What are the products of the hydrolysis of lactose?

December 15, 2025 •

3 min read

According to the National Institutes of Health, approximately two-thirds of the world's population experiences some form of lactose intolerance. This condition is directly related to the body's ability to perform the hydrolysis of lactose, a process that breaks down the milk sugar into smaller, absorbable components.

Quick Summary

Lactose hydrolysis is the breakdown of the milk sugar lactose into the simple sugars glucose and galactose, a process critical for digestion.

Key Points

Primary Products: The main products of lactose hydrolysis are the monosaccharides D-glucose and D-galactose.
The Reaction: Lactose is a disaccharide that breaks apart with the addition of one water molecule in a process catalyzed by an enzyme or acid.
Key Enzyme: The enzyme responsible for lactose hydrolysis in humans is lactase, or β-galactosidase, found in the small intestine.
Lactose Intolerance Cause: Symptoms of lactose intolerance occur when undigested lactose reaches the large intestine and is fermented by bacteria, producing gas.
Industrial Advantages: Hydrolysis increases the sweetness, solubility, and lowers the freezing point of dairy products, beneficial for creating lactose-free items.
Different Methods: Hydrolysis can be achieved enzymatically under mild conditions or chemically using acid and high heat.

Understanding the Core Chemical Reaction

Lactose is a disaccharide, meaning it is a carbohydrate formed from two smaller sugar units, or monosaccharides. The name 'lactose' comes from lact, the Latin word for milk, and ose, a suffix denoting a sugar. During hydrolysis, a water molecule is added to break the glycosidic bond that links the two monosaccharide units together. The overall chemical equation for the hydrolysis of lactose is: $$C{12}H{22}O_{11} + H_2O \rightarrow C6H{12}O_6 + C6H{12}O_6$$.

The Specific Products: Glucose and Galactose

The two simple sugars produced from the hydrolysis of lactose are D-glucose and D-galactose. These monosaccharides are isomers, meaning they have the same chemical formula ($$C6H{12}O_6$$) but a different arrangement of atoms.

Glucose: This is one of the most important simple sugars and is a primary energy source for most living organisms. After absorption into the bloodstream, it is used by the body's cells for energy production.
Galactose: The other product, galactose, is transported to the liver after absorption. The liver then converts the galactose into glucose for use in energy production.

The Mechanisms of Lactose Hydrolysis

Lactose hydrolysis can occur through two primary methods: enzymatic catalysis and acid hydrolysis. While the end products are the same, the conditions and applications differ significantly.

Enzymatic Hydrolysis with Lactase

In the human body, the hydrolysis of lactose is catalyzed by the enzyme β-galactosidase, more commonly known as lactase.

The Role of Lactase: The enzyme is located in the brush border of the small intestine.
The Process: When lactose-containing food is ingested, lactase attaches to the lactose molecule and uses a water molecule to cleave the β-(1→4)-glycosidic bond.
Absorption: Once broken down into glucose and galactose, the monosaccharides are readily absorbed into the bloodstream from the small intestine.

Acid Hydrolysis

Industrial processes can also hydrolyze lactose using inorganic or organic acids, such as hydrochloric acid, at high temperatures. This method is less common for consumer products due to its harsh nature.

Conditions: Requires low pH (around 1–2) and high temperatures (100–150°C), which can damage other components like proteins.
Application: Primarily used for producing lactose-free whey syrup or in other contexts where protein degradation is not a concern.


Feature	Enzymatic Hydrolysis (Biological)	Acid Hydrolysis (Chemical)
Catalyst	β-galactosidase (lactase)	Inorganic or organic acids (e.g., HCl)
Conditions	Mild temperature (30–55°C) and neutral or slightly acidic pH	High temperature (100–150°C) and very low pH (1–2)
Side Effects	Highly specific, preserves nutritional value	Can degrade proteins and form undesirable byproducts
Commercial Use	Production of lactose-free milk, yogurt, and cheese	Primarily for whey processing and other non-dairy applications

The Role in Lactose Intolerance

For individuals with lactose intolerance, the body produces insufficient lactase to properly hydrolyze lactose. This leads to the undigested disaccharide continuing to the large intestine, where it is fermented by gut bacteria.

Fermentation: The bacterial fermentation of lactose produces gases, such as hydrogen, carbon dioxide, and methane.
Symptoms: This process is responsible for the classic symptoms of lactose intolerance, including bloating, gas, abdominal pain, and diarrhea.
Solutions: Lactase supplements and pre-hydrolyzed lactose-free dairy products provide the necessary enzyme to break down the lactose before it can cause digestive issues.

Industrial Significance for Food Products

Beyond mitigating lactose intolerance, the process of hydrolyzing lactose offers several advantages for the food industry:

Increased Sweetness: The resulting glucose and galactose mixture has a higher perceived sweetness than lactose alone, allowing for a reduction in other added sugars.
Improved Solubility: The monosaccharide products are more soluble than lactose, which prevents the grainy texture that can result from lactose crystallization in products like ice cream.
Lower Freezing Point: The hydrolysis process lowers the freezing point of products, leading to a smoother texture in frozen dairy products.

Conclusion

The hydrolysis of lactose is a fundamental biochemical reaction that breaks down the milk disaccharide into its constituent monosaccharides: glucose and galactose. This process is essential for human digestion, particularly for individuals with lactose intolerance, and is also a critical tool for the food industry. By understanding the science behind this reaction, from the natural enzymatic process in the body to its industrial applications, we can better manage dietary needs and innovate in food production. The ability to perform this simple chemical breakdown has far-reaching effects, influencing everything from global dietary trends to the texture of a frozen dessert.

An interesting resource providing more depth on the industrial uses and techniques of lactose hydrolysis is available at the National Center for Biotechnology Information: Advances in Low-Lactose/Lactose-Free Dairy Foods.

Frequently Asked Questions

The chemical formula for lactose hydrolysis is $$C{12}H{22}O_{11} + H_2O \rightarrow C6H{12}O_6 + C6H{12}O_6$$. The products are glucose and galactose, which are isomers with the same formula but a different structure.

The enzyme lactase, or β-galactosidase, acts as a catalyst in the hydrolysis of lactose within the human body. It significantly speeds up the process of breaking the lactose molecule's glycosidic bond into its two monosaccharide units.

Acid hydrolysis is generally not used for producing consumer-grade lactose-free products like milk because the high heat and low pH required can damage milk proteins. Enzymatic hydrolysis is the preferred method.

Lactose intolerance is the inability to properly digest lactose due to insufficient lactase enzyme production. This means the body cannot efficiently perform lactose hydrolysis, leading to digestive issues when consuming dairy.

After lactose is hydrolyzed into glucose and galactose in the small intestine, both are absorbed into the bloodstream. Glucose is used directly for energy, while galactose is converted into glucose in the liver.

The primary application is producing lactose-free dairy products for consumers with lactose intolerance. Other uses include increasing the sweetness and improving the texture of dairy products by preventing crystallization.

No. Most mammals stop producing significant amounts of lactase after weaning, a phenomenon reflected in the genetic prevalence of lactose intolerance in many human populations.