Most of the gleaming steel tanks outside Fage's yogurt factory hold milk. One, however, holds the yogurt byproduct whey. Dan Charles/NPR hide caption

itoggle caption Dan Charles/NPR

Tim Opper, of Cabot Cheese, inspects equipment that separates whey protein from sugar in the company's whey processing plant. Dan Charles/NPR hide caption

itoggle caption Dan Charles/NPR