Chymosin

Other Names

calf chymosinchymosin Achymosin BEC 3.4.23.4EC 3.4.4.3milk-clotting enzymepre-prochymosinpreprorenninprochymosinrennetrennin

Synopsis

Chymosin (Rennin): A Comprehensive Reference

1. Identity and Chemical Characterization

1.1 Nomenclature and Classification

The accepted IUBMB (International Union of Biochemistry and Molecular Biology) name for the enzyme is chymosin. Its systematic name is aspartic protease, it is also commonly known by the alternative name rennin, and it is classified under IUBMB enzyme nomenclature EC 3.4.23.4, with CAS number 9001-98-3. Chymosin is a protease found in rennet and is an aspartic endopeptidase belonging to the MEROPS A1 family.

Proteolytic enzymes can be classified on the basis of their catalytic activity into one of four groups — serine, cysteine, metallo, and aspartic proteinases. Chymosin (rennin; EC 3.4.23.4) is a neonatal gastric aspartic proteinase of commercial importance in cheesemaking. It belongs to the aspartic proteinase family, which is widely distributed in many organisms and tissues with different physiological and functional properties.

1.2 Protein Structure

Structurally, chymosin consists of a single polypeptide chain of 323 amino acids, folding into a β-sheet-dominated structure typical of aspartic peptidases, with two aspartic acid residues in the active site facilitating catalysis at an optimal pH of 4.5–5.0 and a temperature around 40–50°C. Chymosin is bilobular, with Asp 32 and Asp 215 acting as the catalytic residues.

The secondary structure of aspartic acid proteases, to which chymosin belongs, is predominantly a β-chain with several short α-helices. The active site of aspartic proteinases is highly conserved and consists of residues Asp-Thr-Gly from each domain.

1.3 Genetic Variants

Commercial preparations of calf rennet contain two forms of chymosin, A and B, usually in the proportion of about 40% of A and 60% of B. Chymosins A and B have been shown to differ by only one amino acid in the polypeptide chain; the former has an aspartic acid residue at position 286, whereas the latter has a glycine residue at the same position. Chymosin A and chymosin B have pH optima of 4.2 and 3.8, respectively, and both act to initiate milk clotting by cleaving κ-casein between Phe 105 and Met 106. Chymosin A slightly exceeds chymosin B in proteolytic activity, whereas chymosin B is more stable at low pH (below 3.5) than chymosin A.

Natural chymosin may consist of up to six molecular species, corresponding to genetic variants A and B, each of which is a mixture of three forms differing at the N-terminus, with one being three residues longer, and the other two residues shorter, than the mature chymosin.

1.4 Natural Biological Source

Chymosin is naturally synthesized as an inactive precursor, prochymosin, by the chief cells in the abomasum (fourth stomach compartment) of neonatal ruminants such as calves, lambs, and kids, where it aids in curdling milk to facilitate prolonged digestion in the acidic gastric environment. Chymosin is produced, together with pepsinogen, by adelomorphic (or zymogenic) cells at the level of the gastric glands of the stomach mucosa. The production of this enzyme is greater in the neonatal period.

Chymosin has an important job in the digestive system of some very young mammals, notably ruminants such as cows, curdling milk to give it a better chance of being digested. Human young do not produce chymosin, using different enzymes to attack milk proteins.

The availability of chymosin — and ultimately its price — depends upon the demand for veal, since the production of chymosin diminishes, and the production of pepsin increases, as the animal matures and is weaned.

1.5 Commercial and Prepared Forms

Chymosin is commercially available in three primary categories:

Animal rennet (traditional): Chymosin, commonly known as rennin, is the principal milk-coagulating enzyme present in rennet. Rennet, which has a long and extensive history of safe use in making cheese and other dairy products, is commercially prepared by aqueous extraction of dried fourth stomach of unweaned calves. In calf rennet, 5–20% of the milk-clotting activity is due to bovine pepsin, with the amount of pepsin in the rennet being a function of the age, weaning, and diet of the calves whose stomachs are processed during its preparation.
Fermentation-produced chymosin (FPC): Today, most commercial chymosin used in cheese production is produced recombinantly in Escherichia coli, Aspergillus niger var. awamori, and Kluyveromyces lactis. More than 90% of the rennet used is produced by fermentation, and it also has the added benefit of being kosher and halal.
Microbial rennet (non-chymosin coagulants): Scientists discovered that certain molds — Rhizomucor miehei and Rhizomucor pusillus — naturally produce enzymes similar to chymosin, capable of coagulating milk. These are distinct from chymosin proper and are not discussed further here.

