Which parameters influence fermentation

At higher temperature and lower pH, auto-aggregation seems to be less extensive for DSM Similar behavior has been observed in the parental strain ATCC when exposed to more drastic conditions pH 2. It would be interesting in future work to test the survival of DSM under such drastic conditions and simultaneously perform intracellular pH assays to determine if cells manage to keep their pH homeostasis.

Preconditioning or pre-adaptation of cells during fermentation has notable impact on the survival rates during freeze-drying of probiotic bacteria Shin et al. For instance, the pre-adaptation by both heat and acid stress has shown to improve the freeze-drying survival rates in Enterococcus faecium HL7 Shin et al. Regarding the impact of pH on freeze-drying survival, studies performed in L. In our study, the survival rates of DSM after freeze-drying are in the range published for other lactobacilli Martos et al.

According to these authors, the freeze-drying survival rate of the parental strain, as assessed by colony forming units, increases when the pH of the culture is decreased. However, their formulation differed since they used skim milk as lyoprotectant instead of sucrose which should be kept in mind when comparing the results. In a more recent study with L.

These researchers cultivated their strain in MRS and exposed it to various stress conditions, including pH stress 4. Therefore, culturing strain DSM at higher pH could be a useful option to improve survivability and therefore higher number of viable cells in the final product.

The pH of the fermentation culture influenced post-desiccation stresses low-pH stress and bile salt stress survival in an opposite manner as it did for freeze-drying survival per se. A lower culture pH enabled cells to survive exposure to low pH and bile salts better than for cells cultured at higher pH. Preconditioning through exposure to mild pH stress to boost survivability to a more severe pH shock has been observed for many other microorganisms Skandamis et al.

Proteomic studies with another L. Interestingly, similar key proteins were overexpressed in strain ATCC when exposed to purified bile salts Lee et al. This supports our observation that cells grown at lower pH have better survivability to bile salt exposure. In addition, similar finding has been reported for L. These authors found that genes implicated in protection to bile also protect against acid stress in L. Therefore, larger differences in temperature should be tested in future experiments, or alternatively, the biomass subjected to heat shock at sub-lethal temperatures to verify if significant differences are observed in freeze-drying survival rate.

As discussed above and as observed in other studies, preconditioning at low pH improves survivability during a pH and bile salt shock Yadav and Shukla ; Chen et al. Moreover, the stress responses to an acidic pH have become better understood, but apparently these responses are detrimental for freeze-drying tolerance. Instead, the improvement of the latter emerges during growth at higher pH around neutral and might be related to a higher ratio of unsaturated fatty acids to saturated fatty acids in the cell membrane Wang et al.

Thus, responses do occur in the cells that are divergent and not completely streamlined for the series of stress events the L. This demands that an appropriate solution for maximizing survivability over the whole life cycle from production to consumption is found: either i a compromise weighing in all the different encountered stress characteristics, or ii by providing the probiotic bacteria in capsules to avoid direct exposure to bile and acid in the GIT that would also allow for production of more freeze-drying tolerant biomass by growing at higher pH.

A subsequent comparative and functional proteomics and transcriptomics would be the appropriate tool to analyze how the cells in such a process become more tolerant to the freeze-drying stress. In conclusion, culture pH in the range of 4. Survivability to freeze drying is better for cells grown at higher pH, whereas survivability to GIT conditions is better for cells grown at lower pH. Culturing strain DSM at higher pH values could be a useful option to improve survivability and therefore higher viable cell numbers in the final product, however an optimization of the process should be performed to improve the resistance of these cells to the acidic environment in the stomach.

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CL designed and executed the experiments. RS executed part of the experiments. All authors read and approved the final manuscript. This article does not contain any studies with human participants or animals performed by any of the authors. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Google Scholar. Biotechnol , 15, 1— Engan, S. In: Brewing Sciences. Academic Press, London. Kunkee, R. In: The Yeast , Rose, A.

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