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Inspired by this question... when making a sourdough starter or similar, does the flour contribute a significant amount of yeast to the final product [either from the flour's origins in the fields, or from the factory where it is milled], as opposed to yeast that is in the environment (floating in the air or whatnot)? Would two different sourdoughs, both made in the same kitchen, but with different sourced flour, be significantly different due to the yeast (ignoring differences in the flours themselves)? Or would the natural environment dominate the yeast production?

I recognize that some flours might be different than others, so if that is significant (such as bleached flour versus unbleached, more highly processed, etc., please mention that.

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    Thank you for asking that! As said in my answer in the question I linked, I have seen assertions about it being all from the flour (by which people usually mean "from the wheat itself" as opposed to how you defined it) or all from the air, but no firm proof. If somebody can come up with well-sourced evidence, I would love reading about it.
    – rumtscho
    Jan 18, 2023 at 17:52
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    Somebody did an experiment here: thefreshloaf.com/node/37259/… - take from that what you will, or do an experiment yourself.
    – Billy Kerr
    Jan 18, 2023 at 18:26
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    @BillyKerr That sounds like an answer to me! At least, something of one.
    – Joe M
    Jan 18, 2023 at 19:29
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    @BillyKerr That definitely seems like an answer to me!
    – Joe M
    Jan 19, 2023 at 18:09
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    @JoeM - ok, I've turned it into an answer!!!
    – Billy Kerr
    Jan 20, 2023 at 11:39

3 Answers 3

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It Depends, but ...

First, it depends on the flour. Bleached, sterilized, hot-rolled white flour has the least (possibly none) naturally-occurring wild yeast on it. Cold-rolled unbleached organic whole-grain rye flour has the most. Everything else is in between. Clearly, if you're using sterile flour, any yeast is going to need to come from elsewhere.

Second, it depends on your environment. If your starter is being incubated in a open bakery during the rainy season in San Francisco, it absolutely will pick up some yeast from the environment, more from surface contact than from "the air". In my personal experience as a San Francisco sourdough baker, the primary place that environmental yeast in California comes from is the fruit flies that drown in your starter (nobody wants to say this, but it's true, fruit flies are huge yeast carriers). But, if you're creating sourdough in the New Mexico desert or on the International Space Station, you're not going to have much environmental yeast available.

Within those parameters, for a reasonable starter using non-sterile flour in an average kitchen, where is most of the yeast coming from?

The flour.

Per the wild yeast blog:

Yeast grow on grain and arrive with the flour. One gram of flour contains about 13,000 yeast cells. I don’t deny that there are a few yeast in the environment that find their way into the starter, but by and large the yeast that will survive in the starter are the ones that like the menu there, i.e, the ones that have a taste for grain.

Given this, why even bother with the whole open-dish sourdough cultivation if you don't live somewhere with ample environmental yeast? Mostly for the bacteria. Sourdough is a culture of yeast and bacteria, and benefits from your environmental bacteria (as well as those on the flour) if you get the right ones.

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    For what it's worth the ISS doesn't have much diversity in its microflora; only 5 genera of fungi, of which only 1 is a yeast, and it isn't in the baker's yeast genus. paper here. So, I don't think environmental yeast started sourdough would work up there, even if you could work with a particulate like flour.
    – bob1
    Jan 18, 2023 at 23:34
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    FuzzyChef, I must admit that I am disappointed by this answer. I loved that the question was posted exactly because all the information I have seen so far has been random blogs full of assertions of unclear origin, and hoped that our community can turn up trustworthy information. Your first link is a Quora question (already not a high-quality source) and its answer is that bleached flour makes good starter. If we interpret this as evidence for the starter content in the flour (which is a shaky conclusion depending on already knowing the overall answer!) then it would argue for bleached ...
    – rumtscho
    Jan 19, 2023 at 17:09
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    @rumtscho if you can find a serious scientific study on this, post an answer. I searched the DBs I have access to, and turned up nothing to the point.
    – FuzzyChef
    Jan 19, 2023 at 21:10
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    @FuzzyChef sadly, I think this is why I am so haunted by the idea of a definitive answer - I don't know it myself and don't know where to find it :( I don't even want to wake the impression that "yeast comes from the grain" must be wrong - maybe it is right, I am just upset with the quality of the available sources. I will take a look myself, but if you find a source that's locked for you, shoot me the link in the Frying Pan, I have lots of journal access over my work account.
    – rumtscho
    Jan 19, 2023 at 22:41
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    If it's bothering you that much, then just do a version of TheFreshLoaf's test, only much more robustly.
    – FuzzyChef
    Jan 20, 2023 at 4:56
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There's an article in Scientific American which is worth a read here: The Science of Sourdough. It's also archived here in case the link rots.

