate, because these will cool more quickly than deep containers. Foods must be stored at temperatures below 4.44 degrees Celsius to remain food safe. Purchase an inexpensive thermometer to keep in your refrigerator, so you can be sure it is maintaining the correct temperature.
Reheat foods in shallow dishes and in thin layers, for the same reason. During reheating, foods will be in the food safety "danger zone" for the time it takes them to go from 4.44 degrees C to 60 degrees C. For safety, all leftover foods must be heated to at least 73.9 degrees Celsius.
Incorporate leftover foods into casseroles, soups, stews or meat pies to create new meals. Any dish containing leftovers must be heated to an internal temperature of 165 degrees, when tested with an instant-read thermometer.
Cool dishes of leftovers, or containing leftovers, as directed above. Remove uneaten portions to the refrigerator immediately, rather than allowing them to cool at room temperature. Foods cool more quickly if periodically turned or stirred.
Reheat dishes containing leftovers quickly to a temperature above the 140 degree upper limit of the danger zone. Once a dish, or any part of it, has been reheated for a second time, it must be discarded if uneaten. Not only does the quality suffer, the risk of food borne illness increases sharply with each cycle of heating and cooling.
Reheating foods in a sauce or gravy will help minimise drying. If the sauce or gravy is also leftover, it must be heated to boiling before being added. The antioxidant properties of many spices and herbs will help prevent stale flavours, so highly seasoned dishes tend to hold up better under reheating. Quick heating and cooling, and careful temperature control, are essential to food safety with leftovers. Instant-read thermometers are inexpensive and widely available, and should always be used to monitor food temperatures.