This feels like it should be an easy question to find an answer to, but all I'm finding are unreliable sources saying both yes and no.
I'm referring specifically to any kind of store-bought flour used for cooking or baking.
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Flour “contains yeast” in the sense that it is not a sterilised product and contains impurities such as wild yeast cells. The yeast and bacteria contained in flour, in fact, are the genesis of “sourdough starters“.
Flour does not “contain yeast”, in the sense it has nowhere near enough yeast cells to use to make a risen dough. Yeast (or an established sourdough starter) must be added to make dough rise.
This may surprise you, but there are arguments for both a "yes" and a "no" answer, and both are right. My personal preference is on the "no" side.
The arguments for "flour does not contain yeast" are:
The argument for "flour does contain yeast" is that technically, there are yeast spores in flour. So, in a very literal sense, flour contains yeast. But if choose to use this to say "flour contains yeast", then you have to concede that everything contains yeast - flour, sugar, the meadow outside, your own hair, and the very air you breathe in. The statement would be irrelevant, which is a very damning thing to say about a statement!
The first one is unlikely, but I'd like to get it out of the way: Bread mix. These mixes work analogously to cake box mixes - the producer put in everything needed to bake bread, beside the stuff that would make it wet, perishable dough. This includes both flour and yeast, and the package can look deceptively similar to flour.
The second is much more interesting. When you make a sourdough starter, you put a mixture of flour and water at room temperature, and soon you have a thriving colony with lots of yeast inside. Where did that yeast come from? There is a lot of lore among bakers that it (= the yeast that won the "race" and became dominant in the starter) came from the grains of wheat, and stayed on in the flour.
Personally, I prefer to believe the second theory - that the dominant strain more likely came from the air. The air certainly carries a lot of yeast spores, which seems to not be widely known among non-biologists. Also, I remember reading somewhere in the nerdier books on baking that the fact that you tend to get different types of sourdough in different locales is not due to the local yeast strains (as claimed in bakers' lore) but due to the exact starter recipe, including proportions, add-ins and feeding schedule (which, traditionally, were just as regional as recipes) plus the environmental conditions, mostly the circadian temperature cycle in the kitchen.
This does not exclude the presence of yeast within the flour. When you have yeast in the air, you have it sticking to everything that comes in contact with it, including the wheat-grains pre-milling, and the flour post-milling. But when your starter colony has equal chances to stem from your open window as from the wheat (or larger, because nowadays wheat gets fumigated with all kinds of stuff), and when the final taste of the bread turns out to not be dependent on the terroir of your favorite wheat grower, but on what selection pressure you apply during the starter process, then the discussions and fretting about breeding your starter from a flour with the perfect yeast pedigree start looking rather unnecessary.
So, in the end: If you want to be technically right, you can say that "flour contains yeast". If you want to bake something tasty, disregard it and follow a good recipe.
based on does-flour-contain-yeast - it does, but it can be without too (there are alternatives) .
Based on a simple google search it does contain it. So yes, fluor contains yeast.