I’ve listened to top, top bakers – some of the world’s most celebrated, talk absolute nonsense on the subject of sourdough fermentation, so feel it’s my duty to provide clarity on this topic. To be fair, it’s easy to get confused about how sourdough fermentation works. There are so many processes going on, and even in this fairly hefty article, I haven’t delved into the detail of them all. One thing is for sure, it’s not just yeast that makes sourdough rise. Sourdough is much more complicated!
In this article, my challenge was to accurately explain the science behind the sourdough fermentation process, without being too technical. I mean, the subject is simple stuff for biologists, but for the average home baker you could spend a fortnight trying to get to grips with it, and I doubt that you really want to do that. So I’ve done it for you.
Before we start, this is not a guide on best practices, or sourdough troubleshooting, and it won’t answer questions such as, “why is my sourdough bread dense?” -directly anyway. The purpose of this guide is for bakers to understand what is really going on when making sourdough bread. With this knowledge you’ll be able to make better choices and produce better bread. So let’s get into “how does sourdough work” by understanding the basics first.
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What is sourdough?
Leavened bread has been made with sourdough for hundreds, if not thousands of years. A levain, (or “leaven” depending on what country you’re from) is the active ingredient used in bakery products to make them rise. In the case of making bread, the levain produces carbon dioxide which is then trapped by interlocking gluten strands. As more CO2 is produced, the pockets of gluten expand to make the dough expand.
Over the past couple of centuries the bakers levain of choice has changed from natural sourdough to bakers yeast. The particular yeast strain, saccharomyces cerevisiae is used in all varieties of bread baking yeast. But as I’m sure you’re aware, the use of sourdough has been revitalised in recent years. Though not nearly as widespread as yeasted bread, sourdough is extremely popular to make at home and it’s a popular item in stores and bakeries across the world.
Sourdough bread takes longer to rise than yeast-leavened bread. This, (in part) provides the unique flavour, texture and health benefits we associate with authentic sourdough bread, all of which we’ll discuss in this article.
There are lots of supermarket imitation sourdough breads. The real bread campaign in the UK dispels the big brands who attempt to pass products off as sourdough when actually yeast and dough conditioners are more at play.
Sourdough bread doesn’t require extra ingredient additions to make it perfect. Aside from a little malt flour in rare cases, all that is required to make outstanding sourdough bread is flour, water and salt. An experienced baker is able to manipulate their workings to produce unique flavours and extraordinary bread, and, after reading this, I hope that you will be able to do so too.
What is in a sourdough starter?
A sourdough starter is in essence a culture of wild yeasts and lactic bacteria. You can learn how to make a sourdough starter on my recipe page, but I’ll skim over the basics:
Water and flour are combined in a clean jar and left for 24 hours. The next day, ⅔’s of the mixture is replaced with fresh flour and water. The starter is left again and the discarding and refreshment steps are repeated every 24 hours, or when the batter peaks (rises to the top before dropping).
Over time, wild yeast and bacteria are cultured and the starter becomes more and more active. After a couple of weeks a mature starter, ready to make bread is produced.
How does sourdough produce gas?
The wild yeasts and Lactic Acid Bacteria (LAB) found in a starter get introduced primarily in the flour. Yet, some of the microflora is airborne. Over time their numbers increase and, by utilising the simple sugars derived from the breaking down of carbohydrates in the flour, they produce gas and other products which enhance the dough.
There will be a unique blend of different species and strains of yeast and lactic bacteria in each starter. Each one will vary in activity and as the starter matures the species will change as those best suited to the environment become more prominent.
In most mature starters, LAB outnumbers yeast cells by 100 to 1, however gas, although it is produced, is much slower when created when LAB is fermented. This means that LAB and wild yeasts are both responsible for gas production at a 50:50 ratio -approximately.
What is Lactic Acid Bacteria?
Lactic Acid Bacteria (LAB) are rod-shaped (bacilli) or spherical (cocci) bacteria found in many living things, especially decomposing plants and milk products. The main species used in sourdough is Lactobacillus but there are many other lactic acid bacteria genera.
They provide enzymes and catalysts for the sourdough fermentation process to produce lactate alongside other outputs. For lactate to become lactic acid it just takes a readily available hydrogen ion. Because of the similarity between them, both lactate and lactic acid are often used interchangeably.
