Protein powders dont spoil the way meat or dairy does, because a dry environment makes it nearly impossible for microbes to grow, says Bob Roberts, Ph.D., a professor of food science at Penn State University.

So unless youre storing your canisters in a warm, humid place (like the bathroom or your gym locker), youre fine on that front.

As for the muscle-building mojo of protein powder, it does diminish after the use-by date has passed. You can blame a chemical reaction called Maillard browning: The protein reacts with sugar left over from when the whey was extracted from milk, resulting in a gradual breakdown of the amino acid lysine.

If you lose lysine, then the powder will not be as complete of a protein, says Roberts.

Only a day or two past the expiration date? Put a bit of it on your tongue. Another telltale sign of Maillard browning is a fade in flavor, says Roberts. If you taste cardboard, toss it.

And if it always tasted like cardboard, then no wonder the stuff sat around for so long. Maybe its time to go shopping for a new brand, such as Isopure Zero Carb Creamy Vanilla powder ( $46, amazon.com ). It contains 50 grams of protein per serving.