What Happens to Lactase in Milk? A Comprehensive Guide

November 27, 2025 •

3 min read

An estimated 75% of the global population is lactose intolerant to some degree, a condition linked to a natural decrease in lactase production after infancy. For these individuals, understanding what happens to lactase in milk is key to enjoying dairy products without discomfort. Whether naturally occurring in the gut or added during production, the enzyme's function is to break down milk's sugar, lactose, into simpler, more digestible components.

Quick Summary

Lactase is a key enzyme in the production of lactose-free milk, where it's added to break down lactose into digestible sugars. In individuals with lactose intolerance, a natural deficiency of this enzyme prevents proper lactose digestion, leading to symptoms like bloating and gas.

Key Points

Lactase is an enzyme that breaks down lactose. In the human body, lactase splits lactose into the simpler sugars, glucose and galactose.
Lactase deficiency causes lactose intolerance. When the body doesn't produce enough lactase, undigested lactose ferments in the gut, causing digestive issues.
Lactose-free milk is made by adding lactase. Manufacturers add the enzyme to regular milk, pre-digesting the lactose for intolerant consumers.
Lactose-free milk is sweeter than regular milk. The simpler sugars (glucose and galactose) produced by the enzyme are perceived as sweeter by our taste buds.
Lactase can be added to milk at home. Commercially available lactase drops allow individuals to make their own lactose-free milk by adding the drops and refrigerating the milk for 24 hours.
Nutritional value is maintained in lactose-free milk. The addition of lactase does not change the milk's nutrient content of calcium, protein, and vitamins.

The Natural Role of Lactase in Digestion

In mammals, the enzyme lactase is produced in the small intestine, where it plays the vital role of breaking down lactose, the complex sugar found in milk. Lactose is a disaccharide, meaning it is made of two simpler sugar molecules: glucose and galactose. A functioning lactase enzyme cleaves the lactose molecule into these two simple sugars, which are then easily absorbed by the body.

For most of the world’s population, lactase production naturally decreases after childhood weaning. For these individuals, consuming regular milk means that the lactose travels undigested to the large intestine. There, colonic bacteria ferment the lactose, producing gases (like hydrogen, carbon dioxide, and methane) and acids that cause the unpleasant symptoms of lactose intolerance, including bloating, gas, cramps, and diarrhea.

Creating Lactose-Free Milk: A Controlled Process

To create lactose-free milk, manufacturers intentionally introduce the lactase enzyme into the product. This proactive step ensures the lactose is broken down before consumption, making the milk digestible for those with an enzyme deficiency. The process is typically straightforward and consists of several key steps:

Sourcing the Enzyme: Commercial lactase is typically derived from microorganisms such as yeasts (Kluyveromyces fragilis or Kluyveromyces lactis) or fungi (Aspergillus niger).
Adding the Enzyme: After milk is pasteurized, a measured amount of lactase is added to the milk.
Incubation: The milk and enzyme mixture is then held for a period, often at refrigerated temperatures. During this time, the lactase works to hydrolyze the lactose.
Quality Control: The final product is tested to ensure the lactose content has been reduced to an acceptable level for intolerant individuals.

Some manufacturers also use a membrane filtration technique to remove a portion of the lactose before adding the lactase, which results in a product with a more neutral, less-sweet flavor profile.

Impact on Taste, Nutrition, and Texture

The enzymatic breakdown of lactose has some notable effects on the milk's sensory properties, though its nutritional value remains the same.

Comparison Table: Regular vs. Lactose-Free Milk


Feature	Regular Milk	Lactose-Free Milk
Lactase	Not present	Added during processing
Lactose Content	Contains lactose (milk sugar)	Very low or zero lactose
Carbohydrates	Lactose	Primarily glucose and galactose
Sweetness	Mild, less sweet	Noticeably sweeter
Digestibility	Can cause digestive issues for intolerant people	Easily digested by most lactose-intolerant people
Nutritional Profile	Rich in calcium, protein, vitamins	Retains same nutrients
Fat Content	Varies (e.g., skim, whole)	Same as corresponding regular milk

Beyond Milk: Lactase in Other Dairy Products

Lactase is not only used to create lactose-free milk but is also valuable in the production of other dairy products, such as yogurt, cheese, and ice cream.

Yogurt and Kefir: For fermented products, lactase can be added to the milk before the fermentation process begins. The starter cultures also naturally consume some lactose, but adding lactase ensures the final product has a much lower lactose content.
Cheese: Hard, aged cheeses like cheddar and parmesan contain very little to no lactose naturally because it is mostly drained off with the whey and fermented by bacteria during ripening. However, lactase treatment can be used for softer cheeses or to further reduce residual lactose for sensitive individuals.
Ice Cream: By breaking down lactose, lactase can be used to create sweeter, creamier, and more easily digestible ice cream products.

Making Your Own Lactose-Free Milk at Home

For those who prefer to control their own food processing, it is possible to make lactose-free milk at home using commercially available lactase drops.

Purchase Lactase Drops: These are available at most pharmacies and online retailers.
Add Drops to Milk: Follow the instructions on the package to determine the correct number of drops per liter or gallon of milk.
Mix Thoroughly: Shake or stir the mixture well to distribute the enzyme evenly throughout the milk.
Refrigerate and Wait: Let the milk sit in the refrigerator for at least 24 hours. The enzyme will work slowly in the cool temperature to break down the lactose. A longer refrigeration period can increase the conversion rate.
Enjoy: After the incubation period, your homemade lactose-free milk is ready to drink.

For more information on digestive health and conditions like lactose intolerance, consult the Mayo Clinic website.

Frequently Asked Questions

Lactose-free milk has the lactase enzyme added during production to break down the lactose sugar. Regular milk contains intact lactose, which can cause digestive issues for those with intolerance.

No, lactose-free milk does not have less sugar overall. The lactase enzyme breaks the lactose molecule into two simple sugars, glucose and galactose, but the total sugar content remains the same.

No, lactose-free milk is not for dairy allergies. It is still a dairy product made from cow's milk and contains milk proteins, which trigger allergic reactions.

Yes, lactase drops can be added to other dairy products, such as cream or half-and-half, to reduce their lactose content. The process requires refrigeration time to allow the enzyme to work.

Lactose-free milk tastes sweeter because the lactase enzyme breaks the complex lactose sugar into two simpler sugars, glucose and galactose. Our taste buds perceive these simple sugars as sweeter.

Hard, aged cheeses like cheddar and parmesan do not have lactase added. They are naturally very low in lactose because most of it is removed during the cheesemaking process and fermented away by bacteria as the cheese ages.

When using lactase drops to make milk at home, it typically takes at least 24 hours of refrigeration for the enzyme to sufficiently break down the lactose. Waiting longer can increase the conversion.