What is going on in sourdough bread? What is the science behind it? Hopefully I can answer some of your questions with this video. My general script / info I used for the video are below. The references are referenced in [brackets] and the links are a the bottom of the page.
The embedded video is here (the video link is below)
Video Link: https://www.youtube.com/watch?v=RMsvHOs3MjQ
Feel free to check out the other bread science videos and information:
Part 1 - Introduction to Bread Science: Citations and References ; YouTube video
Part 2 - The Bread Making Process: Citations and References ; YouTube video
Part 4 - The Science of Rye Flour and Rye Bread: Citations and References ; YouTube Video
Part 5 - The Science of Salt-Rising Bread: Citations and References ; YouTube Video
Part 3: Flour, Water, Yeast, Bacteria and Salt: Sourdough Bread Science
Slide 2: Pictures
Relationship analogy
When you start getting into sourdough, you’re tapping into a deeper level of bread. You are culturing your own community of yeast and bacteria that can create some of the most flavorful bread of your life. There aren’t really any shortcuts here. You’re responsible for this starter.
Remember, some of this information is more tentative because there’s still a lot we don’t know about sourdough, and we don't yet know all the species of yeast and bacteria that reside in sourdough starters globally. Hopefully as time goes on, we'll learn more.
Slide 3: Outline
In this video, we will primarily be discussing un-enriched (lean) sourdough, which is simply made of flour, starter (flour and water), and yeast.
History:
Sourdough is thought to be the oldest kind of leavened bread. It is linked to ancient Egypt. Some sources say it originated between 4000-3000 BCE[35], others say up to 8000 years ago. It is thought that a mixture of water and flour were left sitting around for a few days. Then bubbles appeared and someone decided to try cooking it and voila, the origins of leavened bread. This concept was also being discovered in different parts of the world, and with different foods.
Sourdough was used for centuries to make bread, and other fermented products such as beer barm (ale yeast) or pieces of old uncooked dough were also used to leaven bread. Even rotten food was used to leaven bread in cases such as Salt-rising bread, which I also have a video on.
Later, much later, in the 19th century, commercial yeast was born.[32] It was designed to work fast and consistently. But it wasn’t developed to make very flavorful bread, unlike with sourdough.[32] I would like to note that you can definitely make tasty crusty bread with commercial yeast, by modifying the amounts you add, changing the rising times, and playing with ingredients.
Slide 4: What is a sourdough starter?
The yeast are called 'wild' yeast because they come from the flour or the environment (which basically makes them wild). Wild yeast is also a phrase used to differentiate sourdough yeast from commercial yeast. The starch and proteins in flour can be food sources for wild yeast and bacteria that live in the starter; hence the attraction.
Furthermore, lactobacillus bacteria (I will refer to as LAB) can also enter the starter from the environment. The bacteria are typically from the lactobacillus genus. This genus of bacteria is also responsible for a lot of other fermented foods.
So, when you make a sourdough starter, you’re essentially creating a little ecosystem. Those that can eat the resources available and tolerate the acid and alcohol are more likely to live there, and those who can’t, will die off. Essentially, it’s natural selection that’s influenced by you![10] Your starter community will change overtime, but it’s important to first build a very strong and active community.
I mentioned in Part 1 that yeast, their enzymes, and the enzymes in flour prefer specific temperatures, usually between 85-105°F; 29-40°C. Lactobacillus also have temperature preferences. In general, keeping the starter at room temperature or slightly warmer will yield an active starter.
Slide 6: Enzymatic Chain:
Now remember in Part 1 when I talked about how the enzymes in flour and yeast can break down starch and sugar into different substances, such as glucose that can be used for fermentation? Well, that process can extract some sugars, break down some starch, and break down some sugars, all of which develops flavor!
Then remember in part 2 where I discussed making a simple bread with commercial yeast, and how you can speed up the process by adding sugar (sucrose)? That added sucrose can quickly be broken down into glucose by invertase in commercial yeast and then that yeast can use the glucose for fermentation (which is a fast process, much faster than sourdough enzymatic breakdown). Well in that case of the commercial yeast bread, we were essentially skipping most of the steps in the enzymatic chain, mainly the activity of amylase and maltase.
