A potato’s cut surface starts to darken as soon as it meets the air; this happens when an enzyme inside the potato reacts with oxygen. Water blocks that contact and stops the browning. Soaking has another benefit: excess surface starch dissolves away, making fried potatoes crispier. On the other hand, soaking for too long makes the potatoes waterlogged.
Potatoes contain an enzyme that becomes active when exposed to air. While the skin is intact, the enzyme cannot do its work; as soon as a knife opens the surface, it meets oxygen and the cut area first turns pinkish, then grayish. The same reaction occurs in apples and avocados. Water stops the process by blocking oxygen; air cannot reach the surface under the water, so its color stays the same. Even a peeled but uncut potato darkens, because the skin is also a protective layer. So put a potato in water as soon as you peel it; even a few minutes on the counter can leave marks on the surface.
The water should be cold and cover the entire potato. Any slice that sticks out above the surface will darken at that spot. Warm water does not slow browning enough and softens the potato’s surface. Choose a wide container and do not pile the potatoes on top of one another. If they will soak for a long time, put the container in the refrigerator; water left on the counter warms up and becomes less effective at protecting the potatoes. Covering the container also helps the water stay cold longer and keeps refrigerator odors from transferring to the potatoes.
Water has a second function. Cutting leaves loose starch on the potato’s surface; this starch burns quickly in hot oil and makes slices stick together. As potatoes soak, the surface starch dissolves and the water turns cloudy. Changing the cloudy water once improves the result further; the fried potatoes are crispier and do not fall apart in the pan. For the same reason, grated potatoes are also placed in water; grating increases the surface area considerably, so the starch dissolves faster.
There is a limit to soaking time. A few hours is fine for most purposes, but if the potatoes will sit longer than 2 hours at room temperature (1 hour above 32°C / 90°F), keep the container in the refrigerator. Beyond that, the potatoes start absorbing water, their texture loosens, and they turn out watery and fall apart when boiled. For a longer soak, add a few drops of lemon juice or vinegar to the water to strengthen the protection. Do not salt the water; salt draws water out of the potatoes and damages their texture. There is no need to soak potatoes for boiling for a long time; the water does its job during cooking. Check how the water smells, but do not taste it; do not use potatoes that have soaked in sour-smelling or foul-smelling water.
Always dry potatoes after removing them from the water; do not skip this step. When wet potatoes go into hot oil, the oil splatters violently, which is dangerous and prevents a crust from forming on the surface. Lay the slices on one clean cloth or paper towel and pat the tops dry too. The same rule applies to oven-baked potatoes: wet potatoes steam instead of browning. Do not rub the potatoes while drying; pat them, or the edges of the slices will get crushed and fall apart in the oil.