Wild yeasts and LAB are aerotolerant anaerobes -meaning they organically multiply in the presence of oxygen. This means that it is essential to allow for some airflow in a new starter by loosening the lid of the jar -though pressure builds in a tightly sealed jar as gas is released which could force a glass jar to explode so it’s still wise to loosely seal the lid of a mature starter to prevent this from happening.
To produce gas, we’ll need a carbon source, and for this, we need simple sugars.
Stage 1: Breaking down starch into sugars
Starch makes up around 70-75% of the flour and is used to supply the yeasts and LAB. In order for the starch to be metabolized, it needs to be broken down into simpler sugars.
Starches are polysaccharides, which are made from chains of monosaccharides – the scientific name for simple sugars. By breaking down polysaccharides we end up with monosaccharides (single sugars) or disaccharides (two single sugars bonded). It’s the monosaccharides that yeast and LAB require and the type of monosaccharides used will vary depending from starter to starter based on the species of yeast and LAB they contain. The most widely used monosaccharides in bread are the 6 cell carbon sugar group called hexes. These include glucose and fructose.
The amylase enzyme is the kickstarter for breaking down starch. It exists in the flour already, but if low volumes are found, the addition of malt flour provides a diastatic boast.
Amylase gets to work right away by breaking down the damaged starch in the flour. Around 10-15% of the starch is damaged in most brands of flour so this is no problem. There are two versions of amylase; ɑ-amylase and β-amylase:
ɑ-amylase takes polysaccharides; amylose (22-25%), and amylopectin (75-78%). It then breaks them down into maltose, maltotriose (chains of maltose), glucose and dextrins. Its peak output occurs between 5-6 pH therefore as the starter matures and becomes more acidic, activity slows.
β-amylase has an important role too, it:
- Breaks the maltotriose chains to produce maltose
- Picks off maltose to be consumed by the yeast or LAB
- Breaks down dextrins into glucose
Dextrins are great and are often added as an ingredient in supermarket bread. These starches soak up water making the dough capable of holding more water to make softer bread with even oven spring.
When acidic sourdough bread is baked in the oven, any dextrins that are not broken down by β-amylase are hydrolysed. Here, the short-chained starch partially rebranch with the degraded starch molecules in the flour to harden and darken the bread as part of the Maillard reaction. This contributes to flavour, colour and crispness in the crust.
Some of the most prevalent starches found in flour are glucofructans. They consist of one fructose molecule chained to at least one glucose molecule and contribute to 1-2% of the dough. Using enzymes produced by the yeast, glucose is removed and eventually, all of the glucose and fructose molecules are independent and available for gas production.
Using sucrose as a glucofructan example:
Sucrose, also known as table sugar is the simplest type of glucofructan. It consists of a bond of just one glucose molecule to one fructose. The enzymes invertase and sucrase break down the bonds to separate the glucose and fructose molecules.
Some yeast and LAB varieties produce an enzyme called maltase which breaks the common disaccharide maltose into two glucose molecules. As with all of these processes, there are variants of these simple sugars which appear -depending on how the starches are broken down. I’ll share an example later on regarding the popular LAB, Lactobacillus sanfranciscensis.
Step 2: Glycolysis breaks down sugars for respiration and fermentation
The enzymes described above produce simple sugars called glucose and fructose. These enter a metabolic pathway called glycolysis where they are converted into pyruvic acid. The first section of this is the Embden-Meyerhof Pathway (EMP).
You will notice ATP and ADP in the diagram. Adenosine triphosphate (ATP) is the energy-carrying molecule found in all living cells. It donates molecules to other cells for reactions to happen. In bread baking, it releases phosphate molecules to become, adenosine diphosphate (ADP).
There are two stages of glycolysis. In the energy-requiring phase, the glucose molecule is rearranged, and two phosphate molecules latch on, transforming it into fructose-1,6-bisphosphate. The phosphates are taken from two ATP energy molecules which converts ATP into the less powerful energy molecule, ADP.
