At least not at the concentrations we normally use in baking.
When I started baking bread 40 years ago, at least when I created yeasted bread, I proofed the yeast and then threw everything into my bowl at the same time, salt and all. So I found it strange that when I started baking again in earnest during the pandemic that so many people on the forums would say to keep your salt and yeast separate because salt kills yeast.
Folks, I’m just going to say it: Salt does NOT kill yeast, nor does it inhibit its activity – at least not in the way most might think. As I indicated at the top, in order for salt to kill yeast, it would have to be in a pretty high concentration to do that. If someone tells you that salt kills yeast, they’re sadly misinformed.
But before I go on, let me give the backstory on why I’m writing this post. Last night, I was flipping through channels trying to find something to watch and ended up just watching reruns of Diners, Drive-ins, and Dives. In this one particular episode where Guy was visiting a Whole Foods-like grocery in Sioux Falls, SD called Looks, the chef making the pizza dough mentioned osmotic yeast as the yeast he uses so he can up the salt content of his dough.
Uh-oh… though I had seen that episode before, for some reason osmotic yeast triggered a geek moment and I proceeded to get on my computer and go down the rabbit hole to understand the term. What I found out was that the term osmotic yeast is actually a bit of a misnomer and that the chef probably made it up. Not a big deal. What he really meant was that he was using yeast that had already gone through osmotic stress so it was resistant to later osmotic stress challenges.
So what is osmotic stress? When yeast encounters saline or even a sweet environment, it goes through what is called osmotic stress as it reacts to the higher saline or sugar concentration; as both salt and sugar leech water from their environments. Yeast reacts to this by producing glycerol to help protect its cell walls from further osmosis – or leeching.
During this period, there is little to no CO2 production, which is why people might think salt or sugar inhibits the yeast. I suppose that the osmotic stress period could be considered an inhibitor period, but once the yeast has protected itself, it goes back to its normal course of business and starts producing gas. How long this period lasts is affected by the ambient temperature.
Now the interesting thing about the yeast producing glycerol is that not only does it protect the yeast’s cell walls from further osmotic stress, but it’s also released into the dough which purportedly aids in the dough’s strength and extensibility (though glycerol’s role in dough development is still being studied). Kind of cool stuff! Now back to osmotic yeast…
Pre-stressed yeast has been proofed in a saline solution to bolster the cell walls and produce glycerol. This means that when it is added to a dough, there is no lag period where the yeast has to build up a tolerance to the saline environment. It has already gone through the stress so it just starts acting.
In my dive into the rabbit hole, I came across this excellent article that provides a mildly-technical layman’s perspective of salt-stressed yeast and how it is used in baking. I encourage you to read it! When I make baguettes next, I’m going to salt-stress my yeast first! Another great thing about that article that I linked to is that it has references to lots of scientific research showing the effects of salt-stressed yeast on fermentation. I read through many of the abstracts and articles and they’re pretty eye-opening! The author also shows how to salt-stress your yeast before you mix! Can’t wait to try it out!
Admittedly, I haven’t read through all the science on this as of yet, so while I understand the basic mechanics, I’m by no means an expert on the subject. So I encourage you to do some research on this subject as it completely busts the myth that salt kills yeast!