NONINVASIVE SPECTROSCOPY FOR MONITORING CELL-DENSITY IN A FERMENTATION PROCESS

Short-wavelength near-infrared spectroscopy in the wavelength range 700-1100 nm is used to monitor cell density in a fermentation process vessel over a wide concentration range (1-60 g/L). The measurement is performed noninvasively through the glass walls of the vessel using a bifurcated fiber-optic bundle to both excite the sample and collect the diffusely backscattered radiation. It was found that, in regions where there was little absorption, the major change was an exponential decrease of the logarithm of the diffusely backscattered radiation with Increasing cell concentration. Various types of models were fit to the data. The newly developed multivariate method of locally weighted regression gave the best results. It allowed noninvasive determination of cen density of yeast fermentations that were being both bubbled and stirred with an average standard error of prediction of 1.6 g/L. The above results were found to he valid Only if the stirring and bubbling rates remained constant during the fermentation. However, experiments with variation of these two parameters suggested correction factors for them could be readily developed.