Abstract
Transition metals, including copper, iron, and manganese, are known to catalyze the generation of reactive oxygen species (ROS) in beer leading to reduced product stability. Metals in beer are generally derived from raw ingredients. The present study aims to evaluate the impact of brewing and dry-hopping using hops treated with copper-based fungicides (CBFs) on the final transition metal content of model buffer solutions and pilot-scale systems of wort and beer. Copper levels in model wort and beer solutions were elevated (105.6% and 230.4% increase, respectively) when CBF-treated hops were used. In laboratory-prepared wort, elevated copper concentrations were not observed when CBF-treated hops were used for boiling. Dry hopping of beer using CBF-treated hops led to significant increases in total copper content (ca. 75 µg/kg vs. ca. 40–50 µg/kg in the control-hopped beer) when yeast was absent from the treated beer, but not when yeast was present. It was observed that manganese levels were significantly elevated in all hopped beers (ca. 495–550 µg/kg vs. 90–125 µg/kg in the unhopped control), regardless of hop treatment. A hop varietal thiol, 4-Mercapto-4-methylpentan-2-one, was spiked into treated beers, and the rate of oxidative loss was monitored during aging. Rates of thiol loss in treated beer samples did not differ across CBF treatments but were significantly lower in unhopped controls in the absence of yeast (p < 0.0001) and correlated significantly with total manganese content of the beers (R2 = 0.4228, p = 0.0006). The rate of staling in hopped beers as measured by the rate of 1-hydroxyethyl radical generation did not differ among hop treatments, suggesting that excess copper content contributed from the hops does not negatively impact the oxidative stability of the beers. These findings suggest that brewers can use CBF-treated hops without any negative implications for the shelf stability of their beers and do not contraindicate the use of CBF in hops production when necessary.
Highlights
Maximizing beer stability during brewing and packaging is paramount for brewers as they are often unable to control storage time or conditions once the finished product leaves the brewery
We have demonstrated that while excess copper found in copper-based fungicides (CBFs)-treat hops is transferred into model brewing solutions, this effect is modulated in real wort and beer
This would indicate that the use of CBF-treated hops for bittering purposes would have no ill effect on the transition metal load of beer compared to hops grown without these fungicides
Summary
Maximizing beer stability during brewing and packaging is paramount for brewers as they are often unable to control storage time or conditions once the finished product leaves the brewery. Ground state (triplet) oxygen is inherently non-reactive and requires activation by electron transfer, achieving a higher, more reactive spin state [3]. This activation can be directly catalyzed by transition metal ions such as Fe2+ , Mn2+ , or Cu1+ or by-products of the Fenton or Haber–Weiss reactions catalyzed by the same metals [4]. While controlling the total oxygen in finished beer is important, minimizing the levels of transition metal catalysts throughout the brewing process is important
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