I've been buying unpasteurized orange juice recently for its richer flavor, as compared to pasteurized OJ. I've had some sitting in the fridge for about a week, and upon tasting it just now I noticed an extra tang that I'm pretty sure was not there when I first bought it. I presume this is due to some new acid in the juice that has formed from the breakdown of other components of the juice as it aged. I quite like this additional complexity of the flavor, but I am curious: What is it? In particular, what acid might I be tasting?
It is almost certainly a fermentation - opened juice does not keep for a week in the fridge. So you got some bacteria in it which are creating tangy byproducts (lactic or acetic acid). It is a method of creating fermented drinks such as cider (or even fruit wine), but without following a tested process, you cannot be sure if some of the bacteria may be harmful. So it is not safe by today's standards.
What is it? In particular, what acid might I be tasting?
Could be all kinds of stuff. In any event, you can be almost certain that a changing flavor of unpasteurized OJ is due to something not very healthy. Example literature.
If you really like the acidic flavors, consider purchasing pasteurized OJ, and then adding Tart Cherry Juice Concentrate. Or even food grade citric acid, a common ingredient in Sherbets.