Abstract
Today’s yeast total biomass and viability measurements during the brewing process are dependent on offline methods such as methylene blue or florescence dye-based staining, and/or the usage of flow cytometric measurements. Additionally, microscopic cell counting methods decelerate an easy and quick prediction of yeast viability. These processes are time consuming and result in a time-delayed response signal, which not only reduces the knowledge of the performance of the yeast itself, but also impacts the quality of the final product. Novel approaches in process monitoring during the aerobic and anaerobic fermentation of Saccharomyces cerevisiae are not only limited to classical pH, dO2 and off-gas analysis, but they also use different in situ and online sensors based on different physical principles to determine the biomass, product quality and cell death. Within this contribution, electrochemical impedance spectroscopy (EIS) was used to monitor the biomass produced in aerobic and anaerobic batch cultivation approaches, simulating the propagation and fermentation unit operation of industrial brewing processes. Increases in the double-layer capacitance (CDL), determined at frequencies below 1 kHz, were proportional to the increase of biomass in the batch, which was monitored in the online and inline mode. A good correlation of CDL with the cell density was found. In order to prove the robustness and flexibility of this novel method, different state-of-the-art biomass measurements (dry cell weight—DCW and optical density—OD) were performed for comparison. Because measurements in this frequency range are largely determined by the double-layer region between the electrode and media, rather minor interferences with process parameters (aeration and stirring) were to be expected. It is shown that impedance spectroscopy at low frequencies is not only a powerful tool for the monitoring of viable yeast cell concentrations during operation, but it is also perfectly suited to determining physiological states of the cells, and may facilitate biomass monitoring in the brewing and yeast-propagating industry drastically.
Highlights
Microbial cultivations play a key role in many different fields, such as in food, drug and bulk chemical production, as well as in waste-to-value concepts [1]
Batch cultivations were performed in a stainless-steel Sartorius Biostat Cplus bioreactor (Sartorius, Göttingen, Germany) with a 10 L working volume, and in an Infors Techfors-S bioreactor
Anaerobic batches were cultivated at 600 rpm and with a 2 to 4 L/min N2 flow
Summary
Microbial cultivations play a key role in many different fields, such as in food, drug and bulk chemical production, as well as in waste-to-value concepts [1] Process monitoring, such as pH, dissolved oxygen (dO2 ) and off-gas analysis, is state of the art in today’s industrial cultivations for guaranteeing product quality and safety. The most important parameter in bioprocesses, the biomass, can only be determined using offline methods or complex soft-sensor applications [2] These control systems are often dependent on inline/online/at-line detection systems, such as high-performance liquid chromatography (HPLC) for metabolites, off-gas balance, and/or dielectric spectroscopy measurements. The VCC is measured using offline measurement principles including marker proteins or fluorescence probes, such as flow cytometry or confocal microscopy [5,6] Because these control and analytical tools are cost intensive, classical bulk food products—such as yeast and beer—are produced in rather uncontrolled environments
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