An interesting effect of temperature on sourdough starter

[Theofilos]

Hello everyone,
I don’t have an incubator, so I put a jar with the sourdough starter on a radiator.
I made the starter from scratch, that is, I took 50 g of whole-grain rye flour (type 2000) and 70 ml of water.
I mixed these two ingredients and put it on the radiator.
After 12 hours it turned out that the starter had increased its volume by 100%.
I measured the temperature on the radiator and it was 35°C (95°F).
I stirred it and placed it next to the radiator, where it was 24°C (75.2°F).

I have a recipe:
Day 1
18:00: 100 g flour and 150 ml water – mix and leave covered for 12 hours
Day 2
6:00: stir the starter and leave it for 12 hours
18:00: feed the starter with 50 g flour and 70 ml water – mix and leave for 24 hours
Day 3
18:00: feed the starter with 50 g flour and 70 ml water – mix and leave for 12 hours
Day 4
6:00: stir the starter and leave it for 12 hours
18:00: feed the starter with 50 g flour and 70 ml water – mix and leave for 12 hours
Day 5
6:00: stir and leave for a few hours – then it can be used for baking.

At the end of Day 2 the starter was still rising quickly, so I fed it at 18:00 and put it in the refrigerator.
On the morning of Day 3 the volume had again increased by 100%, so I fed the starter and put it back in the refrigerator.
After 12 hours it rose again by 100%.


How is it possible that it grows so much at a low temperature?
On Day 4 I took it out of the refrigerator and it stopped rising. I fed it again in the evening and I’m waiting to see what happens.

 

[Norcalbaker59]

Hello everyone,
I don’t have an incubator, so I put a jar with the sourdough starter on a radiator.
I made the starter from scratch, that is, I took 50 g of whole-grain rye flour (type 2000) and 70 ml of water.
I mixed these two ingredients and put it on the radiator.
After 12 hours it turned out that the starter had increased its volume by 100%.
I measured the temperature on the radiator and it was 35°C (95°F).
I stirred it and placed it next to the radiator, where it was 24°C (75.2°F).

I have a recipe:
Day 1
18:00: 100 g flour and 150 ml water – mix and leave covered for 12 hours
Day 2
6:00: stir the starter and leave it for 12 hours
18:00: feed the starter with 50 g flour and 70 ml water – mix and leave for 24 hours
Day 3
18:00: feed the starter with 50 g flour and 70 ml water – mix and leave for 12 hours
Day 4
6:00: stir the starter and leave it for 12 hours
18:00: feed the starter with 50 g flour and 70 ml water – mix and leave for 12 hours
Day 5
6:00: stir and leave for a few hours – then it can be used for baking.

At the end of Day 2 the starter was still rising quickly, so I fed it at 18:00 and put it in the refrigerator.
On the morning of Day 3 the volume had again increased by 100%, so I fed the starter and put it back in the refrigerator.
After 12 hours it rose again by 100%.


How is it possible that it grows so much at a low temperature?
On Day 4 I took it out of the refrigerator and it stopped rising. I fed it again in the evening and I’m waiting to see what happens.

A 100% increase in volume really isn’t that dramatic. Most healthy starters will double or even triple at peak. For example, I fed my own starter about 11 hours ago using a blend of whole wheat and medium-protein white flour, which is actually less nutrient-dense than rye, and it’s already more than doubled. So seeing a starter expand by 100% isn’t unusual.

Rye is nutrient rich, so when you feed 100% rye, you’re creating a more aggressive starter compared to a wheat-based starters.

Rfrigeration doesn’t stop fermentation instantly. It can take a 3 or more hours for the core temperature to cool enough to suppress yeast activity. During that whole cooling period, the starter is still very active. If the flour is nutrient-rich like rye flour, aggressive fermentation continues while the temperature is slowly dropping.

Inoculation matters too. If you’re using a high carryover of mature starter, you already have a large yeast population at feeding. Even as things cool, that existing population will keep producing CO2. That’s momentum, not yeast thriving in the cold.

Rye also keeps feeding the yeast longer than wheat does. It doesn’t necessarily have more starch overall, but it has a much higher amylase (enzyme) load and starch that’s easier for those enzymes to access. Amylase breaks starch down into sugars like maltose, which feds the yeast. While enzyme activity slows in the fridge, it doesn’t shut off. So a rye starter will keep generating fresh sugars for yeast to fed on even at refrigerator temperatures. Wheat has the same enzymes, but at much lower levels, so it runs out of available sugar sooner.