I'll summarise the intersting part.

A team of researchers have conducted an experiment whereby they gave the same flour to 18 bakers around the world to make sourdough starters using identical techniques. They then used DNA sequencing to analyse the yeast and bacterial species. They are still to publish their full findings in a scientific paper, so perhaps best to wait for that for a definitive answer.

But anyway, here's the most relevant quote (bold highlight is mine).

Even though all the bakers started with the same flour, their starters were all different. Most contained various strains of common baker’s yeast, Saccharomyces cerevisiae, along with a host of other yeasts in varying proportions, they found. The starters also contained a wide range of lactic acid bacteria, mostly in the genus Lactobacillus — though once again, the details varied widely from one starter to the next. Most microbes appeared to have come from the flour — a different draw each time­ — though a few also originated with the baker’s hands or kitchen.

This article is from 2020. Might be worthwhile trying to find out if they ever did publish that scientific paper

Update: apparently they did publish the paper. My mistake, the link is in the article.

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  • I am a bit confused about "they are still to publish" and "find out if they ever did publish" - the SA article has a link to a full scientific paper already, and the paragraph you cite describes it.
    – rumtscho
    Jan 20, 2023 at 13:05
  • @rumtscho - the article was from 2020. I didn't know they had published the paper. The article mentions that they had not published it when it was written. Perhaps that link is an update.
    – Billy Kerr
    Jan 20, 2023 at 20:25
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Sourdough is about creating an environment that supports the growth of a desirable bacteria and yeast colony (not yeast only). In fact, this process has less to do with yeast than you think. In a typical starter lactic acid bacteria outnumber yeast cells 100 to 1. The goal is for that desirable community to essentially take over and propagate as nutrients (flours) are added to the mix, creating an active, but also consistent (flavor and activity) product. Starters with different flavor and behavior profiles can be created and maintained, but it is not a function of the yeast only. Grain type and liquid being used during feeding might impact flavor more. Anyway, maybe I am not exactly clear what you are asking, but my starter began with flour and water in a closed container, and is maintained in a closed container. I suppose any yeast and bacteria started with the initial ingredients. I've now had that starter for maybe 10 years. So, what is "the final product?" ...a ready to use starter? The bread I baked last weekend?

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    This isn't really answering the question - I'm asking how much of that yeast (and I'm open to bacteria/etc.) is coming from the air/environment versus how much was on the flour in the bag.
    – Joe M
    Jan 18, 2023 at 19:24
  • @JoeM how much at what point in the process? As I mention in my answer, I began in a closed system. So, probably most came from the flour. Not sure how to pin down what you are asking....or why.
    – moscafj
    Jan 18, 2023 at 19:26
  • It may be that I don't understand your answer, it's fairly dense - are you saying you've made a starter and precluded any environmental factors, so only including "on the grain" yeast/bacteria?
    – Joe M
    Jan 18, 2023 at 19:33
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    @JoeM Yes. That is with relatively freshly milled flour (mail ordered from a mill), as opposed to the flour you might find on the shelf of your grocery store. But again, it is not solely about yeast. That is why I am curious about what you are interested in learning. Why the question?
    – moscafj
    Jan 18, 2023 at 19:35
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    I also don't see how this answers the question. You have a starter, and you know that there is yeast in that starter. But which of all possible sources of yeast started the strain that became dominant in it? I couldn't find any place where you provide evidence for either of the possible options, just some background information on sourdough starters.
    – rumtscho
    Jan 19, 2023 at 17:02

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