Monday, May 13, 2013


This January, one of the CC's friend's mother got burnt while trying to fry something. She was not hurt. It was more distress than trauma, thankfully.

Since we here at this blog tend towards the analytic bent of mind, it behooves the CC to explain what went wrong.

Let us take a quick detour into what cooking actually does. Cooking transforms substances that may or may not be edible in their raw form into edible ones in the presence of heat.

The mechanism of heat transfer matters of course.

If you boil something, by definition, the temperature at the surface of the boiled object can never go higher than the boiling point of water (= 100°C.) With both baking and frying, you can go to temperatures that are a lot higher.

There is a second technical difference. When you boil something, you can't really "lose" water. The cellular structures may burst and release water but you have water all around you. There's nowhere for the water to go. Comparatively, in both baking and frying, the water content of the burst cells is lost in the form of steam.

Once you understand this, you begin to realize why baking and frying have a lot more in common than is obvious. This is why many "low-fat" cookbooks substitute baking for frying.

It's a buncha bollocks though.

They are emphatically not the same — a proposition that is trivially verifiable by simple tasting. The CC doesn't believe that anyone in the entire history of mankind has ever mistaken a deep-fried food for a baked one and vice-versa, claims to the contrary not-withstanding.

Why would that be?

In baking, you are using air to transfer the heat. In frying, you are using hot oil.

The oil enters the surface of the burst cell and stays there. Ditto for the air. You can blot the oil out using paper towels but you can never remove it completely. The fat has flavor and the CC seriously doubts that anyone has ever confused the taste of fat with that of air.

To recap so far, frying is a precise mechanism of dessication (= removing moisture from food) in the presence of high heat with oil as the medium of transfer.

Now, let's dig into the precise mechanics. We will use the humble potato chip as the example.

When you put a chip into the fryer, the cells on the surface are heated to a temperature that is a lot higher than 100°C. The water in the cells heats up, converts to steam and expands; it bursts the cellular wall structures which are not rigid enough to hold it in, and enters the oil. Oil and water are immiscible and so the steam rapidly exits by rising to the surface.

The bubbles you see when you add food to hot oil is steam escaping the system.

After all the water is gone, you have a solid suspended in a medium of high heat but with no water to give up. This is the second phase and it works exactly like baking. You have a hot surface and a solid and you get a classic Maillard reaction going.

This is why you need to fry "past the point of the bubbles". This is also why there is similarity between baking and frying in the first place. (The difference, of course, is that a small amount of oil has entered the burst surface cellular structures in frying.)

Everyone with the CC so far? Onwards we go!

What happens to a more complex object? An arancini, for example. (Leftover risotto with a surface of egg and breadcrumbs.)

A large object cannot heat up very quickly. This is just Laplace's equation in practice. The surface rapidly turns crisp while the interior stays "moist" because the temperature never hits the steam point.

Experienced Sicilian housewives make "large" arancini whereas johnny-come-lately's try to dry out the rice. They are not playing the same game. (You can easily figure out the losers.)

Welcome to the platonic ideal of frying!

The surface must be crisp but the interior still moist since that is what humans desire.

This is also why fried foods are best eaten fresh. Laplace's equation tells you that the interior will continue to cook even when you pull the object out of the fryer because the surface is still at a very high temperature and air is a terrible conductor of heat. The heat is mostly going inwards. The interior temperature eventually hits 100°C; it generates steam and the fried object turns "soggy".

What happens when you have a very small soggy object possibly with some air pockets?

The outside has crisped up and the object is small enough that the interior has hit 100°C so steam is being generated. The air pockets expand dangerously and the pressure makes the object burst. The internal water hits the oil along with the burst air and you get an explosion of dangerously hot oil.

That is what happened to the CC's friend's mom.

If you fry a small object in hot oil, you must as a matter of necessity compress it using your hands to have absolutely no air pockets. You also need to control the size v/s moisture. It's a pretty subtle game.

Amateurs need not apply. Demonstrably, experience without theory means nothing.

To sum up, experienced chefs do one of the following:
  • Pre-dessicate the object either naturally or artificially.
  • Fry a largish object with a dry surface and wet interior.
  • Fry a small object with a dry surface and wet interior which has no air pockets naturally§.
  • Fry a small object with a dry surface and only barely moist interior which has no air pockets by design at a much higher temperature (= faster.)
There are no other choices.

Once you understand this, the tricks start making sense.

Ever wonder why potato chips stay crispy for days but fried food turns "soggy"?

The chips are so thin they have given up all their moisture but traditional fried food still has a ton of water in it.

Ever wonder why you are told that the oil must be piping hot before you fry anything?

The answer is that the interior shouldn't heat up. You want the surface to dessicate and Maillard-ify (to invent a verb) but the interior must not hit 100°C.

Every wonder why experienced chefs "double-fry" french-fries after sticking them in the fridge after round one?

Once you pre-cook and stick in the fridge you have done two things. You have dessicated the surface and cooled off the fries. When you drop them the second time in boiling oil, you get the crisp surface but the interior which is now quite cold will not hit the steam point. The fries will be crisp on the outside and moist on the inside.

There is no magic. There has never been magic. There will never be any magic.

Technique has always mattered at the highest level but what really matters is the science behind the technique.

† Raw spinach is technically poisonous. You'd have to eat a lot to find out though.
‡ e.g. arancini.
§ e.g. french fries.

1 comment:

Rajni said...

Interesting analysis with reference to heat transfer and what is happening inside.