levels of oxygen for biomass-related product formation. This

requires continuous measurement of DO concentration in bioreac-

tors, and a control system that will react quickly when there is a

deviation from the values designed according to the needs of

microorganisms. Because the number of microorganisms in the

fermenter and their metabolic rate changed over time, the oxygen

requirement of the system also does not remain constant during

fermentation [5, 6].

1.1

The

Measurement of kLa

DO concentration can vary during fermentation as a result of

microbial activity. Physical and chemical methods have been pro-

posed for the measurement of the kLa in stirred tank reactors

(STR). In bioprocesses, steady-state and unsteady-state methods

are mostly preferred. These methods have superior advantages for

several cases according to their application.

1.2

The Unsteady-

State (Dynamic)

Method

The unsteady-state (also known as dynamic) technique is the most

used method for measuring DO concentration. It is based on

monitoring the decrease in DO concentration with an oxygen

electrode by cutting off the air (oxygen) that feeds the bioreactor,

and the increase in oxygen concentration by re-feeding the air

(oxygen). Basically, the DO level is foamed with nitrogen or

reduced to zero by adding sodium sulfide. Then, the increase in

DO concentration as a function of time is followed [7].

The unsteady-state method consists of two stages: consump-

tion and absorption. Aeration is stopped and DO decreases due to

cell respiration during consumption stage. In absorption stage,

aeration is resumed, and the DO increases until a steady-state is

reached. It is often difficult to obtain accurate kLa using the

unsteady-state method. Because, first of all the assumptions about

the extent of gas-phase mixing must be chosen correctly. Secondly,

the electrode response time must be fast [7, 8]. However, it is

commonly used because it gives more accurate result.

During the transfer of oxygen from air to water, the actual

resistance occurs in the liquid film layer at the interface [2, 3]. Inside

the bioreactor where oxygen is not consumed, when resistance in

the gas phase is neglected, the time-dependent change of DO

concentration is given by

qOx ¼ kLa C^∗ CL

ð

Þ ¼ OTR

ð1Þ

where kL is the oxygen transfer coefficient (cm/h), a is the gas

liquid interfacial area (cm²/cm³), kLa is the volumetric mass trans-

fer coefficient (h^1), C^∗is saturated dissolved oxygen concentra-

tion (mg/L), CL is the actual dissolved oxygen concentration in the

broth (mg/L), and the qOx is the rate of oxygen transfer (mg O2

L^1 h^1).

18

Aysegul Inam et al.