Sourdough for Science
This project can be completed at home or in the classroom.
About this lesson
Humans have baked bread for over 10,000 years. All over the world, different cultures bake their own unique breads – and have for centuries. Yet we know almost nothing about the microbes that truly make a traditional sourdough bread. In this project, you can create your own sourdough starter from scratch, just by mixing flour and water. Over the course of 10 days, you can take a series of simple measurements to track the growth of your own “microbial zoo”. Your measurements will help solve the mystery of bread, by quantifying how different flours contribute to the growth of each “microbial zoo” – and how those microbes contribute to the taste and texture of bread.
NGSS Middle School Standards
Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms.
Evaluate competing design solutions for maintaining biodiversity and ecosystem services.
- Students design the sourdough ecosystem with different types of flour to see which types has a greater diversity of microbes. Students can also develop their own experiment by changing materials, or the amount of materials used, and can track their data to see growth.
Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.
- Students can draw conclusions about how the changes they made to their design affected the populations of microorganisms by using the starter’s growth as the empirical evidence.
See additional possible curriculum alignment at bottom of page.
NEW AND IMPROVED! (Small-scale, half-pint version)
Original (larger, quart-size version)
How to participate
Add 1 cup of rye flour to a wide-mouth glass jar. Label: “Rye Control.” Add 1 cup of rye flour to another jar. Label: “Rye.” Repeat for each flour type you wish to test (make sure to clean measuring cups well in between use.)
Add 1 cup of dechlorinated water to each jar. Mix each with whisk or spoon until well combined. (Make sure to clean mixing utensil well in between flour types.) Consistency should be that of toothpaste or apple sauce.
Scoop a small amount of your starter into a spoon, and touch one side of a strip of pH paper to the starter. (This allows the starter to soak into the pH paper, but keeps the other side of the paper clean and easy to read.)
To measure the pH, match the color of the paper to the color key on the package.
Mark the total height of the flour and water mixture on the side of each jar with the date.
Measure the height of the flour and water mixture in the jar. Measure the radius (half the diameter) of the jar. Record on data sheet.
Draw a 1x1 cm square on the side of your jar, and count the number of bubbles in the square. Record on data sheet.
Smell your starter, and record a description on the data sheet (i.e. biscuity, fruity, floral, musty, rotten eggs, sour, no smell, etc.)
Place napkin or cloth cover on jars; secure with lid ring or rubber band. Place control jar(s) in freezer. Place all other jars in a warm location, out of direct sunlight. Record temperature of the area and your geographic address. After 24 hours, measure and record height of each sourdough starter. Count the number of bubbles in the 1x1 cm square. Also record any additional observations: for example, is there a layer of liquid on top of the starter? Remove the cover and record aroma. Then mix starter and measure pH (before feeding).
Remove ½ cup of the starter. This can be discarded, composted, or used in a sourdough recipe. Add ½ cup dechlorinated water and ½ cup of the same flour that was used in that jar, mixing well. This is the ‘feeding’ step of making a sourdough starter.
After mixing, record new total height and pH. Make a new mark (with date) for the new height. Cover. Return to location where the jars were stored previously. Repeat measurements and feeding every 24 hours for ten days.
Photos by Lea Shell
Flours (Rye, whole wheat, all-purpose, etc.). Half-pint jar requires 1 cup for a 10-day experiment. Quart jar requires 12 cups for a 10-day experiment. This amount will feed 1 experimental sourdough starter + 1 control.
- Dechlorinated water (could be filtered or tap water that has been left in a clear bottle overnight). Amount varies per day (see directions).
Non-reactive jars of the same size (we recommend either half-pint or quart sized wide mouth mason jars). Minimum one per flour type, plus one extra for the control (freezer) sample. Here is one option you can buy on Amazon.
- Cloth or paper napkins to use as jar covers.
- Sharpie or tape to mark the jars.
- pH paper 0.0-6.0 and 5.5-8.0: (any pH paper that will detect from 3.5-8.0 at the accuracy of at least one decimal place). You can find these on Amazon here.
Measuring spoon or cups for liquid and solids.
- Whisk or long spoon for mixing in the jars.
About the research
The Dunn lab wants to learn the microbial mysteries behind bread. We have already collected over 500 sourdough starters from participants in 17 countries, as part of the global citizen science Sourdough Project.
By growing the bacteria and yeasts that live in each starter, and sequencing their DNA, we have learned a lot about how where you live, and what you feed your starter, affects the microbes that live in your starter.
Here’s where you come in: now that we know the “global trends”, we want you to grow your own starter from a specific type of flour and measure its growth and acidity over time. Your data will help us to figure out how specific flours affect the microbial growth and flavor of sourdough.
Opportunities for Extension
Bake bread: Use Lea Shell’s Sourdough Recipe, and follow the steps in our New Year, New Bread activity.Compare the microbial community of a sourdough starter with just the starting microbial community in flour and water by making bread with the rye starter and the rye starter that was kept in the freezer. Note how the dough rises, or doesn’t rise. Note flavor and aroma differences.
Compare flour types: Test the effect of flour type on sourdough starter activity. Follow the procedure, but using extra jars each with a different flour type (whole wheat, spelt, millet, buckwheat, amaranth, etc.) Fill out a data sheet for each flour.
Data Visualization: As data comes in, you can plot different variables to compare dynamics over time. (We are eventually hoping to develop an interactive online plot in the style of Wunderground; but for now, Erin has doodled an example below.)
Calculations: You can calculate the change in volume of the starter, not just the change in height. Use this formula:
Volume =pi*(radius of jar)^2*(height of total starter)
On what day did the volume start to double?
Experimental optimization: Help us determine how small a sourdough starter can be to still form: Using the ratios we have provided, trying making mini and micro starters (¼ c. flour? 1 tablespoon flour?) Compare your results with the protocol we have provided. What size makes the fastest sourdough starter, as measured by the fewest days to get a starter that is doubling in size? You can modify the container to be appropriate for smaller sizes- perhaps small plastic water cups. Email us your findings!
About the scientists
Dr. Erin McKenney is a microbial ecologist in the Department of Applied Ecology at North Carolina State University. Learn more on her website.
Dr. Anne Madden is a microbiologist in the Department of Applied Ecology at North Carolina State University.
Lea Shell, M.Ed. is a Digital Learning Specialist for the North Carolina Museum of Natural Sciences.
Other possible curriculum alignment
NC Essential Standards
Food provides molecules that serve as fuel and building material for all organisms.
Photosynthesis and cellular respiration are complementary processes. Plants carry on photosynthesis and cellular respiration where food is broken down into energy. The requirements of one process are the products of the other…….
Understand the processes, structures and functions of living organisms that enable them to survive, reproduce and carry out the basic functions of life.
Within cells, many of the basic functions of organisms—such as extracting energy from food, getting rid of waste, movement and secreting waste—are carried out. The way in which cells function is similar in all living organisms. Even the simplest organisms have parts which enable them to move, take in food, to reproduce and to detect the environment they are in.
Understand the properties of matter and change that occur when matter interacts in an open and closed container.
Clarifying Objective 8.P.1.3
Compare physical changes such as size, shape and state to chemical changes that are the result of a chemical reaction to include changes in temperature, color, formation of a gas or precipitate.
Understand how biotechnology is used to affect living organisms.
Clarifying Objective 8.L.2.1
Know: Biotechnology has created benefits and concerns in the areas of medicine, agriculture, genetics, and
Understand: The microbial world has led to the emerging field of biotechnology which has given us many advances and new careers in medicine, agriculture, genetics, and food science.