design an experiment to show that the amount of bromelain in a pineapple increases as the pineapple ripens? I need a plan which, explains how you would do the experiment, identifies the varibles that would affect the results and explains how to keep them constant.
With all due respect, the idea of AS level coursework is that YOU work out these sort of details, rather than just have it given to you! You may need help with the basic techniques, but things like the variables and fair testing you certainly ought to be able to do yourself. In any case, the situation is not straightforward - there are quite a number of ways this experiment could be done. Bromelain is a protease enzyme, and there are many ways of measuring the action of proteases. One example is given in http://www.science-projects.com/Papaine.htm
The protease in this case is meat tenderiser rather than bromelain ('Jello' = jelly - you could also use agar jelly). As gelatine is a protein, protease enzymes will prevent the jelly setting.