Read my lips: buy new chemicals.

Reader Jud writes:

My wife, the household baker, says why should she replace her baking powder {and soda} every six months as we’ve heard recommended on cooking shows if she is pleased with the results? My response is, to paraphrase, if the results were good with “old” baking powder, why wouldn’t you want even better results by simply using fresh baking powder? Can you tell us about what changes occur with the passage of time and how the effectiveness of the baking powder and soda is affected? It makes sense to me to spend the small amount of money to replace every six months to get the best results possible. How important is it, really?

Jud, all men know how hard it can be to win an argument with our betters. However this time the angels are definitely on your side. Baking powders begin losing potency from the moment they’re mixed and canned, for while corn starch does interfere with the baking soda-cream of tartar reaction, it doesn’t stop it completely. And the reaction only accelerates once the can is opened and humid air is allowed to enter. This is why baking powder degrades faster in the warmer months.

Soda does much the same thing as it’s exposed to air. Though it doesn’t have a chemical partner in the same box like baking powder does, it’s continually reacting with molecules that drift in through that hard-to-keep-closed little cardboard flap. Fresh leaveners are crucial to successful home baking. Tell the missus she needs to take a walk on the wild side and drop the two bucks on a fresh can of Calumet. She’ll thank you for it later.

But I also suggest that you thank your lucky stars that you have a miserly spouse. I have one as well, and while I always dread the dressing-down I know I’ll get for buying name-brand trash bags, it’s her save-a-penny instincts that are going to end up sending our girls to good colleges.