The formation of pH gradients in a 700 L batch fermentation of Streptococcus thermophilus was studied using multi-position pH measurements and computational fluid dynamics (CFD) modeling. To this end, a dynamic, kinetic model of S. thermophilus and a pH correlation were integrated into a validated one-phase CFD model, and a dynamic CFD simulation was performed. First, the fluid dynamics of the CFD model were validated with NaOH tracer pulse mixing experiments. Mixing experiments and simulations were performed whereas multiple pH sensors, which were placed vertically at different locations in the bioreactor, captured the response.A mixing time of about 46 s to reach 95% homogeneity was measured and predicted at an impeller speed of 242 rpm. The CFD simulation of the S. thermophilus fermentation captured the experimentally observed pH gradients between a pH of 5.9 and 6.3, which occurred during the exponential growth phase. A pH higher than 7 was predicted in the vicinity of the base solution inlet. Biomass growth, lactic acid production, and substrate consumption matched the experimental observations. Moreover, the biokinetic results obtained from the CFD simulation were similar to a single-compartment simulation, for which a homogeneous distribution of the pH was assumed. This indicates no influence of pH gradients on growth in the studied bioreactor. This study verified that the pH gradients during a fermentation in the pilot-scale bioreactor could be accurately predicted using a coupled simulation of a biokinetic and a CFD model. To support the understanding and optimization of industrial-scale processes, future biokinetic CFD studies need to assess multiple types of environmental gradients, like pH, substrate, and dissolved oxygen, especially at industrial scale. K E Y W O R D S computational fluid dynamics (CFD), dynamic simulation, heterogeneities, lactic acid bacteria (LAB) fermentation, pH gradients, transient CFD simulation 1 | INTRODUCTION Heterogeneities of culture parameters like substrate concentrations, pH, and dissolved oxygen concentrations are regarded as mainly responsible for productivity loss in large-scale bioreactor cultivations.Transport limitations occur at large scale due to insufficient mixing, and cells are consequently exposed to fluctuating conditions. Non-limiting substrate concentrations in the range of 0.3-2 g L −1 are reported in feeding zones during fed-batch processes, whereas there are substrate-limited conditions further away from the feeding Biotechnology and Bioengineering. 2019;116:769-780.wileyonlinelibrary.com/journal/bit

The rapid assessment of cell viability is crucial for process optimization, e.g., during media selection, determination of optimal environmental growth conditions and for quality control. In the present study, the cells' electric anisotropy of polarizability (AP) as well as the mean cell length in Lactobacillus plantarum batch and fed-batch fermentations were monitored with electrooptical measurements coupled to fully automated sample preparation. It was examined, whether this measurement can be related to the cells' metabolic activity, and thus represents a suitable process analytical technology. It is demonstrated that the AP is an early indicator to distinguish between suitable and unsuitable growth conditions in case of a poor energy regeneration or cell membrane defects in L. plantarum batch and fed-batch cultivations. It was shown that the applied method allowed the monitoring of physiological and morphological changes of cells in various growth phases in response to a low pH-value, substrate concentration changes, temperature alterations, exposure to air and nutrient limitation. An optimal range for growth in batch mode was achieved, if the AP remained above 25·10−28 F·m2 and the mean cell length at ~2.5 μm. It was further investigated, in which way the AP develops after freeze-drying of samples, which were taken in different cultivation phases. It was found that the AP increased most rapidly in resuspended samples from the retardation and late stationary phases, while samples from the early stationary phase recovered slowly. Electrooptical measurements provide valuable information about the physiologic and morphologic state of L. plantarum cells, e.g., when applied as starter cultures or as probiotic compounds.