Iron

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Since the investigations by Fenton published in the year 1894, it has been known that iron ions can catalytically promote oxidative reactions. In 1934, Haber and Weiss found final proof for the formation of radicals in aqueous solutions of bivalent iron and copper ions together with hydrogen peroxide and described the strongly oxidative character of these radicals. The formed radicals from both Fenton and Haber-Weiss schemes are extremely reactive and may give rise to radical chain reactions. These reactions ultimately result in the formation of aroma-active carbonyl compounds, directly causing beer stalness.[1]

According to Bamforth and Parsons, hydroxyl radicals (catalyzed by iron, for example) are the most important intermediates in the formation of aged flavor compounds in beer. In a later study, Bamforth further ascertained this thesis by the finding that an addition of peroxides and heavy metal ions to beer led to a very rapid development of stale flavor.[1]

Today, it is generally accepted that molecular oxygen is relatively stable and needs to be activated before developing its damaging impact in bottled beer. The degradation of hydrogen peroxide can be considered the last step of this activation, while heavy metals are catalyzing this degradation. Metals can also catalyze the formation of other radicals in beer without the influence of oxygen (e.g. in the formation of fatty acid radicals). Heavy metal ions are therefore of decisive importance for beer aging.[1]

Iron and copper ions are known to have a negative influence on beer flavor stability. Even concentrations of copper below 50 ppb are reported to cause damage in the final product. The origin of these two metals in beer from raw materials, brewing equipment, diatomaceous earth etc. has been well investigated.[1]

The use of radical scavengers can improve beer flavor stability.

During fermentation, the iron content is strongly diminished.[1]

The investigation of iron species in beer has chemical interest, because the Fe(II)/Fe(III) ions play an important part in the activation of O2 and the initiation of beer aging and staling processes.[2]

The iron content of beer should be as low as possible. Under normal conditions, the iron content of fermented beer is below 0.2 ppm.[2]

Iron content in brewing liquor should be less than 0.1ppm in order to avoid metallic off-flavors, gushing, haze, and oxidation.[3]

Ions of Fe3+ are notorious for contributing negative flavor characters, such as metallic and astringent,16 even at very low concentrations, such as 0.5 mg/L in most beers; Fe3+ can be detectable at less than 0.1 mg/L in more delicately flavored beers.[4]

Iron in the brewing water above 1 mg / l is detrimental to the taste and color of the beer.[5]

Chemical reactions

In the Fenton reaction, iron(II)-ions are oxidized to iron(III) by hydrogen peroxide, forming a hydroxyl radical and a hydroxyl ion. The iron(III) eventually reacts with a further molecule of hydrogen peroxide generating two protons and a superoxide radical. These superoxide radicals react with copper(II)-ions to copper(I) and oxygen in the Haber-Weiss scheme. The copper(I)-ion generated is capable of splitting a hydrogen peroxide molecule into a hydroxyl ion and a hydroxyl radical.[1]

Chapon, Louis, and Chapon (1971) reported that beer sample can reduce a complex of Fe(III) with 2,20-dipyridyl to the Fe(II) complex. So, it seems that iron ions exist in the reduced state, Fe(II), in fresh beer. Therefore, Fe(II) ions dominant in fresh beer are oxidised to Fe(III) ions during the process of beer oxidation. In the presence of molecular oxygen, Fe(II) ions are oxidised to Fe(III) ions, the electron being accepted by oxygen during the formation of superoxide. It seems that these metal-catalysed reactions producing active oxygens may occur in beer during storage and lead to the staling of beer quality. The Fe(II) ions are oxidized into ferric ions such as free Fe(III), weakly bound Fe(III), and strong chelates of Fe(III) (non-heme Fe(III)) during the process of beer oxidation. The non-heme Fe(III) ions increase with beer staling and finally responsible for beer haze.[6]

Because oxygen is a triplet species, it is fundamentally slow to react with organic compounds in their singlet ground states. Consequently, transition metal ions in beer have been proposed as catalysts that activate molecular oxygen.[7]

Irwin et al (1991) showed no correlation between iron levels in beer and flavor stability during bottle storage.[7]

Under normal conditions, the iron content of fermented beer is below 0.2 mg/L. Large amounts of iron can give a metallic taste to beer. Iron salts have a negative action at concentrations above 0.2 mg/L during wort production, preventing complete saccharification, resulting in hazy worts, and hampering yeast activity. The recommended upper limit of iron concentration in brewing liquors and beer is 0.1 mg/L.[6]

In the brewing process, iron ions act as oxidation catalysts to cause darkening, and promote flavor instability and haze.[8] At concentrations of > 1 mg/l iron ions are harmful to yeasts.

