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In essence - to make a sourdough starter:

  1. Put flour and water in a jar, and leave it in a warm place overnight
  2. Divide the remaining mixture in half, throw out half, add half as much flour and water again.
  3. Repeat for 7 days.

I don't see why you couldn't just add all the ingredients required at the start, open it once a day to let more yeast drop in and give it a stir, and keep in a warm place every day.

The vague idea comes to me that throwing out half the mixture every day has a Darwinian element, but it doesn't involve any 'selection', you're simply casting out half, regardless of adaptability.

My question is: Does the process of making a sourdough starter involve Darwinian selection?

EDIT: There is no 'choice' involved in throwing out half the sourdough. It is entirely arbitrary - and both halves look equally the same.

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  • $\begingroup$ How does the half you cast out compare to the half you retain? $\endgroup$ – Bryan Krause May 28 at 7:53
  • $\begingroup$ Dividing it would be closer to genetic drift since nothing the bacteria do has any impact on which half you throw out. Note you don't actually have to throw out any of it is just that the process produces a lot of bulk which is usually too impractical to use unless you own a bakery. $\endgroup$ – John May 28 at 18:11
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Yes, continually diluting the mix like this encourages the organisms (bacteria, yeasts, fungi) to have more constant conditions (maximum nutrients, fewer waste products) and be in log phase growth. The mix selects for particular faster-growing organisms. if you did not dilute, the nutrient levels would change (lower) and waste products accumulate much faster. Then the conditions would be more different day-to-day.

Practically, the dilution adds more nutrients while still allowing the pH to fall. Without the dilution, particular bacteria tend to take over and lactic-acid producers dont get a chance.

refs https://en.wikipedia.org/wiki/Bacterial_growth https://en.wikipedia.org/wiki/Sourdough

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  • $\begingroup$ How is that "Darwinian selection"? $\endgroup$ – Frieke May 28 at 13:04
  • $\begingroup$ I abbreviated the term to selection $\endgroup$ – Polypipe Wrangler May 28 at 13:11
  • $\begingroup$ so step 1 is Darwinian but dividing the culture is not. $\endgroup$ – John May 28 at 18:11
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There absolutly is selection involved. First, you create a selective environment to culture a subset of environmental organisms (yeasts). Then, by infusing fresh media (flour/water) into the cultures each day, you are essentially passaging a live culture or organisms. It's not rational selection, like that practiced by plant breeders, since you aren't hand picking the lineages you want to passage, but it does allow for natural selection to occur in this environment which would otherwise be stunted in a simple batch culture if you didn't add new flour/water mix each day. This passaging allows for more reproductive growth to occur, increasing the probability of adaptive mutations and creating the conditions for such mutation expands via competative exclusion with other clonal lineages. With more generations, organisms in the starter should become more and more specialized to the unique selective pressures in that environment over time. In theory, as they become more adapted to this environment, it will be less and less likely that any new wild strains or spoilage agents will take up residence in the starter. Richard Lenski has been conducting a series of long term E. coli evolution experiments describing how cultured organisms evolve over many (many) generations. Since people are known to keep sourdough cultures alive for years, decades, or even centuries, it seems like an analagous experimental system.

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That is not Darwinian selection, because you are not selecting anything. As you mention yourself, it doesn't matter which half you discard and which half you keep. In fact, you can keep the entire thing. Discarding just makes it more practical.

If all conditions are fine, yeast will grow exponentially, meaning your amount of yeast will double every 2 hours or so. By discarding half of your mix, you simply reduce the volume you are working with. Otherwise you would have to add double the ingredients every time. For example, let's assume by adding fresh ingredients you double your volume. Starting with 250ml you would have 500ml the next day, 1l the day after, 2..4..8.. 16 liters and soon you have a bucket full of yeast.

Handling that volume to avoid overgrowth and insufficient nutrient/oxygen supply is way harder than a small volume. In the lab we grow yeast preferentially on shakers, mixing the solution to ensure all cells get the right amount of nutrients. Otherwise the yeast would die once the nutrients are depleted. So, how do you do that in your yeast bucket? ;) Just stirring it once won't do the job. The yeast would settle and die.

You could keep the halfs in separate smaller containers, but how much space do you have to store these? Here is a video where this issue is explained and what you can do with the discarded half: https://youtu.be/Ye4A2judXck

In the end, what would you do with all that starter? Once you have your sourdough starter ready, you can take half and use it for baking and keep growing the other half. How much bread do you want to bake? Do you need a bucket full of sourdough starter?

Sure, some of the yeast will die and the cells best adapted to the growing conditions will remain. This is a selection process, but it's independent from discarding half of it.

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  • $\begingroup$ Natural selection does not require that a person is rationally selecting something, only that selection is occuring. Taking any group of environmental organisms and placing them in a new environment will absolutly involve selection. Not just initial colonization selection, but also adaptive selection over increasing reproductive generations. It's the same reason lab strains of bacteria or yeast often lack certain phenotypic traits of their environmental ancestors, while environmental organisms sometimes grow poorly in laboratory conditions at first. $\endgroup$ – MikeyC May 28 at 15:59
  • $\begingroup$ The question was if discarding half of the starter involves a "Darwinian element". As I mention in the last paragraph, there is of course always adaptation/selection happening, but that's completely independent of the fact that you are discarding parts of it or not.. $\endgroup$ – Frieke May 29 at 11:10

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