Sourdough Starter
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What is sourdough? An in-depth guide to sourdough technology for bakers

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Sourdough is a fermented mixture of wheat, spelt, or rye flour and water. Sometimes it might also include salt. It is obtained by spontaneous fermentation with environmental lactic acid bacteria and yeasts which make sourdough acidic and give it the ability to leaven bread. To keep sourdough active and in optimum condition, it needs to be refreshed consecutively.

That’s the scientific definition of what sourdough actually is. If you’re an experienced baker then I’m sure you already know that. You might’ve even baked sourdough bread using your own starter before.

My goal with this post is not to give you sourdough bread recipes. I’ve you’re an experienced baker, I’m sure you can bake better sourdough bread than me. There are a lot of online resources that can give you recipe inspirations and teach you artisanal bread baking techniques.

However, there’s a shortage of resources on the internet that look at bread from a scientific and technological viewpoint. That is because a lot of bakers really enjoy baking. I like it too. It relaxes me. But most bakers haven’t been scientifically trained. They are no technologists but artisans. And while they sometimes might drop a science buzzword that they have picked up somewhere, most lack a profound knowledge about the chemical, physical, and biological processes involved in bread baking.

Baker with rye bread
Bakers are artisans that apply scientific and technological knowledge. However, they don’t generate new knowledge. That is the job of the food industry and technologists.

The job of a baker is to produce tasty bread and not to question the science behind it. On the other hand, my goal with this post is to give you information about sourdough bread that you can use to gain a deeper understanding of bread baking. That might not improve your baking technique, but it can help you to better troubleshoot in case of failure and it can help you with the development of new recipes.

The difference between baker’s yeast and sourdough bread

Sourdough makes bread acidic. If you taste a sourdough and baker’s yeast bread side by side, it’s very easy to distinguish them based on their level of acidity. Sourdough bread typically has a pH-value between 3.8-4.6 while Baker’s yeast bread has a pH value between 5.3-5.8. A neutral pH would be 7.

As you can see by comparing the pH values, bread made from baker’s yeast is also acidic. Just as sourdough bread, it also contains organic acids like lactic acid and acetic acid yet in much lower concentrations. The pH scale is a logarithmic scale. A pH value of 4 for sourdough bread is roughly 30 times more acidic than a pH value of 5.5 for yeast bread. We are able to detect a sour taste with our tongue at pH levels lower than 5.

Sourdough bread is known to stale very slowly while yeast bread typically doesn’t last longer than for a couple of days. The sourdough bread is also well protected against any spoilage microorganisms. That is partly due to its lower pH value but also because antifungal compounds are formed by the bacteria in sourdough bread during fermentation. Yeast bread, on the other hand, has a high sensibility to bacterial and mold spoilage.

Selection of rye breads
Sourdough bread is better protected against staling and spoilage than yeast bread.

From a nutritional point of view, it is important to note that doughs made with sourdough have a much higher phytase activity than doughs made only with baker’s yeast. Phytase is an enzyme that cleaves phytic acid. Phytic acid binds the minerals iron, zinc, magnesium, and calcium and thus impairs their bioavailability. Therefore, it is often referred to as an anti-nutrient. If phytic acid is enzymatically cleaved, it releases these minerals and thus they can be absorbed by our body.

The four types of sourdough

Sourdough bread is popular across many countries and cultures. Many different baking technologies have thus evolved. There are countless ways to use sourdough for baking. However, the different kinds of sourdough can be grouped into four major types.

Type I Sourdough

This is probably the type of sourdough you are most familiar with. This type of sourdough is suited to replace baker’s yeast as a leavening agent. It needs to be refreshed at least three times in a row without refrigeration in between so that it has enough leavening power to bake an airy bread. So if you store your sourdough in the fridge, you should always feed it at least 3 consecutive times before baking. Otherwise, your bread will turn out dense.

Flow chart: How to turn a type II into a type I sourdough
How to turn a sourdough starter (mother sponge) into a leavening agent.

Type II Sourdough

This type of sourdough is not suited for leavening bread dough. Instead, it is only added for dough acidification and as a dough improver. It is produced by refreshing a sourdough starter by letting the refreshed sourdough ferment overnight at room temperature. Afterward, the refreshed sourdough starter is stored in the fridge for several days before using it.

