When using enzymes to catalyse industrial processes, like the breakdown of lactose in the manufacture of lactose-free milk, it’s often useful to have the enzyme fixed, or immobilised, to a surface. This saves you from having to separate the enzyme from the reaction mixture, which can be tricky and expensive. It works by pouring the substrate into a column of immobilised enzymes. Inside the column, the substrate binds to the immobilised enzyme’s active site, just as it would in a normal enzyme-substrate reaction. The mixture flowing out of the column will only contain the desired product.