Several of my recipes I've offered on this blog give an option for adding old unfreshed stiff sour dough starter when making bread dough. I do this to add depth of flavor, moistness, and longer shelf life. I always have left over starter after the weekly feeding of my sourdough starter so I freeze it exactly for this use.I would not want to add it to a soft bread dough such as a soft white sandwich loaf or brioche because it makes the texture slightly firmer. But I do add it to most other doughs and I do add it to challah because it makes the dough more stretchy and easier to braid. You need to keep in mind that there is no salt in this starter so you need to add extra salt to balance the flour and water. You may also need to use a slightly larger pan or cut off the equivalent amount or weight of dough and bake it as a roll. If you retard the dough overnight, it will not rise quite as high so you can then use the same size bread pan as the one you would use without the starter. A bread that rises to 5 inches for example will rise to only about 4 1/2 inches if retarded for 8 to 12 hours. Of course you will need to make or purchase a sour dough starter and add enough flour to it to make it the consistency of soft bread dough.
To determine how much starter to use in the dough, multiply the weight of the flour in the dough by 16% and that will be the weight of the starter. For every 75 grams/2.6 ounces of starter add 1/8 teaspoon salt to the recipe. I like to soften the starter by cutting or tearing it in pieces and soaking it in the water used for the dough for 30 minutes before adding the other ingredients. This helps to distribute it more evenly throughout the dough. I am so devoted to this technique I may never write another bread book because it presupposes people will have or make or buy a starter and I feel it would be a serious compromise to omit this! The alternative would be to give the recipe with and without added starter the way I do for recipes on this blog...hmmmmmm