Rennet preparations containing chymosin are sold in both solid tablet and liquid forms. The aqueous extract of calf stomach contains a chymosin precursor, prochymosin, which is subsequently converted to enzymatically active chymosin.

2. Traditional and Historical Use

2.1 Ancient Origins

The use of rennet — the natural source of chymosin from the stomachs of young ruminants — in cheese production has ancient origins, with the earliest unequivocal archaeological evidence coming from Neolithic sites in Poland dating to around 5500 BCE. Pottery sieves containing lipid residues of milk fats, analyzed through organic residue analysis, indicate that early Europeans processed milk into cheese using coagulants like rennet to separate curds from whey.

This practice likely arose accidentally when milk was transported in animal stomachs, leading to coagulation, and spread across Europe and the Near East with the domestication of sheep and goats around 8000 BCE. The step to cheese was likely accidental: milk stored in pouches fashioned from animal stomachs would have been exposed to natural rennet (the enzyme chymosin produced in the stomach lining), causing spontaneous coagulation into curds and whey.

2.2 Ancient Near East and Mediterranean Civilizations

Legend has it that the first use of rennet can be traced back to a traveler who transported milk inside a vessel made from a dried calf or lamb stomach. In transit, the rennet from the stomach transformed the liquid milk into curds and whey — a process that made cheese a nutrient-rich food that transported well and could be kept for years. A more plausible explanation for the origin of cheesemaking using rennet is that Neolithic herders would have been well aware that the stomachs of kids, lambs, and calves, which were slaughtered or died naturally, contained coagulating properties.

2.3 Pre-Modern and Traditional Preparation

Traditionally, cheesemakers obtained rennet from local butchers in the form of vells — abomasums that were cleaned, inflated, and dried. The production of animal rennet in the traditional way involved cutting the dried and cleaned stomach of young calves into small pieces, placing them in salted water or whey with some vinegar to lower the pH. After overnight or several days, the solution was filtered, and the raw rennet — the result of filtration — was used to coagulate the milk. About one gram of the solution can coagulate 2 to 4 liters of milk. This method of rennet production is still used by several cheesemakers in Austria, France, Greece, Romania, Switzerland, and the United Kingdom.

Use of chymosin in cheesemaking has been known since ancient times. Before the 20th century, traditional cheesemakers may not have understood the chemistry of why coagulation methods worked, yet they nevertheless applied centuries of accumulated wisdom and practical know-how.

2.4 Scientific Isolation and the Transition to Recombinant Production

Scientific interest in isolating and characterizing chymosin intensified in the 19th and 20th centuries. In earlier times, chymosin was extracted from dried calf stomachs for this purpose, but the cheesemaking industry expanded beyond the supply of available calf stomachs, which had to be from young calves.

By the 1970s, demand for cheese had outpaced the supply of animal rennet. Scientists in the 1980s isolated the gene for calf chymosin and inserted it into microbes, such as certain bacteria, yeasts, or molds. These genetically modified microorganisms (GMOs) are then fermented in large vats to produce the chymosin enzyme. One of the first industrial enzymes obtained using genetic engineering technologies was cow's recombinant chymosin, which has been considered the standard of a milk-clotting enzyme in cheesemaking for a long time.

The FDA's approval of recombinant chymosin was particularly significant as it marked the first time a genetically engineered product was authorized for human consumption. In 1990, in a precedent-setting decision, the FDA approved the use of FPC in food.

