Impact of Copper Fungicide Use in Hop Production on the Total Metal Content and Stability of Wort and Dry-Hopped Beer

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.

Copper Binding in Sweet Worts Made from Specialty Malts

“…6 Chrisfield et al also showed that excess copper had no negative impact on the radical formation in hopped beers. 5 Copper ions may therefore have competing prooxidative and antioxidative properties, and the overall effect may depend on the actual system. In this study, the pronounced antioxidant effects were observed with worts from the most roasted malts, where radical formation was most extensive due to presence of other prooxidative components (e.g., iron and colored reducing compounds).…”

“…However, Cu­(II) complexes with dipeptides or amino acids can display superoxide dismutase-like activity and thereby have antioxidative properties. , In fact, a decreased spin adduct formation was observed by Jenkins et al when adding up to 2.5 μM Cu­(II) to beer . Chrisfield et al also showed that excess copper had no negative impact on the radical formation in hopped beers . Copper ions may therefore have competing prooxidative and antioxidative properties, and the overall effect may depend on the actual system.…”

“…3,4 However, the impact of copper has recently come into question. 5,6 Malt is the dominating source of transition-metal ions during brewing. 7 Consequently, mashing is the critical step in the brewing process and requires special attention to control the metal levels that are transferred to the beer.…”

“…The stability of beer flavor is affected by all stages of the brewing process as well as the choice of raw materials. Oxidation reactions are some of the main causes for undesired changes, which is the main factor determining the shelf life of beer. , Oxidation can be catalyzed by trace levels of transition-metal ions such as iron, copper, and manganese, and they should therefore preferably be present in minimal concentrations in the final beer as well as during the brewing process. , However, the impact of copper has recently come into question. , …”

“…Copper ions are generally expected to behave as prooxidants in wort and beer. − , Nevertheless, several recent studies have provided contrary evidence. Beers with increased copper concentrations had constant or even decreasing radical formation rates in forced ageing experiments monitored by electron paramagnetic resonance (EPR) spectroscopy. , In a study where 100 ppb Cu­(II) ions were added to cold wort, Zufall and Tyrell reported the resulting beers to behave similar to the reference beer in both chemiluminescence measurements and flavor stability . In sweet worts made from various roasted malts, increased copper concentrations had no effect on the rate of oxygen consumption .…”