Now imagine if instead of taking a shortcut with commercial yeast and sucrose, you decided to utilize the whole chain of sugar-related enzymatic reactions that deepen the bread’s flavor, as well as using an additional microbial community that produces acid and adds even more flavor to the bread dough! This is what happens in sourdough! The long fermentation time and active and diverse microbial community really extract a lot of flavors from the bread in various ways that you don't get with a one-species yeasted bread.[10]
Yeast and bacteria:
One teaspoon of a strong active starter can contain up to 50 million yeast and 5 billion bacteria cells.[36]
Yeast:
Baker’s (commercial) yeast versus sourdough and yeast and LAB in flour:
Remember that baker’s yeast is typically one species (Saccharomyces cerevisiae) and is usually cultivated on an industrial scale. In comparison, sourdough contains many, and in fact we’re not sure how many..., different kinds of wild yeast that are from the air, the environment, the flour, and even your body, and this community differs significantly around the world.
Sometimes “wild yeast” is used to refer to the wild yeast that live in the flour, air, and environment that grow in sourdough.[5]
Furthermore, studies have correlated the bacteria and yeast in the flour you use to make the starters with the communities found in active sourdough starters, meaning the yeast and bacteria that were already present in the flour will likely grow successfully in the starter.[31] Therefore, when starters are made, the first microorganisms with the opportunity to colonize the starter are those in the flour itself.[44] This is pretty understandable because if they can feed on what's in the flour then they can feed on flour IN the starter, unless otherwise inhibited.[31]
Some of these flour microbes come from inside the grain seeds (a.k.a. endophytes) because they were there to help the plant grow, while others can come from the soil where the plant was grown, the outside environment, or colonize the flour during processing.[31,44]
Furthermore, whole wheat or other whole grain flours can be better for making a sourdough starter because they have more nutrients and because the bran and germ may contain more microbes (e.g. wild yeast and LAB) relative to all purpose or bread flour, so whole grain flour may be a preferred food source and a preferred microbe source![43] You might see a faster rise in whole grain starters because of this.[43] In my experience, whole grain starters also yield more tasty bread, and might be linked to more sour flavor as well.
Furthermore, we have the wild yeast and bacteria that colonize the starter from the air, your hands, and other places in the environment.[5]
Relative proportions of what microbes come from the environment vs. flour microbes unclear. Needs more research....
Slide 8: Yeast More:
The wild yeast vary and not all species are known, it’s possible there are hundreds globally.[31] But at least 6 species have been found in sourdough.[31] Current work supports these most known species: S. cerevisiae (primary in commercial yeast), but S. exiguus, Hanensula anomala, and Candida tropicalis have also been found in sourdough.[10]
What they do: along with enzymes in the flour, enzymes in wild yeast can break down starch and sugars. They also perform fermentation with glucose.
To reiterate, these wild yeasts vary, both within a starter and around the world. Therefore, their activity, fermentation rates, enzymes, etc. can vary. Also, the food source and environment you provide them impacts their activity.
Some people think the diversity and regional differences in sourdough contribute to completely different breads and results. See: the sourdough library in Belgium on YouTube or via google.
Wild yeast (e.g., Saccharomyces exiguus, Toluropsis holmii, Candida milleri, S. cerevisiae) are capable of alcoholic fermentation:
Glucose, Fructose, Maltose → Ethanol + CO₂
This fermentation contributes to dough leavening, flavor, and aroma
Slide 9: Bacteria:
The bacteria often found in sourdough are from the lactobacillus genera. This may sound familiar, since lacto- is associated with lactose. For example, lactobacillus are usually found in yogurt, buttermilk, cheese etc. but also in other fermented foods like sauerkraut.
The Lactobacilli bacteria genus contains over 200 species that produce lactic acid as they break down sugars, and so far, at least 60 species have been found in sourdough.[31] Lactobacillus acidophilus is a species found in sourdough, for example.
When we think of commercial yeast, we think of it being all yeast. But with sourdough, the LAB actually outnumber yeast by 100 to 1.[10] When thinking about this ratio, keep in mind though that the bacteria are smaller.
