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Sourdough for Science, Part 7

March 4, 2019

By special guest blogger Erin McKenney, MS, PhD

Reflections on the Sourdough for Science project

Immediately after the taste test (more about that in the next post!), I flew to Chicago to present my citizen science efforts at BakingTech 2019. It was an exciting trip: I learned a lot from industry bakers, and they were excited to hear what we’re learning about sourdough with the help of citizen scientists. Since returning to Raleigh, I’ve been reflecting on the past month of middle school magnificence, as I read through the students’ and teachers’ evaluations. Partnerships between scientists and teachers can be a difficult dance. Teachers want to enrich their curriculum with meaningful activities that empower their students; but schedules aren’t always flexible, and many teachers can’t afford the time to teach a new activity on the fly.

I’ve been incredibly lucky to work with so many adventurous partners: from the teachers and students who literally make the science come alive, to Joshua’s dedication to supplying us with the best materials for the project, I couldn’t be prouder of what we’ve produced together. I have also enjoyed documenting both the science and the process of collaboration. If anyone out there is feeling inspired, I will be posting some suggestions for developing partnerships between scientists and teachers early next week.

Student and Teacher Quotes

“I have worked with a lot of community partners…but truly this experience is one of the tightest I have ever experienced. I really appreciate the structure, progression, vision, and science you bring to the work.

“A parent asked what sourdough had to do with the curriculum …. I was very capable of articulating the connections.”

— Shannon Hardy, 8th grade teacher at Exploris Middle School

“This was a really interesting project, and it was really fun and educating to learn about different microbes and how they turn bread into sourdough bread. I am very glad that I learned this because before I was familiar with sourdough bread and I had eaten it before, but I had no idea that microbes were involved with it being made. During this project, I learned how to work well with a group by using craftsmanship and I learned about the history of different breads and the science behind bread.”

— Avery Riordan

Sourdough for Science: Part 6: Taste Test

February 25, 2019

After I took the starters to Boulted Bread on Sunday, Joshua Bellamy (co-owner of the bakery) fed them extra flour to grow them up overnight. Early Monday morning he built the doughs, mixing each starter with the same blend of flours so that the only differences in flavor would “arise” (haha) from the microbes unique to each starter.

Bread dough in containers

Joshua combined each starter with the same blend of flours, water and salt to bake bread for the middle school students to taste. Photo: Joshua Bellamy, Boulted Bread.

Joshua let the doughs rise at room temperature for four hours, then wheeled them into the walk-in cooler to continue to rise overnight.

The “bulk ferment” at a warmer room temperature helps the microbes to grow quickly and especially favors the bacteria that create acid, giving sourdough its characteristic tangy flavor. The cooler temperature of the “overnight proof” in the walk-in cooler favors yeast, which produce more complex aromas and flavors.


Joshua came back on Tuesday at 4am to bake the bread and give the loaves time to cool. I got to Boulted Bread at 6:45am, and we started slicing. I immediately noticed that the all-purpose and millet starters produced loaves that were flatter than the others. The others showed a more normal rise, though the Red Turkey wheat had a soft, spongy crust. The other breads were sturdy and tough to cut through; but, by comparison, the Red Turkey wheat crust felt almost fragile … I was careful not to crush the loaf while I sliced.

Erin and Joshua slicing bread.

Erin McKenney (left) and Joshua Bellamy (right) slice each loaf of bread into 26 pieces so that middle school students can compare the flavors created by the microbes in six different sourdough starters. Photo: Kevin Jefferies.

At 7:30am I started deliveries while Joshua continued to slice bread. Between drop-offs to middle schools, I came back and wielded a knife once more. We finished slicing at 9am, and by 10am I had finished the final delivery and returned all the transport bins to Boulted Bread.

All that work, and I hadn’t actually saved any bread for myself! Luckily for me, April Shoemaker had leftovers at Ligon middle school, and she shared a few slices of each with me so that I could indulge in my own taste test that night.

For each bread, I first unzipped the plastic bag and inhaled deeply through my nose a few times. After I identified the aromas specific to each bread, I reached in and took a slice. I inspected the air bubbles and felt the texture of the crumb. Finally, I took a few bites — of the crust and the inner part of the loaf — and chewed slowly, exhaling through my nose after each swallow to savor every scent and flavor.

