I read your previous posting on Baker's Percentage.  My question is, given that eggs contain a lot of water, should they be counted in the hydration percentage for bread?

--Eric

About 76% of the total weight of an egg is water.  Of the rest, about 9.5% is fat, 12.5% is protein and 0.4% is glucose.   Technically, that means that for a large egg, weighing approximately 50 grams (1.76 oz), 38 grams are water. Egg whites alone have about 87.6% water.

For a typical white bread recipe, the water in one large egg can represent about about 5% - 6% of the total hydration percentage.  For doughs like Brioche and Challah, the water in the eggs can represent up to 20% or more of the total hydration percentage. 

In the Challah recipe given by Reinhart in The Bread Baker's Apprentice: Mastering the Art of Extraordinary Bread, he calls for 18 ounces of flour, 2 large eggs (3.3 ounces), 2 large egg whites (1.25 ounces), and 7 to 9 ounces of water.  The water in the eggs and whites totals just over 3.6 ounces or 20% of the total weight of flour.  The water represents between 40% and 50% of the total weight of flour, so the overall hydration is 60% to 70% (i.e. total weight of water from all sources/total weight of flour as %).

While I have heard some comments to the effect that we tend to spend too much time focusing on the minutiae of baker's percentages rather than learning how our bread actually behaves when we make it, since my return from the King Arther Flour Artisan Baking at Home class, I have been tracking my recipes and trying to learn how the science and the art come together in the kitchen.  In part, this is necessary simply because the Canadian flours available to home bakers tend to be higher in gluten forming proteins that their American counterparts.  What that means is that a recipe that works for US all-purpose flour will be under hydrated for Canadian all-purpose.  To get the results I want, I am finding I need to increase the hydration by a factor of about 1.03 -- that is, if a recipe calls for 70% hydration, I need to use about 70 X 1.03 = 72.1% hydration.  So far I would only call that a starting estimate, but without understanding where all of the sources of water are that come into a recipe, I can't make the appropriate corrections.  Either that or I try to figure out how to measure 0.03 of 2 whole eggs.