Salt-stressed baker’s yeast has been exposed to osmotic stress caused by a concentrated salt solution. Yeast cells exposed to stress become resistant to a second, more severe stress challenge. Salt-stressed yeast is used in breadmaking because it has a high leavening ability in sweet doughs that contain a high concentration of sugar. Sugar in enriched doughs increases the osmotic stress on the yeast cells and thus slows down the fermentation. Salt-stressing the yeast is a way to speed up the fermentation and improve the physical and sensory properties of sugar-enriched as well as sugar-free doughs.
A widespread myth is that the osmotic stress caused by a moderate salt solution kills baker’s yeast. Way too often I’ve seen American recipes on the internet where people dissolve their dried yeast in water with a pinch of sugar and then insist to not add any salt because salt apparently “kills the yeast”. I don’t know where this myth comes from but it is false.
Salting your yeast before combining all the dough ingredients has many positive effects on bread. In Germany, we even have a name for this technique: “Salz-Hefe-Verfahren” (salt-yeast-method). German bakeries commonly apply the salt-yeast-method for the preparation of sweet yeast doughs.
Salt-stressing baker’s yeast provides many advantages such as:
- a decreased fermentation time
- an improved dough consistency and stability that goes hand in hand with a better proofing tolerance and larger loaf volume
- a softer crumb of the baked loaf
What is baker’s yeast?
The commercial baker’s yeast (Saccharomyces cerevisiae) that we use today was born in the 1920s. It is a fungus microorganism that the Western world has domesticated and optimized to possess a higher carbon dioxide production rate than sourdough or non-baking yeast strains. The thing that makes baker’s yeast special is that it is exceptional at handling environmental stress that occurs during dough fermentation.
Baker’s yeast experiences an osmotic shock during dough fermentation that is due to the salt and sugar in bread dough. The yeast cells dehydrate and shrink due to an osmotic gradient between the inside and outside of the cells. You know this from salting cabbage. Rubbing cabbage with salt or sugar will cause it to “sweat” and release water.
To be suitable for breadmaking, the yeast needs to survive this osmotic stress. If a yeast dies from the osmotic stress that occurs during the breadmaking process, then it suffers from osmotic hypersensitivity and can’t be used to bake bread.
The time it takes for the yeast to adjust to the osmotic stress and develop an increased stress resistance is called the “lag phase” in breadmaking. During the bulk fermentation of bread dough, we can distinguish three phases:
- The lag phase during which the yeast cells respond to the osmotic shock. Only a little carbon dioxide is produced. Also, if carbon dioxide is produced, then it is first dissolved in the aqueous dough matrix before it diffuses to the nucleation gas bubbles incorporated during mixing of the dough and undergoes a phase transition from liquid to gas. This diffusion process can take a few minutes and increase the length of the lag phase. Lag phase = little to no volume expansion.
- The expansion phase during which the production of carbon dioxide is maximal.
- The stationary phase where a balance occurs between the production of carbon dioxide and gas escaping the dough. The dough expansion slows down. If the dough gets over-inflated then it loses stability and might collapse.
What the osmotic stress response of baker’s yeast looks like
If baker’s yeast is exposed to osmotic stress it responds in two ways:
- The sugar trehalose is accumulated in larger amounts if a yeast cell is exposed to environmental stress. Trehalose protects the cell membrane of the yeast and contributes to slowing down the yeast metabolism, thereby promoting the transition to a resting state of cells that allows them to survive in a dehydrated or frozen state. Adding to that, trehalose is an osmotic agent that lowers the osmotic gradient between the inside of the cell and the environment.
- The yeast produces and accumulates glycerol in its cells to lessen the osmotic gradient between the yeast cells and the environment. The lower the concentration gradient between the osmotic agents inside the cell to the osmotic agents dissolved in the environment, the lower the osmotic stress.
Besides glycerol, the amino acid proline is produced and accumulated in yeast strains. The higher the accumulation of proline, the higher the resistance of the yeast to osmotic stress, freezing, dehydration, and heat shock. Proline is produced independently of external stress factors and protects the yeast from dying in stressful conditions by stabilizing proteins and membranes as well as scavenging reactive oxygen species that attack the yeast cell molecules.
What happens to baker’s yeast in high-sugar doughs?
