Influence of yeast drying process on different lager brewing strains viability

Irina C. Bolat, Maria Turtoi, Michael C. Walsh
The potential for the application active dry yeast within brewing industry is supported by the advantages this type of yeast offers. Dried yeast is more robust and stable for transportation, distribution and storage. Further, there is no requirement for skilled laboratory staff as there is for yeast supplied on slope.

During the drying process though, performed at high temperatures, a considerable inactivation of the yeast culture occurs. Lager brewing yeast strains are more sensitive at high temperatures than ale yeast strains, the cell viability being affected at a greater extend as well as their flocculation properties.

The present study focuses on assessing the viability of three lager variant strains that were subjected to drying using the spray-drying approach, with further emphasis on only one variant against the initial mixture of WS34/70. This was part of a larger project concerning WS34/70 lager brewing yeast characterization. The variants analysed within this paper had been previously selected and isolated from the WS34/70 lager brewing yeast and stored at -80°C. The karyotype of the three variants, displayed using the short-run technique for low molecular chromosomes, differ in the ratio of two bands.

Thus the ratio is higher than 1 for the first variant (variant a), is equal to 1 for the second variant (variant b) and is lower than 1 for the third variant (variant c). All the analyses were performed against the WS34/70 lager yeast which underwent the same drying/rehydration treatment as the variants.

The number of the total cells and dead cells, as well as viability, glycogen and trehalose content were evaluated for the dried strains, after their rehydration both in 15P wort and in phosphate buffered saline (PBS) with different amounts of emulsifier. The importance of treating the yeast suspension with an emulsifier before drying was showed by the positive evolution of yeast suspension viability after rehydration. The number of dead cells after rehydration gradually decreased. This phenomenon was obvious when a high amount of emulsifier (10% v/v) was added in the yeast suspension, before drying. A low content of emulsifier (0.05% v/v) was not enough to protect the cell wall from degradation, high amounts of dead cells being recorded after rehydration. As a result the yeast suspension viability is much higher in the first case. Yeast suspension viability was lower when re-suspending the dried yeast in wort than in PBS, both at 1:20 and 1:10 dilution. Variant a displayed the highest increase in viability after rehydration, while the mixture of strains WS34/70 showed to be very resistant to the drying process with high viability values even after 1h of rehydration.
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