Soybean oil is a widely used ingredient, popular in kitchens and various industrial products. The question of whether this common substance is flammable or merely combustible is a matter of scientific classification and safety. While it will not ignite easily at room temperature like gasoline, soybean oil poses a fire risk when subjected to high heat. The distinction between a “flammable” and a “combustible” liquid is defined by specific temperature thresholds. This article explains the physical properties that govern soybean oil’s fire potential and provides steps for prevention and response.
Understanding the Combustion Threshold
The fire hazard of any liquid, including soybean oil, is determined by three specific temperature points: the flash point, the fire point, and the autoignition temperature. Regulatory standards define a liquid as “flammable” if its flash point is below 100°F (37.8°C). Since soybean oil requires significantly higher temperatures to ignite, it is classified as a “combustible” liquid.
The flash point is the lowest temperature at which a liquid releases enough vapor to form a mixture with the air that can briefly ignite when exposed to an external ignition source. Soybean oil’s flash point is typically around 626°F (330°C). This high temperature means the oil is relatively safe under normal handling conditions, but it is achievable on a stovetop if left unattended.
Higher than the flash point is the fire point, the temperature at which the oil produces enough sustained vapor to continue burning after the ignition source is removed. For soybean oil, this sustained combustion temperature is approximately 680°F (360°C). Even hotter is the autoignition temperature, the point at which the oil will ignite spontaneously without any external spark or flame. This temperature ranges from 763°F (406°C) to 833°F (445°C), showing that a fire is unlikely to start on its own unless the oil is severely overheated.
Preventing Cooking Fires Involving Soybean Oil
The primary cause of oil fires is overheating, which occurs when cooking oil is left unattended and its temperature rises past its smoke point and toward its flash point. The smoke point, where the oil begins to break down and release visible smoke, is an early warning sign that the oil is approaching a hazardous temperature. For soybean oil, this point is around 464°F (240°C). If oil begins to smoke, the heat source should be immediately reduced or removed to prevent ignition.
Another danger is the introduction of water or moisture into hot oil, which causes the oil to splatter and violently spread the hot liquid and any existing flames. This happens because water, being denser than oil, sinks to the bottom of the pan and instantly vaporizes into steam. The rapid expansion of steam forcefully ejects the hot oil into the air, potentially spreading fire to surrounding surfaces or igniting nearby materials.
To prevent this, ensure that any food added to the hot oil is as dry as possible, and never pour or spray water near a pan of hot oil. Monitoring the oil’s temperature is the most effective preventative measure, especially during deep frying where large volumes of oil are heated. Use a thermometer to keep the oil temperature well below the smoke point and never leave the cooking area while oil is being heated.
Immediate Response to an Oil Fire
If a soybean oil fire occurs, it is classified as a Class K fire in the United States, indicating it involves cooking oils, grease, or animal fats. The most important rule when dealing with an oil fire is to never use water, as it will cause the fire to explosively expand and spread due to the steam reaction.
For a small fire contained within a pot or pan, the safest method is to smother the flames by sliding a non-flammable lid or metal baking sheet over the container to cut off the oxygen supply. Once covered, the heat source must be immediately turned off. The lid should be left in place until the pan is completely cool to ensure the fire does not reignite.
If the fire is slightly larger but still manageable, baking soda or salt can be poured onto the flames, as these materials help smother the fire by depriving it of oxygen. For larger, uncontained oil fires, a specialized wet chemical Class K fire extinguisher is the correct tool. This extinguisher chemically reacts with the oil to create a foam blanket that prevents re-ignition. If the fire is growing rapidly or cannot be extinguished within seconds, evacuate the area immediately, close the kitchen door to contain the fire, and call emergency services.