Vol. IX · Fats and Oils Plate 8 of 10oil safety, grease fire, boil-over, water and oil, fire blanket, lid, extinguisher, flash point, deep-fryingS

Hot Oil Safely.

A teaspoon of water in hot oil becomes ten litres of steam in an instant. A cup of water on an oil fire throws burning oil across the kitchen.

Three things make hot oil dangerous. Water meeting oil: a drop of water sinks into oil at 180 °C, boils at once and expands about 1 700 times into steam, throwing oil with it — which is why wet food spatters and why a wet utensil or a splash of water can explode a pot. Boil-over: food going in raises the oil level and its steam froths the oil up; a pot filled more than about a third deep can overflow onto the flame. Fire: oil left heating unattended passes its smoke point (IX.3), then its flash point, near 300 °C, where its vapour can ignite, and then its autoignition point, roughly 400 °C, where it bursts into flame on its own.

An oil fire must never be met with water: the water sinks, flashes to steam and hurls burning oil upward and outward in a fireball. Turn off the heat if it is safe to reach; cover the pot with a lid, a baking sheet or a fire blanket and leave it covered until cold; or use an extinguisher rated for cooking oil (Class K in the United States, Class F in Europe). The workshop works out the steam from a splash, the oil level in a pot, and how quickly unattended oil heats.

Below: the depth of oil in the pot and how high it froths when a batch goes in, against the pot's height, and how long unattended oil takes to heat to its flash and ignition points at full power (exact gas law for the steam; frothing and heating are teaching estimates).

What you will learn
Workshop
Steam volume from the ideal-gas law at 100 °C and sea-level pressure. Teaching estimates: frothing raises the oil to about twice its depth with food in; the pot's height is taken as 0.6 of its diameter; heating time is the oil's mass × heat capacity × temperature rise ÷ 1.2 kW, ignoring losses, so real times are somewhat longer. Flash point about 315 °C and autoignition about 400 °C for a typical vegetable oil.

Respect the pot

A single teaspoon of water, about 5 g, becomes about 8.5 litres of steam the moment it reaches hot oil — which is why the oil leaps when wet food goes in, and why a pot of hot oil must be kept away from the sink, the kettle and a dripping lid. In a 22 cm pot, two litres of oil stand 5.3 cm deep, rise to 6.3 cm with 400 g of food and may froth to about 12.5 cm, against a pot about 13 cm high: this pot should hold no more than about 1.7 litres.

Unattended on full gas, two litres of oil reach their flash point in no less than about 15 minutes and could reach self-ignition in about 19; a thin layer of 300 mL in a frying pan could get there in 3 minutes. That is why oil is never left heating while the cook steps away. If it does catch fire: do not move the pot, do not use water; turn off the heat if safe, slide a lid or baking sheet over it from the side, leave it covered for at least half an hour, and call the fire service if it is not out. Keep a fire blanket by the stove.

At the stove

Before you next deep-fry, set out the lid that fits the pot and check where the fire blanket or a baking sheet is. Fill the pot no more than a third deep, and dry the food. Those three habits prevent most frying accidents.

Fire services across the world stage demonstrations of a cupful of water thrown on a burning pan: the fireball reaches the ceiling in a second. Kitchen fires are among the commonest house fires, and frying is their leading cause.

Terms

Flash point
The temperature at which an oil's vapour can be ignited by a flame.
Autoignition
The temperature at which oil ignites by itself, with no flame.
Boil-over
Oil frothing over the rim of the pot when wet food goes in.
Class K or F extinguisher
A wet-chemical extinguisher made for cooking-oil fires.
Never water on an oil fire: cover it and turn off the heat. Fill pots a third deep, dry the food and never leave oil heating unattended.

Sources

  1. Cooking oils have flash points of roughly 290–330 °C and autoignition temperatures of roughly 400–450 °C, depending on the oil and its state (fire-service guidance and NFPA 921, Guide for Fire and Explosion Investigations). Wet-chemical extinguishers rated Class K (NFPA 10, United States) or Class F (EN 2, Europe) are made for cooking-oil fires; water must never be used.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.