PRESSURE HOMOGENIZING

High-pressure homogenizers have been used to disrupt microbial cells for many years. With the exception of highly filamentous microorganisms, the method has been found to be generally suitable for a variety of bacteria, yeast, and mycelia.

This type of homogenizer works by forcing cell suspensions through a very narrow channel or orifice under pressure. Subsequently, and depending on the type of high-pressure homogenizer, they may or may not impinge at high velocity on a hard-impact ring or against another high-velocity stream of cells coming from the opposite direction. Machines which include the impingement design are more effective than those which do not. Disruption of the cell wall occurs by a combination of the large pressure drop, highly focused turbulent eddies, and strong shearing forces. The rate of cell disruption is proportional to approximately the third power of the turbulent velocity of the product flowing through the homogenizer channel, which in turn is directly proportional to the applied pressure. Thus, the higher the pressure, the higher the efficiency of disruption per pass through the machine. The operating parameters which affect the efficiency of high-pressure homogenizers are as follows:

Pressure

Temperature

Number of passes

Valve and impingement design

Flow rate

High-pressure homogenizers have long been the best available means to mechanically disrupt nonfilamentous microorganisms on a large scale. Animal tissue also can be processed but the tissue must be pretreated with a blade blender, rotor-stator homogenizer, or paddle blender. The supremacy of high-pressure homogenizers for disruption of microorganisms is now being challenged by bead mill homogenizers. Still, in terms of throughput, the largest industrial models of high-pressure homogenizers outperform bead mills. The maximum volume of microbial suspension per hour that can be treated by the larger commercial machines is 4,500 liters for high-pressure homogenizers versus about 1,200 liters for bead mills. Even larger capacity high-pressure homogenizers are available but their efficiency in disrupting microbial cells has not been documented. This throughput advantage is diminished somewhat by the fact that most high-pressure homogenizers require several passes of the cell suspension to achieve high levels of cell disruption whereas bead mills frequently need only one.

A familiar commercial high-pressure homogenizer for the laboratory is uses a motor-driven piston inside a steel cylinder to develop pressures up to 40,000 psi. Pressurized sample suspensions up to 35m1 are bled through a needle valve at a rate of about 1 ml/min. Because the process generates heat, the sample, piston, and cylinder are usually pre-cooled. Typical pressures used to disrupt yeast are 8,000 to 10,000 psi and several passes through the press may be required for high efficiency of disruption. Generally, the higher the pressure, the fewer the passes. Pressure cells rated at 20,000 psi maximum come in capacities of 3.7 and 35m1 and there is also a 35m1 capacity cell rated at 40,000 psi.

Most high-pressure homogenizers used for homogenization were adapted from commercial equipment designed to produce emulsions and homogenates in the food and pharmaceutical industries. They combine high pressure with an impingement valve. Those with a maximum pressure rating of 10,000 psi rupture about 40% of the cells in a single pass, 60% on the second pass, and 85% after four passes. Capacities of continuous homogenizers vary from 55 to 4,500 liters/hr at 10-17% w/v cell concentrations. With the larger capacity machines, several passes are needed to achieve high yields of disruption. Considerable heat can be generated during operation of these homogenizers and therefore a heat exchanger attached to the outlet port is essential.