Case Study: The anatomy of a cooking fire investigation: Meier Fire Investigation examines one of the most common types of fire causes and how our fire investigation expert

Cooking Fires are more common than you think

According to estimates by the US Fire Administration, cooking fires account for 45.7 percent of household fires between 2007 and 2011. That’s almost half. As fire investigators, it would therefore be a wise decision to have a good understanding of the phenomenon, of the fuels involved, the source of ignition, fire growth, and subsequent fire spread.

Cooking Fire Investigation: Chemistry and Dynamics

Common occurrences with complex science

To understand how most kitchen fires occur, you need a basic understanding of fire chemistry and fire dynamics of cooking fires and the fuels involved. We’re breaking down the science of cooking fire investigation and we’ll give a few tips on how this affects our cases.

Timing is everything

The time required for a cooking fire to occur once set in motion will vary greatly, depending on a number of variables:

• The size of the burner (diameter, wattage or BTU)
• The heating method (open calrod burner, ceramic glass cooktop, gas)
• The setting of the burner (high, medium, low)
• The size of the cooking pan (diameter, depth)
• The material/fuel in the pan (cooking oil, fatty meat, other)
• The volume and depth of fuel in the pan

Cooktop burners can vary in size for 4 to 12 inches (10 to 30 cm), although 6 and 8 inch (15 and 20 cm) are typical. Common electric cooktops burners range from 800 to 1800 Watts, although larger and smaller are also available. Although ceramic glass cooktops have thermostats to prevent breakage of the glass, most will still produce enough heat to cause ignition. Gas burners in general will produce more heat than the equivalent sized electric burner. Rule of thumb – if it can blacken a fish, it can cause ignition.

What’s in the pot matters

A key component in cooking fire investigation is determining the first fuel ignited. Usually that is what was in the pot. We rely on witness statements and physical evidence to figure out what was cooking at the time of the fire.

High Water Content Foods: Think Soup, Stew, Spaghetti Sauce

If the contents of the pan have a high water content and are low in fats and oils, the time to first dehydrate the fuels can take a number of hours. Until at least a portion of the contents are dried, the temperature of the material will not climb above 212^oF (100^oC), the boiling point of water. Once a portion of the material is dried, however, the temperature can begin to rise again, eventually reaching its ignition temperature. Such a fire typically starts as a smoldering fire, as the burning portions of the fuel can still be covered with wet fuel and there is no mixing action which brings the burning fuel in contact with fresh air.

These high-water food fires produce smoke damage before any significant fire damage occurs. Unfortunately, they can also cause carbon monoxide fatalities if the home is not equipped with an adequate smoke/carbon monoxide detectors. These can be significantly more dangerous overnight, when the occupants are sleeping.

Fatty Foods: You know, the really tasty ones.

A cooking fire investigation involving fatty foods is a different story. A flaming fire will happen quickly if the contents of the pan are high in fats or oils. Little or no evaporation is required to dry the fuel, and the fuel is exposed to fresh air from the start.

The most common cooking oils are soy oil (generically referred to as vegetable oil) and corn oil. These account for approximately 80% of cooking oil produced and used in the U.S. Other types of oil used for cooking are: palm oil; canola (rapeseed) oil; sunflower oil; peanut oil; cotton seed oil; coconut oil; and olive oil.

Some of the characteristics and properties of common cooking oils are:

• Cooking oils have medium to high viscosities which prevents rapid mixing and heat dissipation.
• The smoke point of common cooking oils range between 425^oF and 450^oF.
• The piloted ignition temperature of corn oil is approximately 550^oF.
• Both vegetable (soy) and corn oil have autoignition temperatures of ranging from 625 to 675^oF.
• The boiling point of the oil is higher than the auto-ignition temperature. (Corn oil approximately 685^oF.)

The steps of a common stove fire

Here’s a common scenario of how a fatty food cooking fire can start. A pan of oil is placed onto a heating surface, such as the heater coil on a kitchen range or the flat surface burner of a glass cooktop.

The rate at which the oil heats depends on the size of the pan, the size and power of the burner, the burner setting, and the amount and type of oil in the pan.

The oil will heat relatively quickly at first, but the rate of heating will slow as the temperature increases. This is due to the increase in radiant and convection heat losses as the temperature of the oil increases.

First, The oil will reach its “smoke point”, or the temperature at which visible smoke is emitted from the oil. Even before any ignition occurs, the air can become extremely smoky. The smoke can be quite unpleasant and can cause difficult breathing. Smoking oil may go unnoticed if confined in a separate room or if a vent fan is operating.