The growth of spontaneous flour yeasts is inhibited by using this method so that commercial baker’s yeast needs to be added to the main dough prior to baking. You cannot take your sourdough starter straight out of the fridge and leaven bread dough with it. You need to first transform it into a type I sourdough by refreshing it at least three consecutive times.

Type III Sourdough

Type III sourdoughs are type II sourdoughs that have been dried or preserved in any other way (eg. heat treatment). Many supermarkets carry them nowadays and they are also extensively used by the food industry. If you are too lazy to make your own sourdough, this is a great way to bake sourdough bread.

In general, powdered sourdoughs are less aromatic than liquid or pasty ready-to-use sourdoughs. That is because many volatile flavor compounds, like for example acetic acid, are evaporated during the drying process. I have used pasty sourdough from the store before and the result is indistinguishable from real sourdough. Of course, type III sourdoughs are not suitable for leavening bread. But they give you the same great flavor as homemade sourdoughs. Just remember to add some commercial yeast to them as a leavening agent.

Sponge Dough

Sponge dough for baking bread.
A sponge dough is prepared with baker’s yeast.

Sponge dough is also called pre-ferment. It’s not made from a sourdough starter. To a mixture of water and flour, commercial baker’s yeast is added and the dough is left to ferment between 3 to 20 hours. This pre-ferment is then added to the final dough.

Baking with a pre-ferment improves the loaf volume, taste, flavor, and shelf life of bread. In a technological sense, this is a type of sourdough. So, if you bake bread with pre-ferments, you are technically a sourdough baker.

The defining quality of sourdough is the number of lactic acid bacteria in the dough that produce organic acids. Lactic acid bacteria do grow in doughs made with pre-ferments to a much larger extent than in doughs made without pre-ferments. So, while doughs made with a pre-ferment might not be as acidic as doughs made with a traditional sourdough starter, they still show similarities to a traditional sourdough.

How to make a sourdough starter

There are many ways to obtain a sourdough starter (mother sponge) which can be used to prepare sourdough bread. I want to show you three methods on how you can produce your own mother sponge at home. In German, this mother sponge is referred to as ‘Anstellgut’ or ‘Grundsauer’.

I want to show you the French system, the American system, and a Swiss way to produce the mother sponge.

The French System

The ‘levain’ is a legendary French sourdough starter that is very aromatic with only a mild acidity. To one part of wheat flour, you add about half the amount of water (50-52 % hydration | Dough yield: 150-152) along with a little salt and enzymatically active baking malt. Don’t be surprised: this is a very firm dough. You leave this mixture to ferment for 24 hours at room temperature. This first fermentation step only results in low carbon dioxide release and slight acidification of the dough.

However, the decrease of pH is enough to induce the activity of protein-cleaving enzymes naturally present in flour. Combined with even more protein-cleaving enzymes from the bacteria in the dough, these enzymes break down gluten proteins. Therefore, the dough gets softer during fermentation.

The first step is followed by refreshing the dough simply by adding the same amount of flour as in the first step and a slightly lower amount of water than the first time so that the hydration of the dough is adjusted to 48 % (Baker’s percentage: 148). The main aim of this first refreshment is to introduce oxygen and new fermentable carbohydrates into the system to stimulate microbial growth and activity. This second fermentation is much quicker than the first one because of the higher initial amount of microbes in the dough. Usually, the refreshed dough is left to ferment between 7 to 8 hours.

The refreshment procedure is then repeated several times until the dough volume increases by three to fourfold with respect to the initial dough volume in a timeframe between 7 to 8 hours. The mother sponge obtained by this method is called ‘levain chef’. This mother sponge can then be fed three times to obtain a type I sourdough that can be used to leaven bread.

Flow chart for the production of French-style sourdough starter
Process for obtaining a French-style sourdough starter (levain chef).

Please note that a lot of French recipes call for honey instead of baking malt. That is fine too. Just as malt, honey contains sugars and alpha-amylase enzymes that provide food for the microbes in the flour. Alpha amylases are starch-cleaving enzymes that break down starch molecules into smaller sugar units. This enhances the fermentation and makes sure that the microbes have a steady supply of sugar available to them. This helps the microbes to grow faster and thus you will need fewer refreshment steps to produce your mother sponge. Please take a look at my article about baking enzymes for further information on baking malt.

