Why does fruit juice take so long to freeze?

This is because the sugars in the juice act as an antifreeze, lowering the freezing point of the liquid. In order to freeze fruit juice, you will need to reach a temperature below the freezing point of the juice, which can take longer than it would to freeze pure water. Depending on the type and quantity of sugars present, the freezing point of fruit juice can be significantly lower than that of water, hence requiring a lower temperature for freezing.