MILWAUKEE—Rhonda Dannenberg, a suburban mother of three, stuck her nose in six glasses of beer at the MillerCoors brewery here and swished a bit of each in her mouth. Then she delivered the kind of frank verdict that's shaking up the mens-club world of beer tasting.

"I got a strong bruised fruit," Ms. Dannenberg, 36 years old, said of one of the Miller Lite batches, drawing a few nods from the three other women and two men at the table. "Slight cardboard taste. Oxidized. Unacceptable."

At...