A lot of recipes on the internet call for a higher hydration level to produce a French-style sourdough starter. They often specify to add equal amounts of water and flour. While I know that high-hydration sourdough starters are very common in France, I want to point out that I picked this traditional method for a specific purpose: To teach you about different approaches to sourdough. In the next part, I will discuss high-hydration sourdough in the context of the American system to obtain a mother sponge.

There is a large difference in taste between bread made with a high-hydration sourdough and bread made with a low-hydration sourdough. Sourdough with a lower hydration level favors the growth of acetic acid-producing bacetria and yeasts while a wet sourdough favors the growth of lactic acid-producing bacteria. Acetic acid has a pKa-value of 4.76 while lactic acid has a pKa-value of 3.86. The pKa value is called acid dissociation constant and is a measure for the strength of an acid. The lower the value, the stronger the acid.

Please note again that the pH scale is a logarithmic scale. This means that lactic acid is about 8 times more acidic than acetic acid. But that doesn’t mean that lactic acid tastes more acidic. Lactic acid tastes much milder in taste than acetic acid. Counterintuitively, weaker acids taste sourer than stronger acids. Thus high-hydration sourdough is much more acidic than low-hydration sourdough but tastes less sour.

French baguettes
Tasty baguettes can be baked with a French-style sourdough.

You might know the French ‘pain au levain’. It’s a legendary sourdough bread which is famous for its airiness, crispiness, and aroma. The mother sponge used to bake traditional ‘pain au levain’ is the one I have shown you above. The characteristic strong aroma comes from the low-hydration mother sponge used. You might know that French people are generally very fond of funkier flavors. A lot of the cheese and bread they eat would be considered a fermentation mistake by German standards.

For German and American quality standards, often a mild fermentation is preferred over a partly uncontrolled one that produces more pungent flavors. If a sourdough bread, cheese, or sauerkraut has a mild acidity, it is considered of good quality in Germany. Yet the French eat rotten cheese and funky baguettes and claim them to be a delicacy. Food preferences are different for people from different cultural backgrounds. French bakers typically claim German baguettes to be too mild and lacking in taste. Yet that is how most German’s prefer them: mild and just slightly acidic. In the end, no approach is better. It’s a personal preference. Everyone should get to eat food the way he or she likes it the most.

The American system

Most instructions for sourdough starters on the internet teach the ‘American-way’ to obtain the mother sponge. Of course, this method is not only applied in America but worldwide. The difference to the French-system is that American sourdough starter has a much higher hydration level, often even exceeding a hydration level of 100 %. This sourdough starter is thus ideal to bake mild sourdough bread as you would find it in a German or American bakery.

Fluid sourdough starter (mother sponge)
American-style sourdough has a higher water-content than French-style sourdough.

While in France, Great Britain, and the US, sourdough starters are almost exclusively made from wheat flour, in Germany and the Scandinavian countries a sourdough starter is most often prepared from rye flour. We Germans and the Nordic people love rye bread while it is a niche product almost everywhere else in the world.

For an American-style sourdough starter, one part of wheat or rye flour is mixed with about 1.25 to 1.5 parts of water. This mixture is then left to ferment at 32-35 °C (90-95 °F) for 24 hours to acidify the dough. Then, the same amount of flour and water is added as in the first step. The only thing that changes is the fermentation time. In the second step, the dough is only left to ferment for 8 hours at 32-35 °C (90-95 °F). Then the dough is refreshed for a second time and left to ferment for another 16 hours at 32-35 °C (90-95 °F).

After the second refreshment, the dough gets continuously refreshed in the same way as before and fermented between 8-16 hours at 24-27 °C (75 – 80 °F) until the sourdough has reached a pH value between 3.6-3.8. The whole process takes about 5 days to achieve a stable mother sponge. The mother sponge can either be stored in the fridge or it can be stored at room temperature and be refreshed daily.

Process flow-chart for American-style sourdough
The production process for American-style sourdough. A pH-level between 3.6-3.8 is typically reached after 5 days.

As you can imagine, by feeding the dough consecutively for five days, you will have a lot of mother sponge on hand. The dough weight doubles with every refreshment step. So always start with a small amount of flour. Once you have obtained the mother sponge, you usually discard most of it when you refresh it. For a type II sourdough that is stored in the fridge, typically about 5 grams of mother sponge is added to 50 grams of flour and 50 grams of water. This dough is then left to ferment at room temperature overnight and then used for baking or put back into the fridge for storage.

