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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.

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