All purpose

  • smells roasty
  • flat loaf, medium-hard crust
  • medium bubbles, close crumb, moist and tacky
  • tastes very mild, faint tang


  • smells tangy, a little like beans
  • flat loaf
  • tastes… creamy, like the flavor of grits


  • toasty, a little like spent grain
  • medium bubbles, some large
  • mild grain flavor, tiny tang at the end, almost a roast-coffee flavor in the crust


  • sour and funky, like a farmhouse ale
  • small bubbles, close crumb, tacky
  • bright, almost effervescent, lemony acid


  • grass, grain, and a hint of peanut
  • medium and large bubbles, spongey inside with a hard crust
  • grainy and grassy flavor

Red Turkey wheat

  • roasty, with a hint of hops
  • small/medium bubbles, a little tacky
  • spongey loaf, with a soft/flexible crust
  • coffee, a little sour like yogurt

These are just one person’s opinions, of course. I’m really excited to see what the students thought — of the bread, and the entire starter experience.

Sourdough for Science: Part 5: The Secret Lives of Starters

February 21, 2019

On Friday, having collected the supplies and selected the six best sourdough starters from each middle school, I returned home to sort through the backslop.

What is backslop?

Before you can feed a sourdough starter, you need to “backslop,” or remove a portion of the mature starter, making space for you to add more flour and water. (A mature sourdough starter needs to be fed proportions of flour and water that are equal to the amount of starter. If you didn’t backslop, you would have to increase the amount of flour and water exponentially with each feeding!)  Backslopping also removes a portion of the microbial community, so that the microbes in the starter can continue to grow and divide without running out of nutrients. Backslop can be used to bake bread, or any other sourdough recipe: I add it to waffles and zucchini-nut bread, and I even know people who bake it into crackers for their chickens. If you’re not in the mood to bake, extra sourdough also makes an active compost additive.

Backslop photo by Erin McKenney

Backslop. Photo: Erin McKenney.


After 10 days, the microbial communities in each starter had grown enough to require two feedings a day. In the second week, students had noticed more “hooch” in the mornings, a sure sign that their microbes were going hungry after 24 hours. The good news: this means they had grown thriving microbial communities that could soon be baked into bread. In fact, the communities grew so quickly that I had to feed every starter twice a day (every 12 hours) to make sure the microbes in each starter had enough nutrients to continue to thrive.

It took me two hours to feed the 18 best starters, and another four hours to empty the remaining jars (almost 80 of them). Four gallons of composted backslop later, I had a sinkful of jars soaking in hot water to slowly dissolve two weeks’ worth of flour cement.

Saturday morning, I fed the starters again, and began to notice some interesting differences. (Maintaining 18 starters over a weekend provides comparative insights that you just don’t get from the long-term care of a single starter.) It turns out that the starters fed different flours not only rose to different heights, but did so at different rates. Millet shot up first, within two hours of being fed; but, because there was no “gluten balloon” to capture the carbon dioxide, the bubbles escaped, and the starter fell as quickly as it had risen. Rye was next; and, because that grain contains gluten, its rise was sustained. Emmer and Red Turkey wheat followed, and then einkorn – slow but steady, with bubbles forming a foam on top instead of boosting it up from within. The all-purpose starter dragged in last, more like a jar of thick glue than an active starter, compared to the others. I guess it’s hard to compete with the fresh-ground nutrition packed into a whole-grain flour.

Sourdough for Science: Part 4: How to pick the best sourdough starters

February 21, 2019

By special guest blogger Erin McKenney, MS, PhD

On Friday, February 15, the students entered their 10th day of data, gathered their supplies, and voted to select the best starter grown from each type of flour. The six best starters from each school would be baked into loaves at Boulted Bread for a taste test on Tuesday. But how to pick the best starters?

We first identified which starters had the lowest pH (3.0–3.5, instead of 4.0) and created the most carbon dioxide (as measured by the number of bubbles per cm2 and the total height that the starter grew). These traits indicate the microbial functions you would expect in a mature sourdough starter: bacteria create lactic acid or acetic acid, which lower the pH; and yeasts create carbon dioxide, which forms bubbles and makes the starter rise. The starters with the lowest pH and the most carbon dioxide are therefore most likely to have reached a climax community.

If two or more starters were “tied” for pH and carbon dioxide, we next looked for consistency over time (which, again, indicates a stable climax community) and also compared the smells of each starter. Smell has been an important food factor for humans for millennia: microbes that make us sick tend to create smells that we don’t find appetizing. No one wants to eat something that smells rotten, even if we don’t know which microbes might be making that smell.

