On‐line monitoring of cell concentration of Perilla frutescens in a bioreactor

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Abstract
This article demonstrates the successful in situ real‐time monitoring of the cell concentration of Perilla frutescens in a bioreactor by using a laser turbidimeter. It was found that turbidity measurements at 780 nm with the laser sensor were hardly affected by the red color of the anthocyanin produced by P. frutescens cells, nor by the aeration rate or agitation speed within the ranges investigated. There was an excellent linear relationship, with a correlation coefficient (r2) higher than 0.99, between the sensor's response and the cell concentration. The whole growth stage of the cells, i.e., lag, logarithmic, and stationary phases, in bioreactor cultivations, could be satisfactorily estimated on‐line by means of the in situ turbidimeter. However, during the declining phase of the cells, an apparent deviation was observed between the on‐line estimations and off‐line measurements of cell concentrations by dry cell weight, while the wet cell weight could be estimated by the same turbidimeter system. We found that this deviation was caused by a decrease in the cell density due to an increase of the individual cell volume and a decrease of the cell dry weight during the declining phase. © 1993 John Wiley & Sons, Inc.