Evaporation is a unit operation that eliminates water from a liquid food. If the liquid contains dissolved solids, the concentrated solution can become saturated or oversaturated, with solid crystals deposition.
As fruit juices contain many substances that can be damaged if submitted to high temperatures during relatively long periods, evaporation under vacuum seems to be the logical answer to this problem. When vacuum evaporation is carried out, the boiling point is lowered, and so thermal degradation is minimized.