Now that the best starters have been chosen, I’ll take care of them over the weekend. Boulted Bread will mix them into bread dough on Monday, and bake loaves for the students to taste test Tuesday morning!


The box full of starters. Photo: Erin McKenney.

Sourdough for Science: Student Q&A

February 13, 2019

Questions from Exploris, Ligon and Moore Square middle school students, fielded by Erin McKenney, MS, PhD

Q: Does sourdough smell as bad when baking in the oven as it does as a dough?

A: Sourdough in the oven tends to smell like bread baking, and even mature sourdough starters tend to smell more like bread than what I think you’re smelling now. I think this is because your sourdough starter’s community is still forming. When it reaches the climax community, the starter will probably smell more like something you want to eat.

Q: If there is some alcohol in the dough, can a person become intoxicated from consuming it?

A: Nope. Remember, you bake bread before you eat it – and the heat bakes off all of the alcohol.

Q: Can we see (maybe a recording) Boulted Bread bake our starters?

A: I’m working on a script for a “virtual tour” of Boulted Bread. I’m not sure if we’ll have a video ready in time, but I can ask the bakers if they would take pictures.

Q: Are there any other microorganisms in our starters besides the yeast and bacteria?

A: It’s possible, but we haven’t used DNA sequencing or microscopy to look.

Q: If microbe reproduction slows down in the refrigerator, then why do people get sick in the winter?

A: There are a lot of differences between the microbes that live in sourdough starters and the microbes that make us sick; but here are a couple of reasons.

  • In the winter, humans like to stay warm, so they tend to gather inside together. This means people are more likely to share germs with each other.
  • Microbial reproduction slows down at low temperatures but remember that human bodies are warm — and the microbes that make people sick are only able to do that when they are growing in our bodies.

Q: Can you (scientists) determine the dominant bacteria or yeast in the climax community from the pH or aroma?

A: We know that lactic acid bacteria (LAB) lower the pH of sourdough starter by creating acetic acid and lactic acid, and yeasts are responsible for the aromas. But we don’t yet know enough to be able to identify which bacteria or yeasts are present from the pH or smell of a starter. That is actually part of our ongoing research, and the data you are collecting may help us to answer that question!

Q: Are fungi antibacterial, or is that just mold? Like penicillin?  

A: Mold and yeast are actually both types of fungus, and many molds and yeasts are able to produce antibacterial compounds.

Q: Is the relationship between the yeast and the bacteria commensalistic, mutualistic, or are they competition?

A: Different types of yeasts and bacteria can have different types of relationships. For the global Sourdough Project, we are measuring which different bacteria and yeasts occur in the same starters, to infer what type of relationship they might have. Two microorganisms with positive co-occurrence tend to be found in high abundance together, and we think this means that they work together in a mutualistic relationship. Other microorganisms might compete with each other for nutrients, resulting in negative co-occurrence: the “winner” would be successful and common, while the “loser” would be rare because it was outcompeted. So far, it looks like several species of Lactobacillus bacteria have positive co-occurrence. These bacteria may “get along well” because they are similar: they all make acid and thrive in low-pH habitats.

Yeasts, on the other hand, don’t play so well with others: they tend to have negative relationships with other yeasts, as well as many types of bacteria, possibly because they are competing for the same nutrients. Sacchromyces cerevisiae, the same species of yeast that is used in commercial bread baking, is an exception: it has a positive relationship with a few types of Lactobacillus bacteria.

Q: In a climax community is there one dominant microorganism yeast or bacteria, or are they equal and it is one of each?

A: The climax communities in mature sourdough starters tend to have one or two types of bacteria and yeast that are dominant.

Q: At what temperature does the yeast and bacteria die? HOT?  Does cold ever kill bacteria or yeast or are they just dormant?

A: Most bacteria and yeasts are killed by heating to 160F (71C), especially when the pH is low. Yeasts and bacteria can remain dormant in the refrigerator, but most will die if they are frozen unless we use a special storage technique.

Q: What is the rate of reproduction? 

A: When conditions are optimal (when there’s a lot of food and the temperature is just right), bacteria can divide in as little as 20 to 30 minutes. Yeast take a little longer (once every 1-2 hours). But that is when conditions are perfect; we don’t know the exact growth rates for sourdough starters.

That’s it for now — we’ll add more as more questions come in!

