Eli5: why boiling food makes it soggy but deep frying makes it crispy?


Eli5: why boiling food makes it soggy but deep frying makes it crispy?

In: 6

Properly deep frying food means you get the oil hot enough that water inside the food boils off creating pressure inside the food that keeps oil from penetrating too deeply into the food causing the food to dry out. Boiling food just causes water to soak into the food making it soggy.

Frying improperly can make food soggy, but normally we don’t add the food to the oil until the oil is very hot. That means that while the oil soaks a bit into the outside of the food, it quickly heats the surface of the food to boiling. The water in the food boils rapidly and turns to steam, which creates an outward pressure that doesn’t let the oil penetrate deeply into the food. Only the outside gets oily. That’s why it doesn’t get soggy.

It gets crispy because frying heats the surface of food to very high temperatures and pulls water out. Most foods get harder when cooked at high temperature as proteins stiffen and water is drawn out. (Most fried foods are also coated in batter that, like all breads and bread-related materials, hardens as it cooks.)

This doesn’t apply to boiling because the water in a boiling pot of water never rises above the boiling point, so water in the food boils only very slowly if at all. So the food absorbs, rather than gives up, water.

Boiling is typically a slower process and it occurs at a lower temperature than frying, namely 100 degrees celsius. Boiling water cannot reach a higher temperature than 100 degrees (under normal atmospheric pressure) because beyond that it just turns into water vapor. It’s using the energy from the heat to shift from a liquid to a gas so it can’t actually go above that temperature. This is a much lower temperature than most cooking uses. Frying or oven baking typically use temperatures of around 160-190 degrees celsius.

This is important for two reasons, one is that boiling typically takes longer than frying (boiling meat is typically done for at least an hour, or more, which means that the liquid has plenty of time to seep into the food, but more importantly, boiling occurs at a low enough temperature that the Maillard reaction doesn’t occur. The Maillard reaction is the chemical reaction that turns the outside of foods brown when being cooked, whether in a pan or on a grill or in the oven. It typically happens at around 140-160 degrees celsius minimum and this browning on the outer layer forms a protective barrier that prevents liquids from seeping in, because the exterior of the food turns hard and crispy from soft and porous. In order for that reaction to occur, the food has to be heated up quickly to a high enough temperature.

It is possible to make fried foods soggy if improperly fried, usually by dropping the food in the oil when it’s not hot enough, or dropping too much cold food in hot oil so that it lowers its temperature significantly.

Food contains water, I hope. When you boil it, the energetic water molecules break down the food and let even more water penetrate. I don’t know about you, but I was happy with the amount we had before. So this extra moisture makes things soggy – it’s literally chock full of all the water it can possibly hold, like a sponge.

In contrast, deep frying vaporizes the water already inside the food. Suddenly your crab cake or potato is filling with its own steam, the pressure growing every moment.

That steam would of course love to abscond into the surrounding atmosphere, yet unfortunately it’s surrounding by boiling oil. We all know from grade school that water and oil are [immiscible](https://en.wikipedia.org/wiki/Miscibility), it would take time and energy to have the steam pass through.

We don’t want to give it that time. Instead we fry just long enough to keep the steam trapped inside. Instead of escaping it then turns the food into a deliciously crispy fortress.

Carbohydrates deep fry best, things like gluten or starches in potatoes. The long chains of the carbohydrates link together, and then are shaped by expanding gasses like bubbles or balloons. The water vapour hides inside these as best it can.

That’s how deep frying can produce moist but also crunchy foods. It’s just water following the rules- how to exist while using as little energy as possible. The answer, as in most things in physics when you get right down to it, is [entropy.](https://en.wikipedia.org/wiki/Entropy)

Because oil and water don’t mix.

When you boil something, the water you are boiling with permeates the food and mixes with the water in the food, making it soggy
When you fry something, the oil heats up the water in the food, but doesn’t mix with it, because oil and water do not mix. Hence it is crispy