How Sourdough Bread Fermentation Works

Sourdough bread uses a sourdough starter to raise bread. The starter is an active ingredient that naturally develops yeast and bacteria to condition and ferment dough. Sourdough has a long production time. This helps to generate its distinct twang and deep aromas.

Packed with organic acids, sourdough leavened bread has excellent keeping qualities, flavour, aroma and gas retaining properties.

It requires patience, skill and practice to master making sourdough bread “the king of bread”. It’s a challenge to make the perfect loaf, but once you’ve started baking with sourdough it’s very hard to stop!

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In this post, I’m going to uncover the science behind what goes on in the sourdough fermentation process. Essentially it is a “how sourdough bread is made” guide.

I’ve consciously tried to break down the content so it makes it less scientific – more easy reading. If you don’t have a chemistry degree you should be able to follow along without your head hurting!

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Do I need to understand the sourdough fermentation process?

If you are struggling to obtain consistent results in your sourdough bakes or if you are already skilled but want to understand what is going on learning about sourdoughs fermentation will help you achieve your goals. You can use this information to adapt the timings and methods used in your favourite recipe or troubleshoot your sourdough fails.

Understanding the science of sourdough fermentation will simply help you improve your bakes and be able to tell what helps (and what doesn’t) when making bread. 

And who doesn’t want that??!!

Looking to improve your sourdough bread? View my how to fix dense sourdough bread guide

What is a sourdough levain?

Sourdough bread is made from a sourdough starter that is created in advance by the baker. To make a sourdough starter, flour is added to water and the mixture is left to naturally develop (ferment).

During this time the starter attracts and multiplies wild yeasts whilst developing enzymes to break down complex sugars into simple sugars and acids.

How is a sourdough starter built?

Making a sourdough starter takes time, but is quite easy to do. Mix equal quantities of flour and water together to make a thick batter.

Leave for 24 hours and remove a piece of the starter to mix with fresh flour and water. The remaining starter is then discarded.

Repeat every day, and after a couple of weeks, the flour and water mixture will have developed into a strong levain that’s able to raise bread.

Check my guide on how to make a sourdough starter for accurate measurements and timings.

What is the difference between yeast and sourdough bread?

150 years ago the only levain available to produce bread was sourdough. Brewers yeast was then created a manufactured for use in the brewing industry. Later strains were developed that could be used for making bread.

One strain of yeast bacteria is used in commercial yeasts, Saccharomyces Cerevisiae. It is the basis of fresh, active dried, instant and wet yeast.

When it comes to the sourdough fermentation process is more complicated than yeast made bread.

Multiple yeast strains develop in the starter alongside lactic acids which mature the flour alongside generating leavening properties in the dough.

The natural ecosystem of sourdough

A sourdough starter naturally builds its own ecosystem where the yeasts and lactic acid bacteria feed on separate materials. They become reliant on each other to keep the ecosystem stable and maintain a consistent level of acidity.

This environment makes the sourdough starter incredibly powerful and resilient to foreign bacteria.

How the sourdough fermentation process begins

A sourdough starter begins with tiny organisms which are absorbed and developed by the dough. It takes time and a bit of warmth to build a few simple microorganisms into a potent ecosystem that can be used to make bread.

Where do the microorganisms come from?

Wild yeasts are produced in plants and float around the atmosphere. They are too small to be seen without a microscope, but they are everywhere! Sometimes wild yeasts are found in flour, but there are plenty available in the air.

The starter absorbs the yeasts which then respire by digesting simple sugars and then multiply. The amount of aerobic bacteria that are present in the starter will slow the multiplication growth of the yeast.

What is sourdough fermentation?

Sourdough fermentation is broken down into three key stages; alcoholic fermentation by the yeasts, the breaking down of the protein to provide unique aromatics and lactic acid bacteria (LAB) fermentation to produce organic acids.

Alcoholic fermentation

The process of alcoholic fermentation, often called the yeasts anaerobic respiration works in the same way as yeast made bread.

Many of the sugars in flour contain two cells of simple sugars (disaccharides) or more complex combinations (polysaccharides) often referred to as starches.

