Transition metals
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Although a broad range of transition metals have been reported in beer, iron (III) and copper (II) exist at higher concentrations in beer and have relatively lower reduction potentials which makes them more prone to participate in reduction reactions in beer51. Sources of transition metals in beer have been explored and their fate during the brewing, fermentation, and clarification processes have been examined by several investigators. A concise summary of references pertaining to transition metal content in beer can be found in a recent article by Zufall and Tyrell123. Bamforth and Parsons11 suggest that traces of transition metals (Fe, Cu, Zn, Co and Mn) may cause detriment to beer flavour stability, even when present in bound forms, and should be eliminated at all stages of the brewing process (<50 ppb for Cu specifically)10. This suggestion has been re-iterated by Irwin et al.51, who attest that the rate of beer flavour deterioration is significantly accelerated by trace amounts (<100 ppb) of Cu(II). Specifically, beers containing higher levels of copper II ions (40 to 95 ug/L) exhibit increased stale flavour intensity. This effect is seen to a lesser extent with iron, however excess endogenous iron can result in a metallic off-taste in beer123. High manganese concentration has been linked to the production of a sherry-like off aroma during beer aging. Zufall and Tyrell123 report that Fe, Cu, and Mn exist in malt and wort in an approximate ratio of 6:1:2, respectively. However iron and copper from the raw materials side have relatively little influence on the flavour stability of the final beer. Wort is typically higher in transition metals than beer. Wort boiling can reduce the metal content of the finished beers; chelating, nitrogenous (proteins and amino acids), and polyphenolic compounds originating from raw materials (malt and hops) act to bind and effectively remove a portion of the total metals52,84,85,123. During fermentation, yeast absorbs and intracellularly distributes transition metals to effectively diminish the metal content of finished beer63. This is especially true regarding copper, iron and zinc52,84,123. However, evidence exists to suggest otherwise in regard to manganese. Cellular uptake of manganese by yeast is lower, and thus significant losses of manganese may not be seen during fermentation. Recently Pohl and Sergiel95 investigated Cu speciation and the rate of staling in beer. The authors defined three groupings of Cu species, differing in hydrophobicity and charge: hydrophobic, cationic and residual Cu. The majority (74–82%) of the Cu found in beer exists in the residual fraction. Hydrophobic species accounted for 10–14% of the total copper found in beer, likely present in beer as polyphenolic-bound species, and the cationic species or free Cu contributed 12–13% of the total Cu. Results of these findings corroborated data compiled by Svendsen and Lund110, who found that ~72% of total Cu in beer exists in the non-cationic form. Characterization of transition metal speciation in beer could be of use to brewers; transition metals must be in their free or ionic forms in order to effectively catalyze radical reactions15,20. Brewers should also keep in mind that, beyond playing significant roles in beer off-flavour formation, trace metals have also been associated with beer colloidal instability in the forms of haze and gushing14,42,48,65,84.[1]
Avoidance or removal of transition metal catalysts should slow the reaction rates, lowering the production of free radicals and other reactive species, and reducing the oxidative damage inflicted on wort and beer. Transition metals actively promote staling throughout the whole brewing chain, not only during storage, and are especially reactive during high energy stages (mashing and boiling) (20).[2]
transition metal ions (especially reduced iron and copper ions) are implicated in oxidative beer staling since they catalyze the reductive formation of reactive oxygen species (like the super-oxide anion, hydrogen peroxide and the hydroxyl radical) from triplet oxygen.[3]
zinc, copper and selenium are part of the active site of intracellular anti- oxidant enzyme superoxide dismutase (SOD) and glutathione peroxidase (GSHPx).[4]
With the exception of zinc, the metals in row one of the d-block of the Periodic Table (iron, copper, and manganese) possess unpaired electrons. In other words, they are radicals.[5] These metals can donate an electron to oxygen (O2), to form superoxide (O2–).[5]
The most feasible way of strictly removing the undesirable metal ions whilst brewing would have to be physicochemical, through means of chelation (the act of binding metal ions to other molecules or chelators).[6]
Transition metal ions can originate from all raw materials— liquor, malt, adjuncts, hops, filter-aids, and additives. The brewer should specify practical maximum levels in each case. Simple mass balance analysis would indicate how to achieve metal levels at desirably low levels in the finished product (less than 50 ppb) for copper, the most critical ion. Furthermore, the opportunity for excess uptake from some materials, if they are not used properly, should be recognized (for example, recycling liquor through filter aids can exacerbate iron pick-up). Quantitatively, the most significant sources of copper in beer will be malt and contact with metallics, especially copper-lined vessels. Copper will leak from copper-based brewhouse equipment. Sacrificial copper, or even purposely introduced copper, often are considered to benefit beer flavor by scavenging undesirable sulfidics (73). It may be argued that promoting oxidation upstream through higher copper levels could be advantageous if stale substances can be volatilized and removed at that stage (e.g., in the kettle boil or by wort stripping). Such worts will undoubtedly present lowered reducing power to the ensuing beer, so if they retain any staling precursors, the importance of eliminating oxygen downstream is doubly important. Jacobsen and Lie (43) have shown that wort boiling substantially reduces the copper content of wort by precipitating this ion with hot break. By that stage, however, the damage may be done.[7]
