Rennet
Definition
What 'Rennet' means
Rennet contains chymosin, which cleaves casein to form curds. Traditional animal rennet requires slaughtering young calves, while microbial or fermentation-derived rennet offers a vegetarian alternative. Most industrial cheese now uses non-animal rennet, though some artisanal varieties still rely on calf rennet.
Related terms
In depth
The longer answer
Rennet is a complex of enzymes, primarily chymosin, traditionally extracted from the stomach lining of unweaned calves, lambs, or kids. It is the essential coagulant used to curdle milk in the production of nearly all traditional cheeses. Consumers encounter rennet indirectly whenever they eat cheese, as it is a standard ingredient listed on labels, though its animal origin is often not specified.
In commercial cheesemaking, rennet is added to warmed milk, where its enzymes cleave a specific protein, kappa-casein, on the surface of casein micelles. This destabilizes the micelles, causing them to aggregate into a gel-like curd that traps fat and water. The curd is then cut, heated, and pressed to form cheese. The vast majority of cheese produced today uses either animal rennet, microbial rennet from fungi, or fermentation-produced chymosin (FPC) from genetically modified microbes.
For consumers seeking to avoid animal-derived ingredients, rennet source is a critical detail. Many hard and semi-hard cheeses, especially traditional European varieties like Parmigiano-Reggiano and many Goudas, still use animal rennet. However, a growing number of plant-based and microbial rennets produce identical results. Reading labels for "vegetarian rennet," "microbial rennet," or "non-animal rennet" is essential, though some cheeses labeled "vegetarian" may still use animal rennet in certain regions.
At a glance
Quick facts
| Detail | What to know |
|---|---|
| Primary source | Calf, lamb, or kid stomachs |
| Key enzyme | Chymosin (rennin) |
| Common alternatives | Microbial or fermentation-produced chymosin |
| Label term | Often just "rennet" or "enzymes" |
In practice
What this means for you
- Check cheese labels for "vegetarian rennet," "microbial rennet," or "non-animal enzymes" to avoid calf-derived rennet.
- Many aged European cheeses like Parmigiano-Reggiano and Grana Padano legally require animal rennet.
- Ask at cheese counters whether a cheese uses animal rennet; staff often know the supplier's method.
- Plant-based rennet from thistle or fig sap is rare but used in some artisanal cheeses.
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