In physiology, the term rennet identifies a particular digestive enzyme that operates exclusively on the casein of milk. This enzyme is produced by infants and puppies of mammals in general and targets the caseins of milk, which it breaks up into smaller protein fragments, promoting digestion.
In the dairy industry, rennet is a fundamental ingredient for the production of many types of cheese. The real (and only) rennet is that of animal origin, which - also known as rennet - is prepared from abomasums of young ruminants , generally calves, lambs or kids. Its function is to coagulate the caseins, agglomerating them in semi-solid precipitates which, once drained from the whey and suitably processed / seasoned, will give rise to the cheese. By analogy of function, the term rennet is commonly (and improperly) also extended to other substances with coagulating activity; we speak in particular of vegetable rennet and microbial rennet. In addition to these substances, the simple acidification of milk (spontaneous through the lactic microorganisms present in milk and / or chemical) is able to produce a coagulation - albeit milder - of the caseins.
- rennet coagulation: compact coagulum with an elastic structure; coagulation is rapid (less than one "hour) and takes place at temperatures close to the optimal ones of rennet (35-40 ° C)
- acid coagulation: highly demineralized rennet with a crumbly structure; occurs slowly (from 3 to 24 hours) at relatively low temperatures (18-20 ° C)
Naturally, the two types of coagulation can be combined in various ways to obtain products with intermediate characteristics, also exploiting the effect of heat.
In milk, caseins are found in the form of micelles, lipoprotein particles with a tendency to unite and coagulate; in standard conditions this does not happen for two reasons, the first lies in their electric charge, which at natural pH is negative (negatively charged particles repel each other); the second is related to the presence of the colloidal protector peptide in the C-terminal part of the K-casein. The lack of one or both factors determines the coalescence of these particles, therefore the protein coagulation.
Animal rennet
It is commercially available in solid state (dried or lyophilized powder) or liquid, or in paste; it is commonly available in pharmacies or dairies for the production of homemade cheeses. Their composition is standardized in order to guarantee a final product of constant quality over time. Rennet paste is also referred to as "strong rennet" as it has strong coagulating properties (in addition to a "lipolytic activity useful in the maturation of some cheeses); there is also a lighter rennet, called" sweet rennet "which is instead characteristic powdered calf rennet More generally there is a parameter known as the strength (or strength) of rennet which expresses the quantity of coagulable milk per unit of rennet.
Animal rennet mainly contains two coagulating enzymes: chymosin (or rennin) and pepsin, the proportions of which depend on the age of the animal and the type of diet. The younger the animal and the greater the role of milk in its diet, the higher the percentages of chymosin are; similarly, the abomasum of an adult bovine will contain almost exclusively pepsin. The most important enzyme of the rennet is precisely chymosin, which acts by attacking the K-casein with hydrolysis between the amino acids phenylalanine in position 105 and methionine in position 106. In this way peptide fragments are obtained which destabilize the internal balances of the casein micelles. which no longer repel each other but aggregate, originating the clot.
- PLEASE NOTE: the coagulating activity of the rennet is maximum around 40-42 ° C, while it tends to cancel itself out below 10-15 ° C and above 55-60 ° C. It is facilitated by acidifying substances ( such as citric or tartaric acid: the optimal pH of chymosin is 4.7) and delayed by alkalis. Of course, the activity of the rennet also depends on the concentration of the enzyme; it is also influenced by the degree of pasteurization of the milk
Vegetable rennet
The use of vegetable rennet is maintained in the production of some traditional cheeses, both in Italy and in the Iberian peninsula. It is also suitable, unlike rennet, for the production of cheeses suitable for vegetarians (lacto-ovo-vegetarians).
Vegetable rennet is made up of juices of vegetable parts, mainly Cardo and Artichoke. The table shows a list of some vegetable coagulants; the use of pineapple and sunflower extracts should also be noted.
Video recipe: homemade cheese with vegetable / microbial rennet
Microbial rennet
Food biotechnologies aim to select microorganisms with useful characteristics for production processes; specifically, the researchers succeeded in isolating and transmitting the gene of animal origin responsible for the production of chymosin. Microbial rennets are used in the production of many types of cheese, excluding DOP and typical ones (the use of different types of rennet is regulated by various laws and production regulations).
Homemade Cheese
Cheese - how to make it at home
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Bibliography
- Microbiology and dairy technology. Quality and safety - By Germano Mucchetti, Erasmo Neviani - New Techniques
- Atlas of cheeses - By Giorgio Ottogalli - Hoepli
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