Efficiency of growth and product formation type 3 and 4 For type 3 processes, growth and metabolic activity reach a maximum early in the batch process cycle(Figure 3. 1)and it is not until a later stage, when oxidative activity is low, that maximum desired product formation occurs. The stoichiometric descriptions for both type 3 and 4 processes depend upon the particular substrates and product involved. In the main, product formation in these processes is completely uncoupled from cell growth and dictated by kinetic regulation and activity of cells. Product formation stoichiometry can be used to estimate the upper bounds for product elds in processes. a relatively simple example is the anaerobic fermentation of glucose by yeast. Here, carbon dioxide and ethanol are the only products. Modification of (E 3.9)then becomes Hz >:CO + ch3 oo5 For the ethanolic fermentation described by(E-3. 13), determine the upper bound on the molar vield factor, iey ax From(E-3. 10),YP=1x2 Yields much lower than the upper bound value indicate that there is significant substrate utilisation to support growth, maintenance or synthesis of other products 3.2.1 Product yield considerations We can see that for type 1 processes, high growth rate is obligately linked to a high rate of product formation. Indeed, this is the case for all products produced by a fermentative mode of metabolism, eg ethanol, lactic acid, acetone Chemostat studies ave shown that for most aerobic processes when growth is limited by some nutrient other than the carbon source, the yield of product decreases with increase in specific growth rate (u or D; u=dilution rate(d)in chemostat culture). Conversely, both the biomass yield and the specific rate of substrate utilisation(qs; g substrate g biomass h)increase with specific growth rate growth rates The relationships between specific rate of substrate consumption and dilution rate and and product between yield coefficients and dilution rates are shown in figure 3.2Efficiency of growth and product formation 45 type 3 and 4 For type 3 processes, growth and metabolic activity reach a maximum early in the batch process cycle (Figure 3.1) and it is not until a later stage, when oxidative activity is low, that maximum desired product formation occurs. The stoichiometric descriptions for both type 3 and 4 processes depend upon the particular substrates and products involved. In the main, product formation in these processes is completely uncoupled from cell growth and dictated by kinetic regulation and activity of cells. Product formation stoichiometry can be used to estimate the upper bounds for product yields in processes. A relatively simple example is the anaerobic fermentation of glucose by yeast. Here, carbon dioxide and ethanol are the only products. Modification of (E - 3.9) then becomes: 3 3 E - 3.13 n For the ethanolic fermentation described by (E - 3.13), determine the upper bound max on the molar yield factor, ie Y . - 2x6 =2 3 1x2 From (E - 3.101, Y Ern = Yields much lower than the upper bound value indicate that there is significant substrate utilisation to support growth, maintenance or synthesis of other products. 3.2.1 Product yield considerations We can see that for type 1 processes, high growth rate is obligately linked to a high rate of product formation. Indeed, this is the case for all products produced by a fermentative mode of metabolism, eg ethanol, lactic acid, acetone. Chemostat studies have shown that for most aerobic processes when growth is limited by some nutrient other than the carbon source, the yield of product decreases with increase in specific growth rate (p or D; p = dilution rate (D) in chemostat culture). Conversely, both thq biomass yield and the specific rate of substrate utilisation (qs; g substrate g biomass￾h-’) increase with specific growth rate. The relationships between specific rate of substrate consumption and dilution rate and between yield coefficients and dilution rates are shown in Figure 3.2. growth rates ad Product yields