When a baker uses the term hydration, he is referring to the relative proportions of water (or liquid) to flour in a recipe. Bakers speak of a dough as being such-and-such hydration and other bakers know what the characteristics of the dough are likely to be.
If you refer to the Bakers Percentage section, you will recall that we talked about a dough being 62.5% water or 67% water. This is hydration.
Some breads have well-known hydration numbers.
Classic French Baguettes are 60% hydration.
Classic Italian loaves are 63-65% hydration.
Bagels are usually 56-58% hydration.
Ciabatta is usually 72-74% hydration.
These are for water. Here are some considerations I use for oils, butter, eggs, etc.
Butter, honey and oils are 3/4 liquid.
Large eggs weigh between 63 and 73 grams and are about 3/4 liquid.
Obviously, since these ingredients are added in small amounts in most recipes, you won’t do much damage if you treat them as all liquid.
Dough will handle differently depending on its hydration and the amounts of fats, oils and sugars added to it. A dough with no added fats or oils, a lean dough, will seem to be a bit dryer than a dough of the same hydration but with fats or oils added. A dough with a lot of sugar in it will seem a bit dryer than a dough with no sugar. (The sugar ties up a bit of the water, making it unavailable to the flour.)
Bakers can work doughs below about 65-67% by hand without much trouble. Between 68 and 75%, working by hand is difficult but not impossible. Anything over about 75% is very difficult by hand and gets into the realm of pancake batter; the dough doesn’t develop, it just flows and oozes. One unfortunate side effect of this is that a novice baker may take one look at a really wet dough and get turned off, not realizing that very wet dough can yield really good bread if it is treated properly.
When confronted with a dough that is wet and sticky, many novice bakers throw flour at it and attempt to make the dough dryer so it won’t be so sticky. This is not a good thing to do, as it incorporates new, fresh, unwet flour into the dough and may upset the balance of ingredients in the dough. The better alternative is to start the recipe by holding back a portion of the water and developing the dough with a bit less water. If the dough is too dry, the baker can always add a bit of water later without dire consequences. It’s a bit of a mess, but it works well.
Many times, a baker will start a dough and think it is too sticky or too dry, only to find that the dough changes drastically as it develops. This is another reason to make changes with water. If one were to develop a dough through kneading and then decide to make an addition, any flour he added would be raw flour, and the dough, while seemingly more tractable, would more than likely have nothing more than a surface of raw, new flour, which, if turned into the interior of the loaf, could cause white streaks of raw flour. Not a pretty sight.
As we all learned in grade school, flour and water make paste; no dough is going to be perfectly sticky-free.