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Sourdough for Science: Part 8

May 22, 2019

By special guest blogger Erin McKenney, MS, PhD

Dear readers – sorry for my absence! Where has the time gone?

Have you ever noticed that baking techniques are like parables for life and science? Since my last post, I’ve felt like a batch of dough left to ferment in the fridge;  it takes a long time, but the end product is more complex and ultimately worth the wait. I hope you’ll agree!

Before diving into the student evaluations, I wanted to take a look at their hard-earned results. That data set has been a labor of love, let me tell you! Two sets of ninety-six data sheets each: one for the starters, and one for the bread tasting. And such an exciting story!

Smelly starters

The aromas that the students detected changed over time, together with the height and pH of their starters. Together, these data tell the story of microbial succession (Figure 1: click to enlarge).

Figure 1. Students measured the pH, height, and odor of sourdough starters to monitor the process of microbial succession. As microbial populations grow, yeasts produce more carbon dioxide, which increases the height of the starter, while bacteria produce acid, which lowers pH. Increasing acidity selects for yeasts that produce more appetizing aromas, letting us know that the starter is ready to use to make bread.

Figure 1. Students measured the pH, height, and odor of sourdough starters to monitor the process of microbial succession. As microbial populations grow, yeasts produce more carbon dioxide, which increases the height of the starter, while bacteria produce acid, which lowers pH. Increasing acidity selects for yeasts that produce more appetizing aromas, letting us know that the starter is ready to use to make bread.

  • The flour and water paste is quickly colonized by microbes. Sugar and starch are easy to digest, so the early “pioneer species” quickly multiply. Yeasts in the flour spew carbon dioxide, producing bubbles that cause an early “explosion” in height (see Part 2 of this blog series). These yeasts produce other gases, too, and not all of them smell like something you’d like to eat. In the first week, many students described their starters’ aromas as pungent bodily odors (e.g., farts or stinky feet), or something rotten… one group even described the smell as “death”!
    1. Of the nearly one trillion species of microbes estimated to live on the planet, relatively few help to make sourdough bread smell and taste delicious through the process of fermentation. Many other microbes are decomposers, and produce toxins or other compounds that could make us sick. Luckily, decomposers often also produce rotten smells. Our aversion to those smells may help to protect us from food-borne illnesses.
    2. The bacteria multiply too, and produce sour-smelling acids that lower the pH of the starter, and kill many of the yeast “pioneers.” This “die-off” due to changing environmental conditions causes the height of the starter to drop for a few days.
  • Eventually, a new community grows. These new yeasts are acid-tolerant, and produce the aromas we tend to associate with bread.

When I took a closer look at the data, the story got even more exciting! Each different type of flour appears to feed a unique microbial community, whose yeasts and bacteria behave differently, and grow and change at different rates (Figure 2: click to enlarge).

Figure 2. The power of flour: each type of flour contains a unique combination of nutrients and microbes, which cause the starters to behave differently.

Figure 2. The power of flour: each type of flour contains a unique combination of nutrients and microbes, which cause the starters to behave differently.

Figure 2. The power of flour: each type of flour contains a unique combination of nutrients and microbes, which cause the starters to behave differently.

See how rye and millet peaks rose the highest, but on different days? Those are the day 2 and 3 “explosions” that students recorded in February! It also appears that each starter grows a different “climax community”; pH decreases over time, but plateaus to a different stable point in starters fed different flours.

Taste test

The results of the taste tests were interesting, too: the students described what they smelled and tasted in unexpected, but surprisingly consistent, ways. For example, a LOT of students smelled soy sauce aromas, and many also smelled cheese. In a way this makes sense – soy sauce and cheese are also products of fermentation. In a way, the students’ descriptions indicate which fermented foods they are most familiar with (compared to some of my beer-centric descriptors in Part 6).

But the students picked up on some very specific smells particular to each flour type, too:

  • Red Turkey wheat smelled and tasted spicy.
  • Rye smelled like vanilla.
  • Emmer smelled like pepper.
  • Einkorn smelled like mayonnaise or fish food.
  • Millet smelled and tasted like corn.

Students also seem to agree that, compared to the rest, the bread baked from the all-purpose starters just tasted … bland. This, too, is an exciting, though unintended, result: that the sourdough project may have exposed students to the new and exciting flavors of whole grains.

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