The info came from a truck driver who hauled for the dairy. This was before the milk was filtered & loaded into the truck.
My uncle used the same milk he sold to the processor & was careful to keep it clean.
I wonder if things have changed with modern systems, but milk used to always be filtered just after it left the cow, certainly before it went into the bulk tank. And the tanks were completely sanitized, by heat and chemicals, every couple of days between shipments.
I’m having a hard time getting around systems having got more lax in the last thirty years.