Making sourdough taste more sour is such a well-versed topic that I could have written a newspaper with the number of recommendations I’ve seen. Some of these theories work, but many of these theories aren’t supported by science, so I’ve whittled the good ideas down, so you’ll know what makes sourdough sour and why.
Wherever you are in the world, the ingredients you have and the local environment play a part in what makes sourdough taste sour (or not). But can you make an extra tangy sourdough loaf outside of California? You bet!
A sourdough starter is a culture of wild yeast and acid bacteria strains captured from the local environment or contained within the flour. When combined with moist flour, the yeast and acid bacteria ferment to produce CO2 gas (alongside other compounds) to make bread rise. There are several fermentation routes that occur in sourdough, and thousands of species and combinations of yeast and bacteria can be found in a starter. These collections create unique behaviours, tastes and smells in your sourdough.
Lactic Acid Bacteria (LAB) and wild yeasts multiply in the starter as it matures. They will ferment to produce carbon dioxide and make the starter rise. After regular feedings, a mature starter has been prepared. Once ripe, the acid bacteria and cultured yeasts work together to supply enzymes that break down starch and sugars into simpler hexose sugars. The simple sugars will be fermented by the LAB or yeast cells, where they produce:
These products provide flavour and aroma, but it’s the fermentation routes that produce acetic and lactic acid that makes bread taste sour.
In a moment, I’ll be sharing with you several tips to make your sourdough bread taste more sour. All of them (just about) focus on producing more organic acids (acetic and lactic). It should be noted that acetic acid is produced by heterofermentative LAB fermenting. This fermentation route can also produce carbon dioxide, albeit at a slower rate than in yeast fermentation. A brand new starter becomes acidic and alcoholic after a couple of days. This environment inhibits yeast multiplication which means that the population of yeasts is reduced and remains contained. Because of this, organic acids will outnumber wild yeasts in a mature starter by around 100:1. Therefore, organic acid bacterias (LAB) produce roughly the same amount of CO2 as wild yeasts.
If we deliberately increase the number of acid bacteria in the starter, we speed up the rate at which the bread rises.
Many flavours are produced in the fermentation process of sourdough. These flavours won’t resemble raw flour. Instead, the flour provides the starting blocks for chemical reactions from which they are formed.
Esters are derived commercially for perfumes and food flavourings, but in a sourdough starter they appear naturally. When certain acids and alcohol combine, they can produce esters and aldehydes. The fragrance of an ester is unrelated to the existing smell of the ingredients. If the wrong esters develop, unusual odours such as bananas or pineapple will come from a starter.
An acetone smell is produced where excessive acetic acid occurs. These form a paint-stripper smell which is common in unripe starters.
The smell of yeast adds a beautiful depth of aroma and provides a unique “nose” to your bread with hundreds of strains available. It’s less pronounced in sourdough bread, yet, its aroma is still apparent.
Ethanol appears through yeast fermentation and sometimes when acetic acid is produced. Ethanol, produces mature flavours and dough conditioning factors. The smell of alcohol is distinctive and common in sourdough bread.
Not all sourdough bread has to be sour. Plenty of bakers (and eaters!) prefer their sourdough lightly flavoured. But there are some benefits of making sourdough extra tangy:
The more acids and the more effective they are, leading to a faster rise. Gluten is slowly broken down into individual amino acids by lactic acid. This means that as acidity increases, the dough structure becomes weaker, which makes a well-fermented dough sometimes harder to handle and has a higher risk of collapsing.
The most crucial factor in making your bread sour is how to make a sourdough starter more sour. These points explain just how to do this!
The ash content of the flour is found by burning it —the more products left after burning (the ash), the higher the ash content. The main products that contribute to ash content are protein, bran and wheatgerm. A higher ratio of these products in the flour means more minerals and bacteria for a starter to process.
Flours high in minerals and bacteria provide more matter to consume, so when used to make a sourdough starter, they will take longer to rise. This produces a more robust starter that is more acidic.
Flour that has been finely sifted is whiter and will have a lower ash content than whole wheat flour. Rye flour has a high concentration of minerals, proteins and starches, such as pentosans, which are slow to be broken down.
Bread flour (though not as complex as whole grain flour), has a higher ash content than all-purpose flour, provided by the extra protein in the wheat. High-quality bread flour can rise for longer and make a sourer starter or bread.
Organic flour contains more bacteria due to the lighter cleaning products used in its production. Stoneground flour contains a higher ash content than modern roller-milled wheat. This is more noticeable when using whole wheat flour as it hasn’t been sifted.
Choosing a flour that can enjoy a long rise and has plenty of bacteria to cultivate is essential for an active sour starter. The ideal flour combination for a starter is 80% organic white bread flour with 20% organic dark rye. If you want a wholemeal starter, you can use 100% whole wheat, or even a split of 50% whole wheat, 40% white bread flour and 10% dark rye.
