Salt in dough does more than add flavor: it strengthens the gluten network and gives the dough structure, regulates the yeast so the rise is more even, and darkens the crust. That is why bread without salt comes out flat and pale. Salt should not touch the yeast directly because concentrated salt draws water out of yeast cells and disables some of them, delaying and disrupting the rise. The correct order of adding the ingredients takes only a few seconds.
The measurement is based on the weight of the flour: salt in bread dough is 1.8–2% of the flour weight, or 18–20 grams per 1 kilogram of flour. Below this ratio, the dough becomes slack and spreads on your hands and the work surface; above it, the rise slows noticeably and the bread stays dense. Salt helps the gluten strands wrap around one another more tightly. When the same dough is kneaded with salt, it becomes more elastic and less sticky, and is much easier to shape. Salt also binds some of the water in the dough, delaying a soggy texture, which is why salted bread goes stale more slowly than unsalted bread.
Salt’s second job is to slow the yeast, and this is intentional, not a drawback. Unsalted dough rises quickly, but rapid fermentation leaves little time for flavor to develop, and the dough soon passes its peak and collapses. Salted dough rises more slowly and for longer; during that time, the compounds that give bread its aroma develop. Salt also darkens the crust because it prevents sugar from being used up too early, leaving more sugar in the dough to brown in the oven.
This is why salt should not touch the yeast directly. When dry or fresh yeast comes into contact with concentrated salt, it loses water from its cells and some of the yeast becomes inactive. The dough still rises, but slowly and unevenly. The solution is simple: put the salt and yeast in two opposite corners of the bowl, mix the yeast with water first, and add the salt after mixing it with the flour. Contact after mixing begins is harmless because the salt has been diluted in the water and distributed throughout the dough.
A few cases are different. Do not add salt to the initial mixture used to check whether the yeast is active: foam the yeast with warm water and a pinch of sugar, and leave the salt out of that bowl. For sourdough, salt is usually added after the step where the flour and water rest together. In doughs that use baking powder or baking soda, salt does not affect the rise; it is only for flavor and should be sifted with the dry ingredients to prevent clumps. The proportion is reduced somewhat in sweet doughs, but salt is still added; without it, sweet dough tastes flat and one-dimensional.