Arsthbt

I have a basic sourdough bread recipe that I've made a few times with consistent results, so I decided to try modifying it with different flavors and fillings.

My question is, are there general rules of thumb, specifically regarding how different additions affect overall hydration percentage, baking & proofing times, etc?

One plan is too try a recipe with walnuts, and given their low water content, I assume I can just try adding them in after the first proofing and keeping everything else the same, as I might with seeds and other drier ingredients.

But what if I wanted to add a pure liquid like olive oil, or something with a higher water content like onions? Should I tweak the flour:water ratio? Should I add them in at different times?

And can I add a small amount of an extract, or will the alcohol have some negative effect on the proofing and/or baking process?

As you have already intuited in your question, what you do to adjust is going to vary based on the ingredient, and how much of that ingredient you add to the bread.

I can try to address some of the specific cases you mention:

Walnuts
This generally should not be a problem with adjusting hydration, other ingredients, or cooking times, although if you add too many walnuts (or other fillers) it can make the bread fall apart more easily. If I am adding nuts to a dough, I typically stick to a quarter cup or so for a normal-sized 1 lb loaf.

Olive oil
Adding any oils or fats to your dough will fundamentally change the character of the bread. As you add more fat, the resulting bread will tend to be softer and of course oilier. The crust will also usually tend to be darker when you bake it. A standard white bread loaf might have 1-2 tbsp of butter in it, for example, whereas a brioche might have 4-5 tbsp. If you just want an olive oil flavor in your sourdough, you might consider brushing the surface of your dough before baking (this will also usually cause the bread to darken more while baking) or right after you pull it out of the oven.

Onions
This depends on the hydration of the onions, but when I use fresh, finely chopped onions I do usually lower the amount of water or other liquids in the recipe. I tend to go by the texture of the dough rather than any hard and fast rules here, but is say for a quarter cup of fresh onions I may lower the amount of water for a single loaf by a tablespoon or two. One thing I will advise is that it takes a while for the water in the onions to be incorporated into the dough; I typically add less water to start, mix the dough thoroughly for 5-6 minutes with the onions, then check the texture of the dough to see if I need to make any hydration adjustments.

Extracts
Small amounts (e.g. a quarter teaspoon) do not have a significant impact on the proofing in my experience. In fact, small amounts of alcohol are already created during fermentation, and for a typical bread yeast (or wild fermentation as is the case with a sourdough) the alcohol levels would need to reach over 10% and potentially much higher before fermentation is inhibited in any way. Link Saccharomyces cervisae or "brewing yeast" is the same type of yeast typically used in bread-making, although in sourdough you will usually also find other things that contribute to the character such as lactobacillus and pediococcus.

Stress less?

I have been baking yeast bread for 30+ years. I have never given the slightest care for hydration percentage, heck, I think I got 20 years without even hearing the term. I certainly don't ever work in "bakers percentages" and don't stress about measuring, or particular ingredients.

My process is to dump however much liquid based on the planned size of batch in the bowl, and add flour, etc. until it's dough. Since you have a sense of what dough should look/feel/act like from your current recipe, just add flour until you hit that state. I usually, but not always start with a small amount of warm water and yeast and flour to make a sponge. I'll throw in whatever is around the kitchen that seems like it might work.

It comes out bread every single time. Some better than others, and I do pay attention to that. But it has literally never not been bread. Yeast bread, without the freakouts that are probably a good plan if you are making 500 loaves in a batch and your customers expect the exact same thing every day (how boring...) is one of the easiest foods on the planet to make, IMPE.

Liquids include water, potato or vegetable cooking water, milk, cream, eggs, molasses, pomegranate molasses, maple syrup, corn syrup, olive oil, toasted sesame oil, butter, yogurt, beer, sour cream....a little extract is fine, a bottle of vodka is not fine, as far as the yeast are concerned.

Flour & etc. includes all purpose, whole wheat, bread flour, bean flour, rye flour, barley flour, cornmeal, masa harina, rolled oats, rolled barley, kelp powder, carob, cocoa, coconut, other nuts, sunflower seeds, sugar, brown sugar, rice flour, potato flour, dry milk, dehydrated mashed potatoes (who knows how those got into the house, but they make terrible mashed potatoes, but work fine in bread.) Raisins, other (usually dried) fruits, garlic, etc. all fair game. Cinnamon, cardamon, ginger, other spices as you like.

I regard (normal, not to exclude things like 100% rye, but admitting that they are not the norm) bread as being (wheat) flour, yeast, water and "extras" - salt is purely optional and I haven't used it in 20 years or so. Of course the water in an egg/milk bread will be all from the eggs and milk, but the principle is there. For success and less odds of bricks, I try to keep the "non-wheat flour" dry stuff down to the quantity of liquid, or less, and prefer to lump the whole wheat flour in with that fraction, as the best way to make bricks IMPE is to go for 100% whole wheat. I also don't go too far overboard on the sugar, other than Anadama-eque bread needs a LOT of molasses to be right, to my taste (several batches where I'd add more, since it wasn't there yet in the previous batch.)

Temperature needs to be sensitive to the amount and kind of "extras" - plain flour, water, yeast ( a "lean" dough) can go quite hot, sugar and or eggs/milk (a "rich" dough) lead to more tendency to burn when quite hot, so the more of that there is, the more it turns down - not trying to "be a professional baker" I find a range of 450F on the hot end to 325F on the cooler end works, depending on burnable levels in the loaf. I prefer to roll fruit in when forming the loaf, so none is exposed, as fruit sticking out tends to burn.

Cheese chunks (particularly) and fully-hydrated fruits/veg can result in a small steam crater around the inclusion/addition. This need not bother you, but don't let it surprise you if it happens that all the fresh onion bits are somewhat loose in little pockets when you cut the loaf. Partially drying them or using dehydrated reduces this for something like onions. For cheeses a different/dryer cheese or simply moving the cheese addition phase to "after you cut a slice" rather than "baked in" are the options.

omg I totally love the way you roll. I too do the less stress is better. Thank you for being real :) cheers Kerri