Why do some bread recipes say to let the dough rise once before forming into loaves while others say to let it rise twice?

-- Eileen

There are three basic factors at work in the amount of time that dough is allowed to rise -- ingredients, convenience and flavor.  Doughs that are allowed to rise longer tend to develop more complex flavors.  Enzymes in the dough break down some of the starch from the flour and provide a slow, constant feeding to the yeast.  Because of this, the yeast population is slow to increase, giving more time for fermentation flavors to develop.  At the same time, some bacterial fermentation may also occur, particularly in sourdough breads, which adds even more flavors.

On the other hand, doughs that have sugar, honey or other ingredients that the yeast can feed on easily tend to cause a rapid increase in the number of yeast cells.  The yeast cells quickly devour the sugars, producing a lot of alcohol and carbon dioxide, and then they die off just a quickly.  Breads made in this way can have strong alcohol and yeast flavors if allowed to rise twice before shaping loaves, and may not have sufficient strength left in the yeast after shaping to allow the bread to rise one more time before it is cooked.

Because breads made without sugar, honey or similar ingredients take longer to make and require more attention, they are less popular in the home kitchen than ones made with those ingredients.