Once the starter hits the summit of its rise, it will stay at its peak for an hour or two, sometimes longer. At this point, the LAB microflora population continues to increase. This prevents the multiplication of yeast cells and gas production as it becomes too acidic and alcoholic. So whilst their quantity stays roughly the same, LAB continues to multiply.
There will be acid bacteria just before it collapses, so this is the perfect time to refresh it. Allow your starter to sit at its peak for an hour before feeding. Do this for 3-4 feeds, and you will quickly notice more sour notes in your starter. When you’re ready to bake, remove the piece for the recipe at the same point.
If you wait too long before feeding your starter, it will collapse. Doing this increases the acidic bacteria but does weaken the levain. If this happens as a one-off, your starter will be strong enough to cope, and some say the extra acidic bacteria it produces will enhance its sourness. It’s not recommended to do this too often as the gluten will be damaged and won’t pass as much structural maturity to the dough.
By keeping a sourdough starter, you are essentially cultivating bacteria. For your starter to be at its optimum vibrancy, it needs to be kept at a temperature where the bacteria is most active. Both yeast and bacteria are quicker to operate when they are warm, but if your starter is too warm, the enzymes necessary for breaking down starch are less active. The ideal temperature to keep your starter is between 20-35C (68-100F).
Bacteria and yeasts become more active at different temperatures. Fluctuations in temperature, therefore, upset the balance of the culture. We’ll come back to which temperature you should target in this range to achieve your desired amount of sourness. Whatever temperature you aim for, it’s best to keep it as consistent as possible to ensure your starter has the most vibrant ecosystem it can. Keeping temperature warm and consistent is a gamechanger when it comes to achieving sourdough supremacy!
A home proofer is an ideal solution to controlling the temperature of your starter. With the Brod & Taylor pictured below, you are able to set the temperature of the chamber to your choice, pop your jar of starter in, and forget it. Well, until it’s ready to feed!
Once you have developed a sourish starter, place it in the fridge for a week, the acidic starter is the perfect environment for acid bacteria to multiply. Acetic acid occurs through the oxidation of ethanol, occurring during the PKP fermentation pathway of LAB. It is also argued that if the ethanol produced during yeast fermentation is left for long enough that it can also be oxidised into acetic acid. This means that after a week (or two) of refrigeration, a starter will be more acidic!
For your starter culture to be as powerful as it can be, you want to refrain from introducing any unwanted bacteria. These badies will divert energy in the culture to be broken down to slow down leavening activity. To avoid foreign bacteria, keep your equipment, hands and utensils clean, and regularly clean your container.
Acid bacteria multiply in the presence of oxygen. To add oxygen to your starter you can give it an aerated stir when refreshing, and/or leave the lid on your jar slightly loose (don’t loosen it completely as it can dry out or attract insects).
Over time, a starter will become more and more complex as it becomes efficient at tackling the fresh bacteria. It’s not going to turn sour overnight. It will require regular and consistent treatment to change the ecosystem.
Once you’ve got your starter-smelling sourer, you’ll be itching to get baking. Here’s how to tweak your sourdough bread recipe to make sour bread:
When we covered how to increase the sourness of your starter, we covered how the choice of flour in your starter affects the sourness. Over time, the enzymes in the starter adapt to consume the flour’s bacteria. So if the same flour is used to make bread, the starter will be more efficient at breaking it down. You don’t have to do this to make sour bread, but it will improve the ability of your starter to produce more acids.
Note: Pentosans found in rye flour retain water. This makes bread moister and remains fresh for longer.
If you are specifically trying to make sour sourdough bread, refrain from using any extra ingredients. Ingredients that aren’t necessary will add flavour, which can distract from the sour taste or decrease the acidity of the dough. Once you have mastered making sour sourdough, you can, of course, indulge in whatever addition you wish, but until you do, KISS (Keep It Simple Stupid)!
Although I recommended no further additions, you could add some more acidic ingredients to your dough recipe. Citric acid is also known to bakers as sour salt. It will make your bread sour, and all you need to do is add around ⅛th of a teaspoon per loaf.
Vinegar is diluted acidic acid therefore adding it to your bread will make it sour. Vinegar also has many other benefits in bread dough.
Lemon juice also adds acidity as it contains around 8% citric acid. It can be used to make bread sour, but the flavour of lemon can be overpowering.
The more dough develops, the more flavour will be imparted through organic acid activity. Push the rise during bulk fermentation and proofing for as long as possible to let the acids develop. A great way to do this is by using the fridge.
Enzymatic reactions slow down in cold temperatures. However, dough stored in the fridge will continue to undergo hydrolysis, which is the natural breakdown of starch in water. Placing the dough in refrigeration at the start of bulk fermentation provides a plethora of simple sugars for the yeast and acid bacteria to use for fermentation later on.
