Lactose is a form of sugar that can be found in milk. Lactose is normally broken down into simple sugars - glucose and galactose - inside the small intestine by the lactase enzyme.
Lactase is an enzyme found in the mammalian small intestine that catalyzes the breakdown of lactose (milk sugar) into the simple sugars glucose and galactose making it possible for it to be absorbed via the small intestine and used by the body. If lactose is not broken down, it will pass through the digestive tract without being absorbed.
Lactose intolerant persons cannot generate enough lactase in their small intestine, resulting in undigested lactose reaching the colon and being broken down by bacteria to form lactic acid and gas inside it.
This gas will induce abdominal pains, bloating, and diarrhea.
In order to help lactose-intolerant people, lactose-free milk can be manufactured using lactase
The process to add lactase can be done in-pack or in-batch addition, which can be seen in the picture