What LAB do: They digest sugars in the flours, producing lactic acid and acetic acid.[10] Sources say lactic acid gives the sourdough its mellow rich flavor and acetic acid gives its tang and 'punch'. [34]
This might blow your mind, but some LAB can do fermentation to produce CO₂, somewhat like yeast fermentation! [35]
Slide 10: Lactic acid fermentation (2 major types)
Lactobacillus bacteria community (e.g., L. acidophilus, L. casei, L. plantarum, L. delbrueckii) does this type of Homofermentative reaction:
Homofermentative means that lactic acid is the principal metabolite without production of gas (CO₂) and flavor compounds.[39]
Glucose, Fructose, Maltose → Lactic acid (>90% of total products)
This contributes to acidification and maturation of dough, sour flavor, shelf-life improvement by dropping pH, and improving texture.
2. Another lactic acid bacteria (e.g., L. sanfranciscens, L. brevins, L. fermentum) community does this type of Heterofermentative reaction:
Heterofermentative means that lactic acid is the principal end product of fermentation but technologically significant amounts of one or more of the following metabolites are also produced.[39]
Glucose, Fructose, Maltose → Lactic acid + Acetic acid + CO₂
This contributes to: dough leavening, acetic and lactic acid flavor
Temperature: enzymes necessary for LAB fermentation maintain high activity in temperature around 86°F / 30°C. [38]
Slide 11: Symbiosis, Enzymes
So, let’s start at the beginning of that enzymatic chain.
Amylase: an enzyme found in flour and wild yeast and is the start of the chain I discussed earlier, which starts producing sugars for starch to feed the yeast and the bacteria.[10]
Maltase: Current work suggests that not all wild yeast possess maltase, so they cannot breakdown maltose into glucose that can be used for fermentation! However, bacteria in sourdough may possess maltase.
Invertase: Some wild yeasts can contain invertase that allows them to break down/consume sucrose, glucose, and fructose[10] which explains why adding sugar can enhance some sourdough fermentation.
Proteins can break down into amino acids called peptides, and yeast excrete peptides.[38] These peptides can be sources of food or energy for the LAB and keep the bacteria alive even when all other nutrients, like the maltose in flour, have been depleted.[38]
And the bacteria produce glucose for itself and the yeast. In return the bacteria feed off dead yeast cells (yay!)[1]
Overall, the yeast and bacteria can provide food for each other (gif).
Slide 12: Symbiosis, defense between lactic acid bacteria and yeast in sourdough
However, despite improving the end of the enzyme chain by breaking down more sugars, the LAB also produce acid, which can inhibit the amylase enzyme, thus disrupting the start of the enzyme chain to a degree.[6]
The lactic acid also drives down the pH to acidic conditions, which can inhibit enzymes.
The yeast also produces something that makes the environment inhospitable to some: alcohol. However, similar to how the yeast can tolerate the lactic acid, the LAB can tolerate the alcohol.[10] Funny enough, if the community of yeast and bacteria is left alone too long, each community can actually produce enough alcohol or lactic acid that they can kill each other, and even themselves.[10] which is one of the reasons why it’s important to not let a starter go too long without feeding it (giving it new flour + water).
Fight together: However, the yeast can tolerate these acidic or alcoholic conditions, whereas most pathogenic, or harmful microorganisms cannot.[10]
Slide 13: Together, a symbiotic, but complicated relationship:
The wild yeast and bacteria like different parts of the flour. E.g., yeast eat a range of sugars and starches. Some prefer glucose and fructose, while others prefer maltose.[10]
Slide 14: How a Sourdough Starter is Made:
Making sourdough starter is essentially a long fermentation with continuous feedings to maintain the population and discarding some of it to prevent too many dead cells and alcohol from building up (accumulating).
Sourdough starter is usually a symbiotic relationship between yeast and bacteria, whereas commercial yeast bread contain primarily one type of yeast.
So, how is a sourdough starter made?
1. Day 1:
Combine flour (preferably whole grain flour) and water, keep in a warm place.
