Flavour & Fire
The Sourdough Journal
Starter Science
A Small Ecosystem in Your Jar
A sourdough starter seems almost too simple to be interesting: flour, water and time. Yet as soon as those three come together, a small living ecosystem begins to develop. Several groups of microorganisms work together inside your starter, with wild yeasts and lactic acid bacteria playing the leading roles.
Wild yeasts feed on the available sugars and produce carbon dioxide (CO₂) as they do so. That gas forms the bubbles you see in your starter and is ultimately what helps your bread rise. Lactic acid bacteria feed on sugars too, but mainly produce organic acids, including lactic acid and acetic acid.
Together, these microorganisms shape the character of your starter: how it smells, how it tastes, how sour it becomes and how strongly it can rise. Their balance is not fixed, either. It is constantly influenced by things such as temperature, flour, hydration and your feeding routine.
Where Do They Come From?
You don’t need to add yeast or bacteria to create a sourdough starter. They are already present in the flour and in the environment around us. Once flour and water are mixed together, these microorganisms have moisture and nutrients to work with, and fermentation begins.
This also explains why every starter develops slightly differently. The flour you use, the conditions in your kitchen and the microorganisms already present there all contribute to its own little community. Two starters made on the same day, using the same recipe, can still behave quite differently.
You can think of a starter as something that adapts to its surroundings. It does not follow a fixed programme that produces exactly the same result every day. Instead, it constantly responds to what you give it and to the conditions in which it lives.
From Flour to Food
For the microorganisms in your starter, flour is not simply a source of ready-made sugar. Most of its carbohydrates are made up of starch — long chains of sugar molecules. Naturally occurring enzymes in the grain help break that starch down into smaller sugars that the microorganisms can actually use.
This is one reason different flours behave differently in a starter. Wholegrain and rye flour contain more of the grain itself and therefore bring along more enzymes, minerals and nutrients. A young starter can often respond particularly well to this richer environment.
White flour has been more extensively refined and contains less of those additional components. That does not mean you cannot build a strong starter with it — you absolutely can. It simply helps explain why adding a little wholegrain or rye flour can sometimes give a young or sluggish starter the push it needs.
Why the First Days Can Be So Strange
A new starter is not yet a stable sourdough culture. Many different microorganisms may become active at first, producing bubbles, unusual smells and sometimes even a surprisingly fast rise. A starter that nearly doubles on day three, therefore, is not necessarily strong or mature yet.
As time passes, the environment inside the jar changes. Fermentation gradually makes the starter more acidic. This increasingly favours the microorganisms that are well suited to a stable sourdough culture, while making life more difficult for others.
That is why a young starter can suddenly become much less active after an impressive first burst of bubbles. It does not necessarily mean something has gone wrong. The culture is developing and its balance is changing. Time and regular feeding are what allow that initial chaos to gradually settle into a more stable community.
From Flour to Food
For the microorganisms in your starter, flour is not simply a source of ready-made sugar. Most of its carbohydrates are made up of starch — long chains of sugar molecules. Naturally occurring enzymes in the grain help break that starch down into smaller sugars that the microorganisms can actually use.
This is one reason different flours behave differently in a starter. Wholegrain and rye flour contain more of the grain itself and therefore bring along more enzymes, minerals and nutrients. A young starter can often respond particularly well to this richer environment.
White flour has been more extensively refined and contains less of those additional components. That does not mean you cannot build a strong starter with it — you absolutely can. It simply helps explain why adding a little wholegrain or rye flour can sometimes give a young or sluggish starter the push it needs.
Invisible yeast and quiet bacteria, doing all the actual work.