Factors Affecting the Discoloration of Hard-Cooked Egg Yolks

AN unappetizing and highly objectionable greenish-black discoloration of the yolk in hard-cooked eggs has been frequently reported. There are several remedies offered to lessen this discoloration but very few of them are based on scientific evidence. Since little information was available in the literature concerning discolored hard-cooked yolks, extensive research was undertaken to elucidate the nature, causes and prevention of this problem. REVIEW OF LITERATURE Tinkler and Soar (1920) showed that the discoloration of hard-cooked egg yolks was due to ferrous sulfide (FeS) produced at the interface of the yolk and albumen by the reaction of iron (Fe) from the yolk and hydrogen sulfide (H2S) from the albumen. Heating yolks or albumen separately failed to develop the color. According to Tinkler and Soar (1920) prolonged cooking or slow cooling of eggs in the shell accentuated the color development. Rapid cooling reduced the amount of FeS formed by slowing up the …