with a Y for glucose of 0.5, which is 0.5 g cells g~ glucose, the concentration of glucose needed to obtain 30 g dm"3 cells will be 30/0.5 = 60 g dm"3 glucose. One litre of this medium would also need to contain approximately 3.0 g N, 1.0 g P, 1.0 g K, 0.3 g S and 0.1 g Mg. More details of Y values for different microorganisms and substrates are given by Atkinson and Mavituna (1991b).
An adequate supply of the carbon source is also essential for a product-forming fermentation proces--In a critical study, analyses are made to determine how the observed conversion of the carbon source to the product compares with the theoretical maximum yield. This may be difficult because of limited knowledge of the biosynthetic pathways. Cooney (1979) has calculated theoretical yields for penicillin G biosynthesis on the basis of material and energy balances using a biosynthetic pathway based on reaction stoichiometry. The stoichiometry equation for the overall synthesis is:
«2C6H1206 + 62NH3 + c2Oz + d2H2S04 + e2PAA -» n2Pen G + p2C02 + q2H20
where a2, b2, c2, d2, e2, n2, p2 and q2 are the stoichiometric coefficients and PAA is phenylacetic acid. Solution of this equation yields:
In this instance it was calculated that the theoretical yield was 1.1 g penicillin G g"1 glucose (1837 units mg-1).
Using a simple model for a batch-culture penicillin fermentation it was estimated that 28, 61 and 11% of the glucose consumed was used for cell mass, maintenance and penicillin respectively. When experimental results of a fed-batch penicillin fermentation were analysed, 26% of the glucose has been used for growth, 70% for maintenance and 6% for penicillin. The maximum experimental conversion yield for penicillin was calculated to be 0.053 g g~! glucose (88.5 units mg~'). Thus, the theoretical conversion value is many times higher than the experimental value. Hersbach et al. (1984) concuded that there were six possible biosyn-thetic pathways for penicillin production and two possible mechanisms for ATP production from NADH and FADH2- They calculated that conversion yields by different pathways varied from 638 to 1544 units of penicillin per mg glucose. At that time the best quoted yields were 200 units penicillin per mg glucose. This gives a production of 13 to 29% of the maximum theoretical yield.
The other major nutrient which will be required is oxygen which is provided by aerating the culture, and this aspect is considered in detail in Chapter 9. The design of a medium will influence the oxygen demand of a culture in that the more reduced carbon sources will result in a higher oxygen demand. The amount of oxygen required may be determined stoichiometrically, and this aspect is also considered in Chapter 9. Optimization is dealt with later in this chapter.
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