How to make a starter more sour

Making a sourdough starter more sour is such a well-versed topic that I could have written a newspaper with the amount of recommendations I came across when preparing this article. Some of these theories work, but many aren’t supported by science. So I’ve whittled the good ideas down so I can share with you what makes sourdough sour. In this post I’ll show you how to tweak the flavour of a starter to make less, or more sour bread.

If you don’t know much about how sourdough fermentation works, let’s cover the basics first.

A sourdough starter is a culture of wild strains of yeast and Lactic Acid Bacteria (LAB) to produce several chemical reactions. Fed by fresh flour and water as the starter gets refreshed, the yeast and acid bacteria produce CO2 gas which makes bread rise.

The combination of acids provide many other benefits whilst also determining the flavour of the starter. Before we look at how these can be manipulated to make a starter more sour, here’s the short answer:

To make sourdough more sour, choose organic white bread flour with 20% rye. Keep the starter warm and practice refreshing it after it’s reached peak height for about an hour (just before it collapses). When making bread, use a lot of starter in the recipe and let it rise for as long as possible.

But if you want to understand the topic in detail (which I suggest you do), let’s continue.

What makes sourdough sour? – The components

The sour flavour of sourdough bread comes from lactic and acetic acids. There are several types of lactic and acetic acids alongside wild yeasts strain that can be in a starter.

The collection of organic acids and yeasts create the flavour and aroma of a sourdough starter. There are many variables that change the type and quantities of them. This makes every starter unique! To change the flavour of a starter, we need to change the bacteria and the types of yeast contained.

What does lactic acid smell and taste like?

Lactic acid has mature sour flavour with a creamy linger. Lactic Acid Bacteria (LAB) produces lactic acids which provide gas and flour maturation.

Lactobacillus bacteria produces most of the gas in sourdough bread. Gas production through yeast fermentation is less potent when making sourdough.

Is acetic acid vinegar?

Acetic acid is the sharper vinegary twang that attacks the taste buds. Vinegar is simply diluted acetic acid. The famous sourdoughs of San Francisco taste heavily of acetic acid.

How is acetic acid made in bread?

Acetic acid is produced when ethanol (created through yeast fermentation) is oxidised with acetic acid bacteria. Certain species of Lactobacillus also produce Acetic Acid Bacteria alongside LAB and CO2 during fermentation.

The blend of these acids form the flavour and depth of acidity in the finished bread.

How much of the starters flavour is down to flour?

Of course, the flour will have a big impact on the flavour. It’s the biggest ingredient and the most variable. It impacts the flavour in several ways:

The natural aroma of the flour

The type of grain used and its growing environment or “terroir” enable mills to produce unique flavours and baking qualities in their flours. A bread will take on the flavours and smells of the raw flour, but the flavour of the bread is not simple.

Flavours that develop in fermentation

Flour develops into many flavours through the fermentation process of sourdough. These flavours won’t be like the raw flour. Instead, the flour provides the starting blocks for chemical reactions in which they are produced.

Esters and Aldehydes

When particular acids and alcohol combine they can produce Esters and Aldehydes. Esters are derived manually for perfumes and food flavourings but in this case, they appear without intent. The fragrance of an ester is unrelated to the existing smell of the ingredients but defined by the variety of acids that develop from the flour.

If the wrong esters develop in high quantities they can produce unusual smells such as bananas or pineapple in a starter.

Acetone

An acetone smell is produced where there are excessive acetic acids produced. These form an acetone, or paint-stripper smell which often appears in unripe starters.

The smell of yeast

It’s less pronounced in sourdough bread, yet, the smell of yeast is still apparent in sourdough bread. The smell of yeast adds a beautiful depth of aroma and with hundreds of strains available, provides a unique “nose” to a starter.

Alcohol

And finally ethanol appears alongside carbon dioxide as the yeast undergoes fermentation. Ethanol, like the organic acids, produces mature flavours and dough conditioning factors. The smell of alcohol is distinctive and common in sourdough bread.

Sourdough bread

The amount of bacteria in flour

Not only is the bacteria responsible for the fermented bacteria that is cultured in a starter. The amount of bacteria can also increase the acidity it has. Flours that are high in minerals and bacteria provide more matter to consume so produce a more powerful starter with more acidity.

Organic flour contains more bacteria due to the lighter cleaning products used in its production.

Whole Grains which contain bran, allow LAB to produce more acid in a ripe culture.

Rye flour has a high concentration of complex sugars. These are slower to break down which increases acetic acid to make the starter more powerful, and more sour.

Bread flour, though not as complex as the others, has a higher ash content than all-purpose-flour. This, again, provides more minerals and bacteria for organic acids to develop.

Does a mature starter taste better than a new one?

Over time, a starter will become more and more complex. This is due to it becoming more efficient in tackling the fresh bacteria. When fed, a mature starter breaks down more particles of the flour effectively than a fresh one would. Through culturing enzymes, lactobacillus behaviour is increase to provide a more complex depth of flavour and more rising activity.

After 6 months, a starter pretty much reaches its peak. I have found little evidence to prove that it can improve further after this point. A 5 year old starter is not going to be much better than a 1 year old, in fact it will be about the same.

Regardless, I’m proud to say I’ve had mine since 2016!

How to make a sourdough starter more acidic

We’ve covered how acids create the sour tastes in bread. Now I’m going to explain how acids can be tweaked to produce interesting flavours in sourdough bread. First, let’s look at the ways we can increase the acid levels in a starter:

#1 Ripening the starter to perfection

There are a lot of factors that make a starter sour. Out of all of them, timing refreshments and using it at the right time are the most critical.

