Editing Phenolic compounds

[[Category:Off flavors‏‎]] [[Category:Brewing chemistry]]
[[File:Ellagic-acid.png|thumb|Polyphenolic structure of ellagic acid]]
Phenolic substances (sometimes called tannins or polyphenols) are integral and abundant components of plant cell walls, where they serve as a natural defense system against microbial pathogens, insects, and herbivores, as well as reducing oxidative stress.<ref name=wangas/><ref name=len/><ref name=dabkar>Dabina-Bicka I, Karklina D, Kruma Z. [https://www.cabidigitallibrary.org/doi/pdf/10.5555/20113337376 Polyphenols and Vitamin E as potential antioxidants in barley and malt.] Conference Proceedings of the 6th Baltic Conference on Food Science and Technology FOODBALT-2011; May 5–6, 2011; Jelgava, Latvia.</ref><ref name=schwarz/> During the [[brewing]] process, a wide and complex array of phenolic compounds from [[grain]] and [[hops]] are released into the [[wort]] and therefore are always present in [[beer]]. These compounds play a key role in beer quality since they directly influence the flavor, color, and clarity of beer.<ref name=wangas>Wannenmacher J, Gastl M, Becker T. [https://ift.onlinelibrary.wiley.com/doi/abs/10.1111/1541-4337.12352 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.</ref><ref name=bsp>Briggs DE, Boulton CA, Brookes PA, Stevens R. [[Library|''Brewing Science and Practice.'']] Woodhead Publishing Limited and CRC Press LLC; 2004.</ref> The effects of these compounds are closely related to the amount of [[oxidation]] that occurs. Phenolic compounds readily scavenge oxygen radicals and therefore serve as natural [[antioxidants]], helping to protect grain and hop flavor compounds from oxidation, thereby improving [[flavor stability]]. As antioxidants, they also provide a [[health and safety|health benefit]] when consuming beer and wine. However, as the phenolic compounds oxidize during brewing or beer storage, they add a darker color, add [[haze]], and contribute a variety of off-flavors including [[astringency]] and harsh bitterness. When modified by certain types of yeast or bacteria, phenolic compounds can add distinct flavors such as spicy clove, pepper, or smoke. These types of flavors are usually undesirable and are generally referred to as ''phenolic off-flavor'' (POF). Despite this term, these flavors are an important part of some specialty beer styles such as [[weissbier]] and [[saison]].

The phenolic content of beer depends largely on brewing practice and raw materials.<ref name=aroshe/><ref name=wangas/><ref name=fumgal/> Beer exhibiting high phenolics content (and the related increase in [[antioxidants|antioxidant]] activity) displays better flavor quality, more stable [[flavor stability|flavor and aroma]], improved [[foam]] stability, and longer [[haze|shelf life]] (i.e. improved clarity) as compared with beer with less phenolics.<ref name=narfod>Nardini M, Foddai MS. [https://www.mdpi.com/1420-3049/25/11/2466 Phenolics Profile and Antioxidant Activity of Special Beers.] ''Molecules.'' 2020;25(11):2466.</ref> Due to these beneficial effects, interest is shifting towards preserving the phenolic compounds from the raw materials to the final beer.<ref name=wangas/><ref name=zhache>Zhao H, Chen W, Lu J, Zhao M. [https://www.sciencedirect.com/science/article/abs/pii/S030881460901022X Phenolic profiles and antioxidant activities of commercial beers.] ''Food Chem.'' 2010;119(3):1150–1158.</ref><ref name=zhali/><ref name=yangao/><ref name=sibpla/><ref name=gankub>Ganbaatar C, Kubáň V, Kráčmar S, Valášek P, Fišera M, Hoza I. [https://publikace.k.utb.cz/handle/10563/1006300 Liquid chromatographic determination of polyphenenols in Czech beers during brewing proces.] Potravinárstvo. 2015;9(1):24–30.</ref><ref>Niño-Medina G, Romo-Longoria JD, Ramírez-González IV, Martínez-Reyna OO, Urías-Orona V. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2017-2751-01 Phenolic content and antioxidant capacity level in commercial Mexican lager beers.] ''J Am Soc Brew Chem.'' 2017;75(2):156–158.</ref> With this in mind, brewers have numerous ways to improve the composition of phenolic compounds and the level of antioxidant activity in the finished beer.<ref name=len/><ref name=zhao/> Perhaps most importantly, brewers may choose to [[low oxygen brewing|minimize oxygen exposure]] during the brewing process, resulting in much greater natural antioxidant activity in the final beer while avoiding the negative effects of phenolic compounds. Various other production parameters also have a major impact on the level of phenolic compounds, such as [[milling]] intensity, [[mashing]] temperature(s), [[PVPP]] treatment, and [[filtration]], which can all affect the amount extracted from the malt and carried through to the beer.<ref name=cargui/> The choice and quantity of raw materials (including [[malt]], [[hops]], and [[tannin additives]]) is also important because different ingredients will contribute varying levels of phenolic compounds to the wort.<ref name=piafor>Piazzon A, Forte M, Nardini M. [https://pubs.acs.org/doi/abs/10.1021/jf101975q Characterization of phenolics content and antioxidant activity of different beer types.] ''J Agric Food Chem.'' 2010;58(19):10677–10683.</ref> Lastly, beer storage and [[packaging]] conditions can have a significant impact on the structure of phenolic compounds, which may lead to [[haze]] formation or changes to flavor.<ref name=zhao/>

== Structure and classification ==
[[File:Phenol.png|thumb|Structure of phenol]]
The phenolic unit is an aromatic ring connected to a hydroxyl (OH) group, and this phenolic ring is connected other structural elements to form a variety of phenolic compounds.<ref name=aroshe>Aron PM, Shellhammer TH. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2010.tb00788.x A discussion of polyphenols in beer physical and flavor stability.] ''J Inst Brew.'' 2010;116(4):369–380.</ref> They constitute a very diverse group of substances whose components vary greatly in chemical structure and therefore differ in reactivity.<ref name=yangao>Yang D, Gao X. [https://www.sciencedirect.com/science/article/abs/pii/S0924224421001552 Research progress on the antioxidant biological activity of beer and strategy for applications.] ''Trends Food Sci Technol.'' 2021;110:754-764.</ref> Phenolic compounds exist not only individually, but also in bound form attached to other molecules such as esters, sugars (glycosides), and organic acids.<ref name=flomon>Floridi S, Montanari L, Marconi O, Fantozzi P. [https://pubs.acs.org/doi/abs/10.1021/jf0260040 Determination of free phenolic acids in wort and beer by coulometric array detection.] ''J Agric Food Chem.'' 2003;51(6):1548–1554.</ref><ref name=cargon>Carvalho DO, Gonçalves LM, Guido LF. [https://ift.onlinelibrary.wiley.com/doi/abs/10.1111/1541-4337.12218 Overall antioxidant properties of malt and how they are influenced by the individual constituents of barley and the malting process.] ''Compr Rev Food Sci Food Saf.'' 2016;15(5):927–943.</ref><ref name=Siqueira>Siqueira PB, Bolini H, Macedo GA. [https://www.researchgate.net/publication/49599952_O_PROCESSO_DE_FABRICACAO_DA_CERVEJA_E_SEUS_EFEITOS_NA_PRESENCA_DE_POLIFENOIS 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.</ref><ref name=zhao/> Barley contains hundreds of different phenolic compounds<ref name=zhao/> and the structural classification is very complex,<ref name=cheyni>Cheynier V. [https://www.sciencedirect.com/science/article/pii/S0002916523275115 Polyphenols in foods are more complex than often thought.] ''Am J Clin Nutr.'' 2005;81(1):223S–229S.</ref><ref name=gerbec/> and therefore discussion here will be limited.