Combine equal parts flour and water (e.g. 30 g flour with 30 g water)
My method involves starting with only whole wheat flour and then switching to half whole what and half bread/AP flour for wheat flour, or sticking with whole grain flour.
2. Days 2-4:
Then, every 12-24 hours, discard some (usually 1/2 or 2/3rds of the original starter) and feed until it shows signs of yeast activity (fermentation bubbles, rising, alcohol smell) AND bacteria activity (sour smell)
When feeding, the flour is usually added in a 1:1:1 (1 part starter, 1 part flour, 1 part water, e.g. 30 g:30 g:30 g) ratio. Always measure in grams for consistent results.
The enzymes in the flour and microbes begin to activate, breaking down starches and proteins that act as food sources for the community. Some lactobacilli activity can will make the starter smell slightly sour, or like buttermilk. You can start to get a layer of alcohol (hooch) that builds up and you’ll see bubbles, both of which indicate yeast activity
3. Days 5-7: A stable yeast community begins to develop, and the LAB community strengthens. Each microbe seems to know it’s role and responds well and quickly to being fed fresh flour and water, leading to a consistent rise and fall as it ferments. The alcohol and/or acid smells should start to be more noticeable when you feed the starter.
My tips:
In my experience, making wheat starters in the Northern U.S. and Canada, I usually have some signs of activity on day 1 (some bubbles), and definitely signs of fermentation on day 2 and it’s rising very well, and then a strange pause in activity where the starter stops rising.
This pause in rising can occur on days 3-5, which is why when I build a starter, I’ve started feeding it a 1:2:2 ration every 12 hours starting on day 4 and making sure to keep it at 80-90°F. This REALLY boosts the activity, and a stable yeast community finally forms.
You’ll know your wheat starter is ready when it rises within a few hours of being fed and it passes the float test. It will also smell of acid and alcohol.
All these photos (in the video) are the sourdough starter 24 hours after feeding, you can see evidence of rise and fall.
Taking care of your starter: Feed it at least once every 24 hours in a 1:1:1 ratio: keeps it active and strong at room temperature. Otherwise, you can feed and refrigerate the starter: slows down and can make it less strong, but you can also bring it back to room temperature feed it to build it back up before making bread. Mine is usually in the fridge.
Slide 15: So, what actually happens after feeding?
You discard some and feed your starter. Then what happens?
Well, the yeast and LAB are both reduced in numbers, but they also given new resources to live off of and reproduce.
So, fermentation begins again for both the yeast and LAB. They keep consuming resources (starch, sugars, and proteins, for example), leading to a peak rise when the maximum amount of CO₂ is produced.
But then resources start to run out… the starter has quite a bit of acidity and alcohol. The yeast and LAB are hungry, there are less gases being produced, and the starter starts to fall.
Slide 17: How do wild yeast and LAB affect the dough?
The lactic acid makes sourdough bread more acidic than other breads, for example, a typical sourdough bread pH is between 3.8-4.6 versus the pH of 5.3-5.8 typical of commercial yeast-bread breads.[10]
However, the lactic acid an acetic acid also acts as preservatives after baking, keeping the bread safe from mold for longer than commercial yeasted doughs with sugar or milk products, for example.
Heat – influences enzyme activity
Water – improves enzyme activity and can increase the size of the holes in the bread[6], if gluten is properly developed.
More water can also enhance acidity.[10] Liquid-y starter (near equal balance of flour and water or more water than flour) makes more acetic acid, and your bread will have a more distinct sour flavor.[34] A stiff starter (higher percentage of flour to water, about 2:1) makes more lactic acid, giving your final bread a more mellow, rather sweet taste.[34]
Slide 18: Gluten in Sourdough Bread
Gluten:
The level of gluten breakdown is surprisingly important for sourdough bread. Not only for structure, but also for flavor, browning, and texture of the bread.
For a standard unenriched sourdough loaf, you want to develop the gluten slowly and gently, and the dough ferments for quite a while, long enough for that long enzyme chain to really work its magic to develop a better flavor and texture.
What you can get with long fermentation and gradual, but strong gluten development is nice open crumb structure with different sized holes (picture), and a tight surface area, that when cut, or scored, opens up beautifully while baking.
