The fat went off

Most supplement powders are stable for a year or two. The ones that are not fail through their fat content, and the difference between categories is larger than people expect.

Not every powder is at risk

A pure mineral or amino acid powder has essentially no oxidizable fat and is stable almost indefinitely if kept dry.

Protein powders, meal replacements, seed and nut-based greens blends and anything with an oil-based ingredient all carry fat, and fat oxidizes.

Why whey is worse than pea

This is the specific finding worth knowing. Whey and other animal-based protein powders degrade faster than plant-based powders because of their higher fat content, and over time those fats oxidize to rancidity.

Which inverts a common assumption. A plant protein is frequently the more shelf-stable product, not the more fragile one, and an isolate is more stable than a concentrate because more of the fat has been removed.

So the choice between whey concentrate, whey isolate and a plant protein is a shelf-life decision as much as a nutritional or cost one.

What it smells like

Cardboard, old crayons, or a faint paint-like sharpness. It is subtle at first and customers usually describe it as the flavor being off rather than as rancidity.

They will also, almost always, blame the flavor system rather than the protein.

Slowing it

Choose the material deliberately: isolate over concentrate, plant over animal, where the product allows.

Keep oxygen and moisture out, which is the same barrier discipline that prevents caking. Less headspace, a real seal, and cool dark storage on your side as well as the customer's.

And buy raw protein in quantities you will use. A rancid input makes a rancid product from day one, and it is the material least likely to be checked on receipt.

References