Got leftovers? Tips for safely savoring foods a second time around

With the holidays quickly approaching and along with them massive meals that often leave leftovers in their wake, IFT member Guy Crosby, PhD, CFS, Science Editor, America's Test Kitchen, Adjunct Professor, Department of Nutrition, Harvard School of Public Health, offers tips on safely savoring food the second time around.

Q: Why do leftovers often taste better the next day?
A: Generally because chemical reactions continue to take place that produce more and/or new flavor molecules. Some flavor enhancement may involve the breakdown of proteins to release amino acids such as glutamate and small nucleotides that interact to enhance savory, meaty umami taste, or reaction of amino acids with sugars to produce new flavor molecules by the Maillard reaction (browning), which can occur when the leftovers are reheated.

Q: Is it okay to put hot food directly into the fridge?
A: It is recommended not to place hot foods in the refrigerator because it can warm the refrigerator above 40 degrees Fahrenheit where harmful microorganisms can grow. Plus, if the food is very hot it may take significant time for the food to cool below 40 degrees Fahrenheit giving time for unsafe microorganisms to grow in the food. It is best to let the food cool to 80-90 degrees Fahrenheit before placing in the refrigerator.

Q: What determines how long a leftover meal will last?
A: Two factors, the eventual growth of harmful microorganisms and the development of off-flavors and unacceptable texture. The growth of microorganisms is dependent on the temperature of the food, its moisture level, and the pH level of the food. Acidic pH slows or retards the growth of microorganisms so acidic foods like tomato sauce will generally last longer than a food with a pH above 4.6, such as meat. At pHs below 4.6 bacteria are much less likely to grow. Each food is different. But it’s best to reuse leftovers that have a pH above 4.6 and high moisture within 3-4 days if refrigerated. If in doubt, or if the food will not be reused within 4 days then freeze the food, which stops the growth of microorganisms.

Q: What is the best way to store leftovers?
A: Most leftovers can and should be frozen if they will not be reused within three to four days. Even raw egg yolks can be frozen and reused if a small amount of concentrated sugar syrup is added to prevent damage to the proteins.

Q: If a package of ground beef says that it should be used by a certain date and then you cook it, how many days longer will it last in the fridge?
A: It is probably best to reuse the leftovers within three to four days because ground beef is a high moisture food with a pH above 4.6. One additional day will probably not be too risky for many leftovers, but it is best to use cooked ground beef within four days or less as recommended by the USDA.

Q: How can you reheat leftovers that will have a similar taste and texture as to when they were first cooked?
A: Quickly reheating in a microwave is best for most foods, although microwave ovens can heat food unevenly. Warming up in an oven may be acceptable for some foods like a baked potato, and reheating in a pan also works for liquids like soups. If in doubt use the microwave.

Q: How does cooling down change the flavor of a food?
A: Cooling food changes the flavor in two ways:

1.  Our taste receptors are temperature sensitive and are less responsive below 60 degrees Fahrenheit.
2. Lower temperatures reduce the volatility of aroma compounds. It is estimated that about 85 percent of what we perceive as flavor is due to smell, so there will be less aroma and therefore less flavor at lower temperatures.

Q: Is it safe to refreeze previously frozen leftovers?
A: Yes, as long as the food was not left at room temperature for more than two hours during the first re-thaw, which gives time for harmful microorganisms to grow before the food is refrozen.

Q: Why does citrus keep foods like apples and avocados from browning? 
A: Browning of apples and avocados is due to an enzyme called polyphenol oxidase (PPO), which catalyzes the oxidation of polyphenols present in apples and avocados (and raw potatoes) forming brown pigments. The PPO enzyme is much less active at acidic (low) pHs, so the rate of oxidation is greatly slowed. The enzyme is also deactivated by heat.

 Source: Guy Crosby, PhD, CFS, Science Editor, America's Test Kitchen, Adjunct Professor, Department of Nutrition, Harvard School of Public Health