Ever wondered where the yeast in sourdough starter comes from? Why do people speak about "catching" wild yeast?
Sourdough can be a bit confusing and there are lots of things to consider when bringing your sourdough starter to life.
This article will explain where the yeast in sourdough starter comes from, as well as other factors that can impact the yeast, bacteria and flavor profile of your sourdough starter.
This topic is a fun research question for kids who are learning to bake sourdough.
Let's talk about the science of a sourdough starter!
This is a great post if you're interested in the history of sourdough bread.
Sources of Sourdough Starter Yeast
When you create a sourdough starter using flour and water, the yeast come from three main places:
- flour used to feed your sourdough starter
- environment you're keeping the starter in
- hands of the baker (you!)
Many people talk about "catching a wild yeast" however this is not entirely the case. You're not really catching the yeast so much as cultivating them.
Yeast cells are the micro organisms that digest the flour and water in your sourdough starter, producing carbon dioxide. It's this carbon dioxide that makes your sourdough starter rise.
So let's talk about where the yeast in your sourdough starter come from in more detail:
Yeast Comes From Flour
Majority of the yeast in your sourdough starter actually come from the flour you choose to feed it with.
In fact, given that a sourdough starter is created from adding just flour and water, the first micro organisms to colonise in your jar will actually be from the flour you use.
This is why it's so important to choose good quality, unbleached flour.
Many people agonise over which flour to start their sourdough starter with - and perhaps it is with good reason.
While most all purpose or bread flours will yield a strong sourdough starter, wholegrain flours like whole wheat and rye will often take off faster because of the increased natural yeast and micro organisms found on the flour itself.
For this reason - bleached flour should never be used to make a sourdough starter. Bleaching flour strips it it of its natural yeast and bacteria, making it devoid of nutrients.
It's these yeast and bacteria that are essential to building a sourdough starter.
Yeast From The Air & Environment
A certain amount of the yeast cultivated in your sourdough starter comes from the environment in which it is being maintained.
Yeast is in fact everywhere - it will be in the jar you used, the spoon you stirred it with and the counter top you placed it on.
These yeasts will be unique to your home, town, city and country.
It's for this reason that each sourdough starter is unique in its own way and provides a unique flavor profile.
The best example of this would be San Francisco Sourdough. This sourdough is famous around the world for it's unique sourness and complexity of flavor. This unique flavor comes from the unique lactobacillus san francisco - found only in San Francisco. But also from a unique strain of yeast called candida milleri.
So the unique sour flavor that is so sought after really is only found in San Francisco! It's a product of the flour, air and water used in that particular environment.
It's also interesting to note that if you change a sourdough starter's environment, you will also be changing the yeast and bacteria it contains. Once you move a sourdough starter to a new location (albeit town, city or country) and feed it new flour and water, the yeast it contains will change overtime to reflect it's new local environment.
It's for this reason that even when you purchase a very old starter, it really becomes your own once you reinvigorate it in a new environment.
Yeast Comes From Baker's Hands
Yes, you read that right. A certain amount of the yeast in your sourdough starter actually comes from your own hands!
It's a crazy thought but a part of you is actually living inside your sourdough starter. Maybe that's why we get so attached to these living microbiomes?
This also explains why your sourdough starter is so unique. And why it may or may not act like someone else's sourdough starter.
Many people ask why I don't list times for bulk fermentation in my recipes - and the reason is that bulk fermentation will depend on the behaviour of your unique sourdough starter. I can list the time it takes mine to ferment - but yours will not be the same! And this is part of the reason - your starter is a little bit of you!
There have actually been studies on the bidirectional exchange of yeast and bacteria from baker to starter and vice versa. You can read more about this study here - but in summary they have actually found that bakers often have higher levels of lactic acid and yeasts present on their skin.
It would seem that part of a baker's success with sourdough could actually be attributed to time. That is their hands seemingly handle the dough more easily over time, almost becoming "less sticky".
So perhaps while we are waiting for our sourdough starter to mature, we are also waiting for our "bakers hands" to evolve. It is certainly food for thought!
Do Yeast Reproduce In Sourdough Starter?
Yes, yeast reproduce in sourdough starter. Yeast are a single cell organism that reproduce in the right conditions.
