Until now, that would be the azodicarbonamide, a chemical that adds elasticity in products such as shoe soles, and Subway customers aren't happy.
In fact, after a blogger was able to collect more than 70,000 signatures, the sandwich chain has made the decision to take the chemical out of its recipe.
"We are already in the process of removing azodicarbonamide as part of our bread improvement efforts despite the fact that it is (a) USDA and FDA approved ingredient," Subway said in a statement. "The complete conversion to have this product out of the bread will be done soon."
According to the Center for Science in the Public Interest, the chemical has been used in commercially baked bread from retailers such as Starbucks, McDonald's and Arby's and is meant to strengthen dough.
The successful movement to convince Subway to change is recipe was started by Vani Hari, the blogger behind Food Babe, who began talking about the chain's ingredients in 2012.
Her petition, launched last week, has been signed by more than 70,000 people.
"When you look at the ingredients, if you can't spell it or pronounce it, you probably shouldn't eat it," Hari said. "I commend Subway for finally responding to me and now over 57,000 concerned citizens. Their swift action is a testament to what power petitions and individuals who sign them can have."