Fermentation Design

In the growing of food yeast on wood sugar, one of the most difficult problems is that of foaming. This difficulty has been largely overcome by a fermenter designed for use in the production of yeast from sulfite waste liquor in the Waldhof plant at Mannheim. A modification of the fermenter was used for the continuous production of yeast on wood sugars in America. Fermentation periods of 2.6 to 3 hours were obtained with a strain of Torula utilis. Yields were 38 to 49% of the total sugar. 

Knights, A. J. University of Wales, Ph.D. thesis (1981). Determination of the biological kinetic parameters of fermenter design from batch culture data. 

Therefore, we must make assumptions to be able to arrive at simple models which are useful for fermenter design and performance predictions. Various models can be developed based on the assumptions concerning cell components and population as shown in Table 6.1 (Tsuchiya et al., 1966). The simplest model is the unstructured, distributed model which is based on the following two assumptions ... 

Since the oxygen is sparingly soluble gas, the overall mass-transfer coefficient KL is equal to the individual mass-transfer coefficient KL. Our objective in fermenter design is to maximize the oxygen transfer rate with the minimum power consumption necessary to agitate the fluid, and also minimum air flow rate. To maximize the oxygen absorption rate, we have to maximize KL, a, C - CL. However, the concentration difference is quite limited for us to control because the value of C L is limited by its very low maximum solubility. Therefore, the main parameters of interest in design are the mass-transfer coefficient and the mterfacial area. 

Gas-liquid mass transfer is commonly modeled in terms of a gas film (between the bulk gas and interface) and a liquid film (between the interface and bulk liquid). Hindrance to mass transfer causes soluble gas (e.g., O2) concentrations to decrease across these films. The highest mass transfer resistance usually exists in the liquid film therefore, it controls the overall oxygen transfer rate (OTR). In aerobic fermentation, an effective fermenter design achieves an efficient OTR through intimate gas-liquid contact. OTR is described in terms of oxygen concentration and characteristics of the gas-liquid interface, as follows ... 

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