=== Phenolic acids and simple phenols ===
Phenolic acids are among the most abundant phenolic compounds in beer.<ref name=yangao/> Structurally they contain a single phenolic ring structure along with a carboxylic acid group. Phenolic acids are divided into: benzoic acid derivatives and cinnamic acid derivatives.<ref name=cargui/> Most phenolic acids (the cinnamic acid derivatives in particular) in malt and beer are bound to [[beta-glucans and arabinoxylans|cell wall polysaccharides]], amides (i.e. phenolamides), or other molecules.<ref name=yangao/><ref name=zhao/><ref name=wangas/><ref>Pihlava JM, Kurtelius T, Hurme T. [https://onlinelibrary.wiley.com/doi/full/10.1002/jib.311 Total hordatine content in different types of beers.] ''J Inst Brew.'' 2016;122(2):212–217.</ref><ref name=vanvan/><ref name=narghi>Nardini M, Ghiselli A. [https://www.sciencedirect.com/science/article/abs/pii/S0308814603002577 Determination of free and bound phenolic acids in beer.] ''Food Chem.'' 2004;84(1):137–143.</ref> Volatile phenols are formed when the carboxylic acid group is removed from phenolic acids.<ref name=wangas/> The aromatic alcohol tyrosol is formed by fermentation of the amino acid tyrosine, and is present in beer in significant concentrations.<ref name=gerbec>Gerhäuser C, Becker H. Chapter 12: Phenolic compounds in beer. In: Preedy VR, ed. [[Library|''Beer in Health and Disease Prevention.'']] Academic Press; 2009:124–144.</ref>

{| class="wikitable"
|+ Common phenolic acids
! Benzoic acid derivatives
! Cinnamic acid derivatives
|- style="vertical-align:top;"
|
* Vanillic acid
* 4-Hydroxybenzoic acid
* Syringic acid
* Protocatechuic acid
* Gallic acid
|
* Ferulic acid
* p-Coumaric acid
* Sinapic acid
* Caffeic acid
|}


<gallery>
File:Ferulic-acid.png|Ferulic acid
</gallery>

=== Polyphenols ===
Another main group of phenolic compounds in beer can be broadly termed as "polyphenols", which contain multiple phenolic rings.<ref name=savel/> These consist of both flavonoids (e.g. flavanols, flavones, flavonones, flavonols, and proanthocyanidins) and non-flavonoids (e.g. [[stilbenes]], [[chalcones]], [[lignans]], and hydrolyzable tannins).<ref name=cargui>Carvalho DO, Guido LF. [https://www.sciencedirect.com/science/article/abs/pii/S0308814621020999 A review on the fate of phenolic compounds during malting and brewing: technological strategies and beer styles.] ''Food Chem.'' 2022;372:131093.</ref><ref name=habkos/><ref name=wangas/><ref name=gerbec/> Flavonoids (also called anthocyanogens) are the major free phenolic compounds in barley,<ref name=zhao/> and the main groups of free flavonoids are monomer flavanols (flavan-3-ols) and polymeric flavanols (proanthocyanidins AKA condensed tannins).<ref name=wangas/><ref name=yangao/> For example, catechin (flavan-3-ol monomer) is an abundant free polyphenolic compound in both barley and hops.<ref name=cargui/><ref name=cargon/><ref name=wangas/><ref name=habkos/> Catechin frequently is a component of the over 50 different proanthocyanidins detected in barley grains, including the dimers prodelphinidin B3 and procyanidin B3.<ref name=quibai>Quinde-Axtell Z, Baik BK. [https://pubs.acs.org/doi/abs/10.1021/jf060974w Phenolic compounds of barley grain and their implication in food product discoloration.] ''J Agric Food Chem.'' 2006;54(26):9978–9984.</ref><ref>Callemien D, Collin S. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2008-0215-01 Use of RP-HPLC-ESI (–)-MS/MS to differentiate various proanthocyanidin isomers in lager beer extracts.] ''J Am Soc Brew Chem.'' 2008;66(2):109–115.</ref> Kaempferol and quercetin glycosides and their malonyl esters are notable examples of flavonols extracted from hops.<ref name=wangas/><ref name=habkos/><ref name=aroshe/> Prenylflavonoids, also found in hops, represent a class of flavonoids with at least one prenyl or geranyl group attached to the phenolic ring.<ref name=habkos/><ref name=quival>Quifer-Rada P, Vallverdú-Queralt A, Martínez-Huélamo M, Chiva-Blanch G, Jáuregui O, Estruch R, Lamuela-Raventós R. [https://www.sciencedirect.com/science/article/abs/pii/S0308814614012205 A comprehensive characterisation of beer polyphenols by high resolution mass spectrometry (LC–ESI-LTQ-Orbitrap-MS).] ''Food chem.'' 2015;169:336–343.</ref> Multifidol and multifidol glucosides are other constituents of hops, which can have significant flavor impact even at very low concentration.<ref name=habkos/>

=== Amino phenols ===
Finally, wort and beer contains amino phenols in the form of phenolic [[amino acids]], which can also be part of peptides or [[protein]]s.<ref name=yangao/><ref name=zhao/><ref name=cherai>Cheiran KP, Raimundo VP, Manfroi V, Anzanello MJ, Kahmann A, Rodrigues E, Frazzon J. [https://www.sciencedirect.com/science/article/pii/S0308814619302900 Simultaneous identification of low-molecular weight phenolic and nitrogen compounds in craft beers by HPLC-ESI-MS/MS.] ''Food chem.'' 2019;286:113–122.</ref> The other types phenolic compounds are derived from these phenolic amino acids, specifically phenylalanine and to a lesser extent tyrosine (via the Ehrlich pathway).<ref name=wangas/>

