Although the similar relationships applied to both
growth and acid formation (Fig. 5), the acid formation
kinetics were usually different from the growth kinetics
by a term that represented the lag time. This lag time,
also known as the maturation time, is the period during
which the culture taken up all the ammonium ions but
has not yet started producing citric acid. The rate of
product formation was then said to be proportional to the
rate at which the cells entered the maturation state. The
rate of product formation was expressed as follows:
rPt ¼ krXðt−tmÞ ð5Þ
where rPt is the product formation rate at time t, rX(t−tm)
is the rate of biomass growth at time t beyond the