Submerged liquid fermentation

Fig. 1. Comparison of typical solid-state fermentation (SSF) and submerged liquid fermentation (SLF) systems. A stirred-bed SSF bioreactor of the design of Durand and Chereau [2] is compared with a typical stirred SLF bioreactor. For each bioreactor an expanded view of the microscale is also shown, in order to highlight differences between the micro-structure of the two systems. The relative scales make it clear that mixing is possible on much smaller scales in SLF than in SSF, since in SSF mixing cannot take place at scales smaller than the particle size. Note that particle sizes in SSF are commonly larger than 1 mm...

Table 1. Comparison of solid-state fermentation and submerged liquid fermentation...

The bioreactor is the central point of a fermentation process. It is here that the biotransformation takes place, that a raw material is turned into a desired and valued product Optimization of the rate of formation and yield of product within the bioreactor is a key part of optimizing the production process. Although the field of bioreactor design for submerged liquid fermentation systems is well developed, many of the principles cannot be directly translated to SSF systems. Solid beds and liquid broths are different solid beds are not as easy to mix as liquid broths, and due to poor heat transfer properties of solid substrate beds, heat removal is much more difficult in SSF than it is in SLF. 

The yield of citric acid on sugar varies from process to process and from manufacturer to manufacturer. The theoretical maximum is 112 percent on sucrose. The liquid surface fermentation has a yield of 90 to 95 percent, and the submerged culture fermentation 75 to 85 percent. Improvements have been made by reducing the formation of by-products, mainly oxalic acid, and yields of the submerged culture process are reaching those of the surface culture process. 

Generally, the fermentation proeess involves the addition of a speeifie eulture of mieroorganisms to a sterilized liquid substrate or broth in a tank (submerged fermentation), addition of air if aerobie, in a well-designed gas-liquid eontaetor. The fermentation proeess is then earried out to grow mieroorganisms and to produee the required ehemieals. Table 11-1 lists examples of the proeesses used by fermentation. 

Pretreatment of Substrate. Several different lignocelluloses were pretreated with NaOH. This pretreatment partially solubilizes the hemicelluloses and lignin and swells the cellulose so that the organism can utilize it for its growth and for production of a cellulase system in SSF. The treated lignocelluloses were not washed. The NaOH treatment is done with a minimum amount of water so that, after the addition of nutrient solution and inoculum, the moisture content is less than 80% wt/wt and there is no free water in the medium. More water was added to make suspensions of different lignocellulosic substrates of the desired concentration (1% or 5%) for liquid-state (submerged) fermentation (LSF). 

Since 1993, a two-stage submerged fermentation has been used to produce vinegar of more than 20 percent acetic acid. In the first fermentor, alcohol is added slowly to a total concentration of about 18.5 percent. After the acetic acid concentration has reached 15 percent, about 30 percent of the liquid from the first fermentor is transferred to a second fermentor. In the second fermentor, the fermentation is carried out at a reduced temperature of 18-24°C and continues until the alcohol is almost depleted. 

Submerged fermentation processes are currently considered to be the best methods of vinegar production. The bacteria are active in the liquid, no mother of vinegar is produced, and continuously controlled aeration is guaranteed. This means that temperature control and thus carefully... 

The most common procedure for large-scale aerobic fermentations is to use deep tanks of a stirred liquid medium. Efficient air exchange is obtained by continuous pumping of sterile air into crossed or coiled perforated pipe placed at the bottom of the tank at rates of up to one volume of air per unit of medium volume per minute [62]. This method promotes aerobic growth throughout the medium, which accomplishes large volume production in a limited space. But it also has problems, which relate to the maintenance of sterility and occasionally from foam formation. Overall, however, submerged... 

Submerged fermentation procedures are used almost exclusively today. A few surface fermentation processes (on liquids or solids) are still used. Cost comparisons of labor, air compression, infection, etc., can be made, but modem batch fed, highly instrumented and computerized submerged methods predominate. Submerged methods are also the predominant culture propagation technique. The general principle is to have the fewest possible... 

Figure 3 shows protease production profile in liquid medium by submerged fermentation (SmF) through 8 days. It can be seen that most fungi exhibited maximum protease secretion... 

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