The modified sugar is now unstable and thus splits to form two phosphate-bearing three-carbon sugars. One is DHAP and the other is glyceraldehyde-3-phosphate. Only glyceraldehyde-3-phosphate can pass onto the next stage, but fortunately, the DHAP sugar can easily be converted into a second glyceraldehyde-3-phosphate.
The second stage called the energy-releasing phase actually occurs twice as we now have two three-carbon sugars. Both sugars go through a series of reactions catalyzed by several enzymes which utilise NAD+. This is a coenzyme found in all living cells that’s vital for metabolism.
NAD+’s main function in glycolysis is to transfer hydrogen back and forth. When NAD+ is reduced by gaining a hydrogen molecule it becomes NADH. When NADH passes on the hydrogen, it reverts to NAD+.
Several reactions occur to the glyceraldehyde-3-phosphate molecules to produce pyruvates at the end of the pathway. These include reinstating the phosphate molecules to ADP which produces two more of those energetic ATP molecules and the loss of some water (H2O).
In our bodies, ATP is what gives us the energy to move and is the primary output from this pathway. But in bread making, the pyruvate is the key component for the next stage. By the end of the process, we are left with two pyruvates, 2 ATP, 2 H2O and 2 NADH. The full equation is displayed here:
C6H12O6 + 2 ADP + 2 Pi + 2 NAD+ → 2 CH3COCOO− + 2 ATP + 2 NADH + 2 H2O + 2 H+
*Glucose is C6H12O6
*Pyruvate is CH3COCOO−
Providing there is a continual serving of simple sugars, the action of glycolysis can be repeated. There are other monosaccharides that are produced such as fructose, ribose, gluconate and (possibly) arabinose. These may also follow the glycolysis route.
With the presence of oxygen, yeast is able to respire. This uses the pyruvates previously produced to produce carbon dioxide. It does this by following the Krebs or Citric-acid cycle.
Essentially the pyruvate is decarboxylated to become acetyl. The acetyl is oxidised and undergoes a cycle of reactions that release CO2. At the end of the cycle, energy in the form of ATP is produced as well as acetyl. The acetyl produced can then be re-oxidised and the cycle can be continually repeated.
Carbon dioxide is transported as a liquid to a weak point in the gluten before it is released as gas. The gas is captured by the gluten matrix to make the dough rise.
Step 3: Fermentation processes
Respiration is the preferred way to raise quickly-made yeast-leavened bread. However, when long bulk fermentation and rising times are used in artisan yeast bread or sourdough bread, fermentation is preferred.
When lower quantities of levain or a less effective one (such as sourdough) are used, the bread takes longer to rise. The presence of too much oxygen can lead to the over-oxidation of flour which will destroy the flavour of the bread and weaken its structure. By reducing the amount of oxygen absorbed during mixing by only gently kneading or a no-knead method, we protect sourdough bread from over-oxidating and can ferment the dough for longer.
Without oxygen, the pyruvates cannot respire using the Krebs cycle. They must take a different path in the form of anaerobic respiration. This leads to fermentation being initiated by the yeast in the format of alcoholic fermentation or by lactic bacteria in homofermentative or heterofermentative reactions.
Some of these routes produce CO2 in order to make the bread rise, but not all. Having said that, they are all vital when making terrific sourdough bread.
As discussed in the bread fermentation process article, respiration and alcoholic fermentation are the key functions of gas production in yeasted bread. Alcoholic fermentation is only a handful of steps away from respiration. It is what happens when there is no oxygen available to conduct the Krebs cycle after EMP.
The pyruvate produced in glycolysis latches to an inorganic phosphate. At the same time, the NAD+ produced at the end of glycolysis gains a hydrogen to produce NADH. The pyruvate decarboxylase enzyme that is used causes the pyruvate to release CO2, and in the presence of alcohol dehydrogenase (an enzyme present in bakers yeast), the pyruvate completes its conversion into ethanol. What you are left with is:
Pyruvate + NAD+ + ADP = Ethanol + NADH + C02 + ATP
The NADH can be used in the glycolysis cycle for the next glucose sugar to repeat the cycle. ATP is the energy coenzyme, more powerful than its ADP relative.
NOTE: During glycolysis, one glucose sugar is split into two pyruvates therefore the outputs of this process are doubled. Therefore, from one glucose molecule; two molecules of ethanol, two molecules of carbon dioxide and two ATP are produced.