Next, The temperature of the oil will continue to rise until it reaches its autoignition temperature. The auto ignition temperature is the point where the oil will spontaneously ignites without the aid of an external source of flame. The time it takes for the oil to heat from room temperature to autoignition temperature can vary greatly but, general estimates of within 10 to 60 minutes are quite reasonable.

There may be a flash as the collected vapors above the oil ignite, but then the flames will generally start small. The oil will continue to heat from below. The fire plume interferes with the transfer of heat away from the oil by convection, and some additional radiant heat is added from above. The temperature of the oil begins rise rapidly. The preheated fuel causes the flame height will grow quickly. Depending upon the depth of the pan and the amount of oil in it, the oil may spill over from simple expansion as the oil heats, or boil over when hot enough. The oil can then spread well beyond the pan.

Once the flame plume has developed, it may come into contact with another fuel load above. This may be an overhead microwave oven, and exhaust hood or kitchen cabinets and their contents. The fire would continue to spread as additional fuel loads are encountered. Depending on available fuel loads, fire growth, vents, construction materials and many other factors, it may take anywhere from a few minutes to over an hour for the fire would be noticeable from outside of the house.

Cooking Fire Investigation: What Happens & when it happens

In fire science lingo this is the Ignition Sequence and Fire Spread.

Typically, a kitchen fire starts, grows and spread in the following sequence:

• A pan of oil is placed upon the burner of the cooktop.
• The burner is turned on in preparation of cooking.
• The oil will reach its smoke point, generally 100 to 150^oF (56 to 83^oC) below the auto-ignition temperature.
• The oil in the pan heats until it reached its auto-ignition temperature (600 to 700^oF, 315 to 370^oC).
• Once ignited, the flames reached a height of 4 feet (1.3m) or more, based on the pan diameter.
• The oil may spill out of the pan from expansion, boiling over, or both. The flame height would increase significantly if spill over occurs.
• Convective and radiant heating then spreads to objects above or near the cooktop.

In an accidental kitchen fire, fire patterns are generally speaking, fairly predictable. One might expect a “V” pattern on the wall behind the cooktop, and a semi-circular pattern on the ceiling above. These patterns may become obscured if the fire spreads significantly beyond the cooking area, especially if flashover occurs in the room of origin.

IN your cooking fire investigation, If you find fire patterns that vary greatly from these, consider things that may have affected fire growth and movement. Ventilation from windows, doors and vents can cause patterns to shift. Fuel loads on adjoining countertops can ignite and cause other patterns which obscure the original patterns or seem to indicate a different origin. Firefighting activities can also cause change in the fire patterns. If, after taking all of this into account, the patterns still don’t make sense, you may have to start looking for another origin and cause.

Beyond Cooking Fire Investigation: Other Content

Explosion Investigations.

About Our Cooking Fire Investigation Expert: Richard Meier

Cooking fire investigation expert, Richard J. Meier has been full-time fire investigator since 2011. He is a professional Fire and Explosion Investigator and Analyst, a degreed Mechanical Engineering Technologist, and has been engaged in design and manufacturing engineering, failure analysis, and fire and explosion investigation for thirty years.

Richard Meier is a Certified Fire and Explosion Investigator (CFEI), a Certified Fire Investigation Instructor (CFII), and Certified Vehicle Fire Investigator (CFVI) through the National Association of Fire Investigators (NAFI). He is also a Certified Fire Protection Specialist (CFPS), certified by the Certified Fire Protection Specialist Board and the National Fire Protection Association (NFPA). In addition, he is a Certified Fire Investigator by the International Association of Arson Investigators (IAAI).

Richard Meier served on the Board of Directors of NAFI and taught various aspects of fire investigation at NAFI and other training programs. He has taught for the International Association Fire Investigators, the Broward County Fire Inspectors Association, the Florida Fire Marshals and Inspectors Association, and the National Aeronautics and Space Administration (NASA).

Richard Meier is a member of the Technical Committee for NFPA 901 – Standard Classifications for Incident Reporting and Fire Protection Data (NFPA committee page) representing the National Association of Fire Investigators, and on several task groups for NFPA 921 – Guide for Fire and Explosion Investigation. He continually engages in advanced education in fire investigation, and conducts research in fire investigation and material science as it relates to fire investigation. He has presented his research at international fire investigation, and fire science and technology conferences.