Like most foods, if left out in the open air, cooked mashed potatoes can go bad. Airborne bacteria settles on the food and can quickly multiply. Warm mashed potatoes, with their low acidity and high proportion of carbohydrates, make an ideal place for bacteria to grow. Yet if you store cooked mashed potato safely, you will be able to eat them another day.
Bacteria and Mashed Potatoes
Mashed potatoes have the right pH and nutrient levels for bacteria. In a 2007 report from the "Journal of Food Safety," the bacteria Streptococcus pyogenes grew rapidly in mashed potatoes stored at 77 degrees Fahrenheit. At 98.6 F, the bacteria took just over 30 minutes to become fully established. Harmful bacteria that makes food go bad will grow quickly in mashed potatoes. Even at 44.6 F, the bacteria survived. This shows how important it is to store mashed potatoes properly after cooking.
Leftover food, including cooked mashed potatoes, should never sit uncovered at room temperature for more than two hours, according to the FDA. This puts it in the bacteria "danger zone" of temperatures between 40 F and 140 F. Instead, aim to refrigerate the cooked mashed potato as soon as possible. The refrigerator should be cooler than 40 degrees to prevent bacterial growth.
Potatoes do not freeze well. The flesh often turns mealy and dry. However, cooked mashed potatoes may last better than large potato chunks -- and will keep for up to a month. Ensure that the mashed potatoes are very smooth, then scoop them into silicon cupcake cups or a muffin tin to create individual portions suitable for freezing.
Reheat cooked and stored mashed potatoes to 165 F before eating. This is usually easiest to achieve in a microwave. To be sure, check the temperature in the center of the mashed potatoes with a cooking thermometer. If the potatoes smell sour or rancid after heating, then they're likely bad and should be thrown out.
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