Rennet is a material used to coagulate milk, causing it to separate into solids (curds) and liquid (whey). The active enzyme in rennet is called rennin or chymosin (). The chief use of rennet is in the making of cheese and junket. The enzyme reacts with κ-casein on the surface of the casein micelle particles in the milk. This in the presence of calcium ions causes the coagulation of the casein micelles to form a curd. Commercially, rennet is sold in liquid, powder, and tablet forms.

Rennin reacts specifically with κ-casein, cleaving the protein between the amino acids phenylalanine(105) and methionine (106), producing two fragments. The soluble fragment (residues 106-169) which becomes part of the whey is known as glyco macro peptide and contains the glycosylation sites for κ-casein. The other component (residues 1-105) is insoluble.

The most common source of rennet is the abomasum (fourth stomach) of slaughtered, milk-fed new-born cow calves or other young ruminants such as camels or goats. Traditionally rennet was prepared by washing and salting the stomachs of animals shortly after slaughter. The rennet was then hung up and dried until needed.

Alternative rennets

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Many plants have coagulating properties. Some examples include fig tree bark, nettles, thistles, mallow, and Creeping Charlie. Commercial vegetable rennets usually contain rennet from the mold Mucor miehei. Rennet from thistle or cynara is used in traditional cheese production throughout the Mediterranean.

In recent times, the genes responsible for rennet production have been transferred from calf cells to bacterial cells. The daughter cells of these bacteria produce rennin that is identical to that of calves. Some trade names for this kind of rennet include Chy-Max, Chymostar Classic, and Chymogen. In 1994 Chy-Max was used in the production of over 60% of the United States Cheese Production. Since it is much more cost effective for corporations to use this biotechnology there may be no reason to believe that any U.S. food products contain rennet directly derived from animals any longer.

Newer test results show that chymosin from genetically modified bacteria is not completely identical to the natural product - see: http://lib.bioinfo.pl/pmid:15969502 - so the use of these products can be identified. Same for rennet produced from microbial coagulants. Also the cheese yield might suffer alot so that the cheaper price of these products is often more than compensated by the losses of cheese. Also the development of flavour is very often different from the one that is gained by the use of natural rennet.

Natural Rennet is a limited product anyway - only about 35% of the worlds cheese production can be done with original natural calf rennet so there is a need for artificial coagulants especially for cheap and lower quality cheeses.

See also

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• Pepsin

References

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Carroll, Ricki. Making Cheese, Butter, & Yogurt. Storey Publishing 2003.

"Biotechnology and Food: Leader and Participant Guide," publication no. 569, produced by North Central Regional Extension. Printed by Cooperative Extension Publications, University of Wisconsin-Extension, Madison, WI, 1994. Publication date: 1994.

Tom Zinnen, biotechnology specialist, University of Wisconsin-Extension and UW Biotechnology Center, UW-Madison; and Jane Voichick, professor, Nutritional Sciences, UW-Madison and UW- Extension.

Animal products | EC 3.4.23 | Food industry

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