3. Key Constituents and Active Compounds

3.1 Primary Enzymatic Active Site

Chymosin specifically hydrolyzes the Phe105-Met106 bond in the κ-casein protein of milk, destabilizing casein micelles and initiating the coagulation process essential for cheese production. The aspartic proteinase chymosin exhibits a local network of hydrogen bonds involving the active site aspartates and surrounding residues that may have an influence on the rate and optimal pH of substrate cleavage. Bovine chymosin is synthesized in vivo as preprochymosin and secreted as prochymosin, which is autocatalytically activated to chymosin.

Charge interactions between the histidines on kappa-casein and the glutamates and aspartates of chymosin initiate binding of the enzyme to the substrate. When chymosin does not bind the substrate, a beta hairpin — sometimes called "the flap" — can hydrogen bond with the active site, covering it and preventing further binding of the substrate.

3.2 Precursor Forms: Preprochymosin and Prochymosin

Chymosin is produced intracellularly as preprochymosin. Preprochymosin is shortened by 16 amino acids during secretion and appears in the stomach as prochymosin, which, in turn, is activated to chymosin by cleavage of an additional 42 amino acids. Chymosin activation occurs at low pH (3.0–3.5) and is accompanied by the removal of a 42 amino acid propeptide, resulting in active chymosin.

In the acidic environment of the stomach, chymosin is activated by disruption of the electrostatic interactions between this prosegment and the active site, followed by proteolytic removal of the 42-amino-acid chymosin prosegment. Prochymosin is a substantially inactive form of the enzyme which becomes activated under acidic conditions to the active chymosin by autocatalytic removal of the pro-fragment.

3.3 Species Variants and Comparative Properties

Researchers have obtained and studied recombinant chymosins of sheep (Ovis aries), goat (Capra hircus), water buffalo (Bubalus arnee bubalis), and camel (Camelus dromedarius).

Camel chymosin has received particular research attention. Despite having 85% sequence identity with bovine chymosin, camel chymosin shows a 70% higher milk-clotting activity than bovine chymosin towards bovine milk. Camel chymosin exhibits a 70% higher clotting activity for bovine milk and has only 20% of the unspecific protease activity of bovine chymosin. This results in a sevenfold higher ratio of clotting to general proteolytic activity. The enzyme is also more thermostable than bovine chymosin.

Cow's milk is generally used for investigating milk-clotting activity, but mutual adaptations have occurred between chymosin and casein for any given species. Pig chymosin is 6–8 times more active against sow's milk than against cow's milk; conversely, calf chymosin clots sow's milk with only half of its activity against cow's milk.

4. Mechanism of Action

4.1 Primary Proteolysis: κ-Casein Cleavage

In order to understand how chymosin coagulates milk, one needs to know about milk proteins. The majority of milk protein is casein, and there are four major types of casein molecules: alpha-s1, alpha-s2, beta, and kappa. The alpha and beta caseins are hydrophobic proteins that are readily precipitated by calcium — the normal calcium concentration in milk is far in excess of that required to precipitate these proteins.

Kappa casein is a distinctly different molecule — it is not calcium-precipitable. As the caseins are secreted, they self-associate into aggregates called micelles, in which the alpha and beta caseins are kept from precipitating by their interactions with kappa casein. In essence, kappa casein normally keeps the majority of milk protein soluble and prevents it from spontaneously coagulating.

Chymosin proteolytically cleaves and inactivates kappa casein, converting it into para-kappa-casein and a smaller protein called macropeptide. Para-kappa-casein does not have the ability to stabilize the micellar structure, and the calcium-insoluble caseins precipitate, forming a curd.

4.2 Two-Phase Coagulation

Enzymatic milk coagulation is a two-phase process: a first phase where a proteolytic enzyme — chymosin or pepsin — attacks κ-casein, resulting in a metastable state of the casein micelle structure, and a second phase where the milk subsequently coagulates and forms a coagulum.

When the specific bond between the hydrophobic (para-casein) and hydrophilic (acid glycopeptide) groups of casein is broken, the hydrophobic groups come together and form a 3D network that traps the aqueous phase of the milk.

