The Primary Digestive Enzyme: Lactase
For the human body, the main enzyme that acts on milk is lactase, also known as β-galactosidase. It is produced in the small intestine and is responsible for hydrolyzing lactose, the primary sugar found in milk. This process breaks the lactose disaccharide into two simpler, more easily absorbed sugars: glucose and galactose. For individuals with adequate lactase, this process occurs efficiently, and the sugars are readily absorbed into the bloodstream.
When a person has insufficient lactase production—a condition known as lactase deficiency—they may experience lactose intolerance. This can cause digestive symptoms such as bloating, gas, cramps, and diarrhea because the undigested lactose travels to the large intestine, where gut bacteria ferment it. To manage this, many people use lactase supplements, which contain the enzyme to help break down lactose when consuming dairy. Alternatively, they can opt for lactose-free milk, which is pre-treated with lactase during processing to break down the lactose beforehand.
The Coagulating Enzyme: Chymosin and Rennet
In the dairy industry, a different set of enzymes is used to transform milk, primarily in the production of cheese. The most well-known of these is chymosin, the key component of rennet. Chymosin is a protease enzyme that works to coagulate milk, separating it into solid curds and liquid whey.
How Chymosin Coagulates Milk
- Cleavage of Kappa-Casein: Chymosin specifically targets and cleaves the kappa-casein protein, which normally stabilizes the casein micelles (protein bundles) and keeps them suspended in the liquid milk.
- Micelle Destabilization: By snipping off the water-loving portion of the kappa-casein, chymosin destabilizes the micelles and removes their natural repulsion for one another.
- Aggregation: The now-destabilized casein proteins aggregate together, forming a gel-like substance known as curd.
This coagulation process is a critical first step in making nearly all cheeses, and the type of rennet used can significantly impact the final product's texture and flavor profile. Historically sourced from animal stomachs, rennet is now also widely available from microbial, vegetable, and fermentation-based sources to meet modern demands and dietary needs, such as vegetarianism.
Other Important Enzymes Acting on Milk
Several other enzymes, both native to milk and added during processing, play specific roles that influence dairy product quality, flavor, and shelf life.
Lipase
- Function: Lipase enzymes hydrolyze milk fats (triglycerides) into free fatty acids and glycerol.
- Flavor Development: These fatty acids are crucial for developing the characteristic flavors in certain aged cheeses, such as Swiss and Romano.
- Flavor Issues: Conversely, an overabundance of lipase activity can cause undesirable, rancid flavors in milk and other dairy products.
Proteases
- Function: Proteases break down proteins into smaller peptides and amino acids.
- Role in Milk: Indigenous milk proteases, such as plasmin, can affect the quality of milk and cheese, sometimes contributing to age gelation in UHT milk or developing bitter flavors during ripening.
- Industrial Use: Some proteases are used deliberately to speed up cheese ripening or to create hypoallergenic milk protein hydrolysates for infant formula.
Catalase
- Function: Catalase breaks down hydrogen peroxide into water and oxygen.
- Role in Cheesemaking: It can be used in the production of some cheeses as an alternative to pasteurization, where it removes residual hydrogen peroxide that might inhibit the bacterial cultures necessary for cheese production.
Comparison of Enzymes Acting on Milk
| Feature | Lactase | Chymosin (Rennet) | Lipase |
|---|---|---|---|
| Function | Breaks down lactose (milk sugar) | Coagulates casein (milk protein) | Hydrolyzes milk fats (triglycerides) |
| Target Molecule | Lactose | Kappa-casein | Triglycerides |
| Application | Human digestion, lactose-free milk | Cheesemaking | Flavor development in cheese |
| Origin | Small intestine (humans), microorganisms (industrial) | Animal stomachs, microbes, plants (industrial) | Indigenous milk, animal sources, microbial (industrial) |
| Product Impact | Improved digestibility, increased sweetness | Formation of curds for cheese | Flavor development or rancidity |
Conclusion
The enzymatic action on milk is a complex and fascinating field that encompasses both human physiology and large-scale food production. From the lactase in our small intestine that allows us to digest milk sugar to the chymosin used for centuries to turn liquid milk into solid cheese, enzymes are crucial catalysts. Other specialized enzymes like lipase and protease further influence the flavor, texture, and shelf life of various dairy products. A deeper understanding of what enzyme acts on milk not only sheds light on our own digestive health but also underpins the vast and diverse world of dairy technology. For further scientific reading on the subject, a comprehensive overview is available in this Use of Enzymes in Dairy Industry: A Review of Current Progress article.
