Over many years—we're talking three to five once opened—the powder slowly absorbs moisture and air. It clumps up like a cranky cat refusing to move. The acid strength that makes your cookies chewy and your whipped cream stable fades away. You can still use it, but it's like trying to start a campfire with damp matches. It just won't work the same.
If you open the jar and it smells like a dusty basement? That's fine. No smell is the smell you want. If you see actual bugs or mold—which is rare, because it's so dry—toss it. But usually, the biggest sign is that you pour it out, and it falls in a sad little lump instead of a fluffy cloud.
The Cupboard Test (and a Funny Anecdote)
My mom once had a jar that was older than me. She swore it was "still good" because it had a 1970s label. I tried to use it to stabilize a lemon meringue pie. The meringue looked okay going in, but after baking? It wept like a toddler denied candy. The pie was a beautiful, watery tragedy. Lesson learned: old cream of tartar is a liar.
Cream Of Tartar Expired at Charles Mcclelland blog
So how do you check? Do the vinegar test. Mix a tiny bit of cream of tartar with a few drops of vinegar. If it fizzes like a secret agent's escape potion, it's still active. If it just sits there like a guilt-ridden potato, time to buy a new one.
