Sourdough for Science: Student Q&A

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!