The trouble started when I tried to make a bowl of oatmeal the other day. I measured, stirred, and poured the oats and milk as I always do. But then, two bites into the oatmeal I realized I’d made a huge mistake. The oatmeal tasted salty, a little sour, and just, off. I sniffed the milk I used and it was still fresh; I tasted the nut butter I’d dolloped over the bowl, and it too was just fine. There was no question: my oatmeal was rancid. How? Why? I took to the internet, as I am wont to do in such moments. At first, most food safety resources told me that oats rarely, if ever, will go bad before you’ve finished the container. Oats will stay fresh if kept in an airtight container in a cool, dry place for at least 12 months, they said.

Full disclosure: on that particular morning, I was out of the standard rolled oats I always use and instead made my porridge with another package of fancier, non-commercially processed oats I had in the back of my fridge. I’ll be honest, they were definitely about 12 months old. But I’d used them in cookies the week before and noticed nothing wrong, so I assumed they were fine. Not so much. 

Like most packaged products, oats will have a “best by” date printed on the package. This doesn’t necessarily mean that the oats will go bad after the date, only that the product will no longer be at its optimal freshness following the date; Quaker Oats’s website clarifies that “items beyond their recommended shelf life are safe to consume but may experience slight changes in flavor, color or nutritional value.”

So, why do oats go bad? The answer is twofold. Reason number 1: Oats can go bad more quickly if they’re not commercially processed. Standard packaged oats you’ll find at the store (like Quaker, for example) begin as oat groats that are first dehusked and rolled into flakes, then are treated with wet steam and toasted lightly in order to stabilize the grains. After the oats go through this processing, the oils in the grains will stay fresh for longer. The oats I used the other day were unsteamed (also known as unstabilized), and therefore contained unstabilized oils, which was a likely culprit for why the oats went rancid. 

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Another reason oats can go bad has to do with packaging. If the oats are packaged in paper, airborne yeast and humidity can enter enter the bag and spoil the products. Unlike mold, which is a very obvious indicator of spoilage (and can grow on oats if they’re exposed to moisture), these yeasts are pretty much invisible, and while affected oats will still look OK, their sour flavor will convince you otherwise. 

Preventing oats from going bad is easy, it just requires a bit of thought. If the oats are stored in paper, after opening the package, transfer the grains to an airtight glass or metal container and stash them the fridge or freezer. This is also a great way to prevent other nuisances that can arise in a pantry situation, like mealy bugs (sorry for the nightmares). If the oats are processed, they’ll probably be fine in the fridge for one year; if they’re unsteamed, it’s best to finish them off within a few months.