The roots of the term rennet come from rennet derived from animal parts, but in cheesemaking, with modern manufacturing the term is now used broadly to describe a range of chymosin based coagulant. This specific rennet article reviews the different types chymosin based milk coagulants, general information on is in the Wiki: Rennet.
Animal based rennet is made from the abomasum of un-weaned milk fed calves. The abomasum is also known as the fourth stomach, and in young animals, the “rennet-bag” or “vell”. The tissue secretes acids and the rennet enzymes chymosin and pepsin. New-born calves have 95% chymosin (sometimes referred to as rennin) and 5% pepsin, as the animal ages and its diet changes from milk to grass, the ratio changes such that at about nine months of age, the ratio has reversed to 10% chymosin and 90% pepsin.
Animal based rennet typically contains 90% chymosin and 10% pepsin in purified form and is considered by many cheese makers to be the preferred rennet. Different ratios are available such as 92-85% chymosin and 8-15% pepsin.
Note, calf based rennet does not qualify for some diets and may concern some for animal welfare reasons.
Animal derived rennet is normally available in a liquid, paste, or powder formats with liquid easily being the most common.
Liquid format animal rennet contains, in addition to the enzymes chymosin and pepsin, trace proteins, sodium chloride brine, acetate, propylene glycol, caramel color, and flavour preservatives sodium benzoate, and potassium sorbate.
Powdered format animal rennet contains, in addition to the enzymes chymosin and pepsin, sodium benzoate and sodium chloride.
Manufactured liquid calf based rennet is often shipped in large i.e. 5 US gallon containers, and thus hobby Cheese Making Supply Stores often repackage into smaller containers.
Note, rennet paste is normally animal based and made of ground stomachs and brine. As it uses the whole stomach it is also rich in lipase which results in a piquant cheese such as Feta, Provolone, and Romano. Another choice to achieve this effect is to use liquid refined rennet and dried lipase powder.
Liquid calf rennet is normally produced in a very concentrated form because when diluted, the enzymes become unstable and lose strength.
Liquid calf based rennet is normally available in single, double strength, and triple strength. To complicate matters, single strength in Europe is different from that in USA.
In the mid 1990’s a new standardization measurement (IDF Standard 157:1992) for rennet was adopted based on IMCU/mL or International Milk Clotting Units per ml of milk.
Until 1990, the only source of chymosin was calves. Around 1990, scientists created a system to make chymosin that doesn’t require calves. Using genetic engineering, the gene for chymosin was cut from a calf cell and inserted into the genomes of bacteria and yeasts such as Kluyveromyces lactis. The microbes replicate and grow rapidly, can be grown continuously, and make an exact copy of calf chymosin.
It can be argued that these rennets are not genetically engineered as nothing was changed in the gene.
This product is often certified Halal or Kosher and suitable for vegetarians.
Approximately 70% of the cheese made in the U.S. are coagulated using fermentation produced chymosin.
Fermentation produced rennet is normally available in concentrated liquid and powder formats.
Fermentation produced rennet is 100% pure chymosin however it can be combined with pepsin to make a more natural animal based type product.
Several forms of milk coagulating enzymes are based on microbial fungal sources, the most popular being Rhizomucor miehei, others being Cryphonectria parasitica or Endothea parasitica. Here the mold is grown in large vats and the enzyme extracted and purified and stored with salt.
While microbial rennets are effective at coagulating milk, they are not as good at making aged cheeses as their pathway after coagulation develops poorer flavor and texture than animal based rennet.
In general, microbial based rennets are cheaper than animal based rennets.
Microbial rennet is normally available in concentrated liquid, powder and tablet formats.
Tablet microbial rennet normally also contains microcristaline cellulose and sodium chloride (table salt).
Liquid microbial based rennet is normally available in single or double strength.
Examples include thistle or fig leaf.