== Sources of phenolic compounds ==
All phenolic compounds originate from plant material, which for beer typically includes [[grain]]s and [[hops]].<ref name=wangas/><ref name=len>Lentz M. [https://www.mdpi.com/2311-5637/4/1/20 The impact of simple phenolic compounds on beer aroma and flavor.] ''Fermentation.'' 2018;4(1):20.</ref> Other plant ingredients such as fruit (i.e. in fruit beer) will contribute phenolic compounds if they are used as ingredients. In lightly-hopped industrial beers, barley malt contributes 70–80% of the total polyphenols in beer, with the remainder coming from hops.<ref name=wangas/> However, in the more hoppy styles that are popular among craft and home brewers, much more of the beers' phenolic content is contributed by the hops.<ref name=cargui/><ref name=sibpla/><ref name=borsol>Boronat A, Soldevila-Domenech N, Rodríguez-Morató J, Martínez-Huélamo M, Lamuela-Raventós RM, de la Torre R. [https://www.mdpi.com/1420-3049/25/11/2582 Beer phenolic composition of simple phenols, prenylated flavonoids and alkylresorcinols.] ''Molecules.'' 2020;25(11):2582.</ref> Levels of phenolic content in wort and beer can vary widely.<ref name=borsol/><ref name=zhache/> The final composition of phenolic compounds in beer depends on the characteristics of the raw materials used and the methods of brewing and fermentation.<ref name=Siqueira/>

=== Grain ===
During [[mashing]], phenolic compounds are extracted into wort from the grain. Cereal grain contains phenolic acids and a variety of polyphenols, making up around 0.2% of grain by dry weight.<ref name=cargui/> In [[barley]], the phenolic compounds are predominantly found in the outer layers of the grain (husk, pericarp, testa, and aleurone cells), while their concentration is noticeably lower in the endosperm.<ref name=wangas/><ref name=zhozha>Zhou B, Zhao J, Schwarz PB, Li Y. [https://link.springer.com/article/10.1007/s11694-021-00964-0 Effect of grinding and extraction conditions on the determination of antioxidant activity and phenolic acids in barley.] ''J Food Meas Charact.'' 2021;15:3823–3836.</ref><ref>Nordkvist E, Salomonsson AC, Åman P. [https://onlinelibrary.wiley.com/doi/abs/10.1002/jsfa.2740350611 Distribution of insoluble bound phenolic acids in barley grain.] ''J Sci Food Agric.'' 1984;35(6):657–661.</ref> The total amount of phenolic compounds depends on the genetic variety and growing conditions, although the relative amounts are fairly consistent.<ref name=wangas/><ref name=cargui/><ref name=zhao/><ref name=len/><ref name=zhazha/><ref name=zhozha/><ref name=whield/> The total extractable phenolic content (and antioxidant potential) increases during [[malting]].<ref name=cargui/><ref name=wangas/><ref name=zhao/><ref name=habkos/><ref name=humsan>Humia BV, Santos KS, Barbosa AM, Sawata M, Mendonça MdC, Padilha FF. [https://www.mdpi.com/1420-3049/24/8/1568 Beer molecules and its sensory and biological properties: A review.] ''Molecules.'' 2019;24(8):1568.</ref><ref name=zhazha/><ref name=guicur/><ref name=leimar>Leitao C, Marchioni E, Bergaentzlé M, et al. [https://www.sciencedirect.com/science/article/abs/pii/S0733521012000136 Fate of polyphenols and antioxidant activity of barley throughout malting and brewing.] ''J Cereal Sci.'' 2012;55(3):318–322.</ref><ref name=dorgui>Dvořáková M, Guido LF, Dostálek P, Skulilová Z, Moreira MM, Barros AA. [https://onlinelibrary.wiley.com/doi/epdf/10.1002/j.2050-0416.2008.tb00302.x Antioxidant properties of free, soluble ester and insoluble-bound phenolic compounds in different barley varieties and corresponding malts.] ''J Inst Brew.'' 2008;114(1):27–33.</ref><ref name=simhor>Šimić G, Horvat D, Dvojković K, et al. [https://www.agriculturejournals.cz/web/cjfs.htm?type=article&id=144_2016-CJFS Evaluation of total phenolic content and antioxidant activity of malting and hulless barley grain and malt extracts.] ''Czech J Food Sci.'' 2017;35(1):73–78.</ref><ref name=dvodou>Dvořáková M, Douanier M, Jurková M, Kellner V, Dostálek P. [https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.2008.tb00320.x Comparison of antioxidant activity of barley (''Hordeum vulgare L.'') and malt extracts with the content of free phenolic compounds measured by high performance liquid chromatography coupled with CoulArray detector.] ''J Inst Brew.'' 2008;114(2):150–159.</ref><ref name=korkun>Koren D, Kun S, Vecseri BH, Kun-Farkas G. [https://link.springer.com/article/10.1007/s13197-019-03851-1 Study of antioxidant activity during the malting and brewing process.] ''J Food Sci Technol.'' 2019;56(8):3801–3809.</ref><ref name=dabkar/>

The phenolic acids [[ferulic acid]], p-coumaric acid, gallic acid, caffeic acid, vanillic acid, and sinapinic acid are among the main phenolic compounds in malt.<ref name=cargui/><ref name=wangas/><ref name=schwarz>Schwarz KJ, Boitz LI, Methner FJ. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2012-1011-02 Release of phenolic acids and amino acids during mashing dependent on temperature, pH, time, and raw materials.] ''J Am Soc Brew Chem.'' 2012;70(4):290–295.</ref><ref name=len/><ref name=calcol/><ref name=dorgui/><ref name=marcab/><ref name=szw>Szwajgier D. [https://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2009.tb00376.x Content of individual phenolic acids in worts and beers and their possible contribution to the antiradical activity of beer.] ''J Inst Brew.'' 2009;115(3):243–252.</ref><ref name=socpaj>Socha R, Pająk P, Fortuna T, Buksa K. [https://www.tandfonline.com/doi/pdf/10.1080/10942912.2017.1306550 Antioxidant activity and the most abundant phenolics in commercial dark beers. International journal of food properties.] ''Int J Food Prop.'' 2017;20(sup1):1–15.</ref> Most of the phenolic acids in cereals are insolubly bound to cell wall carbohydrates ([[beta-glucans and arabinoxylans|arabinoxylans]]), while some are bound in the form of glycosides or are in free form and thus more easily extractable.<ref name=wangas/><ref name=cargui/><ref name=calcol>Callemien D, Collin S. [https://www.tandfonline.com/doi/abs/10.1080/87559120903157954 Structure, organoleptic properties, quantification methods, and stability of phenolic compounds in beer—a review.] ''Food Rev Int.'' 2009;26(1):1–84.</ref><ref name=len/> The higher phenolic content in malt has been mainly attributed to the enzymatic release of bound phenolics during malting.<ref name=cargui/> Optimization of the malting process can protect the polyphenols present in barley and also promote the formation of new [[antioxidants|antioxidant]] compounds.<ref name=zhazha>Zhao H, Zhao M. [https://ifst.onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2621.2011.02831.x Effects of mashing on total phenolic contents and antioxidant activities of malts and worts.] ''Int J Food Sci Technol.'' 2012;47(2):240-247.</ref><ref name=Siqueira/><ref>Zhou B, Jin Z, Schwarz P, Li Y. [https://www.mdpi.com/2311-5637/6/2/48 Impact of genotype, environment, and malting conditions on the antioxidant activity and phenolic content in US malting barley.] ''Fermentation.'' 2020;6(2):48.</ref><ref name=cargui/>