Simplifying the sugars

Single cells sugars called monosaccharides are required as they are small enough to penetrate the cell walls of the wild yeasts. This is necessary for the yeast to respire and enables it to create carbon dioxide and ethanol. 

There will be some single cell sugars available in the dough but to feed the yeast sufficiently so that the bread will rise more need to be produced. This occurs by the degradation of the disaccharides and polysaccharides by amylase enzymes found in the dough.

Producing gas to make the bread rise

As carbon dioxide is produced, the gas gets enclosed in the breads gluten structure. As the dough produces more carbon dioxide it stretches the gluten, making the bread rise.

Is ethanol important for the bread?

Ethanol matures the dough by assisting it’s keeping qualities and improving the flavour and aroma of the bread. Although not essential in the production of gas, its role is to produce bread-like aromas and fortify the gluten structure which enables it to retain gas.

Breaking up the protein

Wetting the flour washes away the soluble protein to leave gluten. Gluten is a complex strand of amino acids. As gluten hydrates, lactic acid simplifies the amino acids to create superior alcohols and aldehydes.

The addition of mature flour to a dough enhances its properties so less kneading and/or bulk fermentation are required to develop the gluten. It is this inclusion that is one of the biggest benefits of sourdough bread.

Depending on the formats of the amino acids found in the flour, the dough will generate different flavours in the dough. Much of the flavour of individual sourdough breads are derived from superior alcohols and aldehydes, not from the smell of the flour.

Though I find that smelling the flour is a great indicator to discover the health of the flour and the care taken in production.

Lactic acid bacteria fermentation

As complex sugars are simplified through the use of amylase and various other enzymes they produce various lactic acids, acetic acids and CO2.

It is often thought that the sourdough fermentation process involves solely wild yeasts, however, lactic acids outnumber the wild yeast by up to 100 to 1.

What is lactic acid?

Members of the Lactobacillus family occur in bread production when complex starches and sugars are broken down by enzymes to be used by the yeast. These microorganisms ferment with sugars to create acids.

The acids are vitally important for the dough as the aid maturation. According to an experiment by Bakerpedia, the inclusion of lactic acid benefits the dough in the following ways:

  • Increases acidity
  • Enhances the flavour
  • Adds flavour
  • Prevents mould to increase shelf life
  • Produces dough conditioners
  • Helps to absorb minerals

These help to enhance the activity of the yeast, to produce and retain gas. Lactic acids are an integral part of making sourdough bread.

How lactic acids are used in sourdough

The effect organic bacteria has on sourdough wild yeasts

As oxygen combines with the hydrated flour it aerates the bacteria producing aerobic bacteria which is the majority of the microflora found in the dough initially. 

After day one, aerobic bacteria begins to break down into lactic (mainly) and acetic acids though enzymes. We need the levels of aerobic bacteria to drop so that the wild yeasts can multiply. Whilst weaker strains of yeast start to die off as the acidity of the dough increases, the more powerful ones become more prevalent.

The effect that lactic acid has on the dough as it ferments

It takes 3-4 days (at optimum conditions) for the wild yeasts and lactic acid bacteria (LAB) levels to develop sufficiently to raise bread. Many bakers find that a starter needs to be at least 14 days old before the ecosystem is resilient enough to mature and proof bread effectively.

How are enzymes created?

Enzymes are naturally produced by bacteria to break down the bonds in complex cell structures.

We can select and add enzymes to a dough by using bread improvers to target the degeneration of particular cells. This enhances features in the bread that would already occur.

During the baking process, the enzymes disappear therefore don’t need to be included in the products packaging.

The types of yeast and lactic acids in sourdough

A sourdough can contain 1-4 different strains of yeast and lactic acids, though there are hundreds of varieties available. The ratio of each strain varies to generate flavours and aromas truly unique to each levain. Here are some of the most popular variations of wild yeasts and lactic acids found in sourdough starters:

Yeasts found in sourdough

  • Kazachstania exigua (Saccharomyces exiguous)
  • Saccharomyces cerevisiae (commercial yeast)
  • Candida milleri
  • Candida humilis

Lactic acids found in

  • Lactobacillus fermentum
  • Lactobacillus sanfranciscensis (prominent in San Francisco)
  • Pediococcus pentosaceus
  • Lactobacillus plantarum, and L. brevis

An extensive list of microorganisms found in sourdough can be found on Wikipedia.