Traces of metals such as copper and iron should be eliminated at all stages. A reduced rate of flavor deterioration would be achieved more effectively by preventing hydroxyl radical development through processing than by over emphasizing the direct uptake of oxygen.[8]
Iron (Fe), copper (Cu), and manganese (Mn) ions act as catalysts in the formation of ROS, and their levels should be minimized throughout the entire brewing process.[8] However, other metal ions are required in certain concentrations, for example calcium due to its stabilizing effect on amylase activity, and zinc (Zn) due to its positive effect on fermentation performance.[9] Metal ions in beer can originate from processing or storage, as exemplified by the intake of iron during filtration and the leakage of aluminum in cans.[10,11] Nevertheless, the main source of Fe, Cu, and Mn in beer brewing is the raw materials.[8,12] In a study by Wietstock et al.,[12] malt accounted for over 94% of the total amounts of iron and copper present during the entire brewing process. Notably, the major amounts of these metals did not end up in the sweet wort but were removed together with the spent grains after lautering. This makes mashing the critical step where the metal levels in wort and eventually the final beer are mainly established. Frederiksen et al.[13] reported that an addition of Fe(II) during mashing had no influence on the final iron concentration of a sweet wort made from pilsner malt indicating a possible metal-levelling effect. The malt bill has a large influence on the ionic composition of the resulting sweet wort, and using roasted malts increases the wort concentration of Fe, but decreases the concentration of Cu compared to using pilsner malts.
spent grains have a large capacity to bind Fe(II) from the wort. Other studies have also demonstrated that spent grains can efficiently remove heavy metal ions from water, including Cu(II).[25,26] The binding of metals is generally attributed to complexation with functional groups of lignin, cellulose, hemi-cellulose, and insoluble proteins, the main components of spent grains.[9] Extra transition metals added in the mash don't necessarily carry down stream because they bind to spent grains.
Iron becomes trapped during the mashing.[10]
The packaging and the brewing equipment used can be sources for metal ion entry into the wort and beer.[11]
all by-products (spent grain, hot break, and fermentation sediments) which arose during beer production were collected and their metal ion concentrations were determined after drying and determining the dry matter. From the data, it can be concluded that hot break had the highest metal concentration, with the exception of Zn, where yeast was highest. In beer, the concentrations measured were considerably lower. Taking into account the amount of byproducts released during beer production, the highest proportion of metal ions was discharged during spent grain removal.[11]
- Application of ICP Sector Field MS and Principal Component Analysis for Studying Interdependences among 23 Trace Elements in Polish Beers
- Speciation of Cu, Fe and Mn in beer using ion exchange separation and size-exclusion chromatography in combination with electrothermal atomic absorption spectrometry
See also[edit]
References[edit]
- ↑ Aron PM, Shellhammer TH. A discussion of polyphenols in beer physical and flavour stability. J Inst Brew. 2010;116(4):369–380.
- ↑ Mertens T, Kunz T, Wietstock PC, Methner FJ. Complexation of transition metals by chelators added during mashing and impact on beer stability. J Inst Brew. 2021;127(4):345–357.
- ↑ Wannenmacher J, Gastl M, Becker T. Phenolic substances in beer: Structural diversity, reactive potential and relevance for brewing process and beer quality. Compr Rev Food Sci Food Saf. 2018;17(4):953–988.
- ↑ Fantozzi P, Montanari L, Mancini F, et al. In vitro antioxidant capacity from wort to beer. LWT - Food Sci Technol. 1998;31(3):221–227.
- ↑ a b Bamforth CW, Lentini A. The flavor instability of beer. In: Bamforth CW, ed. Beer: A Quality Perspective. Academic Press; 2009:85–109.
- ↑ Mertens T, Kunz T, Methner FJ. Assessment of chelators in wort and beer model solutions. BrewingScience. 2020;73(May/June):58–67.
- ↑ Bamforth CW, Muller RE, Walker MD. Oxygen and oxygen radicals in malting and brewing: a review. J Am Soc Brew Chem. 1993;51(3):79–88.
- ↑ Bamforth CW, Parsons R. New procedures to improve the flavor stability of beer. J Am Soc Brew Chem. 1985;43(4):197–202.
- ↑ a b Pagenstecher M, Maia C, Andersen ML. Retention of iron and copper during mashing of roasted malts. J Am Soc Brew Chem. 2021;79(2):138–144.
- ↑ Frederiksen AM, Festersen RM, Andersen ML. Oxidative reactions during early stages of beer brewing studied by electron spin resonance and spin trapping. J Agric Food Chem. 2008;56(18):8514–8520.
- ↑ a b Wietstock PC, Kunz T, Waterkamp H, Methner FJ. Uptake and release of Ca, Cu, Fe, Mg, and Zn during beer production. J Am Soc Brew Chem. 2015;73(2):179–184.