Surface Fermentation

The oldest and a relatively simple method for production of complex flavour extracts is the fermentation of flavour raw materials by bacteria and moulds (aspergillus, mucor) on solid nutrient media. The media is placed in simple thermostatic boxes on baking-tray like plates to which the micro-organisms are inoculated. This technique which has originated in Japan for the production of Koiji seasonings is still used today for the production of soy sauce made from rough-ground cereals and soy beans which are inoculated with special moulds. After incubating the nutrient media for several days, it is finally extracted with water to isolate the fermented products contained in the media [5]. For many years the Koiji technique was also the preferred method for the production of enzymes which are secreted by bacteria and moulds into the extracellular environment (the nutrient media). Another example for a traditional surface fermentation process is vinegar production by acetic acid bacteria grown on the surface of wood chips.

2.2.5 Submerged Fermentation

Submerged fermentation is generally applicable for the manufacturing of cell products by propagation of micro-organisms and cell cultures in a fluid nutrient media. The submerged fermenter (diagram shown in Figure 2.64) is normally operated in a sterile manner i.e. all components have to be sealed against the environment by ports which are air-tight and impenetrable by bacteria. The whole fermenter consisting of an agitated tank with thermostatic mantle, a stirrer and several lines for respiratory gases, pH regulation agents, nutrient source etc. has to be autoclaved prior to reaction. Therefore, it must be able to withstand sterilization with overheated steam at 121°C. In the course of aerobe fermentations, the micro-organisms have to be supplied with respiratory gas in the fermenter by an intense aeration system. Depending on the heat balance of the reaction sometimes huge amounts of heat have to be removed by cooling registers which are built into the fermenter. With complex measuring and controlling devices the environmental conditions within the fermenter for pH, temperature, ionic strength and nutrient concentration are controlled with high accuracy.

Submerged Fermentation Images

Stirrer Drive

Fig. 2.64: Diagram of a submerged fermenter

Stirrer Drive

Fig. 2.64: Diagram of a submerged fermenter

As a consequence of the adverse operating conditions (sterilization heat; viscose, particle containing media with high dry matter content; protein containing solutions tending to sedimentation; aggressive cleaning aids; gas-bubble containing media) the sensors for retrieval of the control data are constructed in a complicated and expensive way. The on-line analysis of exhaust gas composition by specific sensors allows the continuous evaluation of the activity of the fermenting micro-organisms. In this way an on-line evaluation of the productivity rate of the micro-organism culture is achievable.

By combination of all data thus obtained the process may be controlled with sufficient precision by a process computer. The culture within the fermenter can be maintained over a considerable amount of time in the state of maximum productivity.

The initial propagation of a culture of micro-organisms for production fermenters with payloads up to 200 m3 is achieved via several steps of one order of magnitude each. One starts with the laboratory scale (1 l) via diverse inoculum fermenters (10, 100 l) to obtain a sufficient count and concentration of living micro-organisms for the fermentation. For any successful fermentation a limiting density of living microorganisms of about 108per ml is required.

Submerged fermentations are mostly operated in batch processes but can also be run continuously in certain cases (continuous fermentation). Batch fermentations may last up to 10 days. Following the fermentation the flavour raw material is extracted from the fermentation broth. In industrial fermentations typically cell counts of 10-30 g/l are obtained. For a profitable cost/efficiency relation a product yield of 20-30 g/l has to be achieved. Aerobe fermentations require oxygen transfer rates to the fermentation broth of about 100 mmol/l per hour. Depending on the viscosity of the media 0.75-2.5 KW stirring power has to be applied for each m3 of fermentation broth.

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  • hagos
    What is surface fermentation?
    2 months ago

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