Year 2017
Aki Wassholm

Mitigation of the impacts of ageing on brewers yeast

Maintaining stable flavor profile in beer is crucial after the beer is stored. The storage conditions may vary greatly inducing loss of flavors and formation of undesired compounds. Yeast is a key component in brewing and potentially contributes to the flavor stability of the beer. Maintaining the yeast viable throughout the brewing process prevents the formation of off-flavors via cell lysis and/or metabolic activity. The major contributor to cellular age under non-dividing state is the accumulation of reactive oxygen species (ROS), often referred to as The Free radical theory of ageing. The major source of ROS is the mitochondria and without a sufficient antioxidant system, these radicals can cause cellular damage.

The purpose of this work was to study whether increasing yeast’s resistance to oxidative stress influences its longevity. A technique called adaptive evolution was applied to one bottom-fermenting (lager) and one top-fermenting (ale) brewing yeast strain, A15 and A224, respectively, to improve their resistance against oxidants. The yeast was adapted in seven different media including YPM and YPM supplemented with acrolein, ethanol, CuSO4, H2O2, menadione and paraquat for approximately 150 cell generations. Isolated adapted variants were then screened for resistance to these oxidants. A copper-adapted variant of A15 was found and it was shown to restrict the uptake of the metal. The original strain on the other hand accumulated quickly toxic amounts of copper. The adapted strain was also more sensitive to ethanol and grew slower in control conditions compared to the parent strain suggesting its growth is copper-limited. The copper-adapted strain was also more sensitive to chronological ageing, revealed by the longevity experiment carried out in this work.

Antioxidant analyses executed during the ageing experiments revealed that the non-enzymatic antioxidant glutathione is the first to appear and be depleted during storage in beer, but not in water. The activities of enzymatic antioxidants superoxide dismutase (SOD) and catalases increased later in the experiment. SOD activity remained relatively low in water-stored yeast while the content of catalases was not affected by the media.

Thesis electronical version can be downloaded from here

This info last modified 27 Apr 2024 by Jukka Kortela