Origin Estimation of the Black Component of "Black Eggs" from Owakudani, Hakone Hot Springs, Japan

Black egg or "Kuro-Tamago", is a well-known souvenir of Owakudani fumarole area of Hakone volcano in Japan. The eggs, which were originally ordinary chicken eggs boiled in a certain natural hot water pond in the area, literally have a black surface, which is formed by some hydrothermal chemical reaction of the hot water in the pond. However, its black component has not yet been determined, although it has been explained as iron sulfide formed on the surface of the egg shell. The black color on the surface eventually changed to a brownish color when left to stand within days scale. The observed phenomena would be problematic for the explanation so far, since iron sulfide is relatively stable in atmospheric air. In this study, we successfully isolated the exterior black film material covering the egg shell in the solid state by immersing it in citric acid solution. We conducted various chemical analysis on the black film material and determined significant amounts of carbon (C), nitrogen (N) and sulfur (S) together with protein-like substances, while no significant iron sulfide was indicated. Given the high carbon and nitrogen contents with biogenic amino acids and fatty acids, the black component may have been produced by the Maillard reaction in the hydrothermal environment. The high sulfur content also suggests the formation of high molecular weight organic polymers by sulfur-mediated cross-linking reactions on the exterior of the egg shell.