The clearest sign red pepper flakes have gone bad is losing their color and shine: fresh flakes have a vivid red color with a slight oily gloss, while stale flakes turn dull and fade toward a pale brownish tone.
How do you spot mold?
Because of their oil content, red pepper flakes are more prone to mold in humid conditions than most other dry spices. If you see whitish, fuzzy spots on the inside of the jar or on the flakes themselves, that's mold, and the whole jar should be discarded — scooping out the moldy part and keeping the rest isn't enough.
Condensation droplets collecting on the inside of the lid also point to a moisture problem, which sets the stage for mold to develop over time. If you notice moisture on the lid, transfer the flakes to a dry, clean container right away.
What does a change in smell and taste tell you?
Fresh red pepper flakes have a sharp, slightly smoky smell; if that smell fades and is barely noticeable, the flakes have lost their freshness. Heat and aroma fade over time in taste too — stale flakes added to a dish won't deliver the expected color and heat, just a bit of visual red.
Higher-oil pepper flakes can also develop a faint rancid smell over time; like stale nuts, this comes from the oils oxidizing, and it's another sign the flakes should be thrown out.
How do storage conditions affect shelf life?
Kept in a cool, dark cabinet in an airtight jar, red pepper flakes can hold their color and heat for 1-2 years; exposed to light and moisture, that can shrink to just a few months. Flakes stored right next to the stove spoil especially fast because of the heat.
If a large bag of flakes will take a long time to use up, freezing part of it is an effective way to preserve freshness; flakes in the freezer fade and lose their smell far more slowly than at room temperature.
Storing flakes in a metal tin instead of glass can make a difference too; metal blocks light completely, but unlike glass it hides moisture better, making it harder to spot condensation inside without opening the lid. That's why glass jars are usually preferred — they make checking easier.
In short, faded color, white mold spots, condensation on the lid, weakened smell, or a rancid odor together mean the red pepper flakes have gone bad; even one of these signs means they shouldn't go into food and should be discarded safely. Compared to whole spices like peppercorns, red pepper flakes have more exposed, oily surface area, so they usually show spoilage signs sooner than other spices sitting on the same shelf. Buying smaller packages and not letting a large stock sit around for too long is the simplest way to keep red pepper flakes fresh. Jotting the purchase date on the jar also makes it easier to remember which of several jars on the shelf should be used up first.