Citric add production

There are several apparent problems which we still need to resolve in this section before we have a better understanding of citric add production. 

Let us consider Figure 5.3 again. Both pyruvate kinase and dtrate synthase (enzymes III and V) are inhibited by elevated ATP concentrations. During citric acid production ATP concentrations are likely to arise (ATP produced in glycolysis) and either of these enzymes could, if inhibited, slow down the process. In fact all of the evidence suggests that both enzymes are modified or controlled in some way such that they are insensitive to other cellular metabolites during citric add production. 

Figure 5.4 summarises the changes occuring in A. niger in citric add production mode when compared to conventional metabolism. It is worth studying the Figure for some time because it explains some of the features necessary for a successful fermentation process. 

Conventional stirred reactors with working volumes of 50 to 150 m3 have been used routinely for citric add production whereas tower bioreactors, currently 200 m and larger (greater than 600 m3) are envisaged. 

Continuous culture is not considered suitable for citric add production the requirement for a multi-tank system to separate growth and product formation would make the process uneconomic. 

The pH has to be controlled - the acid which is produced has to be neutralised maintaining a pH in excess of 6.0. Below pH 3.0 the glucose oxidase is inactivated and in fungal systems low pH encourages citric add production. 

The production of organic acids by micro-organisms, and especially citric acid, is cortsidered in detail in Chapter 4. In this section therefore we will oirfy briefly consider citric add production, from an energetics perspective. 

Normally sensitive to ducose-6-phosphate and ATP. Sensitivify relieved during citric add production. 

During citric add production there is massive generation of NADH but littie demand for ATP. Thus the situation could quickly arise where there is no further ADP available for oxidative phosphorylation widiin the cells. This means that the electron transport chain cannot operate and no further oxidation of NADH can occur. 

Thus, in theory at least, there are two substrates for citric add production ucose and oxygen. The latter has to be supplied at all times because if oxygen supply is interrupted, even for a few minutes, tfien dtric add production drops dramatic y and does not recover, even after a rapid resumption of the oxygen supply. Interestingly, growth is unaffected by a transient cessation of oxygen supply. It has been shown that the conventional electron transport chain is associated with normal growth mechanisms but the second, sped electron transport chain, is sp>edfically involved with dtric add production. 

An understanding and awareness of the biochemistry of citric add production has given us the opportunity to predict suitable medium and environment cfniditions. 

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