Relationship between shear energy input and sedimentation properties of exopolysaccharide producing Streptococcus thermophilus strains
Häffele, F., Mende, S., Jaros, D., Rohm, H., Nirschl, H.
Separation and Purification Technology, 2018, 202, 21 - 26
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 have however a large negative impact on the separation properties of the cells. These EPS can be categorized in cell-bound capsular EPS or in free EPS that are liberated 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 because of shearing off the capsular EPS layer. Shearing media with free EPS resulted in a viscosity decrease and hence also in higher sedimentation velocity as determined with 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 point out that a defined shear treatment of EPS producing bacterial starter cultures leads to improved separation properties and therefore higher bacteria yields. It is our hypothesis that both EPS-types affect separation efficiency of the bacteria cells: free EPS because of increasing media viscosity and capsular EPS because of acting like a friction pad.