Homofermentation of LAB
The homofermentative process uses LAB to convert carbohydrates into lactate (lactic acid). It follows the same Embden-Meyerhof Pathway (EMP) as respiration and alcohol fermentation to produce pyruvates.
Fructose and other hexes
The main starch found in bread is maltose which is produced by a bond between two glucose sugars. This means that glucose is the most prevalent monosaccharide in bread dough, seconded by fructose found alongside glucose in the disaccharide, sucrose.
Any simple sugar can follow the pathway of glycolysis, not just glucose. Fructose will enter the pathway a little later on as fructose-6-p is the second stage of the process.
There is evidence to suggest that fructose operates two pathways. This involves sucrose producing lactate, acetate, ethanol, CO2, and mannitol. It is getting a bit complicated for this article but feel free to explore the article that I’ve linked
Homofermentation is very similar to ethanol fermentation. Instead, the enzyme, lactate dehydrogenase which is produced by LAB, catalyses the pyruvate to produce lactate and ATP. Carbon dioxide is not produced in this process but the lactic acid derived from lactate has many benefits discussed later on.
Again, as the glucose molecule is broken into two pyruvates in the EMP, there are 2 lactate and 2 ATP molecules produced for each monosaccharide used.
Heterofermentation of LAB
Heterofermentative bacteria use the Phosphoketolase Pathway (PKP) instead of EMP. Sugars that take this route produce lactate, carbon dioxide, and, ethanol or acetate.
One glucose is broken down into glyceraldehyde-3-p to produce a pyruvate and finally lactate as it did in the EMP pathway. However, you may recall that the process was doubled as alongside the glyceraldehyde-3-p it also produced DHAP. This easily became another glyceraldehyde-3-p molecule and therefore a second pyruvate is produced to double the output.
In the PKP, a 6 carbon sugar (glucose) is reduced to a 5 carbon sugar (Xylulose) and one carbon is released as CO2 in the process. This produces a 3 carbon molecule (glyceraldehyde-3-p) and a 2 carbon, acetyl. The acetyl can either produce acetic acid or ethanol.
Fermenting other sugars
Up till now, we’ve only considered the use of 6 carbon groups of monosaccharides categorised as hexoses. What is worth knowing is there are also 5 carbon group monosaccharides that also appear in bread, called pentoses. Blocks of pentoses become the starch, pentosans and are readily found in the bran of the flour grain or in rye flour. We’ll discuss the power of rye flour in sourdough fermentation near the end of this article.
These sugars can steal a carbon to enter at the start of either glycolysis pathway. But they are more likely to break down into ribulose-5-phosphate for use in the phosphoketolase pathway. The species and strain of the bacteria impact which sugars can be utilised and which are preferred in fermentation.
For acetyl to produce acetic acid it uses a process called co-metabolism. This is when a carbon backbone (a hexose sugar) and a co-substrate combine to facilitate the formation of acetic acid. If there is no suitable co-substrate, the acetyl turns into ethanol. Possible co-substrates include:
- Short-chain aldehydes
- Oxidized glutathione
But it doesn’t have to stop there. If the dough contains the correct Acetic Acid Bacteria such as the genera, Acetobacter, the ethanol can be oxidised to carbon dioxide and water using the Krebs cycle.
Putting all fermentation pathways together!
Here is a diagram that provides the basic information to consider what the microflora is doing during sourdough production. It only includes the basic inputs and outputs so is not necessarily scientifically accurate.
Starter or bread – What’s the difference?
The processes described above are how the wild yeasts and acid bacteria respire/ferment. This could be in any circumstance, in a starter or in sourdough bread dough. Lactic acid bacteria still appears in yeast-leavened bread when left to ferment for long enough. The use of preferments such as biga or poolish utilise the production of organic acids to enhance the maturity of the bread in a kind of sourdough-yeast fusion.
The lifecycle of a new sourdough starter
A new starter starts with flour, water and lots of potential! Initially, it’s the wild yeasts that heavily populate the starter, however, after a few days, acid bacteria multiply and eventually take over. The fermentation processes introduce ethanol and lower the pH by the production of acid.
