Optimisation of High Gravity and Diet Beer Production in a German Brewery by Flow Cytometry

Analysis of aging in lager brewing yeast during serial repitching

Serial repitching of brewing yeast inoculates is an important economic factor in the brewing industry, as their propagation is time and resource intensive. Here, we investigated whether replicative aging and/or the population distribution status changed during serial repitching in three different breweries with the same brewing yeast strain but different abiotic backgrounds and repitching regimes with varying numbers of reuses. Next to bud scar numbers the DNA content of the Saccharomyces pastorianus HEBRU cells was analyzed. Gene expression patterns were investigated using low-density microarrays with genes for aging, stress, storage compound metabolism and cell cycle. Two breweries showed a stable rejuvenation rate during serial repitching. In a third brewery the fraction of virgin cells varied, which could be explained with differing wort aeration rates. Furthermore, the number of bud scars per cell and cell size correlated in all 3 breweries throughout all runs. Transcriptome analyses revealed that from the 6th run on, mainly for the cells positive gene expression could be seen, for example up-regulation of trehalose and glycogen metabolism genes. Additionally, the cells' settling in the cone was dependent on cell size, with the lowest and the uppermost cone layers showing the highest amount of dead cells. In general, cells do not progressively age during extended serial repitching.

Analysis of livingS. cerevisiae cell states—A three color approach

BACKGROUND

Biosyntheses often fluctuate with the state of the cell in the cell cycle and on the capacity of the cell to access and metabolize a carbon source. Visualization of substrate uptake by individual cells, together with the simultaneous analysis of proliferation activity and the proportion of dead cells, facilitate reliable and quasi-online process optimization.

METHODS

Flow cytometry and Hoechst 33342 staining were used to follow proliferation activity of living Saccharomyces cerevisiae cells, whereas 2-NBD-glucose was employed to analyze the cells' substrate affinity. Propidium iodide was used to determine the proportion of dead cells. Calibration and verification experiments were performed with cells grown batch-wise as well as in transient state regimes.

RESULTS

A new and rapid three-color assay was developed and tested under varying microenvironmental conditions.

CONCLUSIONS

Live/dead cell states and the affinity to 2-NBD-glucose vs. proliferation states were determined during respiratory and/or fermentative modes of metabolism.

Affinity of singleS. cerevisiaecells to 2-NBDglucose under changing substrate concentrations

BACKGROUND

Saccharomyces cerevisiae is a widely employed microorganism in biotechnological processes. Since proliferation and product formation depend on the capacity of the cell to access and metabolize a carbon source, a technique was developed to enable for analyzing the S. cerevisiae H155 cells' affinity to extracellular glucose concentrations.

METHODS

The fluorescent glucose analogue 2-NBDglucose was employed as a functional parameter to analyze the cells' affinity to glucose. Structural parameters (proliferation, neutral lipid content, granularity, and cell size) were also investigated. Cells were grown both in batches and in chemostat regimes.

RESULTS

The 2-NBDglucose uptake in individual cells proceeds in a time- and concentration-dependent manner and is affected by respiratory and respirofermentative modes of growth. The process is inhibited by D-glucose, D-fructose, D-mannose, and sucrose, but not L-glucose, D-galactose or lactose; maltose is a weak inhibitor. The affinity of the individual cells to 2-NBDglucose was found to be high at low extracellular glucose concentrations, and weak at high concentrations. An additional, underlying pattern in the cells' affinity to glucose was detected, illustrated by the recurrent appearance of two subpopulations showing distinctly differing quantities of this substrate.

CONCLUSIONS

A multiparameter flow cytometry approach is presented that enables, for the first time, for analysis of the affinity of individual S. cerevisiae cells to glucose. Besides the adjustment of the yeast cell metabolism to extracellular glucose concentrations by altering their affinity to glucose, at least one further mechanism is clearly involved. Two subpopulations of cells were resolved, with different affinities not correlated with other cellular parameters measured.