ObjectivesDielectric spectroscopy is commonly used foronlinemonitoring of biomass growth. It is however not utilized for biomass concentration measurements due to poor correlation with Cell Dry Weight (CDW). A calibration methodology is developed that can directly measure viable biomass concentration in a commercial filamentous process using dielectric values, without recourse to independent and challenging viability determinations.ResultsThe methodology is applied to samples from the industrial scale fermentation of a filamentous fungus,Acremonium fusidioides. By mixing fresh and heat-killed samples, linear responses were verified and sample viability could be fitted with the dielectric\documentclass[12pt]{minimal}
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				\begin{document}$$\Delta C$$\end{document}ΔCvalues and total solids concentration. The study included a total of 26 samples across 21 different cultivations, with a legacy at-line viable cell analyzer requiring 2 ml samples, and a modern on-line probe operated at-line with 2 different sample presentation volumes, one compatible with the legacy analyzer, a larger sample volume of 100 ml being compatible with calibration for on-line operation. The linear model provided an\documentclass[12pt]{minimal}
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				\begin{document}$${R}^{2}$$\end{document}R2value of 0.99 between\documentclass[12pt]{minimal}
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				\begin{document}$$\Delta C$$\end{document}ΔCand viable biomass across the sample set using either instrument. The difference in ∆C when analyzing 100 mL and 2 mL samples with an in-line probe can be adjusted by a scalar factor of 1.33 within the microbial system used in this study, preserving the linear relation with\documentclass[12pt]{minimal}
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				\begin{document}$${R}^{2}$$\end{document}R2of 0.97.ConclusionsIt is possible to directly estimate viable biomass concentrations utilizing dielectric spectroscopy without recourse to extensive and difficult to execute independent viability studies. The same method can be applied to calibrate different instruments to measure viable biomass concentration. Small sample volumes are appropriate as long as the sample volumes are kept consistent.