Does yeast expire?

The answer

Yes, yeast gradually loses strength, and the best-by date on the package is the producer's guide to how long it should stay at full strength. Red Star Yeast gives that date as 2 years from packaging for its dry yeast, and recommends using opened jars within 4 months. Yeast kept cool and dry may still work somewhat past the date, while heat and humidity weaken it sooner. Test older yeast in lukewarm water with a pinch of sugar and replace it if it barely foams.