There are several types of continuous press used commercially: the belt press for fruit processing, the screw expeller for both fruit processing and oil extraction,and the roller press for sugar cane processing.
Belt presses consist of a continuous belt, made from canvas-plastic composite material, which passes under tension over two hollow stainless steel cylinders, one of which is perforated. Pulped fruit is fed onto the inside of the belt and is pressed between the belt and the perforated cylinder. Juice flows through the perforations and the press cake continues around the belt and is removed by a scraper or auger. They produce high yields of good-quality juice.
The screw expeller consists of a robust horizontal barrel containing a stainless steel helical screw. The pitch of the screw flights gradually decreases towards the discharge end, to increase the pressure on the material as it is carried through the barrel. The final section of the barrel is perforated to allow expressed liquid to escape. Press cake is discharged through the barrel outlet and the pressure in the barrel is regulated by adjusting the diameter of the discharge port. Frictional heat reduces the viscosity of the oil and some types of expellers have supplementary heaters fitted to the barrel to improve yields. In juice extraction the barrel is cooled to reduce the friction heat, generated by the movement of food, which may have an undesirable effect on flavour and aroma. The oil cake has 5–18% (w/w) residual oil, depending on the type of oil-seed and the operating conditions. A modified screw press in which high shearing forces act on the seeds as they are forced through a series of throttle rings located in the barrel.
In the roller press, pulp or sugar cane is fed between heavy fluted metal rollers and liquid flows along the grooves to a collection pan.