Vol. IX · Fats and Oils Plate 7 of 10frying, oil temperature, batch size, recovery, crowding, fryer, thermometer, loadHT

Oil Temperature and Load.

Two litres of oil at 180 °C hold less heat than you think. A big handful of chips takes it to 120 °C in a minute.

A pot of oil is a store of heat: two litres at 180 °C hold about 0.6 MJ more than at room temperature. Food going in draws on that store twice — to warm itself, and above all to boil off its water, which costs 2.26 kJ a gram (VIII.2). A 600 g batch of raw chips will boil off over 200 g of water: half a megajoule, nearly all of the oil's stored surplus. The burner refills the store, but a domestic gas ring delivers only about a kilowatt into the oil.

So the batch sets the temperature. A small batch dents it and the oil recovers while the food fries; an overloaded pot falls towards 120 °C, the steam flow weakens, the food soaks and goes pale and greasy (IX.6). The rules follow: plenty of oil, small batches, a thermometer, and waiting for the oil to come back to temperature between batches. The workshop drops a batch into a pot of oil.

Below: the oil temperature over eight minutes after a batch of raw chips goes in, for a small batch, a large one and yours (stove bench), with the frying range marked.

What you will learn
Workshop
Stove bench: rapeseed oil on a 3 kW gas burner (40 % reaching the pot, run at 90 %), raw chips at 8 °C, 78 % water, losing up to half of it; the heat-transfer coefficient falls as their surfaces dry. A teaching lumped model: one temperature for the oil.

Small batches, plenty of oil

In two litres at 180 °C, 150 g of chips takes the oil down only to 170 °C. A 600 g batch drops it to 121 °C and keeps it below 160 °C for 7.8 of the next eight minutes: the chips sit in oil too cool to keep the steam streaming and come out pale and greasy. The same 600 g in four litres bottoms out at 133 °C. A good rule for a home pot is a batch of no more than about a tenth of the oil's weight.

Practical frying. Use a deep, heavy pot with at least 2 litres of oil and a clip-on thermometer; heat the oil to the top of the range so the batch brings it into the range; dry the food first (wet food costs more heat, VIII.2); fry in batches and wait for the thermometer to recover before the next; keep cooked batches hot and crisp on a rack in a low oven. Electric fryers with thermostats and powerful elements recover faster, which is much of why restaurant chips are better.

At the stove

With a thermometer in a pot of oil at 180 °C, add a small handful of chips and note the lowest reading; let it recover and add a large handful. The second drop is much deeper and the recovery much slower.

Restaurant fryers hold 15–25 litres of oil heated by elements of 10 kW or more, and cooks fill the baskets to marked lines: the ratio of food to oil is a specification, not a guess.

Terms

Load
The amount of food added to the oil at once.
Recovery
The return of the oil to its set temperature after a batch.
Thermal mass
How much heat a body stores per degree: more oil, more stored heat.
Clip-on thermometer
A frying thermometer held in the oil on the side of the pot.
Food cools the oil by warming and boiling off its water. Fry small batches in plenty of oil and let it recover between them.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.