Pistachio Nuts Can Spontaneously Combust?

I do so love a good listicle. These often poorly researched fluff pieces give me never-ending fodder for articles. One such claim that makes the food-related listicle is the alarming fact that pistachios, particularly raw or fresh pistachios, can spontaneously combust!

Well, it’s true. It’s also very rare. And, as well, there is no reason to single out pistachios. Pick on a crappy nut, like the Brazil nut. Yuck.

Spontaneous Combustion in Food Storage

While spontaneous combustion in humans is probably a myth, it’s a real thing with haystacks and tree nuts. It can also happen in grain silos. It can happen anywhere there is a very large amount of combustible material being stored.

As this material breaks down or decomposes, heat is generated. This heat becomes trapped within the material and builds up. If there is also enough air trapped the autoignition temperature can be reached, and the stuff will spontaneously combust.

This is especially true of high-fat foods like nuts. Any high-fat organic substance may tend to undergo such self-heating during the decomposition processes.

If you store enough fresh pistachios in a container, things get very hot as they start to decompose. If conditions are right, they can go up in flames.

However, the same is true of sunflower seeds, peanuts, walnuts, cashews, and those aforementioned Brazil nuts.

In other words, in large-scale food storage and transport (this can happen aboard ship), spontaneous combustion is a thing. So, why is the pistachio nut being singled out so often? I blame Wikipedia. I’m always blaming Wikipedia, so why not now?

Why Pistachios?

The last line of the Wikipedia entry on the pistachio reads: “The improper storage of pistachio products in bulk containers has been known to start fires. Because of their high fat and low water contents, the nuts and especially kernels are prone to self-heating and spontaneous combustion when stored with oil-soaked fiber or fibrous materials.” ¹

assorted nut mix — Though rare, all nuts can spontaneously combust under the right bulk storage conditions, not only pistachios.

All Nuts Can Spontaneously Combust in the Right Bulk Storage Conditions, Not Just Pistachios

There is no reason to mention this as if it is unique to pistachios. No effort is made to explain any further. This is why encyclopedia entries are not usually written by random people who drop by the Britannica office.

The first source used by whoever wrote this line on Wikipedia is not devoted to pistachios but to the self-combustion of high-fat nuts in general. Pistachios are one nut mentioned by the source.

“Because of their tendency to self-heating, pistachio nuts may behave like substances from Class 4.2 of the IMDG (International Maritime Dangerous Goods) Code.” ²

In other words, pistachios behave like OTHER materials that are part of a certain class called ‘class 4.2 of the IMDG Code. Class 4.2 of the IMDG code are ‘substances liable to spontaneous combustion.’ These are not just tree nuts but other things like liquids and mixtures.

Both sources are from the German Insurance Association. The second source is dedicated to pistachios. The second source is a good overview of the spontaneous combustion of pistachio nuts.

However, the same language that is used to describe the tendency of pistachio nuts to self-combust is used by the same source to describe the tendency of brazil nuts to self-combust:

Self-heating of pistachio nuts/brazil nuts is an extremely vigorous process, as the consumption of fatty acids by respiration processes is associated with a considerably greater evolution of heat than is the case with the respiration equation for carbohydrates. Here too, as with cereals, the spoilage process proceeds in a type of chain reaction, because heat and water are formed by the fatty acids consumed by respiration, which in turn contribute to an intensification of the process.

The self-heating of pistachio nuts/brazil nuts requires only a small seat of moisture so that within just a few hours heating may occur at moist points for which weeks or months would be required in goods dry on shipment.

Fresh pistachio nuts/brazil nuts with a high water content tend in particular towards rapid self-heating and may also ignite. Self-heating of pistachio nuts leads not only to a reduction in the utility value of this product (rancid odor and taste) but also has a qualitative and quantitative effect on oil yield. The color and bleachability of the oils are also negatively affected. The oil obtained complicates refining of the crude oils in subsequent processing because a higher free fatty acid content makes decolorization substantially more difficult.

Hydrolytic/enzymatic fat cleavage and respiration may be limited by low temperatures; however, this may only be affected to a limited degree during transport. It is therefore important to ensure storage stability by complying with the limit values for the water content of the goods. ³ ⁴

I could have substituted peanuts, walnuts, sunflower seeds, cashew nuts, or Hazelnuts. ⁵ ⁶ ⁷ ⁸ ⁹

If I had bothered to keep searching, I have no doubt I could have substituted any nut. All of these have the same tendency to self-combust during storage as pistachio nuts.

While nuts may be at particular risk, again, this can happen with many food materials, liquids, and substances. We’ll file self-combustion of pistachios under technically true but misleading statements.