Just about every type of juice can be kosher. The two general types of kosher juice are kosher grape juice — the preparation of which must follow certain rules — and all other kosher juices. The Jewish dietary laws generally allow fruit and vegetable products, so the only reason that any type of juice, other than grape juice, would not be a kosher juice is if it went through some sort of unallowed adulteration during processing. To be sure of a juice's status under the Jewish dietary laws, one should look for a hechsher, which is a mark indicating that it has been certified to meet the laws of kashruth.
The laws of kashruth define which foods are kosher and which are not. They are primarily concerned with meat and fish, defining which meats and fishes are acceptable to eat and how they should be slaughtered. Other laws of kashruth cover the mixing of milk and meat, the correct way to produce milk and how to handle utensils.
As long as a juice contains only juice and no grape products, it is a kosher juice and is eligible for certification as such. Juices can lose their kosher status if they come from vegetables and fruits that have not been carefully washed to remove non-kosher insects. Kosher juice also must not contain any adulterants derived from animals, such as gelatin, which sometimes comes from animal hooves.
Grape products represent a unique challenge because of prohibitions in the laws of kashruth on wines that could have been used in pagan rituals. To be considered kosher, wines or grape juices either must be made exclusively by Jewish people or must have been boiled or heat-pasteurized at some point in their production. Many 100 percent juice products use white grape juice as a sweetener, so many juices that might seem fine can become unkosher.
Ultimately, the best way to identify a kosher juice is to look for a hechsher on its label. A hechsher is a marking that indicates that the juice has been inspected by a kosher certifying authority and that it meets the laws of kashruth. Common hechshers include a K inside of a circle, a K inside of a star, a U inside of a circle and a K inside of a kaf, which is a Hebrew letter shaped like a backward C. Any juice with a hechsher is a kosher juice. Although hechshers tend to be small, they are present on many consumer goods, because the market for kosher items extends beyond the Jewish community to include some Muslims, vegetarians and Seventh-Day Adventists, as well as many consumers who simply believe kosher foods to be of higher quality than non-kosher foods.