As ethanol and acidity increase, many of the original yeast and LAB strains become weaker. Instead, more suitable strains that prefer these conditions begin to thrive. There is a delay between the conditions of the starter changing and starter catching up by populating newer strains which accounts for the varied activity that is often noted in a new starter during the first couple of weeks.
Regular refreshments with the same ingredients make a starter more stable. This is because the same types of yeast and bacteria are populating it. Any foreign materials or unwanted bacteria are digested and consumed by the starter. These weaken the prevalent yeasts and bacteria as they have to use energy to produce new enzymes to consume the foreign materials, but doing so keeps the wanted yeasts and bacteria thriving.
Common mistakes made by new sourdough bakers
One mistake many bakers make is to change the flour that they feed their starter. This change in minerals means the starter has to alter the enzymes it produces thus weakening the starter until the perfect balance is achieved and any unneeded enzymes are not produced.
Another mistake is not keeping things clean. A dirty starter jar can introduce unwanted bacteria into the starter which weakens it. Always replace your starter jar when it starts getting dirty!
A mature starter will be between 3.5 and 4.2 pH. Once the pH drops below 4.2 any harmful bacteria that may have multiplied during fermentation is killed off and the starter is ready to use.
Not seeing any life in your starter? Try my sourdough starter troubleshooting and fixes!
What happens when a starter peaks?
As the starter ferments, it produces CO2 which gets trapped in the gluten and forces the starter to rise. The starter will eventually stop rising when it:
- Runs out of sugars to feed on
- Acidity and ethanol levels rise too high and inhibit the enzymes and fermentation processes
Once the starter reaches the top of its rise it tends to sit there for an hour or two. The starter can’t produce much gas, but the acid bacteria will still multiply. Leaving the starter at its peak before refreshing it will produce a more active and acidity tasting levain. Eventually lactic acid breaks down the gluten strands and the starter collapses.
Types of yeast and lactic acid bacteria in sourdough
A sourdough starter contains around 1-4 different strains of yeast and lactic acids though there are hundreds of varieties available and many types believed not yet discovered. The selection found in a starter is based on the local environment, the ingredients used and the conditions that the starter is kept in. I’ll discuss each of these in a moment, but for now, let’s consider what makes LAB strains different? Lactic bacteria strains fall into one of three categories:
1) Obligate homofermentative
These strains ferment hexose sugars solely through the EMP pathway and almost entirely to lactic acid. Pentoses and gluconate cannot be fermented. This type of LAB are largely found in new starters and not established ones.
2) Obligate heterofermentative
These ferment hexoses almost entirely to lactic acid and, under glucose limiting conditions produce CO2, lactic acid, acetic acid and ethanol. Pentoses can be fermented also.
3) Facultative heterofermentative
These are lactobacilli which can ferment hexoses to lactic acid, CO2, acetic acid and/or ethanol. The pentoses can be fermented to lactic acid and acetic acid. The processes can occur interchangeably, swapping from homofermentative to heterofermentative as required.
The ratio of each strain varies which generates flavours and aromas that are truly unique to each levain. Here are some of the most popular variations found in sourdough starters:
Yeasts found in sourdough
- Kazachstania exigua (Saccharomyces exiguous)
- Saccharomyces cerevisiae (commercial yeast)
- Candida milleri
- Candida humilis
Lactic acids found in sourdough
- Lactobacillus fermentum
- Lactobacillus sanfranciscensis (prominent in San Francisco)
- Pediococcus pentosaceus
- Lactobacillus plantarum, and L. brevis
The impact of different bacteria and yeasts
An established starter cultures the yeasts and LAB that best compliment each other. For example, the most widespread lactobacilli, Lactobacillus sanfranciscensis (now called “Fructilactobacillus sanfranciscensis”) will separate maltose into a glucose-1-phosphate and a glucose molecule.
The glucose-1-phosphate portion enters its heterofermentative pathway to be converted into glucose-6-phosphate, whilst the glucose is excreted to be utilised by the yeast. It also utilises fructose in its heterofermentative activity to produce acetic acid from the acetate. Once fructose supplies deplete it produces ethanol instead.
How fermentation benefits sourdough bread
The basic necessity of sourdough bread production with a starter is simple. It takes an active starter that’s capable of producing CO2 to make the bread rise. But there’s more to fermentation than rising, what about the health benefits of sourdough?, its unique tangy flavour?, or the texture of sourdough bread?
