How a stomach enzyme from nursing calves became the secret to global cheese production

Nature · Published 29 September 2026 Swipe more like this →

In short: Rennet is the essential catalyst that transforms liquid milk into solid cheese. While traditionally sourced from the stomachs of young ruminants, modern cheesemaking has shifted toward fermentation-produced chymosin. This biotechnological marvel now dominates the global market, offering a consistent, efficient, and vegetarian-friendly alternative to the ancient practice of animal extraction.

Rennet is a complex mixture of enzymes, primarily chymosin, which acts on milk by cleaving the kappa casein chain. This process removes the negatively charged glycomacropeptide from the surface of casein micelles, allowing them to cluster and trap fats and minerals into a solid curd. Historically, this was achieved using extracts from the abomasum—the fourth stomach chamber—of nursing calves. Because ruminants produce enzymes specific to their own species' milk, traditional cheesemakers often matched the rennet source to the animal, such as using kid goat rennet for goat's milk.

The limitations of animal-derived rennet, including supply constraints and variable quality, led to the development of alternatives. Since Roman times, various plants like thistles, nettles, and fig juice have been used as coagulants. In the modern era, microbial sources became common, though early versions were often criticized for causing bitterness in aged cheeses. Today, these microbial coagulants have been refined through purification, allowing for high-quality production. Meanwhile, acid-set cheeses like paneer or cream cheese bypass rennet entirely, relying on vinegar, citric acid, or bacterial fermentation to coagulate milk proteins.

The most significant shift in the industry occurred with the advent of fermentation-produced chymosin (FPC). By isolating rennet genes and introducing them into bacteria, fungi, or yeasts, producers can create chymosin that is identical to the animal-derived enzyme but significantly purer. Since its FDA approval in 1990, FPC has become the industry standard, accounting for 90% of the global market share by 2017. Beyond its efficiency, FPC provides superior curd texture and reduced bitterness, while meeting the requirements for kosher, halal, and vegetarian diets. This transition from biological byproduct to engineered precision has fundamentally redefined the economics and scalability of cheese production worldwide.

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