[[Wheat]] and [[rye]] contribute somewhat less total phenolic compounds compared to barley,<ref name=shoden>Shopska V, Denkova-Kostova R, Dzhivoderova-Zarcheva M, Teneva D, Denev P, Kostov G. [https://www.mdpi.com/2076-3921/10/7/1124 Comparative study on phenolic content and antioxidant activity of different malt types.] ''Antioxidants.'' 2021;10(7):1124.</ref><ref name=fogkun>Fogarasi A-L, Kun S, Tankó G, Stefanovits-Bányai É, Hegyesné-Vecseri B. [https://www.sciencedirect.com/science/article/abs/pii/S0308814614009820 A comparative assessment of antioxidant properties, total phenolic content of einkorn, wheat, barley and their malts.] ''Food Chem.'' 2015;167(15):1–6.</ref><ref name=ziekoz>Zieliński H, Kozłowska H. [https://pubs.acs.org/doi/abs/10.1021/jf990619o Antioxidant activity and total phenolics in selected cereal grains and their different morphological fractions.] ''J Agric Food Chem.'' 2000;48(6):2008–2016.</ref> while [[oats]] and [[buckwheat]] contribute the lowest amount of phenolics, much lower than barley.<ref name=ziekoz/>

Factors affecting phenolic compound extraction from grain:
* '''Malt parameters''' - More phenolics are extracted from well-modified [[malt]]s during mashing.<ref name=bsp/><ref name=zhazha/><ref name=chache/><ref name=adb/> Vienna malt, melanoidin malt, and smoked malt provide more phenolic content than other base malts.<ref name=shoden/> Malt color and the use of specialty malts generally has a negligible effect on phenolic acid content of the resulting wort.<ref name=vanvan/><ref name=schwarz/> Smaller kernels generally contain higher levels of phenolics.<ref name=zhozha/> Phenolic acid content is positively correlated positively with the Kolbach index and soluble nitrogen content, and is negatively related to malt extract and viscosity.<ref name=wangas/>
* '''Adjuncts''' - Use of unmalted cereal [[adjuncts]] ([[maize]], [[wheat]], [[oats]], [[rice]], [[buckwheat]], [[rye]], [[millet]], and [[barley]]) results in significantly lower levels of free phenolic acids ([[Congress mash]]ing).<ref name=vanvan/><ref name=fumgal>Fumi MD, Galli R, Lambri M, Donadini G, De Faveri DM. [https://www.sciencedirect.com/science/article/abs/pii/S0889157511000238 Effect of full-scale brewing process on polyphenols in Italian all-malt and maize adjunct lager beers.] ''J Food Compos Anal.'' 2011;24(4–5):568–573.</ref> For example, wort produced with 50% unmalted wheat flakes contains 15–43% less free ferulic acid compared to wort with 100% malted barley.<ref name=vanvan/> There are two exceptions to this: rice increases p-coumaric acid and rye does not alter the ferulic acid concentration.
* '''Milling''' - Fine [[milling]] (causing increased husk damage) increases the amount of phenolic compounds in the wort.<ref name=wangas/><ref name=andski>Andersen ML, Skibsted LH. [https://pubs.acs.org/doi/abs/10.1021/jf0105945 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.</ref><ref name=zhozha/> [[Milling#Conditioning_the_grain|Grain conditioning]] will decrease the total phenolic content in wort, due to higher husk integrity.<ref name=wangas/><ref name=habkos/>
* '''Ferulic acid rest''' - Phenolic acid extraction can be increased by enzymatic activity during mashing.<ref name=vanvan/><ref name=wangas/><ref name=zhao/><ref name=sibpla/> Release of bound phenolic acids is driven by cinnamoyl esterase (also called feruloyl esterase or ferulic acid esterase), an enzyme with maximal activity at 40–45°C (with an extended rest) and pH 5.2–6.6.<ref name=cargui/><ref name=wangas/><ref name=vanvan/><ref name=schwarz/> Enzymatic release declines with increasing temperature up to about 65°C where there is none; at this point the amount of phenolic acid extracted is solely dependent on the free form created during malting.<ref name=vanvan/><ref name=cargui/>
* '''Mash temperature''' - Besides conducting a ferulic acid rest, the total phenolic content tends to increase with increasing mash temperature up to around 70°C where it peaks (mash-out temperature causes a decrease).<ref name=wangas/><ref name=zhazha/><ref name=bsp/><ref name=adb>Narziss L, Back W, Gastl M, Zarnkow M. [[Library|''Abriss der Bierbrauerei.'']] 8th ed. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA; 2017.</ref><ref name=cargui/> The decrease at high temperatures is likely due to a combination of precipitation with proteins and precipitation due to oxidative polymerization.<ref name=cargui/>
* '''Mash duration''' - A longer mash duration extracts more phenolic content.<ref name=adb/><ref name=cargui/>
* '''Stirring''' - The amount of extracted [[beta-glucans and arabinoxylans|arabinoxylans]] and therefore the amount of bound phenolic acids (mainly ferulic acid) is increased by stirring the mash.<ref name=vanvan/>
* '''Oxidation''' - Oxygen exposure during mashing (see [[Oxidation]]) dramatically decreases the concentration of polyphenols during mashing.<ref name=chache/><ref name=adb/><ref name=bsp/> Conversely, [[low oxygen brewing]] and the addition of [[antioxidants]] leads to the production of wort with increased phenolic content.
* '''Added enzymes''' - Exogenous cinnamoyl esterase [[enzymes]] can be used during mashing to increase the phenolic acid content in wort.<ref name=zhao/>
* '''Sparge pH''' - High/uncontrolled pH during mashing or [[sparging]] will increase the extraction of undesirable harsh/astringent compounds, generally thought to be phenols.<ref name=bsp/><ref name=fix>Fix G. [[Library|''Principles of Brewing Science.'']] 2nd ed. Brewers Publications; 1999.</ref><ref name=habkos/> Brewers (especially those that sparge) should control [[brewing pH]], and in particular avoid alkalinity in the sparge water and excessively high temperature.

