The Formation of Ferrous Sulphide in Eggs during Cooking

Abstract

IT is a matter of common experience that on prolonged cooking Qf an egg in its shell (e.g. when it is immersed in boiling water for 15 minutes or longer) a greenish black coloration is produced on the surface of the yolk. It is also well known that if a so-called hàrd-boiled egg" be immersed in cold water immediately after cooking, the green colour is either not apparent at all, or is much less marked than is the case when the egg is allowed to cool slowly. In view of the fact that this coloration is only on the surface of the yolk, it would appear that in its production some chemical action takes place, either between some constituent of the membrane which surrounds the yolk and some constituent of the yolk itself, or that some substance which is produced from the egg white, and which can penetrate this membrane, interacts with some constituent of the yolk. The Nature of the Coloured Substance. It appeared probable at the outset of the investigation that the greenish black coloration was due to ferrous sulphide in a fine state of division, and the various experiments which have been made seem to confirm this view. Thus the coloured substance is readily soluble in dilute acids, the solution so obtained giving positive reactions for iron and evolving hydrogen sulphide on warming, whereas cooked egg yolk showing no greenish colour does not give these reactions. Moreover, the coloured egg yolk is decolorised on prolonged exposure to air or on treatment with hydrogen peroxide. This is also the case with the greenish coloured precipitate of ferrous sulphide when, for example, this substance is precipitated in a fine state of division on a piece of filter paper by the interaction of a solution of ferrous sulphate and am-monium sulphide. It is well known that egg white on prolonged heating evolves considerable quantities of hydrogen sulphide and that the yolk of egg contains iron, probably in combination with proteins. The amount of iron in egg yolk, according to Sherman [1914], is 0*0085 % by weight. A solution of ferrous