Fermentation Chymosin or Recombinant Chymosin
As microbial coagulants were not fit for every type of cheese and as the supply of calf stomachs to produce rennet was not enough to satisfy the strong development of the cheese industry, in the 1980s producers sought replacements. With the development of genetic engineering, some companies tried to insert the calf genes responsible for chymosin production into the DNA of various microorganisms in order to produce chymosin by fermentation. Escherichia coli, Kluyveromyces lactis, and Aspergillus niger var. awamori gave the best results; today, they are used to produce recombinant chymosin. Their production started around 1990. Chymosin was the first artificially produced enzyme to be registered and approved by the US FDA. Now most of the cheese production in the United States is made by recombinant chymosin.
The main difference with calf rennet is that recombinant chymosin contains only chymosin B, while rennet contains both chymosin A and B and bovine pepsins.
Now camel recombinant chymosin is also available, produced by Aspergillus niger var. awamori. It is stated that camel chymosin is less proteolytic than calf chymosin.
Recombinant chymosin is not a GMO, but it derives from a GMO and therefore is not allowed for the production of traditional and PDO (Protected Designation of Origin) cheeses. It is approved halal and kosher and by the vegetarians.