Why are we getting a greygreen coloration to our choux buns
The grey-green discoloration is associated with the formation of ferrous sulphide in the batter. If you examine the internal surface of your buns you may see small black specks. It is most likely that these will give a positive reaction when tested for iron. The iron has probably come from the surface of your mixing bowl which is being abraded by the beater, or from the surface of the container used to boil the roux. The formation of ferrous sulphide is accelerated by prolonged heating or by heating at a high temperature, such as that typically used for the production of choux buns.
The discoloration in your choux buns is similar to the grey-green colour that sometimes forms on the surface of an egg yolk after the egg has been boiled for too long a period or allowed to cool slowly. The reaction is caused by the iron in the yolk combining with hydrogen sulphide from the white of the egg. When making choux buns it is advisable:
• To ensure that the container used to mix the roux is not corroded, and that the mixing utensil does not abrade the surface of the container.
• To ensure that the roux is well cooled before beating in the egg. If cooling is normally done by mixing on a machine, ensure that the mixing tool does not abrade the surface of the bowl. If cooling is achieved by allowing the roux to lie in a metal container, ensure that it is not in contact with iron which may be exposed in a worn tinned bowl.
• To bake at as high a temperature, and for as short a period, consistent with the choux shells being adequately baked.
Continue reading here: Why is powdered ammonium carbonate or vol added to choux paste
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