Substitute by weight or volume?
So I feel like this is a dumb question, but I still don't know the answer. If I want to make a substitution in a recipe, say use some whole wheat flour instead of all white, do I substitute the flour by weight or by volume? Whole wheat flour is less dense than white, so would I replace say 100 grammes of white flour with 100 grammes of whole wheat, or would I replace 100 grammes of white flour with, say, 90 grammes of whole wheat, to try to get the same volume of flour? (I don't actually know the volume-weight conversions for the flours, but you can see what I'm asking). What about liquid ingredients?
Until recently, I've always used volume measurements in baking, and did substitutions by volume too. Now that I sometimes measure by weight, my first instinct is to make substitutions by weight, but that won't yield the same results as substituting by volume unless the things being substituted have the same density. I'm sure what's happening is that the quantities I normally work with are so small, and the ingredients to substitute are so similar in density, that it has not made a difference, but what is actually correct?
Forget volume forever. Always weight. Throw these cups away!
Always substitute by weight, caveat is that different flour will handle hydration differently. Bread flour will take a little bit more water than AP flour for same consistency, strong bread flour will take a little more, whole wheat flour will take even a bit more water. different brands and different types will vary. experienced bakers will adjust hydration by feel after mix. for rest of us, just try to keep that in mind and do small adjustments as we learn.
for your own piece of mind and curiosity, is to just measure as you would your substitution in cups and then weigh the ingredient on the scale and note it along with your thoughts. I've converted a lot of my cup recipes that way and later have them (and tweaks) metric and volume. Many of my conversions are just a little different from standardized tables and so mine are written down on a 5x7 card that flips around in my favourite cup recipes books. I tried pinnng my conversion card inside a cupboard with other everyday stuff but tend to sit down and flip thru cook books elsewhere. (Dont ask me how my table looks right now.) :)
Mini