Rancid foods have the same look and texture as when they were purchased, but their smell and taste have changed. The odor is akin to wet cardboard, oil paint, wood varnish, or play dough. Some people are offended by it and know to throw the product out, while others think that’s just how the product normally smells. What is rancidity exactly? Simply put, it’s the oxidation of fat. Oxygen in the air attacks fat molecules and causes a series of chemical reactions that lead to the formation of new—and decidedly smelly—molecules. These reactions occur even more quickly in the presence of light and heat.

Source: Ricardo