A French fry loses roughly 10% of its crispiness every minute after cooking.

The crispiness of a French fry depends on a dehydrated starch matrix formed during frying. When potatoes are submerged in oil at temperatures between 163°C and 190°C (325°F to 375°F), water in the outer layers evaporates rapidly, creating a porous, rigid crust made of gelatinized starch and protein.The interior remains moist and fluffy while hot. As soon as the fry is removed from heat, internal temperature drops, causing water vapor to condense into liquid. This pressure drop draws moisture from the center toward the surface through capillary action, a process identified by food scientists including J. Kenji López-Alt.Research published in the Journal of Food Science shows that moisture migration is the primary cause of staling and texture loss. Starch molecules in the crust rehydrate and soften, losing structural integrity within minutes. Heat lamps used by restaurants like McDonald's since the mid-20th century delay this process by maintaining surface temperature high enough to slow condensation.Studies demonstrate that a fry can lose up to 50% of its perceived crunch within five minutes of cooling. This is why culinary experts recommend double-frying: the first fry at lower temperature cooks the interior, while the second fry at higher temperature creates a thicker, more resilient starch wall that better resists moisture migration.