Sourdough for Science: Part 3

February 13, 2019

By special guest blogger Erin McKenney, MS, PhD

It’s week two of the Sourdough for Science project! Last year, Exploris students took their “sourdough babies” home to feed over the weekend. This year, to maintain environmental control, we voted for all three schools to store their starters in the refrigerator. This tactic enabled us to “pause” the colonization process, because bacteria and yeast go dormant at cold temperatures.

(Left) The sourdough starters made with rye were bubbly and rose high in their jars, thanks to plenty of nutrients in the flour and gluten proteins that trap the carbon dioxide like a balloon. (Right) Sourdough starters made with millet are runny, and formed a liquid layer over the weekend that smelled like alcohol. This liquid layer is a sign that the microbes inside it are hungry. (Photos: Kristen Kemp/Moore Square Middle School.)

(Left) The sourdough starters made with rye were bubbly and rose high in their jars, thanks to plenty of nutrients in the flour and gluten proteins that trap the carbon dioxide like a balloon. (Right) Sourdough starters made with millet are runny, and formed a liquid layer over the weekend that smelled like alcohol. This liquid layer is a sign that the microbes inside it are hungry. (Photos: Kristen Kemp/Moore Square Middle School.)

On Monday morning, students reported that their starters were very smelly. Most of the starters also had a liquid layer on top of the starter. (Bakers call this liquid layer “hooch” because it contains alcohol. Sourdough microbes only produce alcohol when they run out of food, so this is a sign that the starters are hungry.)

The millet and all-purpose starters so far have had the thinnest consistencies, while the rye and emmer starters have had the thickest.

The viscosity of each starter may depend, in part, on the different nutrients in each type of flour. The gluten in wheat, rye, and emmer flours forms a protein network that helps trap air bubbles and holds the starter together. Millet is gluten-free, so starters made with this grain will lack the protein structure. All-purpose flour is very high in starch, which microbes break down easily, so that highly digested starter tends to be soupy, too.


Want to learn more about sourdough citizen science, and how you can set up your own experiment? I will be presenting the results of our study at the Asheville Bread Festival! Register for my workshop on April 13, 2019.

Sourdough for Science: Part 2

February 8, 2019

By special guest blogger Erin McKenney, MS, PhD

This rye starter was active that the overflow soaked and burst through its paper towel cover. (Photo: April Shoemaker, Ligon middle school.)

This rye starter was so active that the overflow soaked and burst through its paper towel cover. (Photo: April Shoemaker, Ligon middle school.)

On day three of the experiment, students at each school arrived to discover that some of their sourdough starters had overflowed their jars! Hyper-activity is common in the early days of sourdough starter growth: the flour-and-water mixture does very little the first two days, and then on the third day it explodes… only to settle back down for a few more days. This is one of many “typical behaviors” that is common across sourdough starters, but that no one has formally studied before.

Personally, I think it is a natural part of colonization (also known as succession, the ecological process by which a community grows and changes over time).

About Succession

When communities grow “from scratch”, different species join the community and grow in predictable patterns known as succession. Pioneer species are the first “colonizers” to arrive. They usually grow very quickly (“like weeds”) and take over the ecosystem for a short period of time. But in the process they consume readily available resources and change the environment to favor slower-growing, more permanent community members.

Example: Field to Forest
Exhibit: From Forest to Field and Back Again

In the “Mountains to the Sea” exhibit, this panel shows how communities of plants and animals become re-established after a disturbance in a predictable order. Photo: Karen Swain/NCMNS.

In the plant world, weeds eventually give way to shrubs, pine trees, and finally a hardwood forest. This forest is called the climax community.

Sourdough Succession

In sourdough starters, the flour provides plenty of food (i.e. sugar and starch) for the microbes but it takes a couple of days for bacteria and yeast to colonize the mixture and really start to grow. (The “explosion” of sourdough on the third day happens because that’s how long it takes for the first “microbial weeds” to grow.) In the baking world you hear about the overflow phenomenon most often with rye flour.

In fact, bakers recommend creating a sourdough starter with rye because it “jump starts” the growth of the starter.

Why would this happen?

I think this is because rye contains more starch than wheat. It’s like a sugar rush that makes the microbes hyperactive.

So it wasn’t a big surprise when teachers reported on day three that their rye starters had overflowed. The bigger surprise came today: now the millet starters have exploded! This suggests that succession occurs at different rates, depending on what type of flour (and nutrients in that flour) you feed to the starter.

Even more interesting is that all-purpose starters aren’t doing much at any of the schools… unless, of course, they’re just gearing up to overflow tomorrow… Only time will tell!