When you feed your sourdough starter flour and water, enzymes are released in the flour. These enzymes convert long starch molecules into simple sugars. It's these simple sugars that feed the yeast and cause the microbes to reproduce.
This is called alcoholic fermentation because the yeast cells convert simple carbohydrates to carbon dioxide (which causes your starter to rise) and ethanol (this becomes the hooch when then yeast cells are hungry).
You want to use your sourdough starter when the yeast colony is at it's largest - or when it's at its peak.
Does Temperature Affect Sourdough Starter Yeast?
Temperature plays a part in the yeast development of your sourdough starter.
The temperature has to be warmer, rather than cooler, to foster colonisation of sourdough starter yeast and bacteria.
Ideally, you should keep your sourdough starter between 24C - 28C (75F-82F). You can read about the best ways to do this here.
Yeast will go to sleep at temperatures lower than 4C (39.2F) so you really do want to keep your starter warmer for optimal yeast development.
Yeast will produce more carbon dioxide at warmer temperatures which is why your sourdough starter (and indeed bread) will rise faster in higher temperatures.
Best Flour For Faster Sourdough Starter Yeast?
Now that we know that much of the yeast in our sourdough starter comes from the flour which we use, it's important to choose the right flour for your starter.
In most cases, so long as you choose an unbleached flour, you will have success in cultivating a sourdough starter from scratch.
Using a wholegrain flour like whole wheat or rye will often result in a starter that doubles within the first few days. This is a great start, however it doesn't mean that you will be able to bake bread any faster.
A sourdough starter takes at least 14 days to become viable. You can read more about knowing when a sourdough starter is ready to use here.
A good plan is to use a blend of all purpose flour and rye or whole wheat for the first few days to give the yeast a good start. You can then continue on with just all purpose flour for the next few weeks. Only giving it a little boost of rye if you need to.
You can read more about the best flour for sourdough here.
What is the Difference Between Sourdough Starter Yeast and Commercial Yeast?
The yeast used in commercially prepared yeast is a single strain of yeast that has been cultivated to be dependable and rise bread in a very fast manner.
Believe it or not, commercial yeast is actually "Saccharomyces cerevisiae" which is a member of the mushroom family.
These days, you can buy instant dry active yeast, active dry yeast and many other varieties of commercial yeast. They all aim to be a reliable, fast way to rise dough.
Sourdough starter yeast or wild yeast differs from commercial or baker's yeast because it is naturally occurring in the air and in the flour used to cultivate it.
It is also a variety of strains, which may take a lot longer to rise the bread than the specially selected and cultivated strain of commercial yeast.
This being said, sourdough bread is very different from bread risen with commercial yeast because it will have a more complex flavor and aroma. It's also much more easily digested due to the bacteria living in the starter.
You can read my full guide on the differences between yeast and sourdough starter here.
Frequently Asked Questions
The first micro organisms to make a home in your sourdough starter come from the flour itself. Once these have started to colonise, some yeast will also come from the air in your kitchen or whatever environment you are maintaining your starter in.
Not necessarily. While your starter does acquire some yeast from the air, it is also colonising the yeast on your flour and hands. Keeping it in a well ventilated area will be fine. There's nothing to say you will benefit from feeding the starter outside - but of course, there's also nothing saying you can't do it either!
Technically some sourdough yeast will come from the air around your starter, so you are catching some, but majority come from the flour you are using to establish your starter.
I guess you can start a sourdough starter with commercial yeast in that you could add a pinch of yeast to the flour and water when you first start it. However in doing this you are introducing commercial yeast strains which will then be competing with the natural wild yeast strains you're trying to cultivate. It could be said that over time the yeast strains would change, but you are really losing the joy of cultivating that wild yeast from scratch.
Sourdough starter is both yeast and bacteria. In fact, a sourdough starter can be referred to as a SCOBY - a symbiotic culture of yeast and bacteria. The yeast and bacteria are a little living community in your jar. They do not compete with each other for food which is what allows them to live together.
Baking any type of bread, including sourdough, can increase the likelihood of dry, sore hands. One of the biggest reasons for this can actually be washing your hands more than you normally would - particularly when you're just starting out. You'll find many solutions, including natural and DIY, for dry baker's hands here.
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