Fe3+ may be present in humic water supplies, complexed with organic matter, and can produce slime deposits in wells and pipes.2 Iron ions can oxidize polyphenols and produce haze.2 Although they may improve foam stability at 0.5 mg/L, they can have negative direct flavor effects (metallic/astringent) at concentrations as low as 0.1 mg/L.5 Further, they may be implicated in accelerating the development of oxidized flavor characters in long-shelf-packaged beers. Fe3+ is an essential nutrient for yeast acting as a cofactor in redox pigments in actively respiring cells.23[4]

Any level of iron above trace level in water supplies is bad news. Iron ions are very flavor active giving a bloody/metallic off note to beer. Iron also forms slimes in pipework and is toxic to yeast. Iron like the other transition metals, copper and manganese, is also involved in oxidative spoilage acting as a catalyst to oxidation.[9]

Adding certain sugars accelerates Fenton reactions, increasing oxidation during boiling, for example.[10] Isomaltulose and fructose are characterized by the highest radical generation, whereas glucose and sucrose show comparable significantly lower influences on oxidative processes as indicated by the radical generation.

Depending on the malts used, a standard wort has levels of around 100-270 μg/L iron[11]

Iron level is highest at the beginning of mashing.[12]

the content of iron and copper in wort and beer is of great significance in the finished product because both are known to accelerate aging reactions by acting as catalysts in the so-called Fenton and Haber-Weiss reactions (17–19,39,40). Furthermore, Fe in particular is known to play an important role in the formation of chill-haze and permanent haze and, thus, can negatively affect the colloidal stability (4,20,22,23). In contrast, Fe is also known to enhance beer foam stability (6,35) and to be very important and influential for the growth and development of yeast, activation or inhibition of enzymes, glucose metabolism, and formation of fermentation by-products (7).[13]

Hops possess the highest metal ion concentration among the standard brewing ingredients; for example, Fe concentrations found were up to approximately eight times higher than in malt.[13]

See also[edit]

Potential sources

References[edit]

  1. ↑ a b c d e f Zufall, C., and Tyrell, Th. "The Influence of Heavy Metal Ions on Beer Flavour Stability." J. Inst. Brew., vol. 114, no. 2, 2008, pp. 134–142.
  2. ↑ a b Dobrinas, S., et al. "Comparative methods applied for the determination of total iron from beer samples." Ovidius University Annals of Chemistry, vol. 21, no. 1, 2010, pp. 35–40.
  3. ↑ Krottenthaler M, Glas K. Brew water. In: Esslinger HM, ed. Handbook of Brewing: Processes, Technology, Markets. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA; 2009.
  4. ↑ a b Taylor DG. Water. In: Stewart GG, Russell I, Anstruther A, eds. Handbook of Brewing. 3rd ed. CRC Press; 2017.
  5. ↑ Narziss L, Back W, Gastl M, Zarnkow M. Abriss der Bierbrauerei. 8th ed. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA; 2017.
  6. ↑ a b Filik, H, and Derya G. "Cloud point extraction for speciation of iron in beer samples by spectrophotometry." Food chemistry, vol. 130, no. 1, 2012, pp. 209–213.
  7. ↑ a b Irwin, AJ, et al. "The Role of Copper, Oxygen, and Polyphenols in Beer Flavor Instability." Journal of the American Society of Brewing Chemists, vol. 49, no. 3, 1991, pp. 140–149.
  8. ↑ Briggs DE, Boulton CA, Brookes PA, Stevens R. Brewing Science and Practice. Woodhead Publishing Limited and CRC Press LLC; 2004.
  9. ↑ Howe S. Raw materials. In: Smart C, ed. The Craft Brewing Handbook. Woodhead Publishing; 2019.
  10. ↑ Kunz T, Brandt NO, Seewald T, Methner FJ. Carbohydrates addition during brewing – effects on oxidative processes and formation of specific ageing compounds. BrewingScience. 2015;68(7):78–92.
  11. ↑ 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.
  12. ↑ Holzmann A, Piendl A. Malt modification and mashing conditions as factors influencing the minerals of wort. J Am Soc Brew Chem. 1977;35(1):1–8.
  13. ↑ 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.
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