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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!

Sourdough for Science: Part 1

February 6, 2019

By special guest blogger Erin McKenney, MS, PhD

Science of Sourdough: Bacteria, flour and yeast combine to produce acid, CO2 and aromas when baked into bread.

Download the large version of this infographic. Photos: Lauren Nichols. Design: Neil McCoy.

Hello, world! I’m a microbial ecologist working in Rob Dunn’s lab at NC State University, where I’m studying the bacteria and yeast that live in sourdough starters. This spring I’m coordinating a scaled-down version of the Sourdough for Science lesson plan at Moore Square, Ligon, and Exploris middle schools in Raleigh. Over 200 students at these three schools will grow their own sourdough starters to compare the growth rate, gas formation, acidity, and smells associated with six different types of flour[1]. The students’ measurements will help us to link concepts of microbial metabolism to bread characteristics.

Project overview

Students will watch introductory videos and read pop science text to learn about the processes and ingredients used in bread making. (You can see all of our materials[2] in a shared Google folder online.) After completing background reading, the students will create, maintain, and measure their starters for 10 days[3]. At the end of the two weeks, students will analyze their datasets and present their results.

In addition to engaging students in authentic scientific research, we have partnered with Boulted Bread, a local bakery in Raleigh that is dedicated to producing artisanal breads and pastries from locally sourced grain that is milled into flour on site. Boulted Bread not only milled the flour that students will feed to their starters, but they have also agreed to bake bread from the 6 best[4] starters grown at each school, for a taste test. During the taste test, students will evaluate the textures, flavors, and aromas of each bread, and relate these bread attributes back to their experimental results.

I will be presenting the results of this project at the Asheville Bread Festival on April 13 in a workshop for educators or anyone else who wants to learn how to do sourdough citizen science. We also plan to publish the students’ data! In the meantime, I’ll write a few guest blog posts so you can follow our progress and results.


  • January 2831: gather, distribute materials
    • Erin will purchase jars, markers, rulers, measuring spoons and distilled water.
    • Boulted Bread will mill and bag flour in two-pound bags.
  • February 1: Erin will drop off materials at each school.
  • February 415: Students will grow starters and collect data.
  • February 15: Erin will pick up supplies from each school to be cleaned and stored at the Museum.
  • February 1819: Boulted Bread will bake loaves for student taste tests.
  • February 18March 1: Students will analyze data and create presentation videos.
  • March (TBD): Erin and Joshua will film a virtual tour of Boulted Bread.
  • April 13: Erin will present results at the Asheville Bread Festival.


  • NC Museum of Natural Science — Erin McKenney, PhD
  • Boulted Bread — Joshua Bellamy, Sam Kirkpatrick
  • Moore Square — Kristen Kemp
  • Ligon — April Shoemaker
  • Exploris — Jessie Francese, Shannon Hardy

[1] We are using emmer, einkorn, Red Turkey wheat, rye, millet, and all-purpose flours. This combination will enable us to investigate how different grains and nutrients impact patterns of microbial growth and succession.

[2] All activities have been developed to support required curriculum standards (i.e. reading comprehension, math, chemistry, microbiology, and ecology). If you see additional alignment opportunities, please share your ideas!

[3] This project will span two school weeks. All starters will be refrigerated over the weekend to “pause” microbial growth.

[4] The “best” starters grown from each type of flour will be identified as those that grow the most, produce the most gas bubbles, and smell the most appealing.

Sourdough for Science: Part 2

JC Yellowstone Trip Recap and Final Report

June 28, 2018

What happened when 13 Junior Curators and 3 amazing leaders from NCMNS spent 8 days in Yellowstone National Park? Well, it was a wild, whirlwind week of wonder. The trip was so incredibly jam-packed with amazing moments that, after a few days of reflection, a recap is in order.

Day 1, spent in Lamar Valley, set us off on a unbelievable start. Upon arriving in the park that afternoon, we almost immediately spotted a black bear with three cubs, a grizzly bear, and numerous elk and pronghorn.

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Black bear cub climbing a lodgepole pine tree

This bountiful wildlife viewing continued into Day 2, when following a fascinating few hours spent with wolf biologist Kira Cassidy, we were able to see two wild wolves from the Lamar Canyon pack feeding on a baby pronghorn carcass. We also saw two peregrine falcons feeding their chicks, and on the same cliffside a family of bighorn sheep were frolicking. Adding to the already numerous species count was a pika on a talus slope, as well as a boreal chorus frog.

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Pika with a mouthful of grass – making a “haystack” that will help it stay active through the winter

Day 3, our final full day in Lamar Valley, was spent with wildlife photographer Dan Hartman, who shared wisdom about being a nature photographer and led us on a beautiful hike where we saw a bull bison and a ruffed grouse.

Bull bison

Bull bison

At Trout Lake later that day, we witnessed cutthroat trout spawning and soaked in some of the first full sun of the trip. We also managed to hunt down a cow moose along Soda Butte Creek later that day!

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Cutthroat trout at Trout Lake

Sun at Trout Lake

Sun at Trout Lake

On Day 4, we journeyed into the main caldera of the park, stopping along the way for an obligatory moment of sledding in the snow that remained in the high altitudes.

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Noah and Carson decided to stick their heads in the snow…

We learned about the explosive, volcanic geologic history of Yellowstone, which is the reason it has so many unique thermal features. We also continued to view the huge variety of wildlife, including pelicans, a coyote, and more elk with calves, as well as more cutthroat trout jumping in river rapids–a definite highlight for our fish nerds!

