Antioxidants

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Over the past decades, preventing oxygen pick-up during the brewing process and avoiding oxidation of the beer after fermentation has been adopted as efficient measures to resolve the beer aging issue. However, flavor stability remains hard to control despite extensive science and engineering research for years. The flavor stability of beer is increasingly found to be determined to a large extent by the endogenous antioxidant activity of beer itself (Zhao et al., 2010). Actually, beer itself with high endogenous antioxidant activity can prevent the generation of free radicals and these free radicals are formed only after the antioxidants in beer are deactivated. As the major endogenous antioxidants of beer, phenolic compounds are of particular interest to brewers because they play a key role in the brewing process by delaying, retarding, or preventing the oxidation processes.[1]

By definition, antioxidants are compounds which can delay or inhibit oxidative damage to substrates. (Poly-) phenols may act as antioxidants by different mechanisms: free radical inactivation by hydrogen atom transfer (HAT), or single electron transfer (SET) reactions, or by chelating prooxidant transition metal ions.[2]

Antioxidants are compounds that have the ability to delay or prevent oxidation reactions (15) and are, therefore, thought to play a significant role in malting and brewing as inhibitors of oxidative damage. Antioxidants, including sulfites and ascorbic acid, can be added during brewing to successfully produce beers possessing a high level of antioxidant activity (39). However, in recent years, there has been a trend toward minimizing the use of added antioxidants because of consumer demand and stiffening regulations. As a result, attention needs to be paid to the protection and development of endogenous antioxidants during malting and brewing, e.g., phenolic compounds and Maillard reaction products, to produce beers with high levels of antioxidant activity and, therefore, increased resistance to lipid oxidation, an important contributor to beer staling.[3]

Although the total phenolic content in beer is lower than in white wines, the total anti-oxidant activity of beer is higher due to a higher content of some phenolic compounds (proanthocyanidin, epicatechin and ferulic acid).[4] Barley contains considerable amounts of phenolic antioxidants, mainly flavan-3-ol derivatives, phenolic acids and flavon glycosides. Among all classes of barley polyphenols, flavan-3-ol derivatives (anthocyanogens) are repeatedly mentioned in the literature due to their high biological activity and their presence in malts predominantly in free forms. On the other hand, these compounds are prone to transformation into dark-colored anthocyanidins at elevated temperatures, at decreased pH, and in the presence of oxygen.

Beer is the most important source of prenylflavonols of the diet. Prenylflavonols are metabolic compounds present in hops, responsible for various biological effects. Its composition depends on the variety of hops used, and the maturation and storage conditions. According to a study by Gorinstein et al., the concentration of procyanidins, epicatechins and ferulic acid is significantly higher in beer when compared to white wine, giving beer larger antioxidant capacity.[5]

Cu(II) is known to catalyze the oxidation of sulfite to sulfate by oxygen, yet metabisulfite had no effect on the 1-butanol. Clearly, any reactive oxygen species generated in the presence of sulfite is consumed at a much faster rate by unreacted sulfite than by 1-butanol. This property presumably contributes to sulfite's efficacy as a beer flavor stabilizer and suggested that sulfite might scavenge reactive oxygen species in the presence of prooxidants such as pyrogallol. A further model reaction study subsequently confirmed that an equimolar concentration of metabisulfite completely protected 1-butanol against oxidation in the presence of pyrogallol.[6] Furthermore, catechol and catechin were as efficient as metabisulfite in protecting 1-butanol against oxidation in the presence of pyrogallol. Thus, polyphenols such as catechin, which contain 1,2-dihydroxybenzene rings, not only are incapable of acting as prooxidants in coupled alcohol oxidations, but they actively protect alcohols from oxidation and therefore function as antioxidants. Monomeric and dimeric catechins can occur at levels of up to 5.5 and 4.0 mg/L, respectively, in beer (12,34) and should therefore help to stabilize beer flavor. Other dihydroxypolyphenols in beer, such as quercetin and leucocyanidin, are present in even higher concentrations (9) and should behave similarly.[6]

Melanoidins are occasionally invoked in beer-staling mechanisms. These poorly characterized, high molecular weight compounds are formed by reactions between sugars and amino acids and contain reducing groups called reductones. Most authors have found that melanoidins have antioxidant properties.[6] However,. ....The model melanoidin solution is known to contain predominantly unreacted sugars and amino acid, and subsequent control experiments showed that its ability to protect 1-butanol against oxidation in the presence of Cu(II) and pyrogallol was due to those unreacted starting materials. The latter are present at rather high concentrations and effectively scavenge the reactive oxygen species. Consequently, it is not possible to conclude from these results whether reductones have any prooxidant or antioxidant effect on the Cu(II)-catalyzed oxidation of alcohols. There seems to be a weak prooxidant effect with Fe(III).