An example of this long fermentation can be seen in your starter. When I feed my wheat starter it becomes thick and stretchy because there is some gluten formation. When I check on it again 24 hours later it’s fermented so much that it’s very bubbly but almost liquid. So, it’s much less viscous and stretch than before.
However, you can ferment the dough too long, which causes 2 things, 1) too much gluten breaks down and the bread loses its structure. 2) Too much starch and sugar is consumed, and the microbes start feeding on the gluten, disrupting the bread structure.[6]
So, fermenting too long is problematic. Vermeulen et al. 2005 paper [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1266010/]:
Proteinase – enzyme that breaks down proteins into amino acids and peptides
Proteolysis – the name for the process of that protein breakdown
One misconception is that the acid from LAB breaks down gluten. Proteolytic activity provides substrates for microbial activity, e.g., the LAB and yeast, which can then break them down into smaller protein components. The most active proteinases (enzymes that break down proteins like gluten) are mostly found in the wild yeast and flour, and are less common in the LAB.
So, the LAB are less responsible for protein breakdown than once thought, the wild yeast may play a bigger role. However, not all wild yeast can perform proteolysis. Proteolysis and amino acid metabolism contribute to the beneficial effects of sourdough fermentation on bread quality. The optimum pH of these proteinases of is below pH 4 (like in sourdough!). However, this proteolytic activity occurs at low pH despite the presence of lactobacilli bacteria. So overall, these long fermentation cycles can lead to prolonged gluten breakdown, thus creating an array of amino acids, peptides, and undisturbed gluten.
Starch
More starch breakdown = more flavor extracted from the flour, especially during longer fermentation. Also, more sugar is released which contributes to fermentation and the Maillard reaction. Another contributor to the Maillard reaction are the amino acids produced by proteases during long fermentation.
Furthermore, more starch may be broken down with long fermentation sourdough. If that is the case, there is still starch hydration and gelatinization that occur during baking, but the relative proportion of remaining starch for these gelatinization and staling processes is different than a fast or quick bread.
So, the long fermentation leads to a mish-mash of alcohol, glucose, maltose, and intact starches.
Sourdough Staling:
The starches still go through retrogradation, i.e. staling, but the acids help keep the bread from tasting as stale. Some sources state that sourdough, and more specifically the acids, slow down retrogradation, thus slowing down staling.[40] Furthermore, a study showed that the biologically sourced sourness provided by sourdough can slow staling relative to breads made with commercial yeast only.[41]
Amylase enriched flour: Rye flours and freshly milled flours tend to contain more amylase.[6] This definitely impacts the sourdough, and may be why I prefer to use rye flour starters for my sourdough, even if I don't 100% understand why.
Slide 19: Better for digestion?
Always consult genuine medical research and a doctor before taking health advice from a website/YouTube video!
First off, a lot of a components in wheat grain are hard to digest. And the very process of fermentation helps break down starch, making it more digestible,[36] so whether you ferment with commercial yeast or baker’s yeast you are getting a better nutritional yield from the bread. However, a lot of sources argue that sourdough fermentation yields more nutritious bread.[36] Oftentimes, commercially produced bread is fortified with vitamins and minerals to make up for the lack of nutrients.[36]
The long fermentation can break down more gluten, which can help make the bread more digestible, even for those with minor gluten sensitivities. [33]
The long fermentation can also break down more sugars, making the bread easier on blood sugar levels.
The acidification and fermentation can also release more minerals and vitamins from the flour that were otherwise inaccessible. [33]
Phytic acid is an anti-nutrient present in seeds (grains, beans, nuts, and seeds) that binds with minerals in our digestive tract, blocks mineral absorption, and leads to mineral deficiencies. Sourdough fermentation of grains with high levels of phytase (enzyme that breaks down phytic acid) may break down phytic acid and make the nutrients more accessible. [42]
However, this doesn’t just mean you should start consuming an insane amount of sourdough bread, just that it’s possible that sourdough bread a little healthier for you than other types of bread. Again, I am not a medical doctor, check with them.
