Kitchen Science for Kids: Sourdough Starter

Reader Contribution by Anne Marie Bonneau

While I wholeheartedly believe kids need to play outside in nature, they can also learn about entire ecosystems right in their kitchens, using only basic ingredients. This lesson covers sourdough starter. If you and your student want to later bake bread, you can find the recipe here.

Mature sourdough starter. Photo by Anne Marie Bonneau. 


Bacterium (plural bacteria): Microscopic, single-celled organism that lives in soil and water and on plants, animals and other matter. Its purpose in life is to reproduce.

Ecosystem: All the living things in a given area. All things that live in an ecosystem are dependent on one another.

Microbes: Tiny living things that can be seen only through a microscope, such as bacteria and yeast.

Symbiosis: A mutually beneficial and dependent relationship between two groups.

Yeast: Another class of microscopic, single-celled organisms which, like bacteria, live everywhere.

Lesson Plan

Time requirement

The starter should be ready for baking within a couple of weeks, with very little active work done to it.


  • Kitchen scale (optional)
  • Thermometer (optional)
  • Two glass measuring cups or small glass or ceramic bowls
  • Measuring spoons
  • Fork
  • Small breathable cotton or linen cloth
  • Flour
  • Water

What Is a Sourdough Starter?

A sourdough starter contains living bacteria and yeast that transform flour and water into a leavening agent. Filled with gas bubbles, a leavening agent makes bread dough rise during baking. A sourdough starter also adds flavor and aroma to bread. It most commonly consists of only flour and water. This living thing needs regular feeding to keep it alive. When well cared for, sourdough starters can live for hundreds of years. The starter that King Arthur Flour sells dates to the late 1700s.

Sourdough bread fresh from the oven. Photo by Anne Marie Bonneau.


About 6,000 years ago in Egypt, someone baked the first loaf of bread. They likely noticed a neglected mixture of flour and water that had sprung to life, bubbling away in a corner somewhere. That accidental starter would have made the first loaf rise. For thousands of years, all bread was made with a sourdough starter.

In 1857, Louis Pasteur first identified yeast under his microscope. Not long after his discovery, around 1880, industry developed commercial yeast, which contains only one strain of bacterium, Saccharomyes cerevisiae. Commercial yeast produces consistent loaves of bread quickly, which meant bakeries could bake more loaves, more quickly, with fewer workers, resulting in higher profits.

However, bread made the old-fashioned way has never completely disappeared. People bake sourdough bread all over the world. San Francisco’s sourdough bread is so famous that scientists named the main bacterium found in sourdough Lactobacillus sanfrancisensis. They later discovered that this bacterium lives in sourdough bread cultures around the world, but no one has ever found this bacterium anywhere else on the planet except for in a sourdough culture.


1. In a glass or ceramic container, mix 50ml warm water (about 110 degrees) with 50 grams flour (approximately 1/3 cup if you don’t have a scale). Your mixture will resemble thick pancake batter. Cover your container securely with a breathable cloth. Leave your container out on the kitchen counter.

2. Stir your starter several times a day. Depending on your kitchen environment, the starter should start to bubble within 3 to 7 days. At this point, begin to feed it daily.

3. To feed your starter, in a separate container, mix together another 50ml warm water with 50 grams flour. Stir in 1 to 1-1/2 tablespoons of the bubbling starter after you have stirred it down. This new mixture is your new starter. The previous one is discard. TIP: Store your discard in the refrigerator until ready to use for making crackers, pancakes or waffles.

4. Continue to feed the starter daily. After 5 days of feedings, it should begin to double in size after feeding and fall back to its original size. This process will take several hours. It will also smell yeasty, slightly fruity and slightly acidic. The starter is now ready to leaven bread dough.

5. At this point, either leave the starter out on the counter and feed it daily or store it in the refrigerator and take it out once a week to feed it.

The Science Behind Sourdough

Yeast and bacteria are present in the air, in the flour and on your hands. You may want to use your clean hands to mix the ingredients to inject them with more microbes. As the microbes begin to reproduce in the starter, the bread-friendly ones take over and crowd out any unfriendly ones.

Lactobacilli bacteria convert sugars in the flour to lactic acid and acetic acid. These acids give sourdough its distinctive sour flavor. Acid-tolerant yeasts thrive in the starter also. They convert sugar into carbon dioxide and ethanol. The carbon dioxide creates bubbles that make the bread rise. Each microbe eats different sugars and so they do not compete with each another for food.

The principal yeast in the starter is Candida milleri. This acid-tolerant yeast doesn’t consume maltose, a sugar in flour starch. Lactobacillus sanfranciscensis, the bacteria found only in sourdough cultures, can’t survive without the maltose they eat. Of course, microbes do die eventually, but not before they produce their replacements. When yeasts die, they break down into compounds that the lactobacilli eat. This mutually beneficial relationship between two organisms is called symbiosis. (If only all relationships went this well…)


Cooked: A Natural History of Transformation, Michael Pollan

The Biology of Sourdough,” Discover

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