High-hydration sourdough starter can typically be stored in the fridge for about 10 days before it needs to be refreshed. A firmer French-style sourdough, on the other hand, can be stored in the fridge for several weeks up to a few months if the hydration level is lowered to about 43 %. Sourdough can also be frozen although several refreshments are needed to get it active again after thawing.

Swiss-style sourdough

This method is somewhere in between the French and American-style sourdough. There are no hard rules when it comes to obtaining a mother sponge. Really, you can use just about any sourdough starter tutorial on the internet and it will probably work. The only thing you need to consider is the taste of your bread: Do you want to bake more pungent and sour loaves or do you want to bake milder loaves with a perfect balance of flavors? With this in mind, you can adjust the hydration level of the sourdough starter to your preference.

So, without any further explanations, here’s another way to obtain a mother sponge:

Process flow chart for Swiss-style sourdough
Swiss way to obtain a sourdough starter.

The addition of fresh apple juice in the first step serves the same purpose as the addition of malt or honey to French-style levains. Apples also contain amylase enzymes that convert starch into sugar. Just make sure to press the apple juice yourself. It should not be heat-treated as heating inactivates the enzymes present in apples.

Fresh apples
Apples naturally contain starch-cleaving enzymes.

How to determine and modify the physicochemical parameters of sourdough

There are four main physicochemical parameters which are related to the sourdough quality and which can be manipulated and monitored to achieve the desired outcome:

  • Dough yield and temperature
  • Dough acidity
  • Fermentation quotient

Dough yield and temperature

I have already talked about the impact of the dough yield on the microbiota of sourdough. The dough yield is often also called baker’s percentage (‘Teigausbeute’ in German). It’s very easy to calculate:

Dough Yield = Dough Weight x 100 / flour weight

If the other ingredients in a dough (eg. salt, sugar, yeast) are not considered, the dough weight is just the sum of the flour weight and water weight. A sourdough consisting of 100 grams of flour and 60 grams of water thus has a dough yield of:

Dough yield = ((100 grams + 60 grams) * 100) / 100 grams = 160

A firm wheat flour sourdough starter typically has a dough yield of 150-160 whereas a soft/ liquid sourdough starter typically has a dough yield of about 200. A firm dough favors the growth of acetic acid-producing bacteria and yeasts while a soft dough favors the growth of lactic acid-producing bacteria. Acetic acid is a weaker acid than lactic acid, however, it tastes sourer.

Another influence parameter is fermentation temperature. A dough fermented at 35-37 °C favors the growth of lactic acid-producing microorganisms while a dough fermented at 25-30 °C favors the growth of acetic acid-producing microorganisms.

Flavorwise, acetic acid has a much higher impact on the flavor of bread than lactic acid. Type I sourdoughs that are used as a leavening agent are typically fermented at around 25 °C. The concentration of acetic acid in the final dough is highly dependent on the yeast activity as yeasts are the major producers of acetic acid in sourdough. Above 32 °C (90 °F), the growth of yeasts is inhibited and thus the acetic acid production in the dough drops to a minimum.

Yeasts produce acetic acid from ketose sugars. Ketose sugars have a different chemical structure than aldose sugars. Fructose is a ketose sugar while glucose is an aldose sugar. That is why fructose is often added to sourdough bread to enhance the taste of the bread. More fructose for the yeasts = more acetic acid produced.

D-Glucose vs. D-Fructose Structural Formulae V.1.svg
Glucose (on the left) is an aldose while fructose (on the right) is a ketose. Note the difference in the positioning of the carbonyl group (=O).

The formation of acetic acid can also be increased by aerating the dough. This can either be achieved by mixing the sourdough starter a few times during fermentation or by using the slap-and-fold technique during the bulk fermentation of the final bread dough.

Dough acidity

One of the easiest ways to judge the quality of sourdough is by measuring its pH value. The optimal range is between 3.5 to 4.3. Food pH meters are not expensive and can be bought for less than 100 dollars. They are no requirement for home bakers, however, they are important for professional bakers to objectively judge the dough quality. A bread dough that uses 20 % of sourdough in relation to the flour weight should ideally have a pH range between 4.7 to 5.4.