4.3 Role in Digestion

The role of chymosin in digestion is to curdle or coagulate milk in the stomach, a process of considerable importance in the very young animal. If milk were not coagulated, it would rapidly flow through the stomach and miss the opportunity for initial digestion of its proteins. Chymosin efficiently converts liquid milk to a semisolid like cottage cheese, allowing it to be retained for longer periods in the stomach.

4.4 Role in Cheese Ripening

In most cheeses, chymosin is responsible for "primary" proteolysis, which leads to the release of peptides that are later used by lactic acid bacteria for "secondary" proteolysis and flavour formation during ripening. Chymosin hydrolyzes αS1-CN at several sites during the early stages of ripening. In studies of Cheddar cheese ripening, less than 15% of the original αS1-CN remained intact after 90 days. Due to its low water activity, β-CN is highly resistant to hydrolysis by chymosin, but about 50% is degraded during Cheddar cheese ripening after 270 days.

5. Scientific Evidence by Area of Application

5.1 Cheesemaking and Industrial Food Technology

Evidence strength: Extensive, well-established scientific literature.

Just one part enzyme to 15,000 parts milk is enough to achieve coagulation. This exceptionally high potency is well-documented industrially and in peer-reviewed biochemistry literature. Although several proteolytic enzymes will clot milk, the best cheeses have been produced from milk clotted with preparations rich in chymosin. The superiority of chymosin for cheesemaking is probably related to the highly specific manner in which it attacks its substrate, kappa-casein.

5.2 Camel Chymosin in Cheddar Cheese Production

Evidence strength: Peer-reviewed comparative studies; moderately strong for industrial cheesemaking outcomes.

The properties of fermentation-produced camel chymosin were compared with those of calf chymosin; camel chymosin has a 70% higher clotting activity per mol on bovine milk than calf chymosin but only 20% of its general proteolytic activity and hence has a 7-fold higher ratio of clotting to general proteolytic activity. A study published in the International Dairy Journal (2009) specifically compared these two coagulants in Cheddar cheese production. The results showed that adding 30% less IMCU/mL camel chymosin compared with calf chymosin gave a coagulum with the same gel strength at the desired cutting time (45 min). There were no significant differences (p > 0.05) in composition and pH between the 60-day-old cheeses. The results of this study suggest that camel chymosin appears to be suitable for making Cheddar cheese with lower levels of proteolysis but with good flavour.

A structural study published in Acta Crystallographica Section D (2013, PubMed PMID 23633601) examined both enzymes' crystal structures. Despite having 85% sequence identity, camel chymosin shows a 70% higher milk-clotting activity than bovine chymosin towards bovine milk. Bovine and camel chymosin are aspartic peptidases used industrially in cheese production. They cleave the Phe105-Met106 bond of the milk protein κ-casein, releasing its predominantly negatively charged C-terminus, which leads to the separation of the milk into curds and whey.

5.3 Application to Camel Milk Cheese

Evidence strength: Preliminary; small-scale experimental studies, no large clinical trials.

The difficulty in producing high-quality cheeses from camel milk is attributed to larger casein micelle size, long coagulation time, low amount of κ-casein, and the small size of fat globules compared to bovine milk. Production of cheese from camel milk has been challenging due to the lack of coagulants that can specifically cleave camel milk κ-casein. A mixture of Withania coagulans and camel chymosin produced better quality camel and bovine milk cheeses than chymosin alone. An enzyme concentration corresponding to 36 µg/L of milk of W. coagulans plus 50 IMCU/L of camel chymosin, a holding time of 4 hours, and an incubation temperature of 60°C provided the optimal textural hardness for both camel and bovine milk cheeses.

5.4 Pharmaceutical and Dietary Supplement Applications: Gastrointestinal Disorders

Evidence strength: Patent-stage and mechanistic rationale only; no published clinical trials identified.

Beyond its established role in dairy processing, chymosin has been explored for medical applications, particularly in addressing gastrointestinal disorders associated with impaired milk digestion. A 2009 patent describes the use of oral chymosin supplements to prevent and treat conditions such as infantile colic, heartburn, gastro-esophageal reflux (GER), and gastro-esophageal reflux disease (GERD) by facilitating the hydrolysis of caseins in low-acid environments, which is beneficial for infants with developing digestive systems.

