Animal Rennet

The source material is the abomasum (fourth stomach) of domesticated ruminants: Bos primigenius (cattle), Capra aegagrus hircus (goat), and Ovis aries (sheep). Usually, the stomachs to produce rennet come from milk-fed animals: calves, kid goats, and lambs. Some special preparations are obtained from baby beef stomachs. The abomasa are collected in the slaughterhouses and frozen immediately. In some countries, they can be salted at 30%. Only the best-quality stomachs are collected to produce rennet.

The extracts of the milk-coagulating enzymes are liquid preparations, which may be dried to powder and optionally also made into tablets.

Animal rennet is a complex of several enzymes. While liquid rennet and rennet powder enzymes are chymosin A and B, pepsin A, B, and C (called also gastricsin), rennet paste enzymes are chymosin A and B, pepsin A, B, C, and lipase.

Chymosin (IUBMB N° and pepsin (IUBMB N° are hydrolases acting on peptide bonds (aspartic peptidases). The molecular mass for calf chymosin is approximately 36.500 Da (Kumar et al. 2010, BRENDA database), and the molecular mass for bovine pepsin approximately 35.000 Da (Munoz et al. 2004, BRENDA database). Both the enzymes are secreted from the glandular cells of the abomasum in an inactive form: prochymosin and pepsinogen. In the lumen of the stomach, due to the low pH, the enzymes are transformed into the active form chymosin and pepsin by cleavage of a peptide residue.

Their physiological role is to digest the milk, which is the main feed for unweaned mammals. Their technological role is to curdle the milk for the production of cheese.

The milk to be processed is heated at 30°C to 35°C and added with a starter culture of lactic acid bacteria to acidify the milk before the rennet is added. Some cheeses (e.g., Petit Suisse) are produced by acidic coagulation, and they require only a tenth of the normal quantity of rennet used for the other cheeses. In this case, the curdling temperature is between 21°C and 35°C and the coagulation takes a long time.

The main protein of milk, к-casein, is found in the surface of the micelles, small spherical aggregates with a diameter in the order of 150 nm. They are quite unstable structures because they are susceptible to a variety of physical and chemical factors. As long as they are intact, they repulse each other, thus remaining in suspension. The technological role of chymosin and pepsin is to bring about the destabilization of the micelle structure, so that the protein clots and forms the semisolid cheese curd, which can be separated from the liquid whey.

Chymosin is very specific in this regard. It brings about the hydrolysis of a peptide bond at a very particular site in the amino acid chain of к-casein, between amino acid residues 105 and 106 (the molecule of k-casein is formed by 169 amino acids). The micelle structure begins to break down, micelles aggregate, coagulation results, and soon the cheese curd can be cut up and separated from the whey.

Pepsin is less specific. It can hydrolyze a wider variety of peptide bonds, but the desired effect is the same. Moreover, some small peptides are released, and they dissolve in the whey.

The two enzymes are present in the stomach juice from the birth of the animal. However, their relative percentage changes with the age of the animal. The abomasa of animals slaughtered very young contain about 95% chymosin and 5% pepsin, while the stomachs of animal three month old produce about 75% chymosin and 25% pepsin, depending on the diet and the breed. When animals are weaned, pepsin take predominance, going from 80% to 90%, while chymosin drops from 20% to 10%. Therefore, abomasa produce rennet with different ratio of chymosin/pepsin, depending on their quality and age (see Figure

Depending on market demand, rennets with different ratio chymosin/pepsin are available. As a general rule, rennet with high chymosin content assures a strong

Chymosin and bovine pepsin content ratio as a function of the animal's age

Figure Chymosin and bovine pepsin content ratio as a function of the animal's age.

Chymosin and pepsin curling times as functions of the pH of milk

Figure Chymosin and pepsin curling times as functions of the pH of milk.

syneresis, while rennet rich in pepsin is preferred to clot quite acidic milk or when a softer curdle is requested. The activity of pepsin is more influenced by acidity than the activity of chymosin, as can be seen in Figure 1.4.З.4.

Rennet is quite thermo-unstable. This means that in the production of scalded cheeses, its activity is partly or wholly destroyed. But a certain degree of activity is still wanted. Even in Emmental and Grana, we can find peptides that should only come from rennet activity, and traditionally a certain level of enzyme activity is wanted to get fast maturation. The thermal instability is important for several types of cheeses (see Figure For instance, during the manufacture of cheddar cheese, approximately 35% of the rennet activity is destroyed up to the time the whey is drained; 6% remains in the cheese following pressing (Holmes et al., 1977). However, the thermal instability can cause problems during transport and storage of the enzymes.

The method used for the determination of the chymosin: pepsin composition of the rennet is IDF 110/ISO15163 (2012). Figure shows the HPLC profile of rennet enzymes.

As shown in Figure, the method used for the determination of the chymosin:pepsin composition of the rennet is IDF 110/ISO15163 (2012).

The influence of temperature on the activity of rennet

Figure The influence of temperature on the activity of rennet.

Clerici natural rennet power cromatography profile

Figure Clerici natural rennet power cromatography profile.

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