Salt is a widely used preservative, quickly drying out microbes and disrupting cell membranes. So, it is a matter of "common sense" to limit direct contact between yeast and salt when kneading dough. Or chance interfering with the rising process.
On the other hand, yeast -- a hardy, ubiquitous microbe, might simply shrug off a bit of NaCl. And many successful recipes mix salt and yeast into the wet ingredients with abandon.
Does the truth lie somewhere in-between?
The answer is revealed by two experiments. First, we check if yeast-growth is inhibited by salt in a "petri dish", and secondly, if it makes a difference in an actual bread recipe.
Instead of petri dish, we chose a plastic bottle, and dissolved yeast in warm water mixed with a meal of sugar (see footnote below1). To see if the yeast was damaged, we measured how much CO2 the yeast exhaled. And it turns out the yeast is just fine, breathing out the same amount of CO2 whether floating in plain sugar water, or drowning in water tainted by 1.5% (or even 5%) table salt!
Common sense 0, Science 1.
Secondly, we compared four traditional, but different strategies for incorporating salt. These are:
- NO SALT.
- WET MIX- dissolve the salt and yeast directly in water, then add to the flour.
- DRY MIX- combine the salt and yeast into the flour, then mix with the water.
- TIME RELEASE- combine and knead together the flour, water and yeast. After the gluten is developed, quickly knead in the salt.
Salt helps strengthen the gluten matrix, ensuring trapped CO2 bubbles are more stable. The WET MIX guarantees all of the starch and gluten proteins are uniformly coated with their share of salt ions. Not too high a concentration to damage the protein molecules, but enough to hold onto water and build additional structure. However, this method gambles with killing the yeast floating in the salty wet mix. At least that is the theory.
The DRY MIX and TIME RELEASE technique initially keeps the salt localized, until moisture slowly dissolves the salt grains and the ions diffuse into the dough. Theoretically, this might give the yeast a head start reproducing in a salt-free environment. On the negative side, locally high salt levels might create pockets of non-uniformity where CO2 bubbles can easily break through. Plus, the enormously high salt concentrations adjacent to the grains can actually damage the gluten structure2.
We prepared samples by all four methods, and to visually monitor the dough's rise, inserted a log of dough into 2 1/2" diameter clear oiled tubes. Some experiments used warm water, other room temperature water to assure thermal uniformity. Some with dry active yeast, others with cake yeast. While the rise times varied, the results did not.
The bread recipe is adapted from a standard "no-knead" dough:
- 200 gms bread flour
- 135 gms water
- 1 tsp table salt
- 1/2 tablespoon yeast
Here is a typical result- in this case, for room temperature (68F) water and dry active yeast. The image is a double exposure take at the beginning and the end of the experiment:
At first, it looks like NO SALT recipe is the most yeast friendly. After all, it rose the highest. But this has nothing to do with the yeast, which laughs off salt. Rather, the lack of salt means the dough is softer and more easily inflated. Like blowing up a party balloon vs a car tire. And, the lack of salt means less water chemically bonded to the starch, so the dough is wetter. Consequently, this dough, when re-kneaded, deflated the most. And when baked, slumped and formed a flat boule.
The WET MIX dough rose slightly less than the other two methods. Again, not because the yeast was damaged, but because the gluten network is strongest. This dough baked into a nearly round boule with large air pockets and visible sheets of gluten spanning the pockets.
Finally, as expected, the two salt grain methods rose to exactly the same height. With well shaped boules, and decent structure.
In other words, you can add salt to a bread recipe whenever you prefer. I prefer the wet method- it is more uniform and reproducible. Plus, in a quick bread where there isn't time for the salt crystals to melt and diffuse, the dough's structure is more highly developed. But this is one case where common sense fails.