Every yeast-raised bread relies on the four basic ingredients of yeast, flour, water and salt for its rise and structure. When the flour is moistened with water, proteins in the wheat begin to bond together, creating long, stretchy strands called gluten. This gluten acts like a sheet of rubber, trapping carbon dioxide that's excreted by the yeast as it consumes the natural sugars in the grain. Salt helps make the gluten springy and elastic, and it slows the metabolism of the yeasts. This is important because if they're too active, the yeasts produce alcohol, giving the loaf an unpleasantly sour flavor and aroma.

Basic four-ingredient loaves are referred to by bakers as "lean" breads, because they contain no added ingredients to enrich their flavor and soften their texture. Most of the ingredients added for rich or sweet breads have the effect of tenderizing the loaf. For example, if you use milk as your liquid instead of water, the bread will have a softer, finer crumb. Eggs add richness and tenderness to the dough, and sugar sweetens and softens the bread's crumb. So do fats such as olive oil, butter or even shortening, which lubricate the bread's proteins and prevent the gluten from forming long, chewy strands. This is why bakers call all fats "shortening" ingredients.

It's rare in baking for an ingredient to have a single effect. Usually any significant ingredient affects the others, and that's the case with rich, tender bread doughs. The added sugar and fat in these breads can inhibit the activity of the yeast, so rich breads such as challah and stollen usually require extra yeast. They also require more salt to balance the richer, sweeter flavor and ensure that the higher levels of yeast and sugar don't result in a sour, fermented loaf. Rich breads tend to brown more quickly as well, so they are baked at a lower temperature than lean breads.

The aphorism that "you never get a second chance to make a first impression" applies to breads. If your crust is tough and chewy, it sharply affects everyone's impression of the bread underneath. Brushing the bread before it's baked with milk, cream or melted butter makes the crust softer and richer. Glazing with a beaten egg also creates a softening effect and gives an attractive sheen. For the tenderest crust, brush the loaves again with milk as soon as they come out of the oven. The milk absorbs into the crust and leaves no visible residue, but it makes the crust beautifully soft and tender.