Liquid Shear

Liquid shear is the method which has been most widely used in large scale enzyme purification procedures (Scawen et al., 1980). High-pressure homogenizes used in the processing of milk and other products in the food industry have proved to be very effective for microbial cell disruption. One machine, the APV-Man-ton Gaulin-homogenizer (The APV Co. Ltd, Crawley, Surrey, U.K.), which is a high-pressure positive displacement pump, incorporates an adjustable valve with a restricted orifice (Fig. 10.21). The smallest model has one plunger, while there are several in larger models. During use, the microbial slurry passes through a nonreturn valve and impinges against the operative valve set at the selected operating pressure. The cells then pass through a narrow channel between the valve and an impact ring followed by a sudden pressure drop at the exit to the narrow orifice. The large pressure drop across the valve is believed to cause cavitation in the slurry and the shock waves so produced disrupt the cells. Brookman (1974) considered the size of the pressure drop to be very important in achieving effective disruption, and as with all mechanical methods, cell size and shape influence ease of disruption (Wase and Patel, 1985). The working pressures are extremely high. Hetherington et al. (1971) used a pressure of 550 kg cm-2 for a 60% yeast suspension. A throughput of 6.4 kg soluble protein h 1 with 90% disruption could be achieved with a small industrial machine. In larger models, flow rates of up to 600 dm3 h^1 are now possible and operating pressures of 1200 bar are utilized in some processes (Asenjo, 1990). Darbyshire (1981) has stressed the need for cooling the slurry to between 0 and 4°C to minimize loss in enzyme activity because of heat generation during the process. The increase in slurry temperature is approximately propor tional to the pressure drop across the valve. Because of problems caused by heat generation and because cell suspensions can be surprisingly abrasive, it is common practice to operate such homogenizers in a multi-pass

Disc Bowl Centrifuge Diagram

discharge of solids fig. 10.19a. L.S. of disc-bowl centrifuge with nozzle discharge.

discharge of solids fig. 10.19a. L.S. of disc-bowl centrifuge with nozzle discharge.

--Effluent

Vinegar For Your Health

Vinegar For Your Health

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