According to a Mayo Clinic article appearing on CNN, burps that smell like rotten eggs are caused by hydrogen sulfide gas produced as food passes through the digestive system. Hydrogen sulfide in the gas can make it taste and smell like sulphur or rotten eggs. Some people with certain infections or diseases may also experience burps that taste like rotten eggs.
Gas is always present in the intestines. According to the Mayo Clinic, nitrogen, oxygen and carbon dioxide in the body come mainly from air that is swallowed while chewing, swallowing, breathing and talking. Other gases are produced by the fermentation of food in the intestines or the breakdown of food in the stomach.
According to the New Health Guide, foods that contain complex proteins can cause burps that smell like rotten eggs. These foods, which include red meats, dairy products and eggs, produce hydrogen sulfide as they are broken down. Vegetables with a strong flavor, such as garlic, asparagus and broccoli, can also produce burps that taste like rotten eggs.
Even if they have not eaten the foods above, people with intestinal conditions like Crohn's Disease and irritable bowel syndrome may experience burps that smell like sulphur. According to Healthline, certain bacteria and parasites, such as Helicobacter pylori and giardia, can also cause belching.