Science, Tech, Math › Science What Are Century Eggs? Are the eggs soaked in horse urine? Share Flipboard Email Print Kondoruk/Getty Images Science Chemistry Chemistry In Everyday Life Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated December 09, 2019 A century egg, also known as a hundred-year egg, is a Chinese delicacy. A century egg is made by preserving an egg, usually, from a duck, such that the shell becomes speckled, the white becomes a dark brown gelatinous material, and the yolk becomes deep green and creamy. The surface of the egg white may be covered with beautiful crystalline frost or pine-tree patterns. The white supposedly doesn't have much flavor, but the yolk smells strongly of ammonia and sulfur and is said to have a complex earthy flavor. Preservatives in Century Eggs Ideally, century eggs are made by storing raw eggs for a few months in a mixture of wood ash, salt, lime, and maybe tea with rice straw or clay. The alkaline chemicals raise the pH of the egg to 9–12 or even higher and break down some of the proteins and fats in the egg into flavorful molecules. The ingredients listed above are not typically the ingredients listed on the eggs sold in stores. Those eggs are made from duck eggs, lye or sodium hydroxide, and salt. That sounds scary, but it's probably OK to eat. A problem does arise with some century eggs because the curing process is sometimes accelerated by adding another ingredient to the eggs: lead oxide. Lead oxide, like any other lead compound, is poisonous. This hidden ingredient is most likely going to be found in eggs from China, where the faster method of preserving the eggs is more common. Sometimes zinc oxide is used instead of lead oxide. Though zinc oxide is an essential nutrient, too much of it can lead to a copper deficiency, so it's not really something you want to eat either. How do you avoid poisonous century eggs? Look for packages that explicitly state that the eggs were made without lead oxide. Don't assume the eggs are lead-free just because lead isn't listed as an ingredient. It might be best to avoid eggs from China, no matter how they are packaged, because there is still a big problem with inaccurate labeling. Rumors Regarding Urine Many people avoid eating century eggs because of the rumor that they have been soaked in horse urine. There isn't any solid evidence that horse urine is involved in the curing, especially considering the fact that urine is slightly acidic, not basic.