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If you're just adding it for nutritional content, you can usually add up to 2 Tbls ground to the base recipe without changing anything else, or more by using it to replace some of the flour. If they seem dry, just add a little more liquid or oil. If you're trying to use flax as an egg replacer, I've had good luck with mixing 1/4 cup ground flax with 1/2 cup oil, then mixing as usual. The muffins will be denser and not as high as ones with egg, but should taste delicious. I can give you more specific quantity suggestions if you list the recipe. Good luck!
Miranda is a contributor at Food52.
Sounds like you can replace some of the flour with ground up flax seeds pretty easily! :
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