Finally, rye makes the rise look bigger than it really is. I mentioned in another thread about the stickiness of rye starter due to the high pentosan load. That sticky viscose dough traps gas more efficiently than wheat. So less CO2 produces a more noticeable expansion than wheat flour.

Tge expansion of the rye starter in the fridge is from residual warmth, high inoculation, ongoing enzyme activity, and rye’s gas-trapping structure all working together.

 

[Theofilos]

Thank you very much.
In what cases will it increase by 40%, which you wrote about here ?

Unfortunately, after taking it out of the refrigerator on Day 4, the starter no longer rose.
I fed it in the evening. It stood on the radiator overnight (12 hours), but it did not increase its volume at all.
It was going so nicely, I was so happy that it was rising so well, and now it’s not rising at all.

Is it possible to somehow restore the starter’s activity?
In its current state, I don’t think anything can be baked with it.

 

[Norcalbaker59]

Thank you very much.
In what cases will it increase by 40%, which you wrote about here ?

Unfortunately, after taking it out of the refrigerator on Day 4, the starter no longer rose.
I fed it in the evening. It stood on the radiator overnight (12 hours), but it did not increase its volume at all.
It was going so nicely, I was so happy that it was rising so well, and now it’s not rising at all.

Is it possible to somehow restore the starter’s activity?
In its current state, I don’t think anything can be baked with it.

A 40% rise is a minimum rise to look for in a starter that is ready to be fed or used, not an end goal. If an established stater doesn’t rise at least 40%, then something is wrong.

The amount of rise depends on the flour type and whether it is blended. An all–whole grain flour, such as 100% whole wheat, is going to rise less than an all-rye flour starter.

If your starter is no longer rising, it is most likely dead. A temperature of 95°F is extremely high, and leaving a starter at that temperature for 12 hours is detrimental to the fermentation environment, especially with an all-rye starter. That temperature strongly favors lactic acid bacteria (LAB) activity and puts fermentation into overdrive. Yeast can survive in that range, but yeast reproduction and metabolism are no longer balanced. You get very rapid sugar consumption and very rapid acid production, leading to yeast that is stressed, unhealthy, and eventually unable to reproduce if those conditions are sustained.

Moving the starter to 75°F does not meaningfully calm things. That temperature is still in the maximum activity zone for both yeast and lactic acid bacteria (LAB), particularly with rye. As a result, the starter never had a chance to recover and stabilize; it simply continued fermenting at a very high rate.

When you fed the starter and put it in the refrigerator, the temperature of the starter did not drop immediately, so fermentation continued for several more hours while the core temperature was still elevated.

Even once the starter eventually cooled, it did not stabilize because the yeast and lactic acid bacteria populations were already extremely high, and an unhealthy biochemical environment (low pH, high enzyme activity, abundant simple sugars) was already established. Refrigeration slows metabolism, but it does not undo accumulated stress or reverse acidification. This unbalanced environment is not conducive to a healthy starter.

At sustained high temperatures, lactic acid bacteria (LAB) can drive the pH low enough to impair yeast viability. Once yeast cells are damaged, cooling and reheating will not revive them, even if starch breakdown continues.

As I explained in a previous thread, it is not advisable to use only highly nutrient-rich flour to create a starter. A blend of white and rye flour helps regulate the amount of sugar available to the yeast, allowing for more controlled growth. Controlling yeast growth also helps regulate the acid environment.

Once the starter is healthy and reliably rising at a predictable rate, and after it has had time to mature (a couple of weeks at minimum), you can gradually transition it to all rye.

Also, is important to avoid exposing the starter to excessive heat for the reasons outlined above.

 

[Theofilos]

Thank you very much for the answer and the explanations.

So the starter I have needs to be thrown away.

 

[Norcalbaker59]

Thank you very much for the answer and the explanations.

So the starter I have needs to be thrown away.

Yes, you will need to start over. Even if the starter had signs of activity, the environment is most likely to acidic to be healthy.

it’s important to keep in mind that asourdough starter isn’t just yeast. It’s a living ecosystem made up of wild yeast, bacteria, acids, enzymes, and the food they all interact with.

How and what you feed is key to establishing and maintaining a balanced environment where both the yeast and the bacteria can thrive.

I still recommend a blend of rye and all purpose flour to create the starter. After a couple of weeks when it’s reliably rising on a predictable schedule, slowly increase the amount of rye flour while decreasing the amount of all-purpose flour. You’ll need to gradually increase the water since rye flour absorbs significantly more water than all purpose flour.

