Why stir sourdough only with wooden spoons? When reading about sourdough (in german texts at least), they almost always say to stir it with a wooden spoon.

Why do they do that? Does metal or plastic somehow interfere with the sourdough?
This is not true anymore, and modern recipes omit that part.

Back then, when silverware was made either from silver or pre-stainless-steel-alloys the acid in sourdough (and other foods*) would interact with the metal and corrode/color the metal and/or spoil the food.

So put your sourdough in your (stainless steel) mixing bowl and knead it in your kitchen machine as you like.


  *examples (thanks to @Stephie): eggs (there were special egg-spoons made from  horn or tortoise shell and later bakelite or plastic to
  prevent the sulphur in the eggs interacting with the metal),
  marmelade/jam (special spoons, jam would get runny if it got in
  contact with the wrong spoon), potatoes (there is a rule to not cut
  them with a knife).