Freshness Versus Expiration

• Relatively few foods are packaged with hard-and-fast expiration dates, indicating a product that rapidly becomes dangerous. Instead, most are freshness or use-by dates. They're intended to let you know how long a product is expected to maintain its best quality, rather than how long it remains food safe. For example, unopened milk or meats don't usually become unusable until days past that date. In the case of relatively inert substances such as baking soda, you have an even wider margin for error.

What Soda Is

• Bakers in bygone years used a variety of substances to leaven their baked goods, from sourdough yeasts to wood ash and ground-up deer antlers. By the 19th century, manufacturers began to produce a purified chemical -- baking soda, or sodium bicarbonate -- that served the same purpose, but more reliably and without the unwanted flavors and odors generated by some of those primitive leaveners. Baking soda is alkaline, so when it's moistened and mixed with acidic ingredients, it reacts by creating carbon dioxide. Buttermilk, lemon juice, molasses, fruit juice and even cocoa have enough acidity to react with baking soda.

Why It's Dated

• Sodium bicarbonate is a mineral, and as an inorganic substance it doesn't really decay or spoil over time. The idea of a best-before date on the box might seem dubious or even comic, if not for soda's absorbency. It has a well-known affinity for odors and is widely touted as a deodorant for refrigerators. Unfortunately, that's a flaw for baking purposes, and the longer it stays in your pantry, the greater the chance you'll produce a batch of cookies with an unexpected hint of garlic or curry powder.

Humidity and Baking Success

• Soda also absorbs humidity from the air, which is mildly acidic and causes a slow-motion version of the leavening process. If you live in a humid climate, this can cause your baking soda to lose potency over time as its leavening power is increasingly eroded by atmospheric moisture. You can quickly test whether your soda is still usable by heating a mixture of water and lemon juice in a measuring cup. When you stir a spoonful of soda into the hot liquid, it should foam vigorously. If it doesn't, discard it and buy a fresh box.