Topic 21.2 · Supplement
Lactase and Lactose-Free Milk
It’s easy to mix up the enzyme and the sugar it acts on here — lactase is what does the work, lactose is what it works on — and an answer that swaps them or blurs them together loses precision fast.
What lactase does
Lactose is a sugar found in milk, built from two simpler sugars, glucose and galactose, joined together. Lactase is the enzyme that catalyses the breakdown of lactose into those two simpler sugars:
lactose → (lactase) → glucose + galactose
Milk can be treated with lactase so that its lactose is broken down before the milk is consumed. That treatment produces lactose-free milk — the enzyme is acting on the lactose already present in the milk; nothing is being removed from the milk or swapped in as an artificial sweetener.
Why this matters
Some people don’t produce enough lactase themselves, so the lactose in ordinary milk isn’t broken down and digested properly once it reaches them, which is what’s meant by lactose intolerance. Treating milk industrially with lactase does that breakdown job in advance, outside the body, so the milk reaches those people already free of the specific sugar their own digestive system can’t process. That’s also why the product is marketed specifically as “lactose-free” rather than “sugar-free” — the glucose and galactose produced by the reaction are still present in the milk and are digested normally.
Supporting context: immobilised enzymes
Industrially, lactase can be immobilised— held in place, for example on a fixed surface — so that milk can flow past it and be treated continuously while the enzyme itself is retained rather than lost into the product. This is useful supporting context for how the process is run at scale, but it doesn’t change the underlying reaction: lactase still catalyses the same breakdown of lactose into glucose and galactose.
Don’t confuse
Lactase acts on lactose specifically — don’t generalise it to “sugar” in general. And reducing lactose is not the same as removing all sugar from the milk: the glucose and galactose the reaction produces remain behind, which is exactly why lactose-free milk still tastes sweet.