Sterility Aspects

The design of a fermentor vessel has to meet aseptic conditions before it can be used for fermentation batches. An improper fermentor design may result in a nonsterile batch due to ineffective sterilization of dead volume or leakage.

Dead Volume: Organisms or media that are deposited in dead spaces may lead to subsequent contamination. In air filled cavities, sterilization temperature may not be reached. The installation of temperature probes into the fermentor at critical positions enables the monitoring of the actual temperature to ensure that efficient sterilization occurs.

Leakage: Most fermentation vessels are not hermetically sealed, and have a number of ports into and out of the fermentor. The common points of leakage from the fermentor include improper sealing of the manhole, sight and light glass, probes, inlet and outlet feed streams, mechanical seals and harvest or sampling valves. Leakage from the fermentor vessel may be identified by two methods, namely pressure test and sealing test.

Pressure test: The fermentor is filled to 70% of the total volume and mixed at the operating speed. Air or nitrogen is sparged into the fermentor to achieve a vessel pressure of 1.5 bars. All the ports are tightly secured and fermentor is examined for pressure loss. Care should be taken to maintain the mechanical seal at a positive pressure relative to the fermentor. If there is a drop in pressure, then there is a leakage from the fermentor.

Sealing test: The fermentor is filled with water up to the mechanical seal and a vessel overpressure of 1.5 bars is achieved. The temperature (To) and pressure (Po) are recorded after ten minutes and again after about fifteen hours. If the specific leakage rate (L) in terms of grams of air per hour per meter of sealing length is less than 0.05 gm/h m, the vessel is considered tight; if higher, then the vessel has to be sealed effectively at the leakage points.

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