Once the starter hits the summit of its rise you may notice that it stays at the top for a while (maybe an hour or two).

Acid growth in a starter over time

At this point the LAB microflora continues to increase in population which inhibits the yeast cells ability to multiply. So whilst the population of the cultivated yeasts stays roughly the same, the LAB increases. This provides more acid activity in the starter , which will make it taste sour.

To develop more sour notes from a starter, get into the habit of allowing it to sit at its peak for about an hour before feeding. Do this for 3-4 feeds and when you’re ready to bake, remove the piece for the recipe at the same point.

Should I allow my starter to collapse before feeding it?

I’m not saying to wait for the starter to collapse before using it. Doing this routinely will increase the acidic aroma, but weakens the levain. Don’t worry if this is just a once off, your starter will be strong enough to cope, just don’t do it every time.

#2 Use the perfect flour

Choosing a flour that enjoys long rises and has plenty of bacteria for the starter to cultivate is essential for an active and sour starter.

The ideal flour combination for a starter is 80% organic white bread flour with 20% organic dark rye. If you want to make a wholemeal starter you can use 100% wholemeal, or even a split of 50% wholemeal, 40% white bread flour and 10% dark rye.

Try to use organic flour for sourdough starters as they contain more food for the starter, but it’s not essential.

#3 Use temperature as an ingredient

As you are probably aware, by keeping a sourdough starter you are essentially cultivating bacteria. Certain bacterias prefer to operate at different temperatures. Acetic acids prefer warmer temperatures, near 32 C (90F). Lactic acids develop strongest when cooler.

There is more to consider regarding temperature. Such as how temperature affects:

  • The activity of the yeast fermentation
  • The rate at which amylase and other enzymes break down sugars into monosaccharides (simple sugars that provide food for the yeasts and LAB)

But let’s try to not get too complex:

More acetic acid = 32C (90F)

More lactic acid = 25C (77F)

#4 Changing hydration makes sourdough more or less sour

Acetic acid can occur through the oxidation of ethanol. The longer dough ferments, the more acetic acid in the starter.

So what’s this got to do with dough hydration? Well, doughs ferment faster when viscosity allows activity to flow freely. So by using less water in a recipe, we slow down activity, thus lengthening the proofing time. This will increase the amount of acetic acid in the starter.

By making a starter runnier, we increase the amount of lactic bacteria.

So a balance between temperature and hydration is important for flavour.

Here’s a diagram which shares how it works:

Diagram showing the affects of temperature and hydration on the flavour of sourdough

Making the bread – what makes sourdough bread sour?

Let’s say we’ve got a nice and sour starter and we are ready to make bread. To maximise the sourness we can use similar methods that we used in the starter.

Here are a few common tweaks that make bread more sour:

Use the same flour

The flour provides food, so, using high quality bread flour means the dough can rise for longer to become more sour (as LAB increases).

It’s best to use the same flour in the dough that’s in the starter. The starter is already competent in consuming the bacteria the flour contains. Using the same flour in the bread that’s in the starter makes fermentation more effective. It’s not essential, but every little helps!

Starter amount. Should I add less starter for a more sour taste?

No, to increase the sour flavours we need to develop the amount of acids in it. Salt inhibits the growth of the bacteria so we need to increase the amount of acid and acid producers by using more levain.

Usually 25-40% of starter is used in a sourdough bread recipe. To lower the sourness, lower the amount of starter to 10-15%.

  • I know many people say otherwise, but I’ve done a lot of research into this and this is the right way!

A higher proof makes sourdough bread more sour

The more a dough develops, flavour will be imparted through organic acid activity. We don’t want a long rise if we want bread to be sour (but, we’ll come on to this in a minute).

Push the rise during bulk fermentation and proofing to as long as possible to let the acids develop.

The downside of longer proofing time for sourdough

Longer proofs make sweeter sourdough bread. Complex starches deteriorate into simple sugars over this period. This makes the bread taste sweeter but is a problem for sour bread.

As sweetness overpowers the bitterness, the bread won’t be as sour.

Using the fridge to increase the sourness of a sourdough

Lactic Acid Bacteria increases when a starter sits at its peak. We can use the same methodology to extract more sour flavours as our dough rises.

Put sourdough bread in the fridge halfway through its final proof. This slows down the rise and increases the amount of time that it stays at its peak to produce more bacteria and a stronger acidic taste.

I’ve not tried this, (I will though!) but it makes sense when you consider San Francisco sourdough. In San Francisco it can be incredibly warm for proofing bread. To get some control over proofing times, slowing things down so the bread isn’t ready at the same time is handy! To do this, the dough can be retarded (slowed down) in the fridge before baking the following day.

The fridge final rise provides the key features of San Francisco sourdough. It’s world-renowned blisters and mighty sour, yet sweet flavour are due to the local bacterias and fridge proofing.

It’s not essential to use the fridge, but it can give the bread an “extra sour’ that perhaps you crave!

Why can’t I get the sourness I want from my sourdough bread?

The properties of a sourdough starter are governed by the microorganisms in the local environment or the ingredients used to make it. If you haven’t got the same environment or ingredients as someone else, you are not going to be able to recreate the same flavour.

We can use the techniques shown above to tweak the flavour to the best of our ability and our surroundings.

A little experiment…

There are occasions where you are culturing the wrong sort of bacteria in your starter. If this is the case I recommend you divide your starter into 4 clean containers and refresh each one a different way.

Use a different flour in one, temperature in another, hydration in the next one and change the height or size of the feeds in the final one. After a week you should see some progress in at least one of them going the way you want it, if not comment below.

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