Our tongue generally observes foods that have a pH value lower than 5 as sour. Dough with a pH value of 4.7 is roughly 5 times sourer than dough with a pH value of 5.4. With a pH meter, you have a very powerful tool to monitor the bread baking process.

Illustration of the pH scale
The pH scale ranges from 0 to 14. Picture Source: Wikipedia.

The food industry also determines the total titratable acidity (TTA) in sourdough. They simply add a lye solution to the sourdough (which is dissolved in water) until all the acids in the dough are neutralized. The amount of lye solution needed to neutralize the acids in 10 grams of sourdough can vary widely between 30 to 220 mL 0.1 N NaOH solution. In general, the total titratable acidity should be higher for rye flour than for wheat flour.

Fermentation quotient

This is simply the ratio of lactic acids to acetic acids in the dough. As discussed previously, the amount of acetic acid can be increased by for example:

  • Adding fructose (or any other ketose sugar) to the dough
  • Increasing the amount of oxygen in the dough by aerating it
  • Decreasing the dough yield
  • Fermenting the dough at a lower temperature as to not inhibit the activity of sourdough yeasts (no more than 32 °C | 90 °F)

How to apply sourdough to bake bread

I don’t want to cover general bread baking and processing techniques like kneading and proofing in this post. Instead, I want to give you some examples of loaves of bread that are baked with a sourdough starter so that you get an idea of the general process on how to bake your own sourdough bread at home.

Pain au levain

French pain au levain loaf
The French pain au levain is a tasty type I sourdough bread.

For a traditional French ‘pain au levain’ you need a mother sponge (levain chef) with a low dough yield of about 148. This sponge gets refreshed three times to obtain a full sour that can be used to leaven the bread dough. The full sour is added to the main dough at the proportion of 25 % with respect to the mass of the final dough. The final dough needs to rise for just 30 minutes before baking. The result is airy and crispy sourdough bread.

Process flow chart for pain au levain
The production process for a French ‘pain au levain’.

German rye bread

German Rye Bread
German rye bread is very healthy and tasty but denser than wheat bread.

The process for baking German rye bread might seem very complex at first but it is very similar to the French method of producing ‘pain au levain’. The following process is for a rye bread that contains no Baker’s yeast so that you need a strong type I sourdough that has enough power to leaven the bread.

Once you have understood and mastered this method you will encounter it in many German bread recipes. The dough yields and fermentation parameters might be different but in the end, all recipes rely on the same basic principles.

Please note that either the full sour from previous bread making or a mother sponge can be used to obtain the full sour used to leaven German rye bread. The fresh sour (Anfrischsauer) shown in the flowchart below is either made from the mother sponge or from an old full sour. The doughs from the two methods shown are not mixed to obtain the fresh sour. You can choose between methods 1 or 2.

Process flow chart for german rye bread
Process for producing traditional German rye sourdough bread.

American sourdough sandwich bread

Sourdough sandwich bread
Sourdough sandwich bread is an example for a type II sourdough bread.

American sourdough sandwich bread is the perfect example of a type II sourdough bread. The sourdough is added to the final dough to impart a nice flavor but not to act as a leavening agent. That is the job of the baker’s yeast. Most of the sourdough bread that I bake is type II sourdough bread because this type of bread is much less labor-intensive and absolutely foolproof because of the addition of baker’s yeast.

Not only sandwich bread can be prepared using a type II sourdough. You can also bake German rye bread with it if you add yeast to the main dough. Then you only need one overnight fermentation step to refresh the mother sponge. If you don’t have a sourdough starter at home, you can also use a ready-to-use sourdough from the store (type III sourdough). Then you don’t need to refresh the sourdough starter overnight but can add it directly to the main dough according to the packet instructions.

Alternatively, you can prepare a sponge dough with a dough yield of 200 that contains 1 % fresh baker’s yeast in relation to the flour weight (1 part flour + 1 part water + 0.01 parts fresh yeast). This can replace the sourdough starter and is technically the fourth type of sourdough.

Process for american sourdough sandwich bread
Process for the production of American sourdough sandwich bread.

