Does baking soda go bad or lose its effectiveness?
What other food in your kitchen sits in an open box for months on end, with changes in humidity, cooking smells, and who know what else wafting through the open box flap from day to day? The whole point of baking soda (bicarbonate of soda) is to neutralize the acidity or alkalinity of substances it contacts and you can be sure that happens in the open box as well as in your mixing bowl. How could it not lose its effectiveness?
The expiration date stamped on the baking soda box in the Ochef test kitchen is three years off. We assume that means if you opened the box for the first time 35 months from now, it would still be good. But we opened it a month ago, and are quite sure it will have lost a significant portion of its effectiveness if we don't dip into it again for the next 35 months.
The folks that sell the market-leading brand in this country Arm & Hammer say there is no easy test for determining the potency of your aging baking soda, and suggest you err on the side of buying a new box to be sure your baking projects succeed. We'll call that Plan A. (Of course, they are in the business of trying to get you to buy new boxes of baking soda.)
Others find it useful to mix a teaspoon of questionable baking soda in a bowl containing a few tablespoons of vinegar. We'll call that Plan B. If there is a burst of bubbling activity, you can assume the baking soda has plenty of leavening life left in it. Otherwise, hang the cost and proceed to Plan A.
Our humble suggestion, if you go out on a limb and spring for a new box, is to also buy a small, flavorless, odorless, air-tight, plastic container in which to store your new purchase, so that kitchen breezes no longer blow across the surface of your baking soda.