2.5 Measurement of Stoichiometric Coefficients

For the growth reaction given in Eq(2-1), the ratio g/b is called the respiratory quotient, often abbreviated as RQ. It is easily measured in large scale fermentors. In Eq(2-1), if the nature of extracellular product is known (i.e. x,y,z), then it is possible to calculate a,b,g and d from experimental measurement of RQ and one other measurement. If no significant amount of extracellular product is formed, as in simply growth processes, then only RQ or one other measurment is needed to compute stoichiometric coefficients. The example given below illustrates this idea.

Example 2-4


For the reaciton equation representing E. coli growth, RQ was measured as 0.85. Calculate a,b,g and d.

Solution

The solution consists of carrying out elemental balances and then solving them. Here, we can write four elemental balances, C, H, O and N. We have five unknowns: a,b,g and d, a and b. One additional relationship is obtained from the given RQ value, thus making the problem solvable!

Rearranging the above system of algebraic equations:

Solution to the above yields