The role of sourdough as a dough improver

In the last part of this post, I want to talk about how sourdough improves the dough and bread structure. In another post, I’ve already talked about baking enzymes and their role as dough improvers in the food industry. Yet a lot of consumers nowadays prefer to buy food with a ‘clean label’. This means that no food additives have been used to produce bread.

Because of the high demand for bread with a ‘clean label’, food manufacturers have rediscovered sourdough in recent years as a natural dough improver. The three main factors that determine bread quality are:

  • The strength and quality of the gluten network
  • The gelatinization properties of the starch
  • The alpha-amylase activity

There are four sourdough metabolites that have an effect on the dough and bread structure:

  • Organic acids
  • Enzymes
  • Exopolysaccharides
  • Carbon dioxide

Organic acids

A little theory on protein charge and solubility

The primary effect of organic acids produced by the sourdough microbiota is the increased swelling and solubility of gluten proteins. Proteins are zwitterions. They contain positively charged and negatively charged functional groups. At their isoelectric point, proteins have a neutral net charge. The isoelectric point of native gluten proteins is around a pH value of 6.2. If you go below a pH value of 6.2, the net charge of the gluten proteins changes from neutral to positive.

That is because, in acidic environments, positively charged hydrogen ions from the proteins dissociate into the solvent to a much lower extent than above the isoelectric point. That is because acidic solvents already contain a large number of dissolved hydrogen ions. The pH value is by definition a measure of the concentration of free hydrogen ions in a solution. Acids are hydrogen ion donors while bases are hydrogen ion acceptors.

So the organic acids in the sourdough release positively charged hydrogen ions into the water phase of the dough. This decreases the pH value but also changes the charge of the water phase to a more positive value. As you might know, positive charges repulse each other. Thus the hydrogen ions from the protein side chains are discouraged from dissociating into the water phase which leads to a positive net charge of the proteins at low pH levels.

This positive net charge of the gluten proteins leads to a higher intramolecular electrostatic repulsion. Proteins consist of amino acid chains. The protein structure is determined by how these amino acid chains are folded together. If there is intramolecular repulsion between the amino acids (because of a positive or negative net charge), the proteins unfold.

Protein 3-d structure
Proteins have a complex three-dimensional structure (this is not the gluten protein in the picture). Picture Source: Wikipedia.

Charged proteins show an increased solubility in water. Water is a polar solvent which means that the charge is unevenly distributed throughout the molecule. Thus it has the ability to dissolve other polar molecules with an uneven charge distribution. An overall positive or negative net charge increases the water solubility so that at the isoelectric point, the protein solubility is the lowest.

The effect of low pH levels on the dough structure

Organic acids improve the dough structure because they decrease the pH level. On the one hand, the solubility of the gluten proteins is improved because the pH level of sourdough is much lower than the isoelectric point of 6.2. Therefore, the dough needs to be mixed and kneaded for a shorter time. The gluten and starch can also take up more water which increases the elasticity of the dough.

Yet, there are also negative effects. The gluten network in sourdough is weaker and softer than in non-sourdough because of the higher intramolecular repulsion of the unfolded gluten proteins. This makes it harder for the gluten to form strong bonds. Also, the gluten proteins get partly broken down in acidic environments and by gluten-cleaving enzymes occurring in sourdough. If this effect is too pronounced it can lead to a decreased bread volume.

But not just the gluten is affected by low pH levels. Pentosans (indigestible carbohydrates naturally occurring in flour) are broken down by acids and thus their solubility is increased. This is a good thing. In rye flour, water-binding pentosans take over the function in the structure-forming process. Pentosans are to rye flour what wheat gluten is to wheat.

German-style rye whole wheat bread
A gel of water-soluble pentosans gives structure to rye bread that enables it to entrap gas bubbles.

A higher amount of dissolved pentosans gives rye and wheat bread an improved crumb structure and volume (the amount of water-soluble pentosans in wheat flour is much lower than in rye flour but they still have an effect). The water-soluble pentosans don’t form a fibrous network like wheat gluten but they form a gel that has the ability to hold gas. Only the water-soluble pentosans form this gel which is why rye flour always has to be acidified to break down the insoluble pentosans into soluble ones.