A model reaction study confirmed that an equimolar concentration of metabisulfite completely protected 1-butanol against oxidation in the presence of pyrogallol (a pro-oxidant).[6]

Metal-chelating agents and ligands are known to alter the reduction potential and the catalytic efficiencies of copper and iron in reactions with molecular oxygen and hydrogen peroxide. Chelating agents such as EDTA are frequently used as quenchers of metal ion redox activity during biochemical research and as antioxidants or preservatives in various food systems. Indeed, Blockmans et al reported that beer treated with EDTA retained its freshness longer.[6] Transition metal ions are also known to bind amino acids, and it seemed probable that they, too, might affect the oxidation of alcohols and the resulting production of aldehydes in beer.

The Cu(II)-catalyzed oxidation of alcohols is inhibited by EDTA, lysine, metabisulfite, and 1,2-dihydroxypolyphenol species. Lysine and EDTA inhibit the formation of aldehydes in aged beer.[6]

The production of aldehydes in beer by added hydrogen peroxide is inhibited by EDTA, and the addition of lysine reportedly improves the flavor stability of beer.[6] Melanoidins bind Fe(II) and Cu(II) more strongly than does EDTA, and it has been suggested that this ability contributes to their antioxidant properties. Although not well documented, dark malts have been alluded to as producing more flavor-stable beers than pale malts. Perhaps this effect can be ascribed to the higher melanoidin content and hence higher metal-binding capacity of dark malts.

Higher levels of "anti-oxidant" substances in the beer retard deterioration processes. These agents, which originate in the grist, seem to work in different ways. They compete for oxygen by being oxidized themselves, or they inhibit enzymes catalysing oxidations, and/or they "scavenge" free radicals. Anti-oxidants include sulphite and bisulphite ions, polyphenols and reductones, which are ene-diol substances resembling ascorbic acid, which are formed during Maillard reactions. These compounds are found in dark malts, which have long been known to have flavour-stabilizing properties.[7]

Antioxidants have been shown in laboratory studies to increase beta-amylase activity and debranching enzyme activity.[8][9] However, the practical significance of this is unknown, and there have been no reports of the combination of sulfites and ascorbic acid increasing beer attenuation.[10]

The addition of these antioxidants to beer before packaging is likely to have a benefit in terms of flavor stability only if the beer is exposed to air during packaging. If in-pack oxygen is minimized, the addition of these antioxidants to the beer before packaging is unlikely to improve flavor stability. In addition, there is some evidence that both (+)–catechin and ferulic acid can contribute negative attributes to the beer. The presence of (+)–catechin in beer has been linked to colloidal instability, whereas the degradation product of ferulic acid (4-vinylguaiacol) can contribute strong flavor and aroma to the beer that could be considered detrimental.[11]

Ascorbic acid and sulfites are reducing agents that reduce quinones back to the original colorless phenols, or that react irreversibly with quinones to form stable colorless products.[12]

Ethylene diamine tetra acetic acid (EDTA) is a chelating agent that chelates the copper prosthetic group of PPO or reduces the level of copper available for incorporation into the holoenzyme (McEvily et al 1992). 4-Hexylresorcinol is a competitive inhibitor of PPO that has a molecular structure similar to that of the enzyme substrate and thus competes for the active site of the enzyme. 4-Hexylresorcinol has been used to prevent browning in shrimp and in fruits and vegetables (McEvily et al 1992; Vámos-Vigyázó 1995; Ashie et al 1996). Benzoyl peroxide is a bleaching agent that bleaches out pigments such as xanthophylls (Melland et al 1984).[12] 4-Hexylresorcinol (0.05 mg/g of barley flour) is a GRAS food additive, but is not approved for cereal products. Among chemical agents, the use of ascorbic acid resulted in significantly higher brightness of dough sheets at 24 hr for all four cultivars tested. Similar effects of ascorbic acid on retarding discoloration of oriental noodle doughs prepared with wheat flour were reported by Baik et al (1995). The second most effective antibrowning agent was 4- hexylresorcinol, a competitive inhibitor of PPO. Weemaes et al (1999) reported that 4-hexylresorcinol effectively inhibited mushroom PPO activity. However, 4-hexylresorcinol was neither a good inhibitor of grape PPO (Martinez and Whitaker 1995) nor effective in improving the brightness of wheat noodle dough.