Ultimately, only a small portion of phenolic acids in malt are extracted into wort during mashing, the majority remains in the spent grain.<ref name=wangas/><ref name=cargui/> A substantial amount of bound phenolic acids (especially ferulic acid) are extracted, which may have effects later during fermentation or aging.<ref name=len/> Varying amounts of polyphenols are extracted; for example, 100% of flavanol monomers are extracted but only 23% of polymers (4+ units).<ref name=wangas/><ref name=habkos/>

The mashing process is very complex and dynamic, consisting of a balance between the release of bound phenolic compounds, and their degradation.<ref name=cargui/><ref name=langra/>

=== Hops ===
[[Hops]] are very high in phenolic content, containing around 4–14% total phenolics by dry weight.<ref name=cargui/><ref name=wangas/><ref name=calcol/><ref name=len/> The bulk of these phenolics are located within the hop cone plant material (the green part of the hop flower).<ref name=len/><ref name=cargui/> The amount of phenolic compounds depends on the hop variety, growing conditions, and harvest time.<ref name=cargui/><ref name=wangas/> For example, the polyphenol content of hops tends to be higher when they are harvested earlier.<ref name=wangas/> The diverse array of hop phenolic compounds include many that differ from those found in grain.<ref name=wangas/><ref name=cargui/><ref name=keljur>Kellner V, Jurková M, Čulík J, Horák T, Čejka P. [https://www.researchgate.net/profile/Vladimir-Kellner/publication/236144996_January_February_2007_32/links/00b7d5165784fc0249000000/January-February-2007-32.pdf Some phenolic compounds in Czech hops and beer of Pilsner type.] ''Brew Sci.'' 2007;60:31–37.</ref> [[Xanthohumol]] is one such compound notable for its significant health benefits and anticarinogenic properties, and brewers can take certain steps to increase the amount of this compound in the beer.<ref name=cargui/><ref name=wangas/><ref name=calcol/> Hop prenylated flavonoids are extremely stable over the course of beer storage.<ref name=wangas/>

Factors affecting phenolic compound extraction from hops:
* '''Cultivar''' - Among hop cultivars, the lower bitterness (low α-acid) varieties often contain higher levels of phenolic compounds.<ref name=calcol/><ref name=sibpla/><ref name=coljer>Collin S, Jerković V, Bröhan M, Callemien D. [https://link.springer.com/referenceworkentry/10.1007/978-3-642-22144-6_78 Polyphenols and beer quality.] In: Ramawat KG, Mérillon J-M, eds. ''Natural Products.'' 1<sup>st</sup> ed. Springer; 2013:2334–2353.</ref> Magnum hops are a notable exception, a high α-acid variety that also has been shown to contribute high polyphenol content.<ref name=sibpla/>
* '''Product form''' - Differing amounts of phenolics will be extracted into the wort based on the type of hop product used.<ref name=calcol/><ref name=cargui/> Fresh (wet) hops contain the highest levels of phenols, which can help improve flavor stability compared to other hop products. The drying and grinding process (i.e. turning fresh hops into pellets or plugs) causes losses in polyphenol content and antioxidant activity.<ref name=wangas/><ref name=cargui/><ref name=calcol/> Hop pellets are available in 2 types: type-90 and type-45, which contain 90% and 45%, respectively, of raw hop cone plant material (which is where the phenolic compounds are located).<ref name=len/> Therefore the type-90  pellets are much higher in phenolic content than type-45 pellets. Hop extracts are another hop product form, and the polyphenol content of these depends on the extraction solvent used during production.<ref name=wangas/> CO<sub>2</sub> hop extracts contain practically no phenolics.<ref name=calcol/>
* '''Product age''' - The total polyphenol content and antioxidant activity of hops decline significantly during storage.<ref name=wangas/><ref name=coljer/>
* '''Stage of addition''' - Differing amounts of phenolics will be passed to the beer based on the stage of addition.<ref name=calcol/> Approximately 50–70% of the total hop phenolics are extracted when added during wort [[boiling]].<ref name=wangas/> A similar percentage is extracted during dry hopping,<ref name=cargui/> however, much more of the phenolics from this late stage hop addition are retained in the final beer (see below). It takes about 2–3 days for maximum phenolic compound extraction during dry hopping, although it may take more time at lower temperatures.<ref name=cargui/>
* '''Hopping rates''' - Using more hops will increase the phenolic content of the beer (with other factors being equal).
* '''Oxidation''' - Oxidation that occurs after the hops are added will cause removal of some portion of the phenolic compounds that were extracted, especially during the stages of wort [[chilling]] and beer storage.

