The Hidden Ingredient in Your Cheese: What to Know About GMO Rennet and How to Find the Real Thing
- brigittebatte
- Mar 13
- 5 min read
Updated: May 2
For centuries, cheesemaking was a simple, natural process—milk, rennet, salt, and time. But in today’s food industry, shortcuts have become the norm. While traditional cheesemakers rely on time-honored methods, the vast majority of modern cheese—especially in the U.S.—is now made using a lab-engineered ingredient that most consumers don’t even realize they’re eating: genetically modified (GMO) rennet.
But what is GMO rennet, and how does it compare to the real thing? More importantly, why do some cheesemakers refuse to take the shortcut, and how can consumers support them? Let’s break it down.
What Is Rennet and Why Does It Matter?
Rennet is the key enzyme that turns milk into cheese. Traditionally, it comes from the stomachs of young calves, where a natural enzyme called chymosin helps them digest milk. Cheesemakers have used calf rennet for centuries to coagulate milk into curds, the first step in making everything from cheddar to parmesan.
In response to growing cheese demands, scientists found a way to genetically engineer bacteria, fungi, and yeast to produce chymosin in the lab. This GMO-derived rennet, called fermentation-produced chymosin (FPC), is now used in over 90% of U.S. cheese production. It functions identically to natural rennet on a chemical level, but it’s produced in industrial fermentation tanks rather than sourced from an actual animal.
While FPC rennet makes cheesemaking cheaper and more consistent, many artisan and traditional cheesemakers refuse to use it—not because it doesn’t work, but because they believe that cheese should be made the way nature intended.
How Traditional Cheesemakers Are Different
While industrial cheese manufacturers have largely replaced animal rennet with GMO alternatives, many artisanal, Amish, and European cheesemakers stick to traditional rennet for flavor, ethics, and heritage.
Artisan cheesemakers often believe that real rennet contributes to deeper, more complex flavors, especially in long-aged cheeses like Parmigiano-Reggiano or Gruyère. Traditional rennet contains not only chymosin but also other enzymes that influence how a cheese matures over time. Some experts argue that cheese made with natural rennet has a richer texture and depth compared to GMO rennet.
The Amish and small dairy co-ops reject GMO rennet, often using microbial (non-GMO) or animal rennet instead. Amish cheesemaking follows a slower, more traditional process, avoiding industrialized shortcuts.
Some European countries, including France and Italy, ban GMO rennet in traditional cheeses. Parmesan, for example, must be made with calf rennet to carry the official “Parmigiano-Reggiano” label. The French government argues that natural rennet preserves culinary tradition and environmental sustainability.
Should You Be Concerned About GMO Rennet?
GMO rennet is considered safe by the FDA and does not remain in the final cheese product. However, concerns go beyond food safety:
Labeling Transparency – In the U.S., cheeses made with GMO rennet are not labeled as such. If the label says “enzymes” or “microbial rennet,” there’s a good chance it was made with GMO-derived FPC.
Ethical Considerations – Some consumers prefer cheese made without genetic modification. While GMO rennet is technically “vegetarian,” its development relies on genetic engineering, which some people avoid for personal, environmental, or religious reasons.
Flavor & Quality – Traditional cheesemakers argue that natural rennet contributes to better aging and depth of flavor, especially for artisanal cheeses.
Supporting Small Farms – The dominance of corporate-controlled GMO rennet has pushed traditional calf rennet to niche status, making it harder for small producers to access.
For consumers who value tradition, transparency, and flavor, seeking out non-GMO or traditional rennet cheeses is a way to support cheesemakers who refuse to take shortcuts.
How to Find Non-GMO or Traditional Rennet Cheese
If you want to avoid GMO rennet and support real cheesemaking, here’s what to look for:
✓ Choose Certified Organic Cheese – USDA organic standards prohibit genetically modified ingredients, including FPC rennet.
✓ Look for PDO (Protected Designation of Origin) Labels – European cheeses like Parmigiano-Reggiano, Manchego, and Roquefort require traditional rennet by law.
