The Science Behind Why Microwaving Fries Always Leads To Soggy Results - Tasting Table
Briefly

Reheating leftover French fries can often result in a disappointing sogginess when using a microwave. This is due to the way microwaves function, creating steam from the water in food, and preventing it from crisping. Potatoes naturally absorb moisture, exacerbating the problem. Alternative reheating methods, like deep frying, air frying, or using an oven, can restore the fries' original crunch. If the microwave must be used, adding oil and using paper towels can help mitigate the sogginess, but other methods are generally more effective.
Microwaves heat food by causing water to vibrate, generating steam that prevents crisping, leading to sogginess in reheated French fries.
For optimal reheating of French fries, opting for methods like deep frying, air frying, or even oven baking can restore their desirable crispiness.
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