One of the 70 different ingredients in a McRib sandwich 

is azodicarbonamide, "a flour-bleaching agent most 

commonly used in the manufacture of foamed 

plastics like gym mats and the soles of shoes."  But

this ingredient only accounts for its texture; it is not 

what gives the McRib its distinctive flavor.  That

is produced by the “pig bits like tripe, heart, and 

scalded stomach.”