Combined optical measurement of dissolved oxygen tension (DOT), pH value, biomass and viscosity in shake flasks

Shake flasks are widely spread in microbial process development. Characterization of the processes by manual offline sampling is time-consuming, highly laborious and a contamination risk. Online monitoring of key parameters would provide deeper insights, while saving time and effort. In this study, a device for optical online monitoring of dissolved oxygen tension (DOT), biomass, pH value and viscosity in shake flasks is presented. DOT measurement relies on fluorescent oxygen sensitive nanoparticles. The fluorescence intensity signal of the nanoparticles is used to trigger the DOT and scattered light measurements inside the rotating bulk liquid. The scattered light signal (610 – 630 nm) can be correlated to offline measured optical density OD₆₀₀, even at elevated viscosity. The pH value is monitored online by using pH sensor spots, fixed inside the shake flasks. The shift of the angle of the bulk liquid Θ-Θ₀ is correlated to the offline measured viscosity. Detection of the leading edge of the bulk liquid, necessary for viscosity measurement, can be performed either using the fluorescence intensity signal of the oxygen nanoparticles or the scattered light signal.