 

[Theofilos]

All of this looks strange.
I made the starter from scratch again. This time I made two versions.
One contains only whole-grain rye flour (type 2000), and the other contains 50% whole-grain rye flour (type 2000) and 50% wheat flour (type 650).
I made some insulation on the radiator and placed a thermometer there. It was 30°C (86°F).
The starter basically didn’t rise. Instead, a skin formed on top of it.
I removed the skin, stirred the starter, and set it aside for 12 hours.
Nothing rose anymore.
Strange. Earlier, at 35°C (95°F), it rose by 100%, and now it doesn’t.
Instead, a dry layer—a skin—keeps forming on the surface.

 

[Norcalbaker59]

All of this looks strange.
I made the starter from scratch again. This time I made two versions.
One contains only whole-grain rye flour (type 2000), and the other contains 50% whole-grain rye flour (type 2000) and 50% wheat flour (type 650).
I made some insulation on the radiator and placed a thermometer there. It was 30°C (86°F).
The starter basically didn’t rise. Instead, a skin formed on top of it.
I removed the skin, stirred the starter, and set it aside for 12 hours.
Nothing rose anymore.
Strange. Earlier, at 35°C (95°F), it rose by 100%, and now it doesn’t.
Instead, a dry layer—a skin—keeps forming on the surface.

The first starter at 95°F rose quickly because the excessive heat triggered rapid bacterial growth. Most of the gas came from bacteria, not yeast, so the yeast never had a chance to establish itself and died.

The two new starters at 86°F didn’t rise because that temperature is still too warm for balanced yeast and bacteria growth. Rye and whole wheat flours are highly absorbent and need more water than white flours like bread flour. The dry surface is caused by a combination of excessive heat, flour absorption, low humidity, and airflow. A radiator running at 86°F not only overheats the starter but also dries out the ambient air. A dry layer on top is a sign of dehydration from excessive heat. While yeast and bacteria can respire without oxygen, airflow is still needed to allow metabolic byproducts to dissipate. The dry skin blocks that dissipation, concentrating waste in the environment and further inhibiting microbial activity.

The issue is the inhospitable environment you keep creating, which prevents healthy development of the yeast and bacteria.

This will continue to happen if you disregard the fundamental rules for starting a sourdough—temperature, hydration, and airflow. To create a healthy starter, stay around 74–76°F, plus or minus a couple of degrees. If you have to choose between cooler temperatures and placing it on a radiator, choose cooler temperatures. My starter lives in an incubator at 65°F and is fed once a day. The day before I mix dough, I increase the temperature to 74°F and feed twice. I always mix a levain using a portion of the maintenance starter; then reserve 15 g of the maintenance starter to feedand return to the incubator at 65°F. Cooler temperatures will slow the development of the starter, but it won’t kill your starter or create an inhospitable environment. But as you have twice experienced, placing your starter on the radiator will kill it.

 

[Theofilos]

Thank you very much for the explanation and the tips.

On the fourth day (after three full days), when neither starter had risen, I combined them in one jar, fed them, and left them overnight at about 21°C (69,8°F).
In the morning, nothing had risen.
I stirred it and put it on the radiator. After 2 hours it rose by 100%.
I made the dough.
I put it on the radiator for just under 4 hours.
I baked the bread.
Here it is.

It collapsed a bit in the middle.

 

[Norcalbaker59]

It

Thank you very much for the explanation and the tips.

On the fourth day (after three full days), when neither starter had risen, I combined them in one jar, fed them, and left them overnight at about 21°C (69,8°F).
In the morning, nothing had risen.
I stirred it and put it on the radiator. After 2 hours it rose by 100%.
I made the dough.
I put it on the radiator for just under 4 hours.
I baked the bread.
Here it is.
View attachment 5125

It collapsed a bit in the middle.

The loaf collapsed because the starter is overly acidic. When a starter is repeatedly subjected to excessive heat, it produces excessive lactic acid and proteolytic enzymes. Together, the acid and enzymes break down gluten through hydrolysis.

This type of collapse is a classic sign of excessive acidity. Even when baked in a tin, dough with degraded gluten fail because the gluten network has been chemically damaged and cannot support the structure. When the acidity is in check, a dough will not require a tin for support.

I have been clear about the need for a healthy balance of yeast, acids, and enzymes, but I will emphasize it one last time. Repeated exposure to excessive heat accelerates acid and enzyme activity while preventing a stable yeast population from forming. The starter becomes increasingly acidic, and the collapsed loaf you produced is the direct result of that acidity destroying the gluten.

