Hello Kurma,
Can you tell me a bit about rennet in cheesemaking, and it's usual sources, and specifically, where does vegetarian rennet come from, and is it genetically modified?

Sally Baker, Islington, London UK

Hello Sally,
Thanks for your letter about Rennet.

Here's some info:

Cheese is made by coagulating milk to give curds which are then separated from the liquid, whey, after which they can be processed and matured to produce a wide variety of cheeses. Milk is coagulated by the addition of rennet. The active ingredient of rennet is the enzyme, chymosin (also known as rennin). The usual source of rennet is the stomach of slaughtered newly-born calves. Vegetarian cheeses are manufactured using rennet from either fungal or bacterial sources. Advances in genetic engineering processes means they may now also be made using chymosin produced by genetically altered micro-organisms.

The exact processes in the making of cheese varies between different varieties. However, all cheeses are made by essentially the same method. Initially, the milk is usually pasteurised by heating at 72°C for 15 seconds to destroy potentially harmful bacteria. The milk is then cooled to around 30°C and a starter culture of lactic acid bacteria is added to help souring. These convert lactose into lactic acid and help in the coagulation process. In addition, they also have a beneficial effect on the eventual quality, taste and consistency of the cheese. Some cheeses are coagulated entirely by lactic acid bacteria and are known as lactic-curd or acid-curd cheeses. However, some cheeses sold as lactic-curd cheese may have had rennet added. The next stage is the addition of rennet, containing the enzyme chymosin.

Rennet is usually sourced from the abomasum (fourth stomach) of newly-born calves. Here, chymosin aids the digestion and absorption of milk. Adult cows do not have this enzyme. Chymosin is extracted by washing and drying the stomach lining, which is then cut into small pieces and macerated in a solution of boric acid or brine at 30°C for 4-5 days. Pepsin may sometimes be used instead of chymosin. This is usually derived from the abomasum of grown calves or heifers, or less commonly pigs. Pepsin may be mixed with calf rennin. Rennet coagulates the milk, separating it into curds and whey. This is called curdling.

Chymosin breaks down the milk protein casein to paracasein which combines with calcium to form calcium paracaseinate, which separates out. Milk fat and some water also becomes incorporated into this mass, forming curds. The remaining liquid is the whey. The strength of different rennets can vary, though usual strength varies between 1:10,000 and 1:15,000 i.e. one part rennin can coagulate 10-15,000 parts milk.

Other substances may also be added during the cheese making process. Calcium chloride is added to improve the curdling process, and potassium nitrate is added to inhibit contaminating bacteria. Dyes (e.g. annatto, beta-carotene), Penicillium roquefortii mould spores to promote blue veining, or propionic acid bacteria to encourage hole formation may be added.

Following curdling, the curds are cut and drained. The size of the cut and the methods used vary for different cheese varieties. For soft cheeses, the curds are sparingly cut and allowed to drain naturally. For hard cheeses, the curds are heated and more whey is drained off. The curds are then cut into small pieces, placed in vats and pressed.

After pressing, the curds may be treated in a number of ways. They may be moulded into different shapes, soaked in a saltwater solution, be sprayed with mould forming spores or bacteria, washed in alcohol, or covered in herbs.

The final stage is ripening, or maturation. This can vary in length from 4 weeks to 2-3 years, depending on the type of cheese. During ripening flavours develop, the cheese becomes firmer and drier, and special characteristics such as holes, blue veining and crust formation occurs.

Vegetarian cheeses are made with rennets of non-animal origin. In the past, fig leaves, melon, wild thistle and safflower have all supplied plant rennets for cheese making. However, most widely available vegetarian cheeses are made using rennet produced by fermentation of the fungus Mucor miehei. Vegetarian cheese may also be made using a rennet from the bacteria Bacillus subtilis or Bacillus prodigiosum.

Advances in genetic engineering techniques mean that some vegetarian cheeses may now be made using chymosin produced by genetically engineered micro-organisms. The genetic material (DNA) which encodes for chymosin is introduced into a micro-organism which can then be cultured to produce commercial quantities of chymosin. This is done by extracting genetic material from calf stomach cells which acts as a template for producing the chymosin encoding DNA. This can then be introduced into the micro-organism. Once the genetic material is introduced there is no further need for calf cells. Alternatively, the chymosin encoding DNA can be bio-synthesised in the laboratory without the use of calf cells.

The chymosin produced is identical to that produced by calf stomach cells. The development of genetically engineered chymosin has been encouraged by shortages and fluctuations in cost of rennet from calves. It's manufacturers claim that genetically engineered chymosin will end the cheese making industry's reliance on the slaughter of calves.

Chymosin encoding DNA has been introduced into three different micro-organisms. These are the yeast Kluyveromyces lactis, the fungus Aspergillus niger var awamori, and a strain of the bacteria Escherichia coli. All of these have now been approved (in the UK) and cleared for use by the Ministry of Agriculture, Fisheries & Food. There is no legal requirement for manufacturers to state whether a genetically engineered rennet has been used in the cheese making process.

Vegetarian cheeses are widely available in supermarkets and health food stores. A wide variety of cheeses are now made with non-animal rennet and labelled as suitable for vegetarians. No particular type of cheese is exclusively vegetarian. Soft cheeses are as likely to be non-vegetarian as hard cheese.

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