Chymosin for cheese-making

In the 1960s the Food and Agriculture Organisation of the United Nations predicted a severe shortage of calf rennet. It was anticipated that an increased demand for meat would lead to more calves being reared to maturity, and hence less rennet would be available. Over the last 30 years several substitutes for calf rennet have been developed, allowing the supply of enzymes to keep pace with cheese production.

Today there are six major sources of protease for coagulating milk - three from animals: veal calves, adult cows and pigs; and the other three from fungi: Rhizomucor miehei (formerly Mucor miehei), Endothia parasitica and Rhizomucor pusillus (formerly Mucor pusillus). In addition, there are now chymosins derived from genetically-modified microbes (Escherichia coli, Kluyveromyces lactis and Aspergillus niger).

Microorganisms have been modified genetically to yield chymosin that is identical to the enzyme obtained from animals. This can be used to produce better quality cheese than the fungal or other animal (non-calf) rennets. The first scientists to make chymosin in this way in 1981 used bacteria. Numerous groups have since followed their lead, using other microbes, so that chymosin has now been obtained from food yeasts.

In 1988, chymosin was the first enzyme from a genetically-modified source to gain approval for use in food. Three such enzymes are now approved in several European countries and the USA. These proteins behave in exactly the same way as calf chymosin, but their activity is more predictable and they have fewer impurities. Such enzymes have gained the support of vegetarian organisations and of some religious authorities. Chymosin obtained from recombinant organisms has been subjected to rigorous tests to ensure its purity.

Today about 90% of the hard cheese in the UK is made using chymosin from genetically-modified microbes.

Notice that the cheese is NOT made using a GMO, but rather the product of a GMO (the enzyme). Consequently, ALL cheeses on sale are in fact 'GMO free'. Another important point is that the enzyme does not remain in the finished cheese. Like all enzymes it is required only in very small quantities and because it is a relatively unstable protein it breaks down as the cheese matures. Indeed, if the enzyme remained active for too long it would adversely affect the development of the cheese, because it would degrade the milk proteins too much.


Above: How chymosin is made by GM yeast.

First the DNA encoding the protein chymosin is isolated from calf cells. A copy of this DNA is inserted into yeast cells in a small ring known as a plasmid. The plasmid is copied within the yeast cells.

Yeast cells are cultivated in a contained environment, that is, a fermenter vessel, like those used for brewing.

Chymosin identical to the animal protein is then made by the yeast.

As it is comprised of a single protein (rather than a mixture) chymosin produced in this way is purer than the rennet which is traditionally obtained from animal stomachs.


Copyright © The University of Reading, 1999
This page was revised last on: Sunday, May 23, 1999
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