What proportion of the natural yeast in Sourdough comes from the flour?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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?