Dear 100 Hour Board,
How is lactose-free milk made? I understand soy/almond/coconut milks, but I've seen actual cartons of 'normal' milk that is lactose-free, and I am curious as to how they do that?
-My Name Here
They milk lactose-intolerant cows.
What Inklings said.
Also, there are a couple of other ways.
A Little Bit About Digestive Physiology Before I Answer Your Question
First thing to know is that lactose is a two-part sugar, similar to sucrose (table sugar). While sucrose is made from one glucose and one fructose (one-part sugars), lactose is made from one glucose and one galactose (another kind of one-part sugar). Our cells only use glucose to fuel their activities, so our bodies use digestive enzymes (specialized proteins) to extract the glucose and prepare the galactose (and fructose) to be converted into glucose. The first step in the process is to convert two-part sugars into one-part sugars.
Enter lactase. Lactase is an enzyme that breaks apart glucose and galactose (rather similar to the enzyme sucrase, which breaks apart glucose and fructose). However, many people don't produce the lactase enzyme. That means they cannot digest it, which creates unpleasant side effects.
Now to Answer Your Question
One method to create lactose-free milk is to add lactase. This does the job of breaking lactose into glucose and galactose for the lactose-intolerant person. Then the enzyme is destroyed by heating or using chemical means.
Another method is to use super-duper sophisticated filtration systems, such as specialized membranes, that remove the lactose from the milk mechanically. They have to be super-duper sophisticated because sugar molecules are so tiny.
If you are interested, read up more on these processes here.