Ion-specific sensors have been developed to measure NH3+, Ca2+, K+, Mg2+, PO43-, etc. However, none of these probes are steam sterilizable.

The mass spectrometer [4] can be used for in-line analysis since it is very versatile, but unfortunately expensive. It does allow for monitoring of gas partial pressures (O2, CO2, CH4, etc.), dissolved gases (O2, CO2, CH4, etc.), concentrations of volatiles (methanol, ethanol, acetone).

The fluorimetric measurements [5,6] are very specific and rapid, but their use in bioprocess is quasi-limited nowadays. Hence, the measurement of NAD (provided it remains at a constant concentration in cells) would be an ideal method for continuous measurement of microbial biomass concentration.

The biosensor [7] is based on a biological receptor, which is coupled to an electronic transducer that converts the biological signal into an electrical signal by measuring voltage, current, light, temperature. Biosensors can be used to measure the concentration of different substrates / metabolites in the culture broth. In order to avoid the possible effects on growth / product formation (i. e. inhibition), the biological receptor can be immobilized on a separate membrane or on the transducer surface. Enzyme electrodes are the most applied, normally for in-line determinations (no steam sterilizable). The specific enzyme is immobilized on a membrane held in close contact to a pH or oxygen electrode. Also microbial electrodes using immobilized whole cells have been used for determination of sugars, acetic acid, ethyl alcohol, vitamin B, nicotinic acid, glutamic acid and cephalosporin. Generally speaking, the biosensors have been investigated with limited success. Hence, only the glucose biosensors have been fully applied. The main difficulties in developing an on-line biosensor are the thermal stability of the immobilized biomaterial, i. e. inactivation during sterilization, and the limited linear range inherent to biologic species.

It is to consider two most recent directions of development regarding the bioprocess variables monitoring and control [8]: (a) realization of miniaturized sensors for the in situ

measurement of temperature, pH or dissolved oxygen; (b) use of analyzers not yet applied for on-line process monitoring in biotechnology, but of real interest: the continuous air — segmented flow analyzer (CFA); the flow injection analyzer (FIA); and the High Performance Liquid Chromatography (HPLC).