DIELECTRIC PROPERTIES OF BIOMASSES

MADRID RE, FELICE CJ AND VALENTINUZZI ME

Chicago, USA.

Congreso; Chicago 2000 World Congress; 2000

Dielectric properties of cellular suspensions have been studied since the late XIX Century. However, only during the last decade, this concept was applied for the estimation of cellular biomass in fermentation processes. When subjected to ac electric fields, cellular suspensions are characterized by three main frequency dispersion regions called, respectively, a, b and g. Under such condition, the membrane capacity, Cm , of all viable cells takes up electric charge. The higher the applied frequency, the lower the net charge, and the lower the suspension capacity. The most important capacity drop is the b dispersión. Capacity C relates to electric permittivity as, e’ = C ( k / eo ), where k = constant and eo = permittivity of vacuum. Thus, the permittivity change, (De’), is a measure of the biomass content, i.d., higher cellular concentrations lead to larger permittivity changes. Commercial equipments based on this principle make use of Schwan's equation, De’=K P/[1+(P/2)]2, relating permittivity with biomass for high volume fractions P, with K = constant. Herein, instead, we use Grosse's model, within the same b range, following the expression De’=(9/4) P [(2lem-Repsm)2/(ep+2em)(l+Rsm)2], where l and sm are the superficial and medium conductivities, respectively, and R = cellular radius. The term within brackets is constant and, thus, De’ appears a directly proportional to the volume fraction. We have designed two tetrapolar probes to measure medium capacity and so determine biomass of a yeast suspension sample. Calibration is accomplished with a SOLARTRON 1255A impedance analyzer.