In two-stage French fry cooking, the second hot-fat bath is about 190 °C.
The first bath is gentler—around 160 °C in the standard preparation—and cooks the potato through. During that stage, starch gelatinizes and the interior becomes soft and fluffy. The fries are then removed and allowed to drain or cool before the hotter finishing fry.
At about 190 °C, the second bath rapidly drives off remaining surface moisture and crisps the gelatinized starch at the exterior. Browning also develops through Maillard reactions involving amino acids and sugars, producing the characteristic golden colour and roasted flavour.
The temperatures are approximate rather than universal rules: fry thickness, potato variety, oil type and equipment all affect the best settings. The key principle is the temperature contrast. One fry cooks the centre; the hotter second fry builds the crisp shell. A single bath has to compromise between those two goals.