If I had been Louis Pasteur, my first reflection would be that the experiment was able to produce milk with longer shelf-life. From there, I would make inferences as to how it happened. I already have knowledge about microorganisms and how they react or degrade with heat and pressure. So, I would conclude that by subjecting the milk to that specific temperature and pressure, the microorganisms that cause spoilage are destroyed.
Later on, I would also realize that it's not only the spoilage-causing bacteria that are destroyed but also all the other microorganisms present in the milk examples of which are probiotics.