Question for science fair project on apple preservation

I am doing a project to keep apples from browning. I am taking apples and trying 4 different citrus juices, including lemon, and seeing which preserves them the best. I tried taking the Ph levels of the citrus juices, and got that grapefruits are 3, limes are 2, lemon juice is a 2, and orange juice is a 4. I then tested the Ph of apples and got 4. I still can not figure out why this occurs. I was wondering if anyone knew anything about the composition or Ph of apples, grapefruits, limes, lemons, or oranges that would be helpful.

Thank You,

I remember being a student in the science fair and I thought that you could use some advice on this. I just learned about this last year myself. Do some research into the enzyme known as "Tyrosinase". You should be able to find information in most Biochemistry textbooks. Also, I would try Vitamin C (ascorbic acid) tablets dissolved in water. There is a connection between Vitamin C, Tyrosinase, and the brown color from apples.

Dear Ben,

I don't think that any citrus juice would work. Try apple juice and see what happens.

Dear Ben,

I tried the project before, dip apple slices in salt water, IT WORKS!