Enzymes

The pH value also influences the enzyme activity in the dough. An acidic pH level decreases the alpha-amylase activity in the flour. This is another reason why rye bread needs to be baked with sourdough or with the addition of an acidic ingredient like buttermilk. Wheat flour has only a low alpha-amylase activity while the alpha-amylase activity in rye flour is much higher.

Alpha-amylases damage the starch granules. If their activity is too high, the water-holding ability of the starch granules is decreased. Thus the crumb of the bread turns out wet and glibbery (‘glitschige Krume’ in German).

German bread roll with wet crumb
If the alpha-amylase activity in flour is too high (eg. if too much enzymatically active baking malt is added to wheat-based doughs) the crumb can become wet. It’s hard to see on this picture but when you bite into this bread roll the crumb feels sticky.

Yet, in low amounts, alpha-amylases are needed to bake an airy and light bread. Please refer to my blog post about baking enzymes for detailed explanations of how alpha-amylases improve the dough structure.

Some of the enzymes produced by the lactic acid bacteria in sourdough decrease the strength of the gluten network. On the one hand, there are gluten cleaving-cleaving enzymes. And, on the other hand, there is glutathione reductase. Glutathione reductase is an enzyme that reduces oxidized glutathione in the dough to non-oxidized glutathione. This non-oxidized glutathione is a reducing agent that inhibits the cross-linking of gluten proteins through disulfide bridges.

Exopolysaccharides

Some strains of lactic acid bacteria synthesize exopolysaccharides from sucrose (sugar). Exopolysaccharides can bind a lot of water and thus increase the softness of the dough and can also lead to an increased bread volume because softer doughs containing more water have an increased oven sprig.

They also act as an anti-staling agent because they inhibit the recrystallization of starch (retrogradation) after baking. Crystallized starch can hold less water than gelatinized starch and thus the bread crumb loses moisture during storage.

Carbon dioxide

Carbon dioxide is the gas trapped in the dough. In sourdough, it is not only produced by sourdough yeasts but also to a smaller extent by lactic acid bacteria. If baker’s yeast is added to the dough, the gas production by lactic acid bacteria becomes irrelevant because baker’s yeast is a much more powerful leavening agent than sourdough bacteria.

While sourdough yeasts and bacteria don’t produce more carbon dioxide than baker’s yeast, sourdough somehow has the ability to entrap a larger amount of carbon dioxide than non-sourdough. I have no certain explanation for why sourdough improves gas retention. It is probably related to the increased amount of water-soluble pentosans in sourdough. As I told you before, water-soluble pentosans are a structure-forming element in bread dough. So for sourdough wheat bread, you have two structure-forming mechanisms: the gluten network and the gel of water-soluble pentosans. Rye bread doesn’t form a gluten network which is why it is denser than wheat bread. You only have the pentosan gel to provide structure.

Sourdough bread with open crumb
Wheat-based sourdoughs can hold more gas than non-sourdough wheat doughs.

Conclusion

As you can see, sourdough technology is a very complex topic. Nevertheless, I hope that I didn’t confuse you but rather gave you a compact overview of the underlying theory and technology for producing sourdough bread. If you know how a system works, it’s easier to manipulate it to achieve a better outcome.

Maybe your sourdough comes out too mild? Then you hopefully have learned that you can add fructose to the sourdough to increase the acetic acid production or lower the dough yield.

Maybe your sourdough comes out too dense? Then either the yeast activity is too low or your pH value is too low so that the gluten network gets broken down and the dough thus has a low gas-holding capacity.

As you can see, whoever has the knowledge has the power to react and manipulate. I’m going to talk about further aspects of baking in future blog posts. As for today, that’s a lot of knowledge to digest in this post. So I’m going to end it here.

Resources:

Handbook on Sourdough Biotechnology

Sourdough Technology—A Traditional Way for Wholesome Foods: A Review

Effect of Single Strain and Traditional Mixed Strain Starter Cultures on Rheological Properties of Wheat Dough and on Bread Quality

Why sourdough bread resists mold

pH and Taste

Effects of pH changes on functional properties of native and acetylated wheat gluten

2 Comments

  1. Marvellous posting! Comprehensive, yet „entertaining“. Thank’s a lot for all those things I did’t know about sourdough, but that are essential to understand the nature of it.

    • Thanks Thomas! I’m glad you enjoyed the article. I know that it is a very extensive one but I’m happy to hear it that is not overwhelming. – Tim

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