The Technical Differences
The primary difference lies in Enzymatic Specificity (how precise the enzyme is) and Proteolytic Activity (how aggressively it breaks down proteins).
1. Specificity (The "Sniper" vs. The "Shotgun")
- Chymosin: It is highly specific. It targets one specific bond in the milk protein (the Phenylalanine105-Methionine106 bond of kappa-casein). By cutting this single link, the milk coagulates into a stable gel (curd) without damaging the rest of the protein structure.
- R. miehei: It is less specific. While it does cut the target bond to clot the milk, it also attacks other peptide bonds within the casein micelle. This "non-specific" activity can result in a weaker curd structure and the loss of fat and protein into the whey, slightly reducing cheese yield.
2. Flavor Development (The Bitterness Factor)
- Chymosin: Because it stops working after the curd is formed, it produces a "clean" flavor profile.
- R. miehei: Because it has higher general proteolytic activity, it continues to break down proteins during the aging process. When proteins are chopped up randomly, they release hydrophobic peptides, which taste distinctly bitter.
- Note: This is the main historical drawback of microbial rennet. If you use it in a cheese aged for 12 months (like an aged Cheddar), the cheese will likely turn bitter and mushy.
3. Thermostability (Heat Resistance)
- Chymosin: Is heat-labile (it deactivates easily during the heating/cooking steps of cheesemaking).
- R. miehei: The native enzyme is very heat-stable. It can survive the cheesemaking process and remain active in the whey. This is problematic if the whey is processed later, as the residual enzyme can spoil whey protein concentrates. (However, modern "XL" or "thermally destabilized" versions of R. miehei have been developed to solve this).
Part 2: Why is Rhizomucor miehei still used?
If Chymosin (FPC) yields more cheese and tastes better, why does R. miehei exist? The answer lies in Labeling, Cost, and Specific Applications.
1. The "Non-GMO" and "Organic" Market
This is the single biggest reason for the continued use of R. miehei.
- Chymosin (FPC): Most modern chymosin is produced via Recombinant DNA technology. Scientists take the gene from a calf and insert it into a host (like Aspergillus mold or K. lactis yeast). Even though the enzyme is identical to the animal version, the process involves Genetic Modification (GMO).
- R. miehei: This is a naturally occurring mold. The rennet is produced by simply fermenting the mold and purifying the extract. It is not a GMO product.
- The Result: In markets like the EU or specifically "Non-GMO Project Verified" products in the US, producers cannot use FPC. They must use R. miehei to maintain their vegetarian and non-GMO status.
2. Cost Effectiveness
Historically, R. miehei coagulants have been significantly cheaper to produce than FPC. While the price gap has narrowed as FPC technology has matured, R. miehei remains a cost-effective solution for budget-conscious, high-volume manufacturers.
3. Short-Shelf-Life Cheeses
The "bitterness" issue mentioned earlier is a function of time.
- If you are making a fresh cheese (like Queso Fresco, Paneer, or young Mozzarella) that will be consumed within weeks, the enzyme does not have time to break down proteins and create bitter peptides.
- For these cheeses, the superior aging profile of Chymosin is unnecessary. Manufacturers can use the cheaper R. miehei without negatively affecting the taste.
4. Vegetarian Certification (Pre-FPC Legacy)
Before FPC was invented (1990s), R. miehei was the only viable option for vegetarian cheese. Many traditional recipes and production lines were optimized for R. miehei decades ago. Changing enzymes can alter the texture and melt-stretch qualities of the cheese; therefore, some producers stick with it to maintain product consistency.
Summary Comparison
Conclusion:
Producers choose Rhizomucor miehei not because it is a better enzyme functionally, but because it fits specific regulatory (Non-GMO) and economic niches, particularly for cheeses that do not require long aging periods.
