They then have trouble digesting dairy products. The lactase enzyme they can no longer produce for themselves can be added to dairy products before they eat them. That is why it was easy to buy lactase for the experiment. Do not try adding lactase to food without following package directions and without parental supervision. Most enzymes can only do their work within a relatively narrow range of pH values.
Different enzymes work best under different conditions.
One that must work in the acidic environment of the stomach differs in this way from one that operates in the more basic interior of the small intestine. Test a control solution. In the bottom right well of your culture plate, put 5 drops of distilled water and 1 drop of lactose solution. Test the pH of the mixture. Add one drop of lactase solution.