=== Fruit and other plants ===
Fruits tend to contain high levels of phenolic compounds, and therefore the use of fruit (or fruit juice) as a brewing ingredient will likely increase the total phenolic content and antioxidant activity of the beer.<ref name=marcab/><ref name=baiper>Baigts-Allende DK, Perez-Alva A, Ramirez-Rodrigues MA, Palacios A, Ramirez-Rodrigues MM. [https://www.sciencedirect.com/science/article/abs/pii/S0889157521001216 A comparative study of polyphenolic and amino acid profiles of commercial fruit beers.] ''J Food Compos Anal.'' 2021;100:103921.</ref> Many fruits have been studied as brewing ingredients: carnelian cherry juice,<ref name=marcab/><ref name=yangao/> quince,<ref name=marcab/><ref name=yangao/><ref name=zapmar>Zapata PJ, Martínez-Esplá A, Gironés-Vilaplana A, Santos-Lax D, Noguera-Artiaga L, Carbonell-Barrachina ÁA. [https://www.sciencedirect.com/science/article/pii/S0023643819300039 Phenolic, volatile, and sensory profiles of beer enriched by macerating quince fruits.] ''Lwt.'' 2019;103:139–146.</ref> persimmon,<ref name=marcab/><ref name=yangao/> goji berries,<ref name=marcab/><ref name=yangao/> jackfruit,<ref name=marcab/> green pepper,<ref name=yangao/> and grapes.<ref name=marcab/> Many non-fruit plants have also been studied and shown to increase phenolic content. Examples include: sedge,<ref name=marcab/> diamond flower,<ref name=marcab/> agarwood,<ref name=marcab/> thyme,<ref name=yangao/> juniper,<ref name=yangao/> hibiscus,<ref name=yangao/><ref name=marveg>Martínez A, Vegara S, Herranz‐López M, et al. [https://onlinelibrary.wiley.com/doi/full/10.1002/jib.387 Kinetic changes of polyphenols, anthocyanins and antioxidant capacity in forced aged hibiscus ale beer.] J Inst Brew.] 2017;123(1):58–65.</ref>, green tea,<ref name=yangao/> reishi mushroom,<ref name=yangao/> chickling,<ref name=yangao/> lentils,<ref name=yangao/> sweet potato,<ref name=yangao/> buckwheat,<ref name=yangao/> cocoa,<ref name=narfod/> walnut,<ref name=narfod/> chestnut,<ref name=narfod/> licorice,<ref name=narfod/> coffee,<ref name=narfod/> olive leaves,<ref>Guglielmotti M, Passaghe P, Buiatti S. [https://ift.onlinelibrary.wiley.com/doi/abs/10.1111/1750-3841.15318 Use of olive (''Olea europaea L.'') leaves as beer ingredient, and their influence on beer chemical composition and antioxidant activity.] ''J Food Sci.'' 2020;85(8):2278–2285.</ref> and lemon balm.<ref name=yangao/> These ingredients may also positively or negatively affect the other characteristics of the beer (flavor, color, clarity, pH value).<ref name=yangao/><ref name=marveg/> Cinnamon has modest antioxidant capacity<ref>Wijewardhana US. Gunathilaka UGSA. Navaratne SB. [http://irjaes.com/wp-content/uploads/2020/10/IRJAES-V4N1P381Y19.pdf Determination of total phenolic content, radical scavenging activity and total antioxidant capacity of cinnamon bark, black cumin seeds and garlic.] ''Int J Adv Eng Res Sci.'' 2019;4(2):55–57.</ref><ref name=merkun/> and has been used by home brewers during mashing for some perceived benefit without substantial flavor contribution.<ref>[https://www.homebrewersassociation.org/forum/index.php?topic=9230.0 Using cinnamon in the mash.] American Homebrewers Association online forum. 2011. Accessed Jan 2023.</ref><ref>[https://www.homebrewtalk.com/threads/cinnamon-in-the-mash.129279/ Cinnamon in the mash.] Homebrew Talk online forum. 2009–2012. Accessed Jan 2023.</ref> Gluten-free beer made with white [[sorghum]] has shown to contain higher levels of phenolic compounds.<ref name=yangao/> Blue [[maize|corn]] used in brewing also provides high levels of phenolics.<ref name=yangao/>

[[Honey]] (a plant derivative) contains phenolic compounds that can increase the phenolic content of beer.<ref name=narfod/><ref>Annapoorani A, Anilakumar KR, Khanum F, Murthy NA, Bawa AS. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3215355/ Studies on the physicochemical characteristics of heated honey, honey mixed with ''ghee'' and their food consumption pattern by rats.] ''Ayu.'' 2010;31(2):141–146.</ref> Other honey bee products, such as propolis extract<ref name=marcab/> and "bee pollen",<ref name=soliva>Solgajová M, Ivanišová E, Nôžková J, Frančáková H, Tóth Ž, Dráb Š. [https://www.jmbfs.org/80_jmbfs_solgajova_2014_fs/?issue_id=3039&article_id=29 Antioxidant activity and polyphenol content of malt beverages enriched with bee pollen.] ''J Microbiol Biotech Food Sci.'' 2014;3(3):281–284.</ref> have also been shown to be useful for increasing phenolic content.

It's difficult to make recommendations regarding the use of any of these plants specifically for the purpose of increasing phenolic content of beer because they all may impact the flavor profile or other qualities of the beer for better or worse. Brewers may wish to experiment and judge the results for themselves, or simply use a tannin additive as a safer alternative.

=== Wood ===
Aging a beverage in contact with [[wood]] such as a barrel or on wood chips will impart it with phenolic compounds.<ref name=len/> Higher toasted wood will generally contribute more phenolics. See [[Wood]] for more information regarding wood aging.

=== Additives ===
Brewers can add purified sources of phenolic compounds in order to gain the benefits of phenolic compounds without negatively affecting the flavor profile of the beer. [[Tannin additives|Phenolic additives]] such as hop polyphenol extract and tannin extract have been successfully used to improve characteristcs of beer,<ref name=wangas/><ref name=defbra>De Francesco G, Bravi E, Sanarica E, Marconi O, Cappelletti F, Perretti G. [https://www.mdpi.com/2304-8158/9/11/1638 Effect of addition of different phenolic-rich extracts on beer flavour stability.] ''Foods.'' 2020;9(11):1638.</ref><ref name=jasgoi/><ref name=karhan>Karabín M, Hanko V, Nešpor J, Jelínek L, Dostálek P. [https://onlinelibrary.wiley.com/doi/full/10.1002/jib.502 Hop tannin extract: a promising tool for acceleration of lautering.] ''J Inst Brew.'' 2018;124(4):374–380.</ref><ref name=witboe>http://www.themodernbrewhouse.com/wp-content/uploads/2017/02/Officiele_tekst_voor_Brewing_Science.pdf</ref><ref name=hob>Ryder DS. Chapter 10
Processing aids in brewing. In: Stewart GG, Russell I, Anstruther A, eds. [[Library|''Handbook of Brewing.'']] 3rd ed. CRC Press; 2017.</ref> including:
*Improved flavor stability, improved malt and hop characteristics, and perceived freshness, especially when added during mashing
*Reduced bitterness, but prolonged stability of the iso-α-acids (including improved light stability)
*Improved foam stability
*Improved body
*Decreased color pickup
*Improved clarity, especially when combined with proper [[brewing pH|pH adjustment]]
*Improved lautering (when used in the mash)
*Decreased lipid oxidation and lipid carryover to the fermentation vessel (when used in the mash)

Because of these significant benefits, use of a tannin product during mashing is recommended as part of a [[low oxygen brewing]] process. It's important to note that these products are a preventative measure — they do not ''reverse'' [[oxidation]] reactions that have already occurred (including the formation of [[lipids|cardboard-like flavors]] during mashing).<ref name=defbra/>

The most benefit is obtained from adding tannins at the beginning of [[mashing|mash-in]]. However, an extra dose can be added in the [[boiling|boil]] (before any other fining agents) in order to help with [[haze|clarification]]. Tannins added to the boil can bind additional [[protein]]s that have been denatured by the heat and now have binding sites exposed that were unavailable during mashing. Tannins can be added even later (right before the final [[filtration]] before [[packaging]]),<ref name=lug/> however this may be impractical and of limited value.