The bread making process: Sourdough (not in video)
Remember what I discussed in videos 1 and 2? Well gluten development, salt, hydration, starch hydration, enzymatic activity, fermentation, fermentation times and temperatures, starch gelatinization, starch retrogradation, and the Maillard reaction are still relevant for sourdough bread!
*Not everyone makes sourdough bread in the methods outlined below
The starter:
For a standard loaf you’ll oftentimes need to feed your starter ahead of time according to the recipe. And there are so many different names for what this can be called. I won’t go over all of them. Recipes can call for mature starter (starter at its peak rise) or a poolish, which can be the same thing, but sometimes poolishes are made with commercial yeast.
Other recipes call this a pre-dough or levain which can be used to describe a particular starter mixture with a specific hydration or ingredients (e.g., a sweet levain contains sugar; a stiff levain has less water and more flour to control acidity) or discard starter which is starter that’s hungry and needs to be fed (usually the most sour).
Autolyse (optional):
Mix flour and most of the water (with or without the starter) together and let rest a few hours. This is done before adding the starter in some cases and before adding the rest of the water and the salt. The mixing of the flour and water helps develop gluten via hydration and physical movement.
This rest period, called autolyse, allows the flour to absorb the maximum amount of water. Enzymes that break down starch into simple sugars are activated and gluten begins to form, as well as relax.[36]
Mixing the starter with more flour and water activates the microbes and enzymes in both the flour and the dough!
The amount of water
Look up information on how hydration effects dough... (note to self)
Add salt:
I think the purpose for adding the salt later is to allow the enzymes to activate beforehand, and to allow the gluten to form naturally during autolyze beforehnd. Then when the salt is added it helps align the gluten and help it become more cohesive. Salt also helps resist over-fermentation because it can slow down yeast and LAB activity.
Bulk fermentation (several hours):
Let the dough rest for several hours, but every 30, 60, or 120 minutes give the bread a stretch and fold to develop gluten and redistribute enzymes and resources while it ferments. You gently stretch and fold because usually sourdough bread is quite hydrated, and the gluten is delicately developed in order to contain the fermentation bubbles and get a wonderful texture.
By the end of bulk fermentation, the dough should be puffier and active, but not 100% fermented because we can’t bake yet! Also, if you let it ferment too long or make it ferment too quickly the yeast and bacteria run out of resources and can start consuming gluten!
Pre-shape and shape:
Often the dough is pre-shaped, rested, and then shaped. I like to think of the pre-shape as a little training session. You form the dough into a loose ball, so it gets ready to form into a tighter shape later.
Then you rest it.
And shape it so it has a nice tight surface it can expand into during the final rise and while baking.
Overnight rise/proofing:
Typically, you place your dough upside down into a floured banneton, cover it, and let it rise overnight in the refrigerator to significantly slow down fermentation but increase flavor via fermentation and additional lactic acid bacteria activity.
While it’s in the fridge the yeast are less active, but the LAB are more active, creating enhanced sourness!
Baking:
Preheat the oven.
Take the bread out when it passes the poke test and flip it right side up, score it, and bake!
Sourdough loaves are typically baked at a much higher temperature than a typical quick yeasted bread. Usually between 450-550°F for a home oven or even hotter for a wood fired oven.
Steam is crucial for a good standard sourdough loaf with a great crust. It makes the inside soft and the outside rise well (get a good oven spring) and generate a super crispy crust. Some people add steam to the oven, while others rely on the steam generated from the dough while it’s baking. For the latter, a Dutch oven can really help.
Usually, steam is important for the first 20-30 minutes of baking to get a good rise as the crust begins forming. After this time the Dutch oven is usually opened, or oven opened to release steam, and the bread is exposed and allowed to properly brown.
After baking:
These loaves are hot, and are, of course, going through the retrogradation process. So let the bread cool to room temperature before cutting into.
Remember that the acids in this bread help preserve it!
Also, interestingly, sourdough bread can actually taste more sour the longer it rests. For example, sometimes when I dig into a loaf older than 3 or 4 days the sourness really stands out.
LAB can slow down the staling process.[37]
References: