Frying is not really cooking in fat
The intuitive picture of deep frying is that food sits in hot oil and the oil gets in. In a properly run fry, the opposite is happening for most of the cooking time. Water inside the food turns to steam, that steam pushes outwards through the crust, and the outward flow keeps the oil from moving in.
Those bubbles rising off a pakora are not the oil boiling — oil does not boil at frying temperatures. They are water leaving the food. When the bubbling slows dramatically, the surface has dried out, which is the signal that a crust has formed and the item is close to done.
Too cool is the failure that ruins the snack
If the oil is below the range where water flashes to steam quickly, the outward pressure is weak and the food behaves like something being poached in fat. Oil seeps in through the surface, the batter absorbs it instead of sealing, and you get a heavy, greasy article that is limp within a minute of leaving the pan.
Worse, the crust forms slowly or not at all. A crust is a dried, browned layer that is rigid because the starch and protein at the surface have set with almost no water left in them. At low temperature the surface never dries far enough, so what you end up with is a soft skin that will go soggy the moment it meets a chutney.
This is also where the oil absorbs into the interior in a way that cannot be drained off later. Draining on paper removes surface oil. It does nothing about oil that has gone into the crumb, which is why an underheated batch tastes greasy right through rather than merely on the outside.
Too hot fails differently and more visibly
Push the oil too high and the surface browns and sets long before the heat has worked its way to the centre. Browning reactions accelerate steeply with temperature, so the outside can be dark in a minute while the potato in the middle is still raw and the batter next to it is still wet.
This is a particular problem for anything thick or dense. A thin sev or a papdi is happy in very hot oil because there is barely any interior to cook. A stuffed item, a large vada or a whole chilli needs a lower temperature and a longer time, and that is a genuine trade-off rather than a matter of skill.
Very hot oil also degrades faster. Frying oil breaks down over the course of a day, developing off-flavours and eventually a bitter, acrid note, and heat and the water and food particles in the pan all accelerate the process. Straining out the burnt debris between batches is the single thing that most extends a batch of oil.
Why the temperature never stays where you put it
Every time cold food goes into the oil, it takes heat out. The oil temperature drops immediately, and how far it drops depends on the ratio between the mass of the food and the mass of the oil. Put six pakoras into a small pan of oil and the temperature can fall right into the range where everything above goes wrong.
That single fact explains most of the difference between a stall and a home kitchen. The vendor has a large volume of oil with an enormous thermal reserve, sitting over a burner powerful enough to put the heat back within seconds. A domestic pan with an inch of oil on a modest flame has neither the reserve nor the recovery.
The workable response is to fry in smaller batches than feels efficient, use more oil in a heavier pan than seems necessary, and give the oil time to come back between batches. Nobody enjoys this advice. It is still the whole game.
What the batter is contributing
A batter is a sacrificial layer. It has water in it, so it steams and protects what is underneath, and it has starch and protein that set into the rigid crust when that water is gone. Besan behaves differently from a rice-flour batter or a wheat one, largely because of how much water each holds and how quickly each sets.
Adding a little coarse flour or semolina to a batter creates an irregular surface with more area and more places for steam to escape, which reads as crunchier. A double fry works for the same underlying reason: the first pass cooks the interior and drives off moisture, the rest lets the remaining water even out through the item, and the second, hotter pass dries and sets the outermost layer without having to cook anything through.
Common questions
How do I judge the oil without a thermometer?
The old test is sound: drop in a small piece of batter. It should sink briefly, then rise and start bubbling steadily within a couple of seconds. If it sits on the bottom the oil is too cool, and if it browns before it has finished rising the oil is too hot.
Can I reuse frying oil?
A few times, if you strain it while it is still warm and store it away from light and heat. Judge it by smell and colour rather than by count — oil that has darkened noticeably or smells sharp has broken down, and it will pass that flavour straight into the next batch.
Why does my batter fall off?
Usually a wet surface underneath. Batter needs something to grip, so a damp vegetable should be patted dry or dusted with a little dry flour first. Oil that is too cool also lets the batter slump and slide before it has a chance to set.
Consumer editor, Wah Taste
Nadia has written about everyday cooking, street food, spice & pantry for most of the last decade and is unreasonably interested in the detail nobody else checks.