Let’s now cover the key components produced during the fermentation process that make sourdough bread so special.
Lactic acids are vitally important for the dough as they aid maturation. According to an experiment by Bakerpedia, lactic acid provides the following benefits to sourdough:
- Increases acidity
- Enhances flavour
- Adds flavour
- Prevents mould to increase shelf life
- Produces dough conditioners
- Helps to absorb minerals
- Retains gas
Maybe you are familiar with the similarity between acetic acid and vinegar? Well, vinegar is (4%) acetic acid that’s diluted with water. Its production in sourdough fermentation produces CO2, but it also benefits the quality of the bread by:
- Lowering the pH to kill harmful bacteria
- Improving shelf life
- Producing a twangy aroma
- Adding flavour
You can see more about the effects of adding vinegar (acetic acid) to dough here.
Ethanol is utilised in the maturation of the dough, it is not as many claim, a by-product of the fermentation process. After the bread is baked, only 2% of the ethanol remains. It is used to:
- Improve odour
- Add flavour
- Control unwanted bacteria to improve keeping quality.
The adenosine triphosphate that’s produced during the fermentation processes is utilised at the start of each cycle of reactions. It provides energy by donating a phosphate molecule so that the processes of glycolysis can repeat.
As the environment becomes more acidic, cereal enzymes become more active, here are a few of the important ones:
Phytase hydrolyzes fructose, ATP and phytic acid. Flour contains between 1-1.5% phytic acid which binds dietary minerals such as iron, calcium, and zinc together. Hydrolising phytic acid with phytase breaks the bonds to aid mineral absorption when eaten.
Gluconic acid, formed by the acidification of glucose is used in cement! It strengthens the dough and enhances the gas retention properties of the gluten.
If you’ve used an autolyse in sourdough, you may understand the impact of proteases. They break down the peptide bonds between the gluten strands through hydrolysis. This speeds up the rate at which proteins are broken down into polypeptides or single amino acids.
Smaller protein chains make the dough more stretchy (extensible) and the loose amino acids produced make the dough feel softer. In autolyse this, alongside the hydration of the starch and protein, shortens the kneading time. The free amino acids can be processed by the yeast and will also improve crust colour and flavour through the Malliard process.
Pentosans are carbohydrates found predominantly in rye flour where they consist of 20-30% of the grain. They are also contained in the bran of whole wheat grain flour. They store a lot of water initially, diverting it from the protein in the flour.
Pentosans play a central role in creating the structure of dough in the oven at temperatures below 45C. As the temperature increases, starch takes over this role.
As prentosorace enzymes increase they break down the pentosan molecules to release the stored water. This is why rye bread dough becomes stickier as it ferments and (partially) why it is suspect to collapse in the oven.
How to change a sourdough starters characteristics
The levels of ash are measured by burning the flour and measuring what is left. The ash remaining is essentially the minerals that the flour contains.
Minerals slow the production of organic acids, however slowing the rate of fermentation actually increases the number of organic acids produced. Less speed, more haste!
Ash is found in higher quantities in whole wheat, rye and high protein flours. As rye is high-ash, many bakers switch 10-30% of the white flour in their starters for rye flour.
If you add an unfamiliar flour to a sourdough culture, it will upset the balance of the ecosystem somewhat. This is due to it needing to produce more of some enzymes to break down the sugars, and less of others. The result is the bread may take longer to rise.
Try to use the flour in your starter to make your bread. It’s not the end of the world if you switch up your flours, but if you run into problems it is something to consider when troubleshooting your sourdough bread.
Malted barley/ malt flour
Malted barley flour is sometimes added to sourdough bread recipes. By malting barley, you produce a powerful source of ɑ-amylase which is then dried and sold as malt flour. It can be used to compensate flour that has reduced enzyme activity.
I must highlight that I often see sourdough bakers attempting to follow recipes that contain activated malt flour. The purpose of this ingredient in sourdough bread fermentation is to enhance the flour. For most situations, the best solution is to instead select a high-quality flour and add malt flour if required. The overproduction of maltose that this potentially produces leads to a gummy crumb that’s most unpleasant.
