I have several out of date cake mixes, probably about two years old. I assume they would not rise properly as the baking powder in them has lost its potency. Would they work if I added fresh baking powder? What would be the proper amount, if so? Thank you, although I suspect the answer will be, "try it and see."--Geri
You are likely right that the potency of the baking powder will have lessened during the time the cake mixes have been sitting on the shelf. How much potency has been lost depends on a number of factors, so it would be hard to estimate how much fresh baking powder would be needed to bring things back to where they should be.
But that may not be the only reason that the cake mixes are no longer good.
Pretty much all cake mixes contain fats that will go rancid with time. It goes rancid through the oxidation of fat molecules. This happens easiest in unsaturated fats. Saturated and trans fats are less prone to oxidation but can undergo chemical changes that break the fat down into simpler molecules that can then oxidize. Rancid fats have an unpleasant flavor and a sharp stinging odor that ruin the taste of foods, and may not be healthy to eat.
Another problem is that the flavor compounds such as vanilla, lemon, chocolate and so on, that are used in the cake mixes lose their potency over time. An old cake mix may well taste more bland than a fresh one.
So, this time I wouldn't recommend a "try it and see" approach. For food safety reasons, the motto "when in doubt, throw it out" comes more to mind in this case.