Why does sourdough taste different even in different places?

The ratio of and the varieties of the yeast and lactic acid bacteria create flavours during the dough sourdough fermentation process. The structure of the amino acids which are broken down by the lactic acid provides a unique flavour from the flour.

The flavour qualities of a sourdough starter are formed by three main components:

  1. The flour
  2. The quantity of microorganisms in the local air
  3. The temperature it’s fermented at

When there is a change in either of these factors the flavour and aroma of the sourdough will alter. As the sourdough is built on an ecosystem of different parts pulling together. If a component is slightly altered a grand change in the flavour profile is likely to occur.

How to change the rate of sourdough fermentation

Once we have developed an active starter we can change the time and temperature of the fermentation to adjust the rate of fermentation. There are plenty of factors that come into play, so let’s cover each one.


Temperature is the most common way to tweak the rate of sourdough fermentation. This allows us to time our sourdough production so we have it ready to bake at (roughly) the time that suits us. Temperature alters the flavour of the bread.


The level of fermentation increases with the length of time that the dough has to ferment. Increasing the length of fermentation allows the sourdough to develop more flavour and gas retaining features. Too much fermentation can result in the gluten breaking down excessively and the bread collapsing or over oxygenation of the flour.


Ash is found in higher quantities in wholemeal, rye and high protein flours. The levels of ash are measured by burning the flour and measuring what is left. The ash that remains is essentially the minerals contained in the flour.

The minerals slow the production of organic acids, however slowing the rate that the ph level drops actually increases the number of organic acids produced, less speed, more haste!

This is the reason why many bakers supplement a portion of white flour with wholemeal or rye flour in their starters.


The amount of mineral activity and levels of acid in the water used to make sourdough can have an effect on the rate of fermentation. Scientifically it can, but finding levels where a significant deterioration or advantage are discovered are rare.

By all means, if you are troubleshooting your sourdough you might want to switch to bottled water but it can be quite a costly and unnecessary solution.


Most scientists say it takes around 7 days to build a sourdough from scratch yet you’ll discover many sourdough bakers insist it takes longer. It’s often recommended that a new sourdough is fed for 14-21 days before there is enough activity for bread making.

You can often tell if the sourdough is ready by the smell. Yeast creates nice aromatics which will be noticeable once the aerobic bacteria levels diminish.

How to adjust the temperature of sourdough fermentation

Taking regular readings of the dough with thermometers is the best way to control the levels of fermentation in regular production. You can adjust the temperature of the dough to consistently achieve your range.

Find out how to regulate temperature in the desired dough temperature and bread proofing temperature articles.

Mastering the process

Should I add yeast to my sourdough bread recipe?

Adding yeast to a sourdough recipe will speed up the process and is a sneaky trick that ensures the success of a new starter. Many sourdough labelled bread in supermarkets contain yeast. The additional yeast must be small as I wouldn’t want you to lose the goodness of the sourdough bacteria.

Preferably to speed up the production of sourdough bread I prefer to add some malt flour (produces amylase) and knead the dough well. This method produces food quickly for the yeast and ensures a fairly strong gluten structure.

Should I autolyse sourdough bread?

You can do! Many sourdough bakers autolyse their flour before kneading the dough. It helps the extensibility of the gluten which improves the oven spring – though it’s not essential!

Should I knead sourdough bread?

Absolutely you can knead sourdough. The presence of prefermented flour helps the dough to mature quickly as well. A short 3-5 minute knead is all that is required and is usually followed by a 4-6 hour bulk fermentation. If the kneading time is increased the bulk fermentation stage should be reduced.

Do no knead sourdough recipes work?

Yes, very well. No knead recipes increase the length of the bulk fermentation which gives more time for any broken or weak gluten to repair and strengthen. This enhances low protein flour so it can be used for sourdough bread.

How the length of fermentation changes the properties in the bread diagram

By not kneading (or lightly kneading) we reduce the amount of oxygen added to the dough which allows us to bulk ferment for long with a lower risk of over oxygenation.

There is no right or wrong in sourdough baking!

Should I stretch and fold sourdough?

