Prevent the dough from "oxidizing"? I was practicing and trying to make an apple pie depending on this source: http://allrecipes.com/howto/perfect-pie-crusts/detail.aspx

In the Liquid section, it's said: "A little bit of acid--vinegar or lemon juice--helps tenderize the dough and prevents it from oxidizing."

What's "Oxidizing"? First time I read/hear this term in cooking?

Please help me learn, I'm still a beginner!
Thanks in advance
I'm not a chemist so I'll let wikipedia do it for me:
http://en.wikipedia.org/wiki/Oxidation

As far as it's culinary effects in crusts- I have seen unreliable reference to the flour oxidizing and developing a off color.

I have never seen this personally and I am skeptical of it. Pie crusts can be made just fine without vinegar. Vinegar does significantly tenderize the crust as well as add an interesting flavor.