Milk-Clotting Enzymes

The use of milk-clotting enzymes in cheese making is one of the oldest enzymatic applications used by man and dates back at least 8000 years. There are different sources of milk-clotting enzymes (animal, microbial, plant), but the main origin has been an extract of suckling-calf abomasa (rennet). Rennet contains both chymosin and pepsin, and the ratio of these in the abomasal mucosa is very much influenced by both the feeding regime and the age of the ruminant, where milk-feeding and young age stimulate chymosin production. Today, milk-clotting enzymes (i.e., chymosin) are also produced via fermentation by use of recombinant DNA techniques. The most specific milk-clotting enzyme is chymosin, which has a high milk-clotting activity and a low general proteolytic activity. Chymosin of different species vary regarding the specific milk-clotting activity, which also is true for all the other milk-clotting enzymes (coagulants). Since calf rennet (i.e., 80–90% chymosin) from the beginning has been used in the development and production of most cheese varieties, the use of other coagulants will give different flavours of the cheeses compared using a preparations dominated by chymosin. For the analysis of the total milk-clotting activity of rennets and coagulants there are today international standard methods and units, replacing old methods and units such as Soxhlet. These topics are all reviewed in this chapter.