Can I substitute some (or all) of the white flour in this recipe with whole wheat flour?
If you want to keep the characteristics of the original recipe but just have a little more whole grain flavor, I recommend starting by substituting no more than 25% of the white flour (by weight) with whole grain flour. if that isn't sufficiently whole grainy for you, you can add more next time.
What should I expect if I do substitute whole wheat flour for white flour?
It certainly is harder to get a fluffy, light loaf when you use whole wheat flour, so expect it to be denser. The bran in the flour also works likes little razor blades, cutting through the gluten strands, so don't expect to get as open a crumb.
What is the difference between bread flour and all-purpose flour?
Bread flour (also known as strong flour) is higher in protein than regular all-purpose flour. Using it can lead to better gluten development, but it also creates a tougher dough.
And soft flour? Type 55 flour?
Soft flour is closer to what is called cake flour in the United States. It is lower in protein and creates a softer, crumbly crumb.
Type 55 flour, which is often used for French baguettes, is fairly close to American all-purpose flour.