When we talk about over proofed dough and poor rise, we are really describing the effects of hydrolysis on the gluten network. By subjecting your starter to excessive heat and creating an imbalance of lactic acid and enzymes, you are effectively building hydrolysis into the dough from the very start.

Acidity control is so important that professional sourdough bakeries use pH meters to monitor both starter acidity and fermenting dough.

Until the starter is kept within a temperature range that allows balanced fermentation, this outcome will continue. No recipe or technique can compensate for a starter that is acidic enough to chemically break down the dough.

 

[Theofilos]

Thank you very much for your reply and valuable tips.

On the last day of the previous sourdough, I made a new one and kept it overnight at 30°C (86°F). It grew by 80%. In the following days, I kept it (and still keep it) at 21°C (69.8°F).

It smells nicely of yeast with a hint of sourness.

Now it is not growing as much.

We will see how the bread turns out – probably tomorrow.

 

[Norcalbaker59]

Thank you very much for your reply and valuable tips.

On the last day of the previous sourdough, I made a new one and kept it overnight at 30°C (86°F). It grew by 80%. In the following days, I kept it (and still keep it) at 21°C (69.8°F).

It smells nicely of yeast with a hint of sourness.

Now it is not growing as much.

We will see how the bread turns out – probably tomorrow.

I am not sure why you keep subjecting your starter to excessive heat. There are no starters or doughs, whether for bread, pastry, pretzels, or muffins, that are intentionally fermented at gluten destroying temperatures.

Bakers make a significant effort to keep the dough temperature in the appropriate range for a healthy dough. Desired Dough Temperature (DDT) for most doughs is in the 74°F to 76°F range. DDT is a calculation of four contributing factors to dough temperature: flour temperature, room temperature, and friction factor from mixing. These values are used to determine the required water temperature so the finished mixed dough is a specific temperature that supports balanced yeast activity, controlled acid production, and proper gluten development.

When a slightly higher dough temperature is used, it is restricted to the final proof, typically no higher than 80°F, and only in a humidity controlled proofer for the last 30 to 45 minutes before baking. The excessive temperatures you are using are simply not used in starter and doughs.

Edit:
It typically takes 10 to 14 days to create a starter WHEN it is fed on a regular schedule and kept at an appropriate, stable temperature. Ten days is the minimum needed to establish a starter.

Days 1 to 3 usually show little to no visible activity.

Days 4 to 7 often show activity, but it is inconsistent and unreliable.

Days 8 to 10 the starter begins to show more consistently and rises within a predictable time frame.

Days 11 to 14 the starter becomes reliably predictable and may be ready to use.

Even then, a starter is still young. It takes several additional weeks, often a couple of months, of consistent feeding and stable temperature to become fully established and resilient.

Early activity is not a sign of yeast activity stability, strength, or balance. In the early stages, a broad range of microorganisms are present, many of which are not compatible with stable sourdough fermentation. As fermentation progresses and acidity increases gradually, these organisms are suppressed or eliminated, allowing wild yeast, enzymes, and lactic acid to establish a stable, symbiotic culture.

Sourdough starter is not a fast process.

 

[Theofilos]

The bread I baked, whose photo I sent earlier, is good—quite fluffy and light.

The starter I used today was begun at a higher temperature, but only for 12 hours.
Then I continued at a lower temperature.
Since I had already started, I wanted to finish it.
I will make the next starter at 21°C (69.8°F).
Today I baked bread with this starter, which in the following days did not rise particularly well.
It was indeed more sour than the previous one.
I added less water to the dough so it wouldn’t be too runny.
I baked the bread. It didn’t collapse at all—unlike the previous one.

Unfortunately, it is more sour, but it rose normally.
It is low because I made two batches and put less dough into the pan.

 

[Norcalbaker59]

The bread I baked, whose photo I sent earlier, is good—quite fluffy and light.

The starter I used today was begun at a higher temperature, but only for 12 hours.
Then I continued at a lower temperature.
Since I had already started, I wanted to finish it.
I will make the next starter at 21°C (69.8°F).
Today I baked bread with this starter, which in the following days did not rise particularly well.
It was indeed more sour than the previous one.
I added less water to the dough so it wouldn’t be too runny.
I baked the bread. It didn’t collapse at all—unlike the previous one.

View attachment 5126
Unfortunately, it is more sour, but it rose normally.
It is low because I made two batches and put less dough into the pan.