If you choose to bulk ferment in the fridge, you need to be careful about adding too much oxygen to your dough. This means less kneading should be done to develop gluten. Instead, a gentle mix is sufficient and stretch and folds are used throughout the first rise.
Note: Longer proofs make sweeter sourdough bread. Complex starches deteriorate into simple sugars over this period. This can make the bread taste sweeter, which is a problem for sour bread. Sweetness can overpower bitterness, meaning the bread won’t be as sour. This is a trade-off that you will have fun experimenting with!
This is one of the best tips! As mentioned, acidity increases as a starter sits at the peak of its rise, so we can use the same methodology to extract more sour flavours in our bread.
Put your sourdough bread in the fridge when it is 50-75% proofed (halfway through its final proof) will slow down the rise, and (if your timing is right), the dough will sit at the peak of its rise (fully risen) for a couple of hours. It means acidic bacteria will multiply, and a stronger sour taste develops.
Many San Francisco bakers utilise this method, perhaps without realising! As it is incredibly warm for proofing bread they store their dough in the fridge overnight. Because the dough is warm when it goes in it rises. By the morning, it will have sat at its fully proofed height and developed extra acidity.
The fridge’s final rise provides many of the key features of San Francisco sourdough. Its world-renowned blisters and mighty sour, yet sweet flavour are due to the local environment and fridge-proofing.
As you will likely be pushing your dough to the maximum rise level in bulk fermentation, you should be confident in your shaping skills. The dough can be weak and full of gas, so you should be careful not to push too much air out, whilst also creating tension so the dough holds its shape when it rises.
As dough ferments, lactic acids weaken the gluten network. And, a weak gluten structure can lead to the dough collapsing before or during baking. High-hydration recipes require the gluten to be at peak development when it rises, so using a super-acidic starter can end in a flat frisbee.
To increase the sourness of sourdough bread, we want to add as much acid as possible. This means that around 30-50% of sour starter should be used to make sour-tasting bread. To lower the sourness, lower the amount of starter to 10-15%.
Salt inhibits the growth of the bacteria, so trying to boost acidity in bread with a less acidic starter is naturally a challenge! A sour sourdough starter is a must when making sour sourdough bread. If your starter is low in acidity, you can use an extended autolyse without salt, but you risk weakening the gluten by too much exposure with this approach.
The sour taste of sourdough bread comes from lactic and acetic acids which are produced by Lactic Acid Bacteria (LAB) and, at a minor level, Acetic Acid Bacteria (AAB). There are thousands of species of acid bacteria that can be found alongside wild yeasts in a starter. This makes every starter unique!
Lactic acid has a mature sour flavour with a creamy linger. Lactic Acid Bacteria (LAB) produce lactic acids as they ferment the sugars in the dough. During this process, they can also produce carbon dioxide, acetic acid, ethanol and energy.
Acetic acid is the sharp vinegary twang that attacks the taste buds. Vinegar is simply diluted acetic acid. The famous sourdoughs of San Francisco taste heavily of acetic acid.
To change the flavour of a starter, we need to alter the acids it cultivates. To do this, we can change the bacteria source by changing the flour or the environment it sits. But this is relatively impractical and relies on a hit-and-miss scenario. The best thing to do is to alter the balance of lactic and acetic acids by changing the temperature or the viscosity of the starter. Here’s how it works:
When LAB ferments in ideal conditions, it prefers to utilise homofermentative lactic bacteria. Heterofermentive bacteria (producing acetic and lactic acid) will continue to ferment at these temperatures, yet homofermentative bacteria ferments at a much faster rate and more lactic acid is produced. When a starter is cooler and less runny, there is less free water, and fermentation is much slower. In these conditions, homofermentative bacteria struggles, so heterofermentative lactic bacteria takes sole responsibility for fermenting the flour.
Keeping the conditions of your starter the same for several feeds will lead it to adapt to these new acid bacteria, making it have more or less acetic acids.
In warmer temperatures, enzymes will break down the preferred sugars that yeast and homofermentative LAB requires to ferment.
More lactic acid = 30-35C (86-95F)
More acetic acid = 20-25C (68-77F)
Doughs ferment faster when viscosity allows activity to flow freely. Water is a carrier between cells, so when there is less water available, fermentation is slowed, increasing the ratio of acetic acid.
Now you have the science behind a sour starter learned, I hope you are now itching to make your starter more to your taste. Be careful, don’t make too many changes at the same time. You can always separate your starter into several jars and make different changes to each to discover the impact! This way, you can have a choice of flavours to choose from when you next make a loaf! To learn more about sourdough fermentation, click the link. Share with me your success stories in the comments below, or ask any questions.
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Hi, I’m Gareth Busby, a baking coach, lecturer and bread fanatic. My goal is to help you become a better baker.
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