"Eu beau suc de mandarine."

Translation: I drink mandarin juice.

[TyMor95]

In this case, does mandarină have to be in the plural?

[sebastianmolin]

Yes, Romanian logic goes like this: When you make juice (be it mandarin juice, orange juice, apple juice, etc.) you use multiple fruits thus "juice of (mandarins/apples/oranges) :-)