Fermentations

Although a consideration of the stoichiometry of respiration gives an appreciation of the problem of oxygen supply, it gives no indication of an organism's true oxygen demand as it does not take into account the carbon that is converted into biomass and products. A number of workers have considered the overall stoichiometry of the conversion of oxygen, a source of carbon and a source of nitrogen into biomass and have used such relationships to predict the oxygen demand of a fermentation. A selection of such equations is shown in Table 9.1. From these determinations it may be seen that a culture's demand for oxygen is very much dependent on the source of carbon in the medium. Thus, the more reduced the carbon source, the greater will be the oxygen demand. From Darlington's and Johnson's equations (Table 9.1) it may be seen that the production of 100 grams of biomass from hydrocarbon requires approximately three times the amount of oxygen to produce the same amount of biomass from carbohydrate. This point is also illustrated in Table 9.2. However, it must be remembered that the high carbon content of hydrocarbon substrates means that high yield factors (g biomass g 1 substrate consumed) are obtained and the decision to use such substrates is based on the balance between the advantage of high biomass yield and the disadvantage of high oxygen demand and heat generation. These points are discussed in more detail in Chapter 4.

Darlington's, Johnson's and Mateles' equations only include biomass production and do not consider product formation, whereas Cooney's and Righelato's equations consider product formation. Ryu and Hospodka (1980) used Righelato's approach to calculate that the production of 1 g penicillin consumes 2.2 g of oxygen.

However, it is inadequate to base the provision of oxygen for a fermentation simply on an estimation of overall demand, because the metabolism of the culture is affected by the concentration of dissolved oxygen in the broth. The effect of dissolved oxygen concentration

Table 9.1. Stoichiometric equations describing oxygen demand in a fermentation

Equation

Terms used

Reference

6.67CH20 + 2.102= C392H65Ol<)4 + 2.75C02+ 3.42H20

7.14CH2 + 6.13502 = C392H650, 94 + 3.22C02+ 3.89H20

C3.92H6.5°1.94 is 100 g (dry weight) of yeast cells; CH20 is carbohydrate

CH2 is hydrocarbon

A = Amount of oxygen for combustion of 1 g of substrate to C02, H20 and NH3, if nitrogen is present in the substrate B = Amount of oxygen required for the combustion of 1 g cells to C02, H20 and NH3 V= Cell yield (g cells g-1 substrate) C = g oxygen consumed for the production of 1 g of cells x = biomass concentration t — time

Y— g biomass g"1 carbon substrate m = maintenance p = allowance for antibiotic production

Darlington (1964) Darlington (1964) Johnson (1964)

Yq/p = g oxygen consumed g-1 glucose Yp/G = 8 sodium penicillin G produced g~ X = g cells (dry weight) produced P — g sodium penicillin G produced glucose

Y0 = g oxygen consumed g_1 cells produced Y = Cell yield (g cells g_1 substrate) M = Molecular weight of the carbon source C, H and O = Number of atoms of carbon, hydrogen and oxygen per molecule of carbon source

Righelato et al. (1968)

Cooney (1979)

on the specific oxygen uptake rate (Q0l, mmoles of oxygen consumed per gram dry weight of cells per hour) has been shown to be of the Michaelis-Menten type, as shown in Fig. 9.1.

From Fig. 9.1 it may be seen that the specific oxygen uptake rate increases with increase in the dissolved oxygen concentration up to a certain point (referred to as Ccrit) above which no further increase in oxygen uptake rate occurs. Some examples of the critical oxygen levels for a range of micro-organisms are given in

Table 9.3. Thus, maximum biomass production may be achieved by satisfying the organism's maximum specific oxygen demand by maintaining the dissolved oxygen concentration greater than the critical level. If the dissolved oxygen concentration were to fall below the critical level then the cells may be metabolically disturbed. However, it must be remembered that it is frequently the objective of the fermentation technologist to produce a product of the micro-organism rather than the organism itself and that metabolic

Table 9.2. Oxygen requirements of a range of micro-organisms grown on a range of substrates (After Mateles, 1979)

Substrate Organism Oxygen requirement Reference

Glucose Escherichia coli 0.4 Schulze and Lipe (1964)

Methanol Pseudomonas C 1.2 Goldberg et al. (1976)

Octane Pseudomonas sp. 1.7 Wodzinski and Johnson (1968)

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Dissolved oxygen concentration

Fro 9 1- The effect of dissolved oxygen concentration on the Qq2

of a micro-organism.

disturbance of the cell by oxygen starvation may be advantageous to the formation of certain products. I Squally, provision of a dissolved oxygen concentration lar greater than the critical level may have no influence on biomass production, but may stimulate product formation. Thus, the aeration conditions necessary for the optimum production of a product may be different from those favouring biomass production.

Hirose and Shibai's (1980) investigations of amino acid biosynthesis by Brevibacterium flauum provide an excellent example of the effects of the dissolved oxygen concentration on the production of a range of closely related metabolites. These workers demonstrated the critical dissolved oxygen concentration for B. flavum to be 0.01 mg dm ~3 and considered the extent of oxygen supply to the culture in terms of the degree of 'oxygen satisfaction', that is the respiratory rate of the culture expressed as a fraction of the maximum respiratory rate. Thus, a value of oxygen satisfaction below unity implied that the dissolved oxygen concentration was below the critical level. The effect of the degree of oxygen satisfaction on the production of a range of amino acids is shown in Fig. 9.2. From Fig. 9.2 it may be seen that the production of members of the glutamate and aspartate families of amino acids was affected detrimentally by levels of oxygen satisfaction below 1.0, whereas optimum production of phenylalanine, valine and leucine occurred at oxygen satisfaction levels of 0.55, 0.60 and 0.85, respectively. The biosyn-thetic routes of the amino acids are shown in Fig. 9.3, from which it may be seen that the glutamate and aspartate families are all produced from tricarboxylic acid (TCA) cycle intermediates, whereas phenylalanine, valine and leucine are produced from the glycolysis intermediates, pyruvate and phosphoenol pyruvate. Oxygen excess should give rise to abundant TCA cycle intermediates, whereas oxygen limitation should result in less glucose being oxidized via the TCA cycle, allowing more intermediates to be available for phenylalanine, valine and leucine biosynthesis. Thus, some degree of metabolic disruption results in greater production of pyruvate derived amino acids.

An example of the effect of dissolved oxygen on secondary metabolism is provided by Zhou et al.'s (1992) work on cephalosporin C synthesis by Cephalo-sporium acremonium. These workers demonstrated that the critical oxygen concentration for cephalosporin C synthesis during the production phase was 20% saturation. At dissolved oxygen concentrations below 20% cephalosporin C concentration declined and penicillin N increased. The biosynthetic pathway to cephalosporin C is shown in Fig. 9.4, from which it may be seen that there are three oxygen-consuming steps in the pathway:

(i) Cyclization of the tripeptide, a-amino-adipyl-cysteinyl-valine into isopenicillin N.

(ii) The ring expansion of penicillin N into deace-toxycephalosporin C (DAOC).

(iii) The hydroxylation of DAOC to give deacetyl-cephalosporin C.

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Table 9.3. Critical dissolved oxygen concentrations for a range of micro-organisms

(Riviere, 1977)

Organism Temperature Critical dissolved oxygen concentration (mmoles dm-3)

Organism Temperature Critical dissolved oxygen concentration (mmoles dm-3)

Azotobacter sp.

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