Stretch and folds realign the gluten and allocate the food for the yeast to feed. Some stretch and fold sourdough techniques develop gluten better than others.

Stretch and folds speed up the bulk fermentation process and create a stronger dough.

How long should sourdough bread fermentation last

The length of the fermentation time is relative to temperature, the activity of the levain, flour and the skill of the baker. The process of making sourdough bread typically lasts from 4 -36 hours.

Generally the longer the duration, the more flavour is generated. 

How long should the final rise last in sourdough bread?

The final rise is around 3 hours for a standard sourdough bread at 28C (82F) however this may have to be extended at cooler temperatures or if the starter is not fully ripe.

Can sourdough bread be made quickly?

The process of making sourdough bread can be accelerated by kneading, using dough improvers and warm temperatures. A vibrant starter is a key driver in the process.

Baking time tables

Below are typical baking schedules for sourdough baking. Note that the fermentation time is increased or lowered when kneading and fermentation in the fridge is used. A proofing temperature in the region of 28C (82F) is used in these examples except the heavy knead method where a proofer set at 32C (90F) is used.

How long does bread take to make?

No knead – fridge final proof

  • Autolyse 30 mins
  • Kneading/incorporation 2 mins
  • Bulk fermentation 4 hours
  • Shaping 30 mins
  • Fridge final proof 8 hours
  • Bake 35 mins

Total 13 hours 37 minutes

No knead – fridge

  • Autolyse 30 mins
  • Kneading/incorporation 2 mins
  • Bulk fermentation 3 hours
  • Fridge fermentation 6 hours
  • Shaping 40 mins
  • Final proof 4 hours
  • Bake 35 mins

Total: 14 hours 47 minutes

Lightly knead

  • Autolyse 30 mins
  • Kneading 6 mins
  • Bulk fermentation 4 hours
  • Shaping 30 mins
  • Final proof 5 hours
  • Bake 35 mins

Total: 10 hours 41 minutes

Heavily knead

  • Autolyse 15 mins
  • Kneading 10 mins
  • Bulk fermentation 2 hours
  • Shaping 30 mins
  • Final proof 3 hours
  • Bake 35 mins

Total: 6 hours 30 minutes

Knead & in fridge

  • Autolyse 20 mins
  • Kneading 6 mins
  • Fridge Bulk fermentation 12 hours
  • Bulk rise 2 hours 30 mins
  • Shaping 30 mins
  • Final proof 4 hours
  • Bake 35 mins

Total: 20 hours 01 minutes

Adjusting flavour of the sourdough

Flavour and texture of sourdough bread change depending on the method used. Sweet, rich tones are offered in long cool fermentation, whereas, fresh and lighter flavours prevail in short fermentation.

Levels of acidity will also change. Bread that has been proofed at a warmer temperature will generate more lactic acid flavour – more sour and creamy yoghurt flavour.

Bread that is fermented cooler will have a more vinegary flavour, due to increased amounts of acetic acid.

View more about changing flavour in the sourdough starter troubleshooting and fixes article.

Is sourdough bread better than yeast bread?

There are less active yeast components in sourdough which slows the process of fermenting the bread. Because of the extended production time and the fermentation provided my lactic acid a more flavoursome and healthier product is produced.

Sourdough bread typically has a lower amount of gluten (low gi), less starch and keeps fresher for longer than yeast bread. It is also less likely to contain further fortification through artificial dough conditioners as it conditions the dough itself.

Why is sourdough bread rubbish in some stores?

One of the reasons why sourdough is such an artisan craft is the amount of variability in the levain itself. Controlling the sourdough to behave in exactly the same way every day takes enormous skill and equipment. Getting a reliable sourdough recipe at home is a challenge and for industrial producers often a complete headache!

Commercial producers will often use dough conditioners or enzymes to provide some consistency. These will also affect the bread negatively as they degrade the sourdough bread’s natural fermentation process.

Further reading

The combination of prefermented flour and the high levels of acids being added to a fresh dough enhance it to levels that cannot be reached elsewhere.

Sourdough bread has the most superior structure and flavour. See my sourdough bread recipe for beginners to get started right away.


The Taste of Bread – Raymon Calvel

Sourdough Fermentation – Bakerpedia

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