The main applications of expression are in the extraction of components plant materials either for direct consumption (for example fruit juices) or for use in subsequent processing (for example sugar, grape juice for wine and vegetable oils). These materials are located within the cell structure of the plants and it is necessary to disrupt the cells in order to release them. This is achieved either in a single-stage,
which both ruptures the cells and expresses the liquid, or in two stages (size reduction to produce a pulp or flour, followed by separation in a press). In general the single-stage operation is more economical, permits higher throughputs and has lower capital and operating costs, but for some products that are especially hard (for example oil bearing nuts) a two-stage expression is more effective. Better extraction is achieved by heating oilseeds or flours to reduce the oil viscosity, release oil from intact
cells and remove moisture. There is an optimum moisture content for each type of oil seed to obtain a maximum yield of oil. In fruit processing, the press should remove the maximum quantity of juice, without substantial quantities of solids, or phenolic compounds from the skins which cause bitterness and browning. This is achieved using lower pressures and fewer pressings.
The factors that influence the juice yield from a press include:
• maturity and growth conditions of the raw material
• extent of cell disruption
• thickness of the pressed solids and their resistance to deformation
• rate of increase in pressure, the time of pressing and the maximum pressure applied
• temperatures of solids and liquid and the viscosity of the expressed liquid.