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White pelicans have a 9-foot wingspan!

We stopped by some of those famous thermal features, including amazing Dragon’s Mouth Spring, and saw a large group of bison by the boardwalk (as well as some tourists who were questionably close to the herd). Day 5’s excitement began near Yellowstone Lake at the Lake Hotel, when an elusive pine marten ran across its front porch! As we traveled towards Old Faithful, we viewed several more thermal features, including Black Sand Pool, Daisy Geyser, and, of course, Old Faithful itself. Viewing Grand Prismatic Spring, the third largest hot spring in the world, from the overlook was a beautiful end to the day.

Daisy Geyser

Daisy Geyser

On Day 6, we began by watching Beehive Geyser erupt, an explosion that turned out to be a fan favorite, and we also had an up-close look at a coyote. After that, we ventured out of Yellowstone to visit Grand Teton National Park for the day. The sun finally blessed us with its presence that afternoon and the weather made for a beautiful mountain hike to several lakes.

Jcs taking in the scenery at Taggert Lake in the Tetons

Jcs taking in the scenery at Taggert Lake in the Tetons

Day 7, our final full day in Yellowstone, was punctuated by viewing many more amazing thermal features in the caldera before returning to the Lamar Valley area to hike around Mammoth Hot Springs with Ranger Matt.

Ranger Matt helped us sample the pH of the water at Mammoth Terraces - it was slightly acidic.

Ranger Matt helped us sample the pH of the water at Mammoth Terraces – it was slightly acidic with a pH around 6.

That night after dinner, some of us went out on one last evening hike and spent a quiet moment listening to elk bugle and birds chirp, just appreciating and remembering all the beauty that we had witnessed over the past week. At our group meeting earlier that afternoon (which featured a guest appearance by black bears in the hills), many of us had described our experience in Yellowstone with words such as “rejuvenating” and “indescribable,” and lying out in the sagebrush that evening watching the dusky pink sky behind the mountains demonstrated that unbelievable, calm beauty perfectly.

But the adventure was not quite over, because on the morning of Day 8 before we departed, we were lucky enough to watch two more wolves crossing the road in front of us, and just a few minutes later a group of five wolves from the Junction Butte pack traveling over hillsides. It seemed the park was giving us one last show before we left.

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Our final Yellowstone sunrise was stunning

A wolf from the Lamar Canyon pack

A wolf from the Lamar Canyon pack that crossed the road in front of us

But soon our time in the park was indeed up and we had to head for the airport. We arrived back home in Raleigh at 1 AM, exhausted and a bit jet lagged, and thoroughly amazed by all that we had seen over the past week. We all agreed that that Yellowstone is an astounding, wild, unforgettable place and we were so fortunate to have spent our time there in the company of each other and our knowledgeable, fun, and all-around marvelous leaders, Melissa, Erin, and Kurt. Many of us hope to return someday, and with any luck any future trips will be just as spectacular as this one was!


Roumi found what might have been a grouse egg on one hike


Olivia at the Grand Canyon of the Yellowstone

Noah and Robbie realize just how strong elk are to carry around such large antlers

Noah and Robbie realize just how strong elk are to carry around such large antlers


The Jcs decide to try to reassemble a bison skeleton

Another moment of reflection when we visited the overlook for Grand Prismatic

A moment of reflection when we visited the overlook for Grand Prismatic

by Olivia Slack

Junior Curator Yellowstone Trip Day 7

June 21, 2018

On our final full day in Yellowstone, we explored the last stretches of the boardwalks around Old Faithful. We saw many beautiful and colorful hot springs and geysers, including Anemone Geyser and Castle Geyser.

JCs watching Anemone Geyser

JCs watching Anemone Geyser


Castle Geyser

After that, we departed for Mammoth Hot Springs. We met up with Ranger Matt Ohlen, who specializes in teaching visitors about the history of the park. He shared a ton of stories about the springs; one fascinating story he told was about Ole Anderson, who made souvenirs in Mammoth Hot Springs in the late 1800s. He dipped horseshoes in the springs, where they were coated with travertine, the mineral deposited by the springs which gives the landscape its distinctive bright white color. We explored the mounds and featured hot springs. We learned the difference between mud pots, hot springs, thermal vents, and geysers thanks to a helpful visual demonstration from Ranger Matt.


Mammoth Terraces

Once we headed back from the springs, we got back to our cabins, had a reflection meeting during which we spotted two black bears, three pronghorns, countless ground squirrels, and nest building swallows. The highlight of the day was yet to come, after dinner: ICE CREAM!!!!!!


Our last sunset

Junior Curator Yellowstone Trip Day 6

June 20, 2018

Today we spent the majority of our time in Grand Teton National Park, but before we left Yellowstone for the day we went on a “wild geyser chase” to follow up our “wild moose chase.” We made it just in time to witness Beehive Geyser erupt, with a maximum height of 200 feet.


Beehive Geyser eruption

As we were running to the geyser, we took a quick detour to get an amazing close view of a coyote.

Coyote in Upper Geyser Basin

Coyote in Upper Geyser Basin

After that we piled in the vans to head to the Tetons. The change in the scenery was stunning, with jagged, snow-capped mountaintops that started out shrouded in clouds, but by afternoon were glowing in some of the fullest sunlight we’ve had on the trip. We went for an almost 5-mile hike and saw two mountain lakes, cascades, and wildflowers. As we walked, we made a treacherous journey through the “land of mosquitoes,” but the views were totally worth it!


Hiking to Taggert Lake


Amazing view of the Tetons at Taggert Lake