David Jenkins (1), David Cook (1), Frieda Dehrmann (2), Sue James (2), Katherine Smart (3); (1) University of Nottingham, Nottingham, U.K.; (2) SABMiller, Woking, U.K.; (3) SABMiller, U.K.
Yeast, Fermentation, and Microbiology
Brewers spend a significant amount of time and effort ensuring that their product meets the desired flavor profile. However, during the time it takes for the product to reach the consumer this profile can change, either through the loss of positive attributes or the development of stale characteristics. The mechanisms of flavor instability are the subject of significant current research and involve both oxidative and non-oxidative routes. Metal ions can catalyze the oxidative instability of beer by acting as prooxidants within the system. Molecular oxygen, itself relatively unreactive, can be activated by the action of transition metal ions that act as electron donors. Atmospheric oxygen can be reduced to the superoxide anion and subsequently hydroperoxyl radical, which can form hydrogen peroxide. From hydrogen peroxide the highly reactive hydroxyl radical can be formed that may ultimately lead to the generation of staling compounds. The involvement of metal ions, particularly copper and iron, in these early reactions (through Fenton and Haber-Weiss reaction pathways) can accelerate the process significantly. In the present research we applied the Electron Paramagnetic Resonance (EPR) forced aging assay to investigate the direct impact that these key metal ions have on the formation of Reactive Oxygen Species (ROS) in a lager beer. The most common metrics extracted from this assay are the lag-time and EPR signal intensity at a designated time point (commonly the T150). These are often related to the antioxidant potential and oxidative stability of the beer, respectively. When dosed directly into the assay, iron, copper, and manganese ions impacted the resulting EPR intensity time course, and consequently the metrics derived from it, in distinctly different ways. Iron reduced the antioxidant potential and oxidative stability of the beer. Although copper also reduced the antioxidant potential its impact on the oxidative stability was less clear, whilst manganese had little overall impact. Metal ions may impact the flavor at any point in the process, but particularly in packaging provided there is oxygen present. Although there are substantial amounts of all of these metals in malt and hops, a relatively small proportion is carried through to wort. Yeast is responsible for the removal of a proportion of metal ions during fermentation, depending on their bioavailability, whilst compromised yeast may also leak metals back into the system. Metal ion pickup may also occur subsequent to fermentation, such as during kieselguhr filtration. In this research we utilised small-scale fermentation systems to screen several brewing yeast strains, both Saccharomyces cerevisiae and Saccharomyces pastorianus, and demonstrate the difference between their ability to sequester metal ions during fermentation. We compare this data with the oxidative stability assessment of the resulting beers and postulate the relative importance of this sequestration on the metrics discussed above.
David Jenkins received a B.S. degree in applied biology from Cardiff University in 2006. He then completed a Ph.D. degree at the University of Nottingham, investigating the “Impact of Dehydration and Rehydration on Brewing Yeast.” Between 2013 and 2015 he worked as a research scientist at SABMiller’s research brewery (Nottingham, UK). He is currently a SABMiller-sponsored Barry Axcell Research Fellow in Brewing Science, at the University of Nottingham, where he is researching factors influencing the flavor stability of beer.