Is any amount of salmonella-tainted flour dangerous, or is there a lower threshold?
There was recently a recall for certain flour, due to risk of salmonella contamination. The CDC announcement says not to cook with the flour. Naturally they are going to be conservative, and they might have in mind uses like cakes and cookies where flour is a major ingredient.
I don't know if my flour was from the affected lot (I no longer have the packaging), but every week I feed my sourdough starter and then bake with it, and I've fed it from that flour. To feed sourdough starter, you take a quantity of the starter and mix in more flour and water to keep the fermentation going. (And then you bake with or discard some of that, else the starter would take over the kitchen eventually.) Every time you feed the starter you "dilute" the flour you started with with new flour.
Can I safely dilute my existing starter through several feedings of safe flour and then use it, or is any amount of the suspect flour a health risk? Suppose I start with 25g of starter, feed it, discard half, feed it again, and do that a couple more times. I'd be down to a starter that contains only a few grams of the original. Is that safe? Or do I need to build a new starter from scratch (or get some from a friend)?
It's been a few weeks since I bought the flour in question, so I've already baked from it with no apparent ill effects, but I'm in pretty good health to begin with and some of my guests aren't. If there's a meaningful risk I'll toss it and start over, but maybe a couple grams of tainted flour in a loaf of bread is below the threshold where it could do harm. On the other hand, a comment points out that it's possible for the salmonella to multiply in the starter if the acidic environment doesn't kill it, so I'd still have larger quantities. I'm not a doctor, chemist, or culinary professional, and I don't know how to evaluate these possibilities.
(Not a full answer, but as @MonicaCellio asked in comments, I'll convert my comment into an answer)
First off, the finished bread should be fine, prolonged heat during baking will kill off the salmonella, see e.g. this page from the Robert Koch Institute (the German institute responsible for disease control and prevention): https://www-rki-de.translate.goog/DE/Content/Infekt/EpidBull/Merkblaetter/Ratgeber_Salmonellose.html?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en#doc2374560bodyText14 (different countries will recommend slightly different temperatures and durations, but a baked bread should be on the safe side for all of them)
I think you neither endangered your friends nor yourself by serving baked goods from this flour.
IF the dough is infected, there might be a risk to infect other food in the kitchen, which might not necessarily be cooked so well as the bread.
And that's the hard part of the question - to which I don't know the answer. It will depend on whether or not salmonella can survive and grow in the acidic environment of the sourdough and the temperature you store it.
IF they can survive and grow there, then diluting won't necessarily help as the salmonella can multiply between feedings and thus diluting won't necessarily decrease the population. I suggest to focus on the question if sourdough is a habitable environment for salmonella instead of focusing on the amount of original flour that's still in there.
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