exploited in separating seeds which may be of similar size and shape, or in the cleaning More detailed information on solid-solid separations is provided in Chapter 9, Separation from the solid matrix Many plant materials contain valuable liquid components such as oils or juices in the cellular structure. These may be separated from the pulped raw material by the use of presses, in a process known as expression. Batch type hydraulic systems or continuous roller, screw or belt systems are available for different applications such as fruit juice, wine and cane sugar production, or extraction of oil from seeds. Expression of fruit juices may be aided by the use of enzymes to improve efficiency of expression and to control the pectin level. Some of the physical properties related to expression processes are discussed by Schwartzberg(1983) An alternative system to recover components from within a solid matrix is extraction which relies on the use of differential solubilities for extraction of soluble solids such as sugar from sugar beet, coffee from roasted ground beans, juices from fruit and vegetables and from materials during the manufacture of instant tea. The most common extraction material is hot or superheated water. However, organic solvents are used, e. g. hexane for oil extraction and methylene chloride to extract caffeine from tea and coffee. The use of supercritical fluids such as carbon dioxide is covered in detail in Chapter 2. Extraction processes as equilibrium stage processes are covered in more detail by Brennan et al 1990), Loncin and Merson(1979), Perry and green(1984). Many oil extraction processes employ expression, followed by solvent extraction,to obtain a high recovery of oil. The crude oil is then subjected to a series of refining processes, involving degumming, decolorisation and deodorisation to remove undesirable Water, the most common component of most foods, can be removed from sol the process of dehydration; in this case thermal energy is required to effect evaporation of the water, and this is usually supplied by hot air. Hot air drying is classified as liquid phase drying and results in shrinkage and case-hardening and loss of some volatiles of foods, Types of drier include overdraught, throughdraught, fluidised bed and pneumatic driers. These are described in more detail by Brennan et al.(1990), Mujumdar(1987) Freeze-drying, whereby the food is frozen and then subjected to a vacuum, provides a method which reduces shrinkage, case-hardening and flavour loss. Sublimation occurs during freeze-drying. Here conditions are controlled such that water is removed directly from its solid phase to its vapour phase, without passing through the liquid state. To achieve this, the water vapour pressure must be kept below the triple point pressure (4.6 mm Hg)(Mellor, 1978; Dalgleish, 1990) The removal of air from fruit and vegetables, prior to heat treatment in sealed to prevent ex 如mm ng. This is accomplished by blanching, using steam mised using steam (SelmanSeparation processes - an overview 9 exploited in separating seeds which may be of similar size and shape, or in the cleaning of tea. More detailed information on solid-solid separations is provided in Chapter 9. Separation from the solid matrix Many plant materials contain valuable liquid components such as oils or juices in the cellular structure. These may be separated from the pulped raw material by the use of presses, in a process known as expression. Batch type hydraulic systems or continuous roller, screw or belt systems are available for different applications such as fruit juice, wine and cane sugar production, or extraction of oil from seeds. Expression of fruit juices may be aided by the use of enzymes to improve efficiency of expression and to control the pectin level. Some of the physical properties related to expression processes are discussed by Schwartzberg (1983). An alternative system to recover components from within a solid matrix is extraction, which relies on the use of differential solubilities for extraction of soluble solids such as sugar from sugar beet, coffee from roasted ground beans, juices from fruit and vegetables and from materials during the manufacture of instant tea. The most common extraction material is hot or superheated water. However, organic solvents are used, e.g. hexane for oil extraction and methylene chloride to extract caffeine from tea and coffee. The use of supercritical fluids such as carbon dioxide is covered in detail in Chapter 2. Extraction processes as equilibrium stage processes are covered in more detail by Brennan et al. (1990), Loncin and Merson (1979), Perry and Green (1984). Many oil extraction processes employ expression, followed by solvent extraction, to obtain a high recovery of oil. The crude oil is then subjected to a series of refining processes, involving degumming, decolorisation and deodorisation to remove undesirable components. Water, the most common component of most foods, can be removed from solids by the process of dehydration; in this case thermal energy is required to effect evaporation of the water, and this is usually supplied by hot air. Hot air drying is classified as liquid phase drying and results in shrinkage and case-hardening and loss of some volatiles of foods. Types of drier include overdraught, throughdraught, fluidised bed and pneumatic driers. These are described in more detail by Brennan et al. (1990), Mujumdar (1987). Freeze-drying, whereby the food is frozen and then subjected to a vacuum, provides a method which reduces shrinkage, case-hardening and flavour loss. Sublimation occurs during freeze-drying. Here conditions are controlled such that water is removed directly from its solid phase to its vapour phase, without passing through the liquid state. To achieve this, the water vapour pressure must be kept below the triple point pressure (4.6 mm Hg) (Mellor, 1978; Dalgleish, 1990). The removal of air from fruit and vegetables, prior to heat treatment in sealed containers, is of paramount importance to prevent excessive strain on the seams during the sterilisation and subsequent cooling. This is accomplished by blanching, using steam or hot water. Nutrient losses due to leaching are minimised using steam (Selman, 1987)