High-sugared doughs with 18-30 % sugar cause hyperosmotic conditions for the yeast. It experiences an extreme osmotic shock. Modern baker’s yeast can overcome this shock by adapting to hyperosmotic conditions by the stress response that I have outlined to you above. However, this takes time.
The lag phase in high-sugared doughs lasts longer than in low-sugared or sugar-free doughs. Adding to that, the yeast produces less carbon dioxide which results in a loaf with a smaller volume.
Salt has similar effects as sugar. However, the salt concentration in bread dough is typically 2 % or lower so that it doesn’t cause the same severe osmotic stress that a high concentration of sugar causes. The lag phase lasts only a few minutes in sugar-free doughs.
Even after the lag phase, salt and sugar slow down the yeast metabolism and carbon dioxide production rate to an acceptable level. If you wouldn’t add salt or sugar to bread dough, then it ferments excessively. The lower the osmotic stress for the yeast, the shorter the lag phase and the higher its carbon dioxide production rate.
How does the salt-yeast method work and what are its advantages?
The salt-yeast method is simple:
1. Dissolve all the salt the recipe calls for in 7-10 times the amount of water.
2. Whisk all the yeast the recipe calls for into the 10-14 % salt solution and leave to sit at room temperature or in the fridge for at least 20 minutes and up to 2 days.
3. Add the yeast-saltwater dispersion to your final bread dough
The hyperosmotic stress from the salt causes the yeast to adapt and become osmoresistant. The amount of glycerol accumulated in the yeast cells increases to counteract the osmotic pressure.
What is happening here is the same as during the lag phase that we can experience in high-sugar doughs. The dough doesn’t rise during the lag phase because the yeast minimizes its carbon dioxide production and focuses on the production of trehalose and glycerol to counteract the osmotic pressure.
When we add salt-stressed yeast to bread dough, then the dough ferments and rises quicker because the yeast has already accumulated the trehalose and glycerol needed to counteract the osmotic pressure. We experience no lag phase. From the first minute of fermentation, the yeast can focus on producing carbon dioxide and organic flavor compounds.
Besides speeding up the fermentation, salt-resistant yeast also increases the amount of gas the dough can entrap. It is still unknown why glycerol increases the bread volume. About 30 % of the glycerol entrapped in the salt-stressed yeast cells gets released into the dough during fermentation. Glycerol is an extremely hydrophilic substance that can bind a lot of water. Therefore, the viscosity increase that glycerol causes might be responsible for the increased dough strength, stability, and proofing tolerance. However, this has not been proven scientifically and is just a guess!
What is also unclear is if salt-stressed yeast improves the flavor of bread. Some studies say it does, others say there is no effect. To be honest, I think it’s a hoax that salt-stressed yeast has a major impact on the bread flavor. I’ve never heard of anyone or read anywhere that flavor is an argument for the salt-yeast method. Why should bread made from salt-stressed yeast be more aromatic? I can’t think of any reason.
When to apply the salt-yeast method?
The salt-yeast method provides the most benefits in doughs that have been enriched with at least 8 % sugar. For low sugar doughs, there is no long lag phase before the dough begins to rise because the osmotic stress is lower for the yeast. The main purpose of the salt yeast method is to reduce the dough fermentation time. This is achieved by performing the lag phase beforehand by salt-stressing the yeast.
But even for sugar-free doughs, the salt yeast method helps to improve the dough consistency. Salt-stressing the yeast leads to the accumulation of a high amount of glycerol within the yeast cells. Part of this glycerol then gets released from the yeast cells into the bread dough. Glycerol increases the dough’s stability, water absorption capacity, gas retention capacity, and proofing tolerance. Adding to that, glycerol has a softening effect on the crumb of the baked loaf.
In German baking books, they usually tell you to salt-stress the yeast between 4 hours to 2 days. However, as little as 20 minutes are already enough to increase the dough strength and shorten the fermentation time. Having said that, it is beneficial to salt-stress the yeast for at least 4 hours. That is because the accumulation of glycerol increases over time if the yeast is exposed to hyperosmotic stress. The more glycerol gets produced by the yeast, the better the dough strength and the higher the carbon dioxide production rate by the yeast.
To conclude: Salt-stressing the yeast is recommended for high-sugar and high-hydration doughs because it eliminates the lag phase for sugar-enriched doughs and improves the dough’s stability and gas retention which helps to increase the hydration and achieve a more open crumb in baguettes or ciabatta.