The patent is directed to medical or dietary treatments comprising administration of a zymogen such as chymosin or other rennins, with embodiments directed to treating infant colic, heartburn, GER, GERD, irritable bowel syndrome, and recurrent abdominal pain. The invention is also directed to improving absorption of calcium from milk for osteoporosis patients.

The treatment options for neonatal colic are limited, and those for gastro-esophageal reflux are, in a significant number of patients, often disappointing. The patent claims that milk with added chymosin represents an important treatment alternative with no significant side effects.

It is critical to note that these claims originate from patent documentation, not from peer-reviewed human clinical trials. No randomized controlled trials or systematic reviews evaluating chymosin as an oral dietary supplement for these gastrointestinal indications were identified in the published scientific literature. The evidence for these therapeutic applications must therefore be considered speculative and unverified until clinical trials are conducted and published.

5.5 Chymosin as a Research Tool and Pharmaceutical Model

Evidence strength: Well-established in biochemistry research; indirect pharmaceutical relevance.

In pharmaceutical research, chymosin serves as a structural model for designing inhibitors of aspartic proteases due to its homology with therapeutically relevant enzymes. The broader family of aspartic proteases includes HIV-1 protease, renin, and beta-secretase (BACE1), all of which are validated drug targets, and chymosin's well-characterized crystal structure provides a template for inhibitor design research.

6. Body Systems and Health Areas Associated with Chymosin

6.1 Gastrointestinal System (Physiological Role in Young Ruminants)

Chymosin is produced by newborn ruminant animals in the lining of the abomasum to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. This physiological role is well established. Chymosin is similar to pepsin in being most active in acidic environments, which makes sense considering its mission in the neonatal stomach.

6.2 Proteolytic and Digestive Biochemistry

Chymosin also exhibits proteolytic activity that is significantly lower than that of pepsin. These dual activities — specific milk coagulation and limited broader proteolysis — allow the use of chymosin as a key coagulation enzyme in cheesemaking.

6.3 Potential Applications in Human Gastrointestinal Health

The proposed approach for oral chymosin supplementation leverages chymosin's specificity for κ-casein cleavage to mimic natural gastric proteolysis, potentially aiding in cases of hypochlorhydria where stomach acid production is insufficient for effective protein breakdown. As noted in Section 5.4, this remains at the patent/hypothesis stage and has not been substantiated by clinical trials in the published literature.

7. Dosage Forms and Reported Dosages

7.1 Industrial Food Processing

In published Cheddar cheese research, camel chymosin was added to cheesemilk at a level of 0.04 IMCU/mL milk (approximately 3.5 mL per 100 L milk), and calf chymosin was used at 0.06 IMCU/mL milk (30 mL per 100 L milk), with both chosen to achieve the same curd strength.

In another comparative Mozzarella cheese study, both bovine and camel chymosins were used at 2 levels: 0.05 and 0.037 international milk clotting units (IMCU)/mL.

In camel milk cheese research, an enzyme concentration of 50 IMCU/L of camel chymosin combined with an incubation temperature of 60°C provided optimal textural hardness for both camel and bovine milk cheeses.

7.2 Proposed Oral Supplement Dosages

In the patent framework for oral supplementation, infants may be given several drops of a composition containing chymosin prior to every feeding, regardless of whether they are bottle or breast-fed. The composition containing chymosin can also be incorporated into a food product such as drinking milk or baby formula. No specific mg/kg or unit dosages for human oral supplementation are established in the peer-reviewed literature, as no clinical trials have been completed and published.

7.3 EFSA Dietary Exposure Estimates

In the context of European regulatory safety evaluation of chymosin as a food enzyme in dairy products, dietary exposure was estimated to be up to 0.73 mg total organic solids (TOS)/kg body weight per day in European populations. These figures reflect background exposure from chymosin-processed dairy products, not intentional supplementation.