See [[Tannin additives]] for product information.

== Fate of phenolic compounds ==
Phenolic compounds are extracted into the wort and beer from the sources mentioned above, but the story doesn't end there. Many of the extracted phenolic compounds are removed or modified before the beer is consumed.<ref name=wangas/><ref name=fumgal/> During mashing, boiling, and chilling, a great proportion of malt phenolics will be lost through precipitation or degradation.<ref name=calcol/><ref name=wangas/><ref name=gornov/><ref name=cargui/><ref name=sibpla/><ref name=fumgal/> Precipitated phenolics are removed along with the [[spent grains]] or the [[trub]] material. Fermentation and lagering result in further decreases in phenolic substances, since they are adsorbed to yeast cells or stabilizing agents, or precipitate due to oxidation.<ref name=wangas/><ref name=fumgal/> Polyphenols are more easily removed than monophenolic compounds.<ref name=wangas/><ref name=Szwajgier/> Keep in mind that removal of phenolic compounds is accompanied by a decrease in [[antioxidants|antioxidant]] activity and reduced [[flavor stability]].<ref name=wangas/><ref name=gornov/>

Phenolic compound removal (at all stages):
* [[Oxidation]] during mashing, boiling, and chilling promotes polymerization, which leads to precipitation and removal with spent grains or trub.<ref name=wangas/><ref name=habkos/><ref name=guicur/><ref name=karhan/><ref name=stephenson/> Oxidative polymerization occurring during beer storage further decreases phenolic content and causes [[haze]].<ref name=wangas/><ref name=habkos/><ref name=siqbol>Siqueira PB, Bolini HMA, Macedo GA. [https://www.researchgate.net/profile/Helena-Bolini/publication/267193879_Polyphenols_and_antioxidant_properties_in_forced_and_naturally_aged_Brazilian_beer/links/55a7b27508aeceb8cad6794a/Polyphenols-and-antioxidant-properties-in-forced-and-naturally-aged-Brazilian-beer.pdf Polyphenols and antioxidant properties in forced and naturally aged Brazilian beer.] ''J Brew Distilling.'' 2011;2(3):45–50.</ref><ref name=marveg/><ref name=coljer/> Oxidation not only causes precipitation, but also degrades some phenolics to other types of compounds, such as quinones.<ref name=calcol/> Lower pH tends to increase polymerization.<ref name=sibpla>Šibalić D, Planinić M, Jurić A, Bucić-Kojić A, Tišma M. [https://link.springer.com/article/10.1007/s11696-020-01276-1 Analysis of phenolic compounds in beer: from raw materials to the final product.] ''Chem Zvesti.'' 2021;75(1):67–76.</ref>
* [[Protein]]s bind and remove phenolic compounds.<ref name=cargui/> Interestingly, less oxidized polyphenols tend to precipitate more readily with proteins.<ref name=wangas/> Boiling for longer periods (over 60 minutes) causes increased elimination of phenolic compounds due to increased precipitation,<ref name=wangas/><ref name=sibpla/> possibly due to increased binding with proteins.
* Stabilizing agents (such as [[PVPP]]) bind and remove phenolic compounds.<ref name=wangas/>
* Yeast cells can bind phenolic compounds, although they may be released later.<ref name=wangas/><ref name=pasame/><ref name=cargui/><ref name=fumgal/> Despite some removal of phenolic compounds at this stage, antioxidant capacity is generally unaffected by fermentation.<ref name=yangao/><ref>Leitao C, Marchioni E, Bergaentzlé M, et al. [https://pubs.acs.org/doi/abs/10.1021/jf104094c Effects of processing steps on the phenolic content and antioxidant activity of beer.] ''J Agric Food Chem.'' 2011;59(4):1249–1255.</ref>

Changes during boiling:
* A minor portion of cinnamic acid-derivatived phenolic acids are transformed by the heat to more flavor-active compounds during stages of high temperature, including kilning, boiling, whirlpool, and pasteurization.<ref name=wangas/><ref name=schwarz/><ref name=calcol/><ref name=vansai/><ref name=langra/><ref name=cargui/><ref name=vandel/> Most importantly, ferulic acid is converted to 4-vinylguaiacol (4VG),<ref name=vanvan/> and when boiling for an extended period (e.g. 3 hours), the level of 4VG may approach sensory threshold concentration (0.3 mg/L).<ref name=len/> (See [[#Phenolic_flavors_(volatile_phenols)|phenolic acid derivatives]] below)

Changes during fermentation:
* Ferulic acid may increase during fermentation, which is attributed to feruloyl esterase activity in yeast liberating ferulic acid from dissolved arabinoxylans, especially from wheat.<ref name=wangas/><ref name=langra/><ref name=cargui/><ref name=calcol/>
* Cinnamic acid-derivatived phenolic acids can be transformed by certain microbes to more flavor-active compounds. This typically occurs via phenylacrylic acid decarboxylase (Pad1-enzyme) and ferulic acid decarboxylase activity of top-fermenting "phenolic off-flavor positive" (POF+) ''Saccharomyces cerevisiae'' yeast strains.<ref name=vanvan/><ref name=len/><ref name=vansai/><ref name=cargui/> In beers brewed with POF+ strains of ''S. cerevisiae'', 4VG is the most prevalent volatile phenol, as ferulic acid dominates the wort cinnamic acid-derived phenolic acids and ferulic acid is preferentially degraded by yeast.<ref name=len/><ref name=calcol/> Some [[contamination|contaminating]] microorganisms like wild ''Saccharomyces'' spp., ''Brettanomyces'' spp., or ''Enterobacteriacea'' can have the same effect, either by Pad-1 activity or phenolic acid decarboxylase activity.<ref name=calcol/><ref name=vansai/><ref name=len/> When POF+ yeast are used, transformation of ferulic acid occurs at a constant rate through fermentation and then decreases sharply during secondary fermentation.<ref name=calcol/> The flavor-active vinyl compounds such as 4VG can be further oxidized or reduced into smaller molecules like vanillin, 4-ethylguaiacol, guaiacol, and 4-ethylphenol through chemical reactions or through the activity of wild microbes (especially by ''Brettanomyces'').<ref name=calcol/><ref name=vandel>Vanbeneden N, Delvaux F, Delvaux FR. [https://www.sciencedirect.com/science/article/abs/pii/S0021967306021029 Determination of hydroxycinnamic acids and volatile phenols in wort and beer by isocratic high-performance liquid chromatography using electrochemical detection.] ''J Chromatogr A.'' 2006;1136(2):237–242.</ref> See [[#Phenolic_flavors_(volatile_phenols)|phenolic acid derivatives]] below for more information.

