Rennet: enzymes used to coagulate milk and make cheese

Composition and mechanism

The principal active enzyme in many traditional preparations is chymosin (also called rennin), a protease that specifically cleaves kappa-casein on the surface of casein micelles. That action destabilizes the micelles and, together with calcium and appropriate temperature and acidity, leads to aggregation into a gel. Other proteases such as pepsin may be present in crude stomach extracts and influence flavor by further protein breakdown.

Sources and production

Rennet preparations come from several origins. Animal rennet is extracted from the lining of the fourth stomach of young ruminants. Microbial rennets are derived from certain fungi or bacteria that produce proteases. Fermentation-produced chymosin (FPC) is made by genetically engineered microorganisms that synthesize chymosin identical or very similar to the natural enzyme. Vegetable or plant-based coagulants, obtained from thistle, fig sap or nettles, have been used in regional traditions and by vegetarian producers. For more on the traditional sources and modern alternatives, see specialist references.

Uses, examples and importance

• Primary use: coagulation in cheese production (soft, semi-hard and hard cheeses).
• Choice of rennet affects texture, yield and flavor development during aging.
• Small-scale and artisanal cheesemakers may prefer animal or plant coagulants for traditional character, while industrial producers favor FPC or microbial enzymes for consistency.

Rennet is part of a broader enzyme family used in food processing; its selection is guided by desired cheese type, dietary rules and labeling requirements. Understanding rennet helps explain many differences in cheese texture and flavor around the world.

Dietary and regulatory notes: consumers seeking vegetarian, kosher or halal products should check labels, since rennet origin affects acceptability. Many jurisdictions require producers to declare the type of coagulant used.