In recent years, many endogenous antioxidants in beer, such as phenolic compounds, Maillard reaction products (MRPs), sulphite and chelating agents, had been reported responsible for flavour stability of beer (Vanderhaegen et al., 2006; Aron & Shellhammer, 2010). Among antioxidants mentioned above, phenolic compounds are of particular interest to brewers because they are described as antioxidants in vitro that possess antioxidant and antiradical properties as well as other biological effects (Rice-Evans et al., 1997; Gorinstein et al., 2007).[13]

The release of phenolic compounds and the formation of Maillard reaction products (MRPs) might be responsible for the increase in antioxidant activity during mashing. However, MRPs with antioxidant activity formed by the elevated temperatures during mashing might make major contribution to the increases in antioxidant activity at the stage of enzyme inactivation, because a rapid decrease in TPC at this stage was observed in this study. Indeed, Araki et al. (1999) observed increases in antioxidant activity levels during boiling for lager production and also attributed them to the formation of MRPs. Moreover, previous studies indicated that black beer with higher content of MRPs showed higher reducing power than lager beer (Lugasi & Ho´va´ri, 2003). Kilned and roasted malts also exhibited strong antioxidant properties because of the contributions of phenolic compounds and MRPs (Samaras et al., 2005). Therefore, phenolic compound and MRPs released from malt or formed during mashing process might make major contribution to antioxidant activity of mashes.[13]

Antioxidants are compounds that have a positive effect on human health. They are able to remove reactive oxygen and radicals from the organism and in this way prevent oxidative damage, such as ageing and degenerative diseases,cancer and cardiovascular disease.[14]

Beer is a homogeneous liquid wherein all components are evenly distributed throughout the medium. The efficiency of potential antioxidants therefore depends on how well these compounds compete with other components in beer as reaction partners with deleterious reactive species (radicals and reactive oxygen species). Polyphenols are present only in very low concentrations, which makes it kinetically impossible for polyphenols to efficiently trap highly reactive radicals and thus act as efficient antioxidants before these radicals react with other and much more abundant compounds in beer (especially ethanol).[15]

ascorbic acid and sulfites act in competition to polyphenol oxidation and are preferentiallv oxidized.[16] If very little ascorbic acid is used, it can be verified that the tannoids remain constant until the ascorbic acid is practically all oxidized. When the concentration of ascorbic acid tends toward zero, protection is no longer complete; and when all of the ascorbic acid has disappeared, the velocity of disappearance of the tannoids is slower than that of the control. Here there is found again a phenomenon similar to that cited in the subsection above; the oxidizing system appears to undergo destructive oxidation in the course of its functioning as an oxidation catalyst.

In complex food systems like beer, different classes of antioxidants (polyphenols, Maillard reaction products, SO2, antioxidant amino acids, hop resin substances, and so on) interact with each other both synergistically and antagonistically. Apart from that, the presence of metal ions and pH are important factors influencing total antioxidant capacity.[2]

Malted barley can have impact on beer stability due to the presence of pro-oxidant and antioxidant activities (1). Malt contains various compounds, originated from barley or formed during the malting process, which can play a significant role in malting and brewing through their antioxidant activities (1). Malt and barley contain, among others, phenolic compounds, phytic acid, ascorbic acid, melanoidins, and several enzymes which can be responsible for part of this total antioxidant power (2). Of these, melanoidins and polyphenols are the most significant sources of natural antioxidants, which can be originated from the malting process or already present in barley (3).[17]

The increase in the level of antioxidant activity observed during fermentation is likely because of the production of reduced nicotinamide adenine dinucleotide (NADH) by yeast, which is a by-product of the conversion of glucose to pyruvate during glycolysis. During fermentation, when sugars are abundant, their conversion to pyruvate/acetaldehyde may be more rapid than that of acetaldehyde to ethanol, leading to an excess of NADH in solution, and thus increased antioxidant activity levels in the beer. This is supported by Saha (35), who observed that NADH reaches a maximum after 120 hr of fermentation, indicating that levels would still be very high at the end of fermentation.[3]

Sulfite is also a natural product of yeast metabolism and can act as an antioxidant (4). Sulfite is first detected in beer after 15–20 hr of fermentation (24). Levels have been reported to reach a maximum after 100 hr and to remain constant after this time (13), suggesting that sulfites may also be playing a role in antioxidant activity.[3]

