Cheese is one of the most beloved foods in the world, a staple in countless diets, and an essential ingredient in everything from burgers to charcuterie boards. But what if I told you that the majority of cheese in the United States contains a hidden genetically modified organism (GMO) ingredient? One that isn’t required to be labeled? And what if I told you that this ingredient is produced by none other than Pfizer?

That’s right—around 90% of U.S. cheese is made using genetically engineered chymosin, a type of microbial rennet designed to separate curds from whey during the cheesemaking process. While rennet itself is nothing new (it’s been used for centuries to make cheese), the shift to genetically modified chymosin is relatively recent. The lack of transparency surrounding it raises serious questions: if GMOs like chymosin don’t have to be disclosed, what else are food manufacturers hiding?

For those of us who want to avoid GMOs as much as possible, this issue deserves more attention.

What Is Chymosin and Why Is It Used in Cheese?

Chymosin is an enzyme that plays a critical role in cheesemaking. Traditionally, it has been sourced from the stomach lining of young calves, where it helps them digest milk. Cheesemakers discovered long ago that this enzyme could be used to coagulate milk, separating curds from whey to form cheese.^[1-2]

For centuries, this process relied on animal rennet, which was extracted from the stomachs of calves. But as industrial cheesemaking expanded, manufacturers looked for cheaper and more consistent alternatives. This led to the introduction of microbial and genetically engineered rennet.

In the 1990s, scientists developed a way to produce chymosin using genetically modified bacteria, fungi, and yeast. Instead of harvesting rennet from animal stomachs, they inserted calf genes into these microorganisms, which then produced chymosin in large quantities. This GMO-derived chymosin was approved by the FDA in 1990, and since then, it has quietly taken over the cheese industry.^[3]

Why Is Most U.S. Cheese Made with GMO Chymosin?

The switch to genetically engineered chymosin happened for several reasons.

• Cost efficiency: Producing chymosin in a lab is significantly cheaper than harvesting animal rennet. Since it can be made in controlled environments, manufacturers get a consistent product without relying on livestock.
• Vegetarian appeal: Some consumers prefer cheese made without animal-derived rennet, even if that means eating a GMO ingredient instead.
• Scalability: Large-scale cheese production benefits from the predictability and efficiency of lab-produced chymosin, which allows for mass production with minimal variability.

But the problem isn’t necessarily that this ingredient exists—it’s that it’s used in about 90% of U.S. cheese, and there is no requirement to label it as genetically modified.

Pfizer’s Role in GMO Chymosin Production

Pfizer may be best known for pharmaceuticals, but the company has a long history in the food industry as well. For decades, Pfizer’s subsidiary, Gist-Brocades, was one of the primary manufacturers of microbial rennet and genetically engineered chymosin. Today, DSM (which acquired Gist-Brocades) and other biotech companies continue producing it, but Pfizer’s involvement in its initial commercialization remains an important piece of the puzzle.

This raises concerns for those wanting more transparency about their consumption. If a pharmaceutical giant developed an ingredient in nearly all non-organic cheese and isn’t required to be labeled as a GMO, it’s worth asking: what else in our food supply is hidden from us?

Why GMO Chymosin Isn’t Labeled

Most people assume that if a food contains GMOs, it will be labeled as such. Unfortunately, that’s not the case with many food-processing ingredients.

Because chymosin itself is an enzyme rather than a whole-food ingredient, the FDA does not require it to be labeled as genetically modified. This means that you could be eating GMOs without knowing it.

Labeling laws in the U.S. allow for these kinds of loopholes, especially regarding food-processing aids. Unlike whole GMO crops like corn or soy, processing enzymes like chymosin fall under a different set of regulations, which means food manufacturers can use them without informing consumers.

How to Avoid GMO Cheese

If you prefer to avoid genetically modified ingredients in your food, there are ways to make sure your cheese is free from GMO chymosin.

• Choose organic cheese. Organic certification prohibits the use of genetically engineered ingredients, including chymosin. Look for USDA-certified organic cheeses to ensure you’re avoiding GMOs.
• Look for cheese made with animal rennet. Traditional cheese made with calf rennet will not contain genetically modified chymosin. However, not all cheese labels specify the rennet source, so you may need to check with the manufacturer.
• Find cheese made with non-GMO microbial rennet. Some cheesemakers use microbial rennet that is not genetically modified. This is different from the GMO-derived chymosin that dominates the market. If the label doesn’t specify, reach out to the company for more information.
• Support artisanal cheesemakers. Many small-scale, traditional cheesemakers use non-GMO rennet and are transparent about their processes. These cheeses may cost more but are often made with higher-quality ingredients.

Final Thoughts: Demand Transparency in Your Food

The issue with genetically engineered chymosin isn’t just about GMOs—it’s about transparency. The fact that 90% of U.S. cheese is made with a genetically modified ingredient that doesn’t have to be disclosed raises concerns about what else might be hiding in our food.

Even if you aren’t strictly anti-GMO, you deserve to know what you’re consuming. Food manufacturers and regulatory agencies should not have the power to make these decisions without consumer knowledge.

For those who care about avoiding GMOs, the best course of action is to choose organic, research brands, and support cheesemakers who use traditional or non-GMO methods. The more we demand transparency in food production, the more likely companies will be to prioritize honesty and quality over hidden ingredients.

References:

1. Nicosia, Fabrizio Domenico, et al. “Plant Milk-Clotting Enzymes for Cheesemaking.” Foods, vol. 11, no. 6, Mar. 2022, p. 871.
2. History of Cheese.” NHCC Museum and Green County Welcome Center, https://nhccmuseum.org/history-of-cheese/. Accessed 12 Feb. 2025.
3. Genetic Engineering Has Been Applied to Cheese Making: No Reason to Be Cheesed Off.” Office for Science and Society, https://www.mcgill.ca/oss/article/health-and-nutrition-technology/genetic-engineering-has-been-applied-cheese-making-no-reason-be-cheesed. Accessed 12 Feb. 2025.