Changes during storage:
*Flavor-active monophenols (such as 4VG) transform during storage, changing the flavor (see [[#Phenolic_flavors_(volatile_phenols)|phenolic acid derivatives]] below).<ref name=wangas/>

== Antioxidant activity and oxidation ==
Phenolic compounds (especially polyphenols) have long been considered the primary natural [[antioxidants]] in brewing raw materials and beer.<ref name=wangas/><ref name=mikhra>Mikyška A, Hrabak M, Hašková D, Šrogl J. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2002.tb00128.x The role of malt and hop polyphenols in beer quality, flavour and haze stability.] ''J Inst Brew.'' 2002;108(1):78–85.</ref><ref name=guidol>Guido LF, Boivin P, Benismail N, Gonçalves CR, Barros AA. [https://www.researchgate.net/profile/Luis-Guido-2/publication/226777056_An_early_development_of_the_nonenal_potential_in_the_malting_process/links/54e318780cf2de71a71df4ce/An-early-development-of-the-nonenal-potential-in-the-malting-process.pdf An early development of the nonenal potential in the malting process.] ''Eur Food Res Technol.'' 2005;220:200–206.</ref><ref name=walhea>Walters MT, Heasman AP, Hughes PS. [https://www.researchgate.net/profile/Paul_Hughes8/publication/268296190_Comparison_of_-Catechin_and_Ferulic_Acid_as_Natural_Antioxidants_and_Their_Impact_on_Beer_Flavor_Stability_Part_2_Extended_Storage_Trials/links/552ce2640cf2e089a3acff3f.pdf 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.</ref><ref name=aroshe/><ref name=vanvan/><ref name=vansai/><ref name=guicur>Guido LF, Curto AF, Boivin P, Benismail N, Gonçalves CR, Barros AA. [https://pubs.acs.org/doi/abs/10.1021/jf0623079 Correlation of malt quality parameters and beer flavor stability:  multivariate analysis.] ''J Agric Food Chem.'' 2007;55(3):728–733.</ref><ref name=zhali>Zhao H, Li H, Sun G, Yang B, Zhao M. [https://onlinelibrary.wiley.com/doi/abs/10.1002/jsfa.5824 Assessment of endogenous antioxidative compounds and antioxidant activities of lager beers.] ''J Sci Food Agric.'' 2013;93(4):910-917.</ref><ref name=marcab>Martinez-Gomez A, Caballero I, Blanco CA. [https://www.mdpi.com/2218-273X/10/3/400 Phenols and melanoidins as natural antioxidants in beer. Structure, reactivity and antioxidant activity.] ''Biomolecules.'' 2020;10(3):400.</ref><ref name=yangao/>
In other words, phenolics are the main defense against the [[oxidation]] of other sensitive substances (such as [[lipids]]).<ref name=mikhra/><ref name=bsp/><ref name=zhao>Zhao H. [https://www.sciencedirect.com/science/article/pii/B9780124046993000640 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.</ref><ref name=calcol/><ref name=gouhug>Goupy P, Hugues M, Boivin P, Amiot MJ. [https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1097-0010(199909)79:12%3C1625::AID-JSFA411%3E3.0.CO;2-8 Antioxidant composition and activity of barley (''Hordeum vulgare'') and malt extracts and of isolated phenolic compounds.] ''J Sci Food Agric.'' 1999;79(12):1625–1634.</ref>
It has been shown that [[malt]], [[wort]], and [[beer]] with higher total phenolic content display increased antioxidant activity, thus improving [[flavor stability]].<ref name=mikhra/><ref name=zhazha/><ref name=Irwin>Irwin AJ, Barker RL, Pipasts P. [https://www.agraria.com.br/extranet_2016/uploads/AgromalteArquivo/o_papel_do_cobre,_oxigenio_e_polifenois_na_instabilidade_do_flavor_da_cerveja_%E2%80%93_en.pdf The role of copper, oxygen, and polyphenols in beer flavor instability.] ''J Am Soc Brew Chem.'' 1991;49(3):140–149.</ref><ref name=dadbel/><ref name=walhea/><ref name=aroshe/><ref name=zhazha/><ref name=jurhor>Jurková M, Horák T, Hašková D, Čulík J, Čejka P, Kellner V. [https://onlinelibrary.wiley.com/doi/full/10.1002/jib.35 Control of antioxidant beer activity by the mashing process.] ''J Inst Brew.'' 2012;118(2):230-235.</ref><ref name=defbra/><ref name=fanmon>Fantozzi P, Montanari L, Mancini F, et al. [https://www.sciencedirect.com/science/article/abs/pii/S0023643897903418 ''In vitro'' antioxidant capacity from wort to beer.] ''LWT - Food Sci Technol.'' 1998;31(3):221–227.</ref><ref name=hablon/><ref name=pasame>Pascoe HM, Ames JM, Chandra S. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-61-0203 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.</ref><ref name=shoden/><ref name=gornov>Gorjanović SŽ, Novaković MM, Potkonjak NI, LeskoŠek-Čukalović I, Sužnjević DŽ. [https://pubs.acs.org/doi/abs/10.1021/jf903091n Application of a novel antioxidative assay in beer analysis and brewing process monitoring.] ''J Agric Food Chem.'' 2010;58(2):744–751.</ref><ref name=zhache/><ref name=zhozha/><ref name=jasgoi/><ref name=sibpla/>
The protective effect of phenols is particularly important during [[mashing]],<ref name=mikhra/><ref name=calcol/><ref name=jasgoi/> where [[lipids|lipid]] concentration is high, oxidative enzymes are active, and oxygen exposure is difficult to avoid entirely. Due to their antioxidant activity, higher levels of phenolic compounds can help with faster lautering.<ref name=karhan/> Phenolic antioxidant activity also helps to protect yeast viability against the stress generated by high levels of ethanol, enhancing the fermentation process and increasing attenuation.<ref name=borsol/> Traditionally, the highest phenolic content in wort (and corresponding antioxidant activity) occurs after boiling due to the addition of hops,<ref name=Szwajgier/> although phenolic compounds may peak later depending on the amount of dry hopping. However, there are a large amount of ''insoluble'' phenolic compounds present in the grain, so the antioxidant effect of phenolic compounds may actually be higher during mashing.<ref name=guicur/><ref>Maillard M-N, Berset C. [https://pubs.acs.org