Why can soaps made from olive oil or soybean oil become rancid or discolored?

Soaps made from olive or soybean oil are susceptible to rancidity and discoloration primarily due to the presence of unsaturated fats within these oils.

When these fats are exposed to air, they undergo a process called autoxidation, where the unsaturated components react with oxygen to form hydroperoxides. These compounds subsequently break down into volatile substances—such as aldehydes, esters, alcohols, and ketones—which produce the unpleasant odors associated with rancidity.12

This oxidative degradation is a chain reaction that can be accelerated by exposure to light or the presence of trace metal catalysts. Because these reactions involve the breakdown of fat molecules into various volatile byproducts, the process often results in undesirable changes in both the scent and the appearance of the soap. To mitigate these effects, manufacturers may incorporate antioxidants, such as tocopherol or BHT, which terminate the oxidative chain reaction by reacting with free radicals.21

Are you interested in learning more about how specific antioxidants are used to stabilize different types of consumer products?