A central step in the production of starter cultures is the separation of the cells from the fermentation medium, which is usually achieved by disk centrifuges. In case of microorganisms which produce exopolysaccharides (e.g., various strains of lactic acid bacteria), the properties of the respective exopolysaccharides may interfere with this separation step. By using six strains of Streptococcus thermophilus the hypothesis was tested that a shear treatment of the fermented culture medium improves subsequent cell separation markedly. Depending on the type of exopolysaccharides (freely present in the medium, or as capsules around the cells) an energy input of up to 2.5 kJ/mL generated with an Ultra-Turrax affected cell chain length of the strains and viscosity of fermentation medium differently. For bacteria producing capsular exopolysaccharides, space-and time-resolved centrifugation experiments revealed an increase of sedimentation velocity after shear treatment. In general, viability of the microorganisms, detected by flow cytometry measurements and fermentation experiments, was not affected by the shearing procedure. The results therefore indicate that strain-targeted shearing is helpful to improve the separability of cells from the fermented media.

Besides fermentation, the production of bacterial starter cultures includes another crucial step, namely, the separation of the bacteria cells. This separation is most commonly carried out with disc stack separators and needs to be adjusted to the respective strain to obtain a high cell recovery rate. Exopolysaccharides (EPS) produced by several starter cultures, however, have a large negative impact on the separation properties of the cells. These EPS can be divided into cell-bound capsular EPS or free EPS that are released into the surrounding fermentation medium. To improve the separation step, shear forces were applied after fermentation with a gear ring disperser to simulate the impact of a homogenizer and the influence on the separation properties of six Streptococcus thermophilus strains was examined. In case of capsular EPS, the sedimentation velocity of the bacteria increased due to shearing off the capsular EPS layer. Shearing media with free EPS resulted in a viscosity decrease and, hence, in a higher sedimentation velocity, as was determined using a disc centrifuge and a LUMiSizer. Sediment compression as measured with the LUMiSizer was also affected by the shearing step. The results of this study suggest that a defined shear treatment of EPS producing bacterial starter cultures leads to improved separation properties and, hence, higher bacteria yields. We assume that both EPS types affect separation efficiency of the bacteria cells, free EPS because of increased media viscosity and capsular EPS because they act like a friction pad.

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Die Auswirkungen einer mittels Hochdruck‐Homogenisator induzierten Scherbehandlung auf die Separationseigenschaften von Exopolysaccharid‐bildenden bakteriellen Starterkulturen wird untersucht. Neben Änderungen seitens der Medienviskosität und der Sinkgeschwindigkeit der Bakterienzellen konnten auch deutliche Unterschiede bei der Kompaktheit des Sediments festgestellt werden. Zusätzlich wird die Skalierbarkeit der im Labor mittels analytischer Zentrifugation gewonnenen Ergebnisse in den Technikums‐Maßstab mit einem Tellerseparator gezeigt.