Old, unpasteurized orange juice has extra tang 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.