[Nora L]

There’s not likely to be an answer that would pass your criteria (“scientific”) because these days scientists are not studying important, practical dilemmas, they are too busy developing new drugs. And back when science was an objective search for truth, there was no such thing as kibble.
The reason why kibble should not be combined with raw food has nothing to do with bacteria, but the speed with which various foods are digested does play a part. Kibble, being a complex mixture of different types of foods, some digestible and some not, and being mis-combined in such a way as to render some of the constituents indigestible for that reason alone, digests very slowly. Anything that is manufactured with the intent that it will be sitting on a shelf for a few months is necessarily going to be difficult to digest. Raw meat has no such properties so it decomposes AND digests much quicker, particularly when consumed by an animal biologically adapted to its consumption, like a dog. I think there are grey areas in the rule that they should not be fed together, and that’s what allows some people to get away with it. I have had many occasions where the combination caused a great deal of digestive upset to the dog, and to account for the difference I think you have to look at the types of food, what may have already been in the gut, the age and condition of the dog, etc. When people are transitioning their dogs, I highly recommend a full day of fasting between the last kibble meal and the first raw meal, because this practically eliminates the possibility of digestive issues. Raw meat sitting on top of kibble will decompose if it’s not digested and the process of putrefaction produces toxic by products that the body will attempt to eject. If the encounter happens in the stomach, vomiting may be the result and if it happens further down in the intestine, it will be diarrhea. Since it’s always iffy to combine the two I have a pretty strict policy of never mixing them for the dogs that I feed. I don’t think there’s any situation where feeding kibble is necessary, because when raw feeding is done properly it is no more expensive or inconvenient than kibble, particularly when preventable vet bills are factored in. There is more info about raw feeding and various related topics on my website http://www.NoMoreVetBills.com.