Leftovers can be a breeding ground for bacteria if they are not handled properly, and it is important to observe food safety when handling leftovers. The most important rule of thumb to remember with leftovers is that if they seem at all questionable, you should discard them. Food borne illness is rarely worth the small amount of food. You can also make leftovers more safe by handling and packaging them in ways which will resist bacterial contamination.
Always observe food safety precautions when preparing food. This makes the food safer to eat, and reduces the risk of creating leftovers which might make someone ill. Remember to wash your hands, use separate cutting boards for meat and vegetables, and to cook food thoroughly to eradicate any bacteria which may be present. Also keep the kitchen clean, and try to thoroughly scrub it down with bleach or another antibacterial product at least once a week.
When leftovers are generated from home cooked food, refrigerate them promptly. Contrary to popular opinion, food should not be left out to cool. If leftovers are not put into the fridge to chill within two hours, they should be discarded. Particularly large batches of food should be broken up into a series of smaller containers to ensure that the food cools quickly. If you put hot leftovers directly into the fridge, which is the best practice, leave the lid off until the food cools down. The same rules follow for foods brought home from restaurants and delis.
Try to keep your fridge at 40 degrees Fahrenheit (4.5 degrees Celsius). This temperature discourages bacterial growth on leftovers and all food. Make sure to check the temperature with a calibrated thermometer, and confirm the temperature level frequently. Do not overload the fridge or freezer with hot foods, as this can make the temperature unstable. It is also an excellent idea to clean the fridge and freezer regularly, discarding foods which have gone bad and wiping down the surfaces inside.
Make sure to date or label leftovers clearly, as food is not generally good after three to five days in the fridge. If the food is not going to be used within that amount of time, freeze it, if possible, or discard it. When you reheat leftovers, reheat them thoroughly so that the food is hot and steaming, with no cold spots. If you need to defrost food, do the defrosting in the refrigerator, never on the counter, or use cold running water to accelerate the process.