If your sourdough starter is tasting overly sour, keep it at a moderate temperature, around 70–74°F. When a starter is showing signs of stress, I wouldn’t keep it below 70°F. And please stop with the excessive heat. You are damaging your starter and undermining your bread journey.

Feed it twice a day, about 12 hours apart, for several days, and don’t try to bake with it during recovery. A strong sour flavor means the starter is out of balance, with to much acid production and poor yeast activity. That’s a classic sign of a stressed, unhealthy starter.

Consistent feedings and moderate temperatures will usually bring the yeast back into balance and lower the acidity levels. Stability of temperature and regularly scheduled feedings are key to salvaging it.

A healthy starter should smell lightly sweet and yeasty, like fresh bread dough, with just a mild tang. When I want that sharper San Francisco–style sour, I stress the levain on purpose. My maintenance starter, though, is always kept balanced and healthy.

 

[Theofilos]

Earlier you wrote that for the first 3 days the temperature should be 80°F, and then 75°F.
However, all right—2 days ago I prepared a starter and from the very beginning I have been keeping it at 70°F.
During the first 12 hours nothing rose.
I stirred it.
After the next 12 hours it rose by about 100%.
I fed it.
After another 12 hours it rose to about 150%.

 

[Theofilos]

It’s interesting that earlier (a few starters ago), using the same flour (the same package) and the same water proportions, and also at a temperature of 70°F, the starter did not want to rise.

That is why, based on the suggestion of an 80°F temperature, I kept the starter at a higher temperature, and then it rose a little.

This current starter, after rising to 150% as I wrote above, collapsed after about 5 hours. It is now about 70% higher.

 

[Norcalbaker59]

It’s interesting that earlier (a few starters ago), using the same flour (the same package) and the same water proportions, and also at a temperature of 70°F, the starter did not want to rise.

That is why, based on the suggestion of an 80°F temperature, I kept the starter at a higher temperature, and then it rose a little.

This current starter, after rising to 150% as I wrote above, collapsed after about 5 hours. It is now about 70% higher.

You haven’t established a starter, let alone one that is healthy and stable. Early on, you put it on a radiator at very high temperatures around 95°F and 86°F, then just hours later moved it into the refrigerator. You mentioned at one point that you “only” left the starter for 12 hours at higher temperatures. You don’t seem to understand that 12 hours of excessive heat is enough to kill off the yeast. This is why starter and dough are never subjected to temperatures above 80°F.

You also mistake early activity for actual development. That initial activity is not growth; it’s the process of killing off microbes that aren’t suitable for the starter.

As I have explained, a stable starter takes about two weeks to develop, and it requires consistent temperatures and regular feedings throughout. That consistency is not optional. When the environment and feeding schedule are erratic, the yeast and bacteria never have a chance to establish.

This process requires patience and discipline. If you continue to ignore the basic protocol and take an erratic approach, you will keep getting the same poor result. I’ve explained the process multiple times in this thread, and I don’t have anything new to add, so I won’t be responding to further comments or questions here.

 

[Theofilos]

In this thread I wrote about three different starters.

Starter No. 1
At the beginning it was indeed at 35°C (95°F), and later at 24°C (75.2°F).
When you wrote that this was too high a temperature, I made another starter.

Starter No. 2 (December 27, 2025)
The temperature was close to what you mentioned in another thread - 80°F for the first 3 days, and then 75°F.

Then you wrote that this temperature was too high and that it should be 74–76°F, and if I didn’t have the ability to maintain such a temperature, it should be lower rather than higher.
So I made another starter.

Starter No. 3 (January 1, 2026)
From the beginning, the temperature was maintained at 70°F.
After 12 hours it did not rise.
After the next 12 hours it rose by 100%. I fed it.
After the following 12 hours it rose by 150% (relative to the initial volume).
After 12 hours it fell back to the starting point. I fed it.
After another 12 hours it did not rise.

 

[Theofilos]

Each time, I took your guidance into account and followed the recommended temperatures.
The last starter (No. 3) was kept at 70°F the entire time, in line with your suggestions.
As I wrote, it only rose at the beginning.
So today I made the dough. Unfortunately, it is not rising.
All the previous ones did rise, but this one does not.
Therefore, it seems that the temperature was indeed too low, since in all the previous cases the dough rose, and now it does not.

 

[Theofilos]

From the history of these three starters, it follows that the temperature should be 80°F at the beginning and then 75°F afterward.
Starter No. 2 was made at those temperatures, and from it I baked the best bread.