Preservative processes vacuum

Bethell Also known as the Full-cell process. A method for impregnating timber with a creosote preservative. The wood is first degassed under partial vacuum and then impregnated under a pressure of up to 10 atm. See also Rueping. 

In this process liquid air is used for condensing the vapours in order to preserve the high vacuum carbonic acid has been used by other investigators, and Levene and van Slyke have recently employed sulphuric acid, cooled by a freezing mixture, as an absorbent for this purpose. 

Lyophilization is a process in which a solution of a drug is frozen to a solid and the solvent, usually water, removed by sublimation on exposure to a vacuum. The process has been studied intensively because it can be applied to the preservation of labile drugs or materials such as proteins which would otherwise be adversely affected by the solvent over a period of time. 

The most satisfactory nonpressure process is known as thermal treatment. The wood to be treated is immersed in a preservative at an elevated temperature. This causes the air in the wood cells to expand so that when the wood is transferred into a preservative bath of lower temperature, the air contracts forming a partial vacuum and atmospheric pressure forces the liquid into the wood. 

FIGURE 1.1. Electron micrographs of liver tissue processed differently for transmission electron microscopy. (A) Processed by the conventional method. Note superior quality of ultrastructural preservation compared with that obtained with microwave heating. (B) Rapidly processed by vacuum microwave heating. The whole process from tissue fixation to resin embedding was completed in 2 hr. The quality of ultrastructural preservation is satisfactory. Magnification 6,21 Ox (B). (B) courtesy of Richard T. Giberson. 

Distillation processes exploit the low volatility of cholesterol compared to the major triacylglycerols of milk fat for removal of cholesterol. Vacuum and short-path molecular distillation processes can efficiently remove cholesterol but it may be achieved at the expense of removing some low-molecular weight triacylglycerols and flavor components of the milk fat. Vacuum steam distillation is commonly used for refining fats and can also be used to refine milk fat. Cholesterol-reduced milk fat, which was produced by steam distillation, has been used successfully to formulate butter, cream and ice cream (Schroder and Baer, 1991, Elling et al., 1995, 1996). If the flavor of milk fat is to be preserved, the flavors can be trapped and re-incorporated into the milk fat that has been stripped of cholesterol (Boudreau and Arul, 1993). 

The vacuum processes involve putting the wood under a vacuum to draw out part of the air. The wood may be subjected to a vacuum alone or to steaming and a vacuum before being submerged in a cold preservative. These methods are used to a limited extent in the treatment of lumber, timber, and millwork. 

In the full-cell process, the wood in the cylinder first is subjected to a vacuum of not less than 22 in. Hg for 15-60 min, to remove as much air as possible from the wood. The cylinder then is filled with hot treating liquid without admitting air. The maximum temperature for creosote and its solutions is 210°F, and for water-borne preservatives it is 120-150°F, depending upon the preservative. Then the liquid is placed under a pressure of 125-200 psi, and the temperature and pressure are maintained for the desired length of time, usually several hours. After the liquid is drawn from the cylinder, a short vacuum is applied to free the charge of surface-dripping preservative. 

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