How do kosher food preparation requirements compare with what is known by modern science to make food safe for eating?
While some of the kosher rules are food-safety rules developed by the Israelite priests (or given by God), the prohibition against mixing meat and milk is because of an ancient ritual that involved cooking  the meat of a slaughtered animal in its mother's milk, a religious practice forbidden in Exodus 34:26 "... You shall not boil a kid in its mother's milk", part of a passage sometimes known as the ritual decalogue that prescribes the destruction of artifacts of other religions, forbids the creation of idols, and lays out other religious responsibilities.

The laws regarding separation of meat and milk come from the concern that a person might accidentally break this law and boil a kid in its mother's milk if they purchase both meat and milk and prepare them together, or if a residue of milk is left on cooking utensils from a previous dish, which then gets into a dish involving the meat of a slaughtered kid.