✓ Buy from Artisan Cheesemakers – Many small dairies and Amish cheese producers use natural or microbial (non-GMO) rennet. Brands like Jasper Hill Farm, Cowgirl Creamery, and some Amish cheese co-ops avoid industrial rennet.
✓ Check Labels Carefully – If a cheese is made with “microbial enzymes,” “vegetable rennet,” or “vegetarian rennet,” it could be either GMO or non-GMO. To be sure, look for brands that explicitly state “Non-GMO rennet” or “Traditional rennet.”
✓ Ask Your Cheesemonger – Local cheese shops and farmers’ markets often carry cheese made with traditional rennet. Don’t be afraid to ask!
Choose Cheese with Integrity
For most people, cheese is a comfort food—but few realize how much modern cheese has changed due to industrial shortcuts. While GMO rennet makes mass cheese production cheaper and easier, many artisan cheesemakers, Amish dairies, and European traditions hold firm in keeping cheesemaking authentic.
Every choice tells a story, and food is no exception. The label on a wedge of cheese can reveal more than just ingredients—it can hint at where it came from, how it was made, and who put in the work to bring it to your table. What you do with this information is entirely your call. But if you ever find yourself wondering, the information is there—you just have to look.
Resources
Shelburne Farms – Webb, S. (2020). Demystifying rennet, a key ingredient in cheesemaking. Shelburne Farms Blog. Describes types of rennet and notes that 80–90% of US and UK cheeses used fermentation-produced chymosin as of 2008, following Pfizer’s 1990 development of GMO chymosin
Taylor & Francis / Modern Industrial Microbiology – Okafor, N., & Okeke, B. (2017). Chymosin. States that over 90% of chymosin used today is produced by genetically engineered microbes, and that this recombinant chymosin is preferred for its purity and consistency
Vegetarian Resource Group (VRG) – Yacoubou, J. (2008). An Update on Rennet. Vegetarian Journal, Issue 3. Provides statistics on U.S. cheese rennet usage: by 2008 about 70% of U.S. cheeses used FPC (GMO rennet), ~25% used microbial, and <5% used calf rennet
vrg.org. Confirms that organic cheesemakers opt for non-GMO microbial rennet.
Ohio State University Commodity Chain Project – LeJeune, E. (2015). Rennet. Explains the supply shortage that led to GMO rennet use in the 1990s and notes France’s ban on GMO rennet (viewing it as better for environment and taste) and the lack of GMO labeling for FPC rennet in many countries.
Simply Cheese (Amish Cheese retailer) – Amish Traditional Cheese (product page) and blog post The Microbial Rennet in Our Amish Cheeses (2024). Describes an Amish cheese cooperative in OH/PA that makes non-GMO, kosher cheeses using vegetable/microbial rennet instead of animal rennet, highlighting the sustainable and ethical aspects of microbial rennet use.
Food Renegade – Hardy, K. (2013). “Is Your Cheese Made With GMOs?” Discusses the prevalence of GMO rennet in cheeses and FDA’s stance. Notes that FPC chymosin was deemed substantially equivalent to calf rennet by FDA (no labeling required) and clarifies labeling terms (e.g. “vegetarian rennet” often refers to FPC). Also mentions European policies on GMO rennet
Sacco System / Food Chemistry Journal – Camina, F. et al. (2019). Assessing the authenticity of animal rennet using δ¹⁵N analysis of chymosin. Food Chemistry, 274, 545-549. Confirms that fermentation-produced chymosin functions similarly to animal rennet but is much cheaper, which is why it’s the most competitive alternative. Also notes PDO cheeses like Parmigiano-Reggiano mandate animal rennet
PMC Journal (Bioresources) – Khassenov, B. et al. (2021). Constitutive expression of Camel chymosin in Pichia pastoris. Reports that “more than 90% of rennet utilized today is produced by fermentation” and highlights that FPC rennet is accepted as kosher and halal
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