Why put flour in Oven Bag?
See this 1972 NYT article from back when these bags were a recent invention and the explosion problem was still current news. https://www.nytimes.com/1972/04/08/archives/why-cooking-bags-explode.html
In recent years, news was made of the dangers of boiling water in the microwave in something like a Pyrex measuring cup, because water heated in that manner could potentially become superheated (hotter than the normal boiling point, but not bubbling, steaming, or visibly boiling) if the surface of the container is so smooth that &quot;nucleation sites&quot; don't form. It then sits there, hotter than boiling but looking still (as if it weren't boiling yet) -- until something disturbs the surface (like the next ingredient, or your hand jostling the cup), at which point it can violently and simultaneously boil, seeming to explode.
Apparently the same thing was thought to be happening in these cooking bags due to water getting trapped under a layer of grease from the meat, and the flour would help those nucleation sites to form (called &quot;boiling chips&quot; in the NYT article) and also bind with some of the fat and water to form a gravy instead of letting the water remain separated. I am not a scientist but I don't think I've said anything egregiously wrong.
Justin Wilson mentioned the explosion risk and the need to use flour in one of his cooking shows back in the day while demonstrating the bag, but didn't go into the science of it, just mentioned that the flour was needed and went on with the show. It's in an episode called &quot;Cajun Meat and Potatoes&quot;.