Macerated strawberries demonstrate osmosis at work. Sprinkle sugar on the cut strawberries, and watch how a puddle of syrup collects on the surface. The high amount of sugar outside the strawberry’s cells, combined with sugar’s ability to attract water, causes the water to leach out of the fruit. The same thing happens to yeast cells—sugar, as well as salt, puts osmotic pressure on the yeast cells, making it harder for them to grow and causing fermentation to take longer. Too much of either can have a crippling effect on the cells.
Think of osmosis as the chemical version of water seeking its own level. Imagine that some very salty water is separated from less salty water by a permeable barrier of some kind, such as the wall around a yeast cell. The water molecules will try to even things up by diffusing from the less salty side to the saltier side until the two solutions are equally salty. It is as if there were a pressure pushing on the solution with the higher concentration of water molecules (and thus the lower concentration of salt)—and, in fact, scientists do talk about the osmotic pressure created by a difference in concentrations between adjacent solutions.
Osmosis can occur in any liquid medium and with any dissolved compound, not just in salty water. Sugar in a sweetened dough will also exert osmotic stress on the yeasts, for example. Mix salt or sugar into dough, and it dissolves in the watery part of the dough and creates an osmotic pressure that tends to suck water out of the yeast cells. The cells, which have unusually low water activity to begin with, try to hold on to what they have by activating networks of genes that produce glycerol, thus creating an osmotic pressure in the opposite direction. The strategy works up to a point—and the glycerol that salt-stressed yeasts produce can actually be useful in certain bread recipes—but the response diverts energy away from reproduction.
In general, when you make yeast-leavened breads, the more salt or sweeteners added to the flour (which already contains more than enough sugars for yeast to feed on), the slower the yeast activity, unless the yeast strain has been developed specifically to tolerate these ingredients. The net effect is that yeast doughs generally take longer to rise when they contain a lot of added salt or sugar.