8. Safety Considerations and Regulatory Status

8.1 Regulatory Approvals

Fermentation-derived chymosin was the first food ingredient produced from a genetically modified microorganism to be approved for use in food by the FDA, and it is now included at 21 CFR 184.1685. Regulatory authorities in Australia/New Zealand, France, the EU, Canada, and JECFA have all determined that various fermentation-derived chymosins are safe and acceptable for use in food processing.

Regulators approved this approach on the basis that the final enzyme is chemically identical to natural chymosin and contains no living genetically modified material. In the United States, the FDA granted FPC generally recognized as safe (GRAS) status after reviewing compositional data and limited toxicological studies, including a 90-day rat feeding trial.

8.2 EFSA Toxicological Evaluations

The European Food Safety Authority (EFSA) has conducted multiple peer-reviewed safety evaluations of different fermentation-produced chymosins. For chymosin from genetically modified Kluyveromyces lactis: the genetic modifications do not give rise to safety concerns. The food enzyme is free from viable cells of the production organism and its recombinant DNA. Genotoxicity tests did not raise a safety concern. The systemic toxicity was assessed by means of a repeated dose 90-day oral toxicity study in rats. The Panel identified a no observed adverse effect level (NOAEL) of 1,000 mg TOS/kg body weight per day, the highest dose tested, which when compared with the estimated dietary exposure, results in a margin of exposure of at least 1,300.

For chymosin from genetically modified Trichoderma reesei (EFSA, 2025): genotoxicity tests did not indicate a safety concern. The systemic toxicity was assessed by means of a repeated dose 90-day oral toxicity study in rats. The Panel identified a NOAEL of 1,000 mg TOS/kg bw per day, the highest dose tested, which when compared with the estimated dietary exposure results in a margin of exposure of at least 76,923.

Based on the data provided, the EFSA Panel concluded that this food enzyme does not give rise to safety concerns under the intended conditions of use.

8.3 Allergenicity

A search for the homology of the amino acid sequence of chymosin to known allergens identified matches with five respiratory allergens and one injected allergen. The Panel considered that a risk of allergic reactions upon dietary exposure to the food enzyme cannot be excluded.

The Panel considered that under the intended conditions of use the risk of allergic sensitisation and elicitation reactions by dietary exposure, although unlikely, cannot be excluded, particularly for individuals sensitised to cedar pollen allergens.

With respect to milk-allergic individuals consuming chymosin-processed cheese: κ-CN is cleaved by chymosin during the primary stage of cheesemaking at the peptide bond F105-M106. The C-terminal 106–169 fragment — known as glycopeptide because all the glucides originally present in κ-CN are retained — is released and lost in the whey. Therefore, cheese is devoid of any glycosylated major component. Nonetheless, cow milk is a common allergenic food, and cow milk-derived cheese retains an appreciable level of allergenicity.

8.4 WHO/JECFA Evaluation History

Health aspects of rennet as a food ingredient were reviewed and evaluated at the fifteenth meeting of the Joint FAO/WHO Expert Committee on Food Additives in 1972. Subsequent JECFA evaluations have reviewed chymosins produced by genetic engineering in multiple microbial host organisms, consistently finding the preparations safe for use as food processing aids.

8.5 Absence of Viable Recombinant DNA in Final Product

The genetic modifications of the production organism do not give rise to safety concerns. The food enzyme is free from viable cells of the production organism and its recombinant DNA. It is intended to be used in milk processing for cheese production and for the production of fermented milk products.

8.6 Interaction with Milk Proteins During Ripening — Implications for Allergenicity

During cheese ripening, milk proteins are degraded by chymosin and milk-derived and bacterial proteases. Commercial allergen-detection methods are not validated for the detection of residues in fermented or hydrolyzed products. This creates a practical consideration in food allergen management, as standard ELISA-based detection kits may underestimate residual allergenic protein in extensively ripened cheeses where chymosin has carried out prolonged proteolysis.

References

Health Conditions

Health conditions that Chymosin may help support.

No conditions available.

Body Systems

Body systems that Chymosin may help support.

No body systems available.