What would make orange juice not kosher?
Curious.
Many food products profess to be "pure ____" but in actuality, are not. They contain minute amounts of various substances to enhance their appearance, taste, or shelf life. These substances may not be kosher; they may be derived from such non-kosher sources as pork or insects. According to KosherQuest, "...Some of the problems that the kosher consumer must be aware of concerning drinks are: flavoring, oil, glycerol, glycerine, gelatin [derived from non-kosher animals], grape juice [which was not kosher-supervised during processing], processing on non-kosher equipment [which was used for non-kosher food items as well]..." (clarifications within brackets are mine)
Insects are also considered non-kosher. This is why fruits and vegetables to be used by a kosher consumer must be carefully examined for the presence of often minute bugs, which must then be eliminated by washing off. Oranges can be infested with scale insects, which are very small. The subsequent processing of these infested oranges can then put insects into the juice--a big no-no.
So, even if it's supposed to be pure OJ, it may not be, and therefore requires rabbinical supervision to merit that little mark (hechsher) on the box. BTW, the O-U is far from the only such mark; there are hundreds of different marks world wide, each representing a different kosher supervisory body. O-U just happens to be the most widely seen.
Non kosher equipment usage.
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