The use of salt impacts the strains of yeast and bacteria in a sourdough starter or dough. By diverting water from the yeast and bacteria cells, salt creates osmotic pressure. Some levain strains will prefer the environment more than others which changes the levains characteristics.
By slowing down the rate of transfer between the yeast and the water cells (or bacteria and water), the rate of fermentation is also slowed.
Aside from fermentation changes, salt, being one of the basic senses is a flavour enhancer. It also helps to keep products fresh.
The gluten structure is also tightened with the addition of salt. The strengthening allows the dough to contain the carbon dioxide produced during fermentation better which improves loaf volume during oven spring.
The amount of mineral activity and acidity in water can affect the rate of sourdough fermentation. Higher levels of minerals found in hard water areas produce bread with a slower, yet higher rise and more flavour. The format is similar to the popularity of beer brewers in Buxton, UK.
Scientifically this happens, but finding a significant deterioration or advantage is rare to most bakers. Providing the water is drinkable and not overly chlorinated it is perfect for making sourdough. I don’t even filter mine!
If you suspect that your tap water is bleached or chlorinated at high levels, leave it out for 30 minutes before using it, or try switching to bottled water.
Regularity of refreshments
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.
From the moment a starter is fed to the point that it peaks, the population of acid bacteria increase. This means that a starter that is regularly fed when it reaches its peak will be more acidic, flavorful and powerful.
How temperature changes sourdough
Temperature is the most common way to tweak the rate of sourdough fermentation. Warming up a starter increases the rate of fermentation and also, the rate at which the enzymes break down the sugars so the yeast and bacteria can be “fed”.
We can cool the temperature of the dough to slow down fermentation. This method is used to overnight “retard” sourdough bread to be baked the following day, or by weekend bakers who store their starter in the fridge so they don’t have to feed it as often. Yet timing isn’t the only reason that the temperature of sourdough should be considered. Temperature changes the flavour of sourdough too.
Warmth is required for all types of sourdough fermentation. Each of the three fermentation routes also prefer wet dough. What we can do to alter the flavour of the sourdough bread is lower the activity of certain fermentation types. This will then boost others to produce more lactate, acetate or ethanol which tweak the flavour.
Here’s a table that explains what occurs when water and temperature are increased or decreased together:
|Fermentative bacteria||Result when water and temperature is increased||Result when water and temperature is decreased|
|0. Homofermentive||More lactic acid is produced||Lactic acid production slows|
|0. Heterofermentive||Lactic acid and alcohol production increases||Acetic acid production increases|
|F. Heterofermentative||Only lactic acid is produced||Only acetic acid is produced|
|Combined results:||More lactic acid||More acetic acid|
A dryer environment means the transfer of cells is slowed down which slows all three fermentative bacteria. As homofermentative bacteria only produce lactic acid, less viscosity leads to slow lactate production, allowing acetic acid production to increase in the heterofermentative reactions.
Both heterofermentative bacteria types produce more acetic acid in a starter/dough when water and temperature are decreased. But really it’s a case of a lack of water slowing all acid production routes down and cooler temperatures boasting acetic acid production.
A decrease in temperature doesn’t boast acetic acid production. The fermentation routes actually operate faster at a warmer temperature! The reason for the increase in acetic acid is based on the supply of sugars available that the bacteria needs for fermentation.
To produce acetic acid, acetyl needs to be co-metabolized, often by fructose. Which is primarily produced by enzymes supplied by the yeast to break down sucrose into glucose and fructose. These enzymes continue to operate when the dough is colder, albeit at a slower rate. This means more fructose is available to produce acetic acid whilst lactic bacteria ferment less.
The result is less glucose is available for the yeast or bacteria, but fructose utilising pathways can continue to operate and the ratio of acetic acid production begins to outnumber lactic acid and ethanol production.
Wet bread dough proofed at warmer temperatures generates more lactic acid – which is a creamy-sharp yoghurt-like flavour.
Dough that is dryer and fermented cooler will have a more vinegary and acidic flavour.
Fermenting sourdough in the fridge
Placing a ¾ proofed sourdough bread in the fridge overnight slows down gas production through fermentation. Yet as there will be a large concentration of lactate already, they continue to multiply thus making the bread more acidic.
