Why?
Having a control group is necessary because it is important to know how long the food samples will take to turn brown without any liquid added.
The food’s skin protects the inside “meat” of the fruit or vegetable from damage and debris. When a fruit or vegetable is dropped and the skin is poked or broken, the food often goes bad faster. The reason fruits and some vegetables go brown when they are cut is because the part containing the oxygen-reactive enzyme is exposed. There is then a lot of surface area for the air to come in contact with the food. For the most part, brown fruits and vegetables still taste fine, they just do not look very appetizing.
Acids prevent browning because they react with the oxygen that comes into contact with the surface of the sample. Once all the acid (or whatever else is covering the surface) has reacted with the oxygen or the acid hasdegraded or washed off, then the sample will start to brown again. Stronger acids, like lemon juice, can even denature the enzyme. This means that the enzyme can no longer perform its original function because of its environment.
Digging Deeper
Why doesn’t dried fruit turn brown? What about the fruit drying process prevents enzymatic reaction with oxygen?