The cheese in your fridge almost certainly contains a genetically engineered enzyme that was never disclosed to you. It does not appear on the label by name. It is not mentioned anywhere on the packaging. The word on the ingredient panel is simply "Enzymes." That is the entire disclosure.
That enzyme is fermentation-produced chymosin, commonly called FPC. It was created by Pfizer in the late 1980s by splicing the gene that makes chymosin — the milk-curdling enzyme found in a calf's stomach — into bacteria, yeast, or mold. In 1990, it became the first genetically engineered ingredient ever approved for the United States food supply. Today it is present in an estimated 90% of all cheese manufactured in the United States.
How traditional cheese rennet works
For roughly 7,000 years, cheesemakers used animal rennet: a complex mix of enzymes extracted from the stomach lining of young ruminants. The dominant enzyme in animal rennet is natural chymosin. When you add it to warm milk, the casein proteins coagulate, curds form, whey drains off, and you get cheese.
The result is a product made from four ingredients: milk, salt, starter culture, and animal rennet. Cheese made this way needs nothing else to hold together, and the residual enzyme mix — including small amounts of pepsin and lipase alongside chymosin — appears to contribute to better flavor and digestibility.
What Pfizer changed in 1990
In the late 1980s, Pfizer's biotechnology division isolated the specific DNA sequence in calf cells that codes for chymosin. Using recombinant DNA technology — gene splicing — they inserted that sequence into a host microorganism, either a bacterium, a yeast, or a mold depending on the production strain. The microorganism was then fermented in industrial vats, and the chymosin it produced was extracted, stripped, filtered, and bottled as a liquid enzyme solution.
The FDA approved this product in March 1990 based on what it called "substantial equivalence" — the logic that if the enzyme produced by the GMO was chemically identical to the calf enzyme, it should be treated the same way. On the strength of that argument, Pfizer's FPC was granted GRAS status (Generally Recognized as Safe), which exempted it from the standard pre-market safety review required for new food additives.
The clinical safety data behind the approval was a single 90-day rat study. No long-term human trials were conducted before the ingredient entered the food supply.
Why it is in 90 percent of cheese
FPC costs roughly half as much as animal rennet. It produces consistent results at industrial scale. It also allows dairy companies to market their cheese as vegetarian-friendly, since no calf stomach is involved.
The FDA determination that FPC did not require labeling as a GMO ingredient sealed its adoption. Manufacturers are not required to tell you whether they used animal rennet, microbial rennet, or FPC. The label says "Enzymes." That word covers all three. You have no way to tell from the packaging alone.
The American Cheese Society has confirmed this directly: "FPC rennet is a genetically modified organism (GMO). According to the culture companies, 90% of North American cheese is made with FPC rennet. But ingredient labels do not distinguish between this type of microbial rennet and the original non-GMO type."
Brands whose cheese is routinely made with FPC include Kraft, Sargento, Tillamook, Cabot, and Kirkland. This covers the overwhelming majority of the cheese aisle in any US grocery store.
The safety questions that remain open
The FDA's GRAS designation essentially transferred safety responsibility from the agency to the manufacturers. As former FDA Deputy Commissioner for Foods Michael Taylor put it in 2014: "We simply do not have the information to vouch for the safety of many of these chemicals."
Several specific concerns have been raised in the peer-reviewed literature since FPC entered the food supply.
The first is contamination from the host organism. The enzyme is produced by fermenting a genetically modified microorganism. The final product is purified, but traces of the host — including its cellular byproducts — may remain. Food science researchers have documented that "genetically modified food enzymes are currently produced from GMOs. Safety concerns have been raised regarding potential contamination of food with bacterial toxins or mycotoxins, allergens, or uncharacterized extraneous substances as impurities." A living genetically modified microorganism has been found in at least one enzyme preparation in the scientific literature.
The second concern is allergenicity. The mold or yeast used to produce FPC is itself a potential allergen. The European Food Safety Authority has noted that allergic reactions to the host microorganisms cannot be excluded. Industry workers exposed to these enzymes in manufacturing settings have shown measurable rates of respiratory sensitization.
The third is long-term unknown risk. FPC has been in 90% of American cheese for 35 years. There is no 35-year human safety data. "Currently, the companies themselves are responsible for the quality control of their products," according to food safety researchers reviewing the regulatory framework.
What the label does not tell you
When you pick up a block of cheddar, the ingredient panel will list the rennet as one of three vague terms: "Enzymes," "Microbial rennet," or "Vegetable rennet." All three can refer to FPC. None of them signal that the enzyme was produced by a genetically modified organism. The FDA does not require that disclosure.
The terms "vegetable rennet" and "microbial rennet" are particularly misleading. True vegetable rennet comes from plants like cardoon thistle. Real microbial rennet comes from naturally occurring mold strains that are not genetically modified. When manufacturers use these terms to describe FPC, the consumer has no way to know the difference.
The Non-GMO Project classifies FPC as a high-risk ingredient. USDA Organic certification prohibits it entirely — if a cheese carries the USDA Organic seal, it was made without FPC. That is the clearest signal available at retail.
How to avoid FPC in your cheese
The most reliable options:
USDA Organic certified cheese. Organic rules prohibit GMO-derived processing aids, including FPC. Cheese sold as USDA Organic must use animal rennet, traditional microbial rennet, or plant-based rennet.
Non-GMO Project Verified cheese. The Non-GMO Project tests for FPC and will not verify products that use it.
Imported European cheese. Many European cheeses, particularly from Italy and France, are made under rules that restrict or prohibit FPC. Parmigiano-Reggiano, for example, must by law be made with animal rennet.
Small-batch domestic cheesemakers. Many artisan cheesemakers still use animal rennet for the flavor advantage and tradition. Ask directly.
The rest of the cheese aisle — the standard American slices, shreds, blocks, and cream cheeses — almost certainly contain the Pfizer enzyme. It has been there since 1990. The label has never told you.
Scan what is actually in your cheese
This is the kind of unlabeled ingredient that Tallow was built to surface. Point your phone at any cheese barcode and Tallow will flag what the label buries, rate the ingredient quality, and show you cleaner alternatives. The word "Enzymes" on a package carries more information than most people realize. You should have the tools to read it.