The way you make commercial enzymes like this is isolate the gene sequence responsible for enzyme production, splice it into a plasmid, and then introduce the plasmid into any of the approved organisms (in the US approved by FDA). After that, you take the new organism, purify it, propagate it, and see if it passes all the tests for stability, viability, purity of enzyme production, etc. Then you apply with the FDA to have the new organism and resulting enzyme be added to the GRAS list.
In other words, in the case of modern rennin production, the rennin producing gene is spliced into a plasmid, and that plasmid inserted into bacteria or yeast, which then make a biosynthesized form of rennin.
