A new method to obtain genetically stable Saccharomyces pastorianus lager brewing yeast variants was developed by scientists at VTT. The method involved repeated exposure of yeast to high ethanol conditions and then high osmotic stress conditions and selecting yeast cells exhibiting tolerance to both stresses. Fermentation experiments at 15 °P wort strength (typical in breweries) showed that the primary fermentation time was shortened by approx. 50% compared to the original strain, without any significant effect to the sensory quality of beer.
Many of the brewing yeast strains in use today have been isolated at a time when fermentation condition were far removed from those in use in today's industrial breweries. Especially the original gravity (corresponding roughly to the amount of dissolved sugars in the wort) has increased in industrial breweries. To improve fermentation performance in these challenging conditions a strain selection method was used by Ekberg et al. to a yeast strain originally isolated from a brewery. An ethanol-tolerant strain (which had previously been derived from the brewery strain by using mutagenisation and selection in ethanol-containing medium) was subjected to serial re-culturing in high osmotic strength medium. The performance of the variant was then compared to the parent strain in experimental wort fermentations, mimicking industrial conditions. Sugar consumption, ethanol production and yeast storage carbohydrate levels were monitored during the experiment. The gene expression levels between control and variant strains were compared using the TRAC method (originally developed at VTT) in collaboration with research partner PlexPress Oy. Sensory analysis was also performed on the conditioned and bottled beers.
With the variant strain 74% attenuation (% of carbohydrates consumed) was reached in 49h. In comparison, it took 42h longer for the parent strain to reach the same attenuation degree. Accumulation of storage carbohydrates was four times less in the case of trehalose and two times less in the case of glycogen. Transcriptional analysis of the variant strain revealed an increased transcription of the genes responsible for α-glucoside (mainly maltose and maltotriose) uptake and metabolism, explaining the faster fermentation rate. Sensory analysis showed no significant difference between the beers. The amount of diacetyl (butterscotch flavour) in the variant beer was somewhat increased, but remained at manageable level. Overall, the results prove that sequential selection for both ethanol tolerance and rapid growth at high osmotic strength can produce strains with enhanced fermentation performance. Furthermore, the dual osmotolerant-ethanol-tolerant nature may also be advantageous with very high gravity worts, offering cost-savings to the industry. An automated system for rapid evolution of micro-organisms has been developed at VTT and is expected to greatly increase the functional diversity of organisms used industrially.
For more information:
1. J. Ekberg, J. Rautio, L. Mattinen, V. Vidgren, J. Londesborough, B. Gibson, Adaptive evolution of the lager brewing yeast Saccharomyces pastorianus for improved growth under hyperosmotic conditions and its influence on fermentation performance. FEMS Yeast Research 13 (2013) 335 - 349. DOI 10.1111/1567-1364.12038.