Think Twice Before Storing Kirkland Signature Pistachios In The Pantry

When you buy the large three-pound bag of Kirkland Signature in-shell pistachios, your first instinct may be to store it in the pantry once opened. But you might want to reconsider as the label on the back of the package recommends storing the pistachios in an airtight container and in the fridge. If you find this recommendation a bit odd, it makes a lot more sense once you learn that pistachios can actually combust under the wrong conditions, one of which is high temperature.

Because pistachios contain over 50% fat, they can begin self-heating. This makes them a potential fire hazard, especially during transport. There are three main factors that can contribute to the spontaneous combustion of the pistachio: weight, moisture, and temperature. When pistachios are transported and stored in large quantities, the pressure of over-stacking can accelerate self-heating. The little green kernels are also great at absorbing moisture from the environment, which is an issue when it comes to both spoilage and flammability. Even a little bit of moisture can cause overheating inside the pistachio. Finally, there's the temperature. Apparently, the ideal temperature for storing pistachios is 32 degrees Fahrenheit, but anything up to 77 degrees Fahrenheit is still considered safe. Never store pistachios near heat sources or in direct sunlight, though.

Knowing all this, Costco's recommendation for storing in-shell pistachios begins to make sense. Storing them in an airtight container keeps the pistachios safe from moisture, and placing them in the fridge assures they're staying at a low temperature.

Interestingly, Kirkland Signature shelled pistachios don't come with the same storage guidelines that in-shell pistachios do. There's actually no storage recommendation on the label at all, so presumably you're fine keeping those in the pantry. One possible reason for that is the smaller size of the package — the shelled pistachios are only sold in 1.5-pound packages, making them less crowded. They're also without the shell, which could slow down the overall self-heating because there's no external insulation like there is with the shell. Finally, the shell is a dry material and could pose a bigger hazard in the event of the nuts overheating.

There is a good reason why pistachios are mostly sold in the shell, too. The shell acts as a protective layer around the inner green kernel that's actually a seed. Many people also have a personal preference for in-shell pistachios, saying that it makes them eat more slowly, or even that they enjoy the shelling process, which can admittedly be somewhat meditative. And when it's not, we know of the foolproof way to open stubborn pistachio shells.