We ran a couple of kitchen tests to see if really old cocoa powder was still fine to use.
Some see a box of unsweetened cocoa powder six years past its expiration date and think “trash.” In the test kitchen, we think “testing opportunity.” Wondering if cocoa spoils or loses its flavor over time as spices do, we made hot cocoa and chocolate butter cookies with the six-year-old cocoa and a freshly opened box of the same product. When we tasted them side by side, only about half the tasters noted a difference, calling the samples made with the geriatric cocoa “duller,” “weaker,” and “more mellow.” No one noted any off-flavors.
When we repeated the test using high- and low-fat cocoa powders one to two years past their expiration dates, again comparing them with samples made with fresh cocoa, tasters could not differentiate between the samples. The compounds that give cocoa powder its flavor are less volatile than those in ground spices, which lose much of their flavor and aroma after about a year. The more volatile the molecule, the more rapidly it evaporates and degrades.
THE TAKEAWAY: If you come across cocoa powder that's past its expiration date–even by a couple of years—it's fine to use.