The antioxidant activity of malt and the corresponding wort is due not only to the phenolic compounds, but also to the products of the Maillard reaction.[18]

During fermentation the antioxidant activity decreased in some cases or showed no changes.[19]

Wort with dark specialty malts contains increased antioxidant activity.[20]

See also

References

  1. ↑ Zhao H. Chapter 64: Effects of processing stages on the profile of phenolic compounds in beer. In: Preedy V, ed. Processing and Impact on Active Components in Food. Academic Press; 2015:533-539.
  2. ↑ a b 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.
  3. ↑ a b c Pascoe HM, Ames JM, Chandra S. Critical stages of the brewing process for changes in antioxidant activity and levels of phenolic compounds in ale. J Am Soc Brew Chem. 2003;61(4):203–209.
  4. ↑ Szwajgier D. Dry and wet milling of malt. A preliminary study comparing fermentable sugar, total protein, total phenolics and the ferulic acid content in non-hopped worts. J Inst Brew. 2011;117(4):569–577.
  5. ↑ Siqueira PB, Bolini H, Macedo GA. O processo de fabricação da cerveja e seus efeitos na presença de polifenóis. (The beer manufacturing process and its effects on the presence of polyphenols.) Alimentos e nutrição. 2008;19(4):491–498.
  6. ↑ a b c d e f g Irwin AJ, Barker RL, Pipasts P. The role of copper, oxygen, and polyphenols in beer flavor instability. J Am Soc Brew Chem. 1991;49(3):140–149.
  7. ↑ Briggs DE, Boulton CA, Brookes PA, Stevens R. Brewing Science and Practice. Woodhead Publishing Limited and CRC Press LLC; 2004.
  8. ↑ Evans DE, Wallace W, Lance RCM, MacLeod LC. Measurement of beta-amylase in malting barley (Hordeum vulgare L.). II. The effect of germination and kilning. J Cereal Sci. 1997;26(2):241–250.
  9. ↑ MacGregor AW, Bazin SL, Macri LJ, Babb JC. Modelling the contribution of alpha-amylase, beta-amylase and limit dextrinase to starch degradation during mashing. J Cereal Sci. 1999;29(2):161–169.
  10. ↑ Sulfites and attenuation. The Modern Brewhouse website. 2020. Accessed November 2020.
  11. ↑ Walters MT, Heasman AP, Hughes PS. Comparison of (+)–catechin and ferulic acid as natural antioxidants and their impact on beer flavor stability. Part 2: Extended storage trials. J Am Soc Brew Chem. 1997;55(3):91–98.
  12. ↑ a b Quinde-Axtell Z, Powers J. Baik BK. Retardation of discoloration in barley flour gel and dough. Cereal Chem. 2006;83(4):385–390.
  13. ↑ a b Zhao H, Zhao M. Effects of mashing on total phenolic contents and antioxidant activities of malts and worts. Int J Food Sci Technol. 2012;47(2):240-247.
  14. ↑ Jurková M, Horák T, Hašková D, Čulík J, Čejka P, Kellner V. Control of antioxidant beer activity by the mashing process. J Inst Brew. 2012;118(2):230-235.
  15. ↑ Andersen ML, Skibsted LH. Modification of the levels of polyphenols in wort and beer by addition of hexamethylenetetramine or sulfite during mashing. J Agric Food Chem. 2001;49(11):5232–5237.
  16. ↑ Chapon L, Chemardin M. The dissolving and oxidation of malt tannoids on mashing-in. Proceedings from the Annual meeting of American Society of Brewing Chemists. 1964;22(1):244–258.
  17. ↑ Guido LF, Curto AF, Boivin P, Benismail N, Gonçalves CR, Barros AA. Correlation of malt quality parameters and beer flavor stability:  multivariate analysis. J Agric Food Chem. 2007;55(3):728–733.
  18. ↑ Shopska V, Denkova-Kostova R, Dzhivoderova-Zarcheva M, Teneva D, Denev P, Kostov G. Comparative study on phenolic content and antioxidant activity of different malt types. Antioxidants. 2021;10(7):1124.
  19. ↑ Koren D, Kun S, Vecseri BH, Kun-Farkas G. Study of antioxidant activity during the malting and brewing process. J Food Sci Technol. 2019;56(8):3801–3809.
  20. ↑ Gąsior J, Kawa-Rygielska J, Kucharska AZ. Carbohydrates profile, polyphenols content and antioxidative properties of beer worts produced with different dark malts varieties or roasted barley grains. Molecules. 2020;25(17):3882.
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