The Science Behind: creating a sourdough starter


If you’ve eaten bread at a sandwich shop before, you know that sourdough is a tasty bread option.  You might even know that sourdough is made with an active starter.  But ‘sourdough’ bread is not where sourdough stops.

In this edition of “The Science Behind,” we’ll discuss:

  1. What is sourdough starter and how does it differ from regular yeast?
  2. Why is lactobacilli so important?
  3. What can I use it for?
  4. How do I acquire a sourdough starter?

I hope you’ll learn not to be scared of sourdough starter – it’s a tool for bakers, not a burden or hindrance!   And it will transform your bread, I promise.

  1. First off, what is a sourdough starter?

Wait – back up.  Actual first off: what is commercial yeast?  Yeast that you buy in the grocery store is a strain of yeast called Saccharomyces Cerevisiae that’s been dried up and broken into little pieces.  By adding water and flour, you wake it up and give it a food source.  Yeast like to eat the sugars that are created when enzymes break down the starches in the flour.


As the yeast eat the sugars, they release carbon dioxide and ethyl alcohol.  The carbon dioxide bubbles are trapped by the gluten net and rise your dough.  The ethyl alcohol provides a little acid that ferments your dough and gives it a nice, complex flavor.

Now, what makes sourdough starter different from commercial yeast?

Commercial yeast is a single strain of yeast. That’s it. Sourdough starter, in contrast, is a combination of a few different strains of wild yeast as well as a few different strains of lactobacilli, an acid-loving bacteria.  Both feed on different sugars from flour. As the yeast strains eat the sugars they like, they release ethyl alcohol.  As the bacteria consume the sugars they like, they release lactic acid.  This means the dough is slightly more acidic than it would be if it relied solely on commercial yeast.  This acid gives sourdough breads their stereotypical tang, although I should note that not all sourdough starters create tangy bread.

What contributes to the sourdough taste?

Different wild yeast and lactobacilli strains are responsible for the different flavors you might coax out of your breads using a particular starter.  Different locations have different strains of wild yeast in the air.

This post from the Encyclopedia Brittanica website explains: “The yeast-bacteria combination that emerges to dominate the starter is largely a function of temperature and nutrient availability. In the case of yeast, the ability to tolerate an acidic environment is especially important.  Continuous yeast and bacterial growth between 20 and 30 °C, in which wild yeasts populate the starter, is often dominated by L. sanfranciscensis bacteria. Other sourdoughs, particularly those made using industrial processes, involve the addition of baker’s yeast (Saccharomyces cerevisiae) and are typically maintained at temperatures above 30 °C. Although L. sanfranciscensis sometimes grows in this type of sourdough, other strains frequently emerge.”

What this means is that, if you want a more acidic starter, you need to keep your starter in a warmish environment, about 68-86 degrees F.  For a more milder taste, keep it at a warmer environment, above 86 degrees but no warmer than 140 degrees (after which yeast die).


How frequently you feed your starter also determines if your starter will have a stronger or more mild taste, since the acidity build up over time as the yeast eat more and more of the sugars in the flour.  But more on that in another post.

2. Why is lactobacilli so important? 

As I’ve said before, sourdough starter is made up of wild strains of yeast and lactobacilli bacteria.  Each have a type of sugar that they like to feast on, and both produce an acidic waste product that contributes to a more acidic PH level in the dough. Enzymes (amylases) in the flour help break down the starches present in flour into sugars to make them more available to the yeast and bacteria.  But if too much starch is broken down into sugars, it can impact the structure of your dough, making it too gummy.


Enzymes slow down if their environment is too acidic.  By creating a more acidic environment, the wild yeasts and lactobacilli regulate enzyme activity and slow it down so all the starch isn’t broken up.  This is particularly important in breads made with flour that is high in amylase enzymes, like whole wheat and rye.

  • By eating more different types of available sugars, sourdough starters create a bread with a lower glycemic index than breads made with commercial yeasts.
  • By creating a more acidic environment, they regulate enzyme activity in the bread.
  • The acidic environment produces benefits after baking as well: the higher acidity retards the growth of mold and other bacteria on your finished loaf.

3. What can I use sourdough starter for? 

Sourdough starter is great in almost any baked goods, whether to give them a little tang or for leavening purposes.

For tang: waffles, pancakes, quick breads, muffins, tortillas!  In breads that use baking soda, the sourdough starter replaces the acid in the buttermilk.

The Perfect Loaf

King Arthur Flour

Cultures for health

For leavening: you can use starter in any bread that calls for yeast, although if you have a tangy starter you might not want to use it in sweet or rich doughs where the sour taste will be off-putting.  Since starters take longer than commercial yeasts to leaven dough, you also want to make sure to add it to a bread that can take longer to rise than most typical enriched doughs with eggs and milk.

Where sourdough starter really shines is in rye breads, which have high amylase enzyme activity and low gluten.  The starter regulates the amylase enzymes and vastly improves the bread’s texture.  The transformation is truly amazing.


this bread is 50% rye, 50% AP flour.  It doesn’t even look like rye bread, it’s so light and airy thanks to the sourdough starter.

4. How do I acquire a sourdough starter?

The easiest way to get a sourdough starter of your own is the find someone who has an active starter already and ask them to give you some.  They are unlikely to refuse, since starters must be dumped out or used up and refreshed frequently.

The next easiest way is to make it yourself.  You could just make a slurry of water and flour, about 75-85% hydration, and leave it uncovered overnight.  Don’t make it too big- I’m talking 75g of flour and 60g of water each feeding.  Dump half of it out and feed it twice a day until you start seeing activity.  Some sourdough aficionados even use apple or pineapple juice to get things going more quickly, providing a food source but increasing the acidity to ward off bad bacteria.8504b-bread2betc-2b002

Don’t forget to continue discarding most of your starter.  This is super, super important!b1283-dscn4055

Creating a starter can take 1-2 weeks before you see enough activity to rise a loaf of dough.  If you don’t see any activity after two weeks, throw it out and start over.493ec-starters

Once your starter is lively, you can begin baking with it.  My favorite basic sourdough recipe is my 1-2-3 sourdough:

1-2-3 Sourdough

125g of active sourdough starter

250g of water

375g of flour

7-10g salt

Get it?  It’s called 1-2-3 bread because it’s 1 part (by weight) starter, 2 parts water, and 3 parts flour.

  1. Mix up all the ingredients and let rise overnight or for 12 hours.  Depending on how warm your kitchen is, this might take more or less time.  In the heat of summer, this bread has finished rising in five hours.
  2. Punch it down and shape it into a boule.  Place in a couche seam-side up or on a piece of parchment seam-side down for about an hour while you preheat the oven to 500 degrees F with your dutch oven inside.
  3. Bake with the dutch oven lid on for 15 minutes at 500 degrees F, then knock it down to 375 degrees F for the second 15 minutes, then remove the lid and continue baking a third 15 minutes at 375 degrees F.
  4. Remove your finished loaf and let cool completely, or at least 30 minutes, before slicing.

Now that you’ve got active starter, check out how to maintain it here.

3 thoughts on “The Science Behind: creating a sourdough starter

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