It allows us to time our sourdough production to have the bread ready to bake at a time that suits us.
Find out how more on how to regulate temperature in the desired dough temperature and bread proofing temperature articles.
How gluten changes during the sourdough fermentation process
One thing we’ve only touched on so far is the impact on the gluten as the sourdough ferments. After the starter is added, the sourdough will undergo its first rise (or bulk fermentation) period. Some bakers autolyse the dough with or without the starter, this topic is best discussed on the sourdough autolyse page.
As the dough bulk ferments, the starch and proteins in the flour are hydrated. The moistened starch is then ready to be attacked by enzymes released by the starter or already existing in the flour itself. These enzymes break down the starch into simple sugars and are used in glycolysis as discussed already.
The moistened protein washes away the soluble proteins to reveal gluten in the form of glutenin and gliadin. These glutens start off all coiled up, but as they hydrate they are able to stretch out and lengthen. Cereal enzymes such as proteases get to work and begin to soften the dough and it becomes softer and stronger over time.
One of the main roles of gluten is to contain the gas produced during the respiration and fermentation processes. To do this, gluten forms strong bonds between each other to produce a gluten matrix. Kneading at the start of mixing is a way to enhance the absorption of water and the rearranging of the gluten structure.
Yet kneading heavily incorporates lots of oxygen into the dough. In sourdough bread, this can lead to over-oxidation which is not good for the bread. Instead of kneading to develop the gluten in sourdough, stretch and folds are often used instead. They rearrange the ingredients in the dough to accelerate the fermentation, alongside stretching the gluten to make it stronger. Stretch and folds are used either throughout bulk fermentation or just in the first half.
Certain sourdough fermentation products such as lactic acid, acetic acid and ethanol benefit the gluten structure by enhancing its ability to stop gas escaping the matrix and stretch so it can capture more to rise higher. These natural dough enhancers are the reason why sourdough bread has a higher volume after baking and can produce an open or aerated crumb.
Incidentally, the irregular open crumb of sourdough bread is thought to be produced by a combination of; Lots of gas produced, a light degass when shaping to retain the gas, acids forming the gluten together, a mature dough and a hot oven with a fully preheated base or baking stone.
Over proofing sourdough
There are many benefits of extending the fermentation of sourdough but alongside the ups, there are also downsides and the risk of over-proofing sourdough.
As fermentation continues, the dough will absorb more oxygen. This provides an initial boost in the glutens elasticity but as time goes on it destroys the carotenoid pigments and the dough also becomes weaker.
What also occurs when the dough becomes over-proofed is that the yeast and bacteria exert all available sugars. “This means that the bread will just stop rising, right?” Well, no, not really! Instead of breaking down starch, the levains start to consume gluten proteins. This weakens the structure of the gluten and the result at best is big bubbles in the bread but can lead to dough structure collapsing. See why did my bread collapse? to learn more.
Sourdough Baking Timetables
These are typical baking schedules for sourdough baking. Note that the fermentation time is increased or lowered when the dough is kneaded. A proofing temperature in the region of 28C (82F) is used in these examples except the heavy knead method where a proofer set at 35C (95F) is used.
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
- 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
- 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
Conclusion: Is sourdough bread better than yeast bread?
There are less active gas-producing components in sourdough which slows down the rise. Because of this extended production time, 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.
Sourdough fermentation process frequently asked questions
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. Starters are likely to contain the same strain of yeast anyway, so it can be done but you’ll lose some of the benefits of lactic bacteria.
Should I add yeast to speed up my sourdough?
To speed up the production of sourdough bread I prefer to add malt flour. This adds amylase which breaks down sugars quickly so they can be consumed by the yeast and lactic acid bacteria.
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. It’s also a wetter dough that requires gentle handling, making it unsuitable for many industrial machines. Commercial producers end up cheating by adding extra ingredients to make the dough more machinable and reduce production times.
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 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 2 -18 hours. Over a longer duration, more flavour is typically 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 was 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.
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 kneading time is increased the bulk fermentation stage should be reduced.
Do no knead sourdough recipes work?
No knead recipes have a longer bulk fermentation which gives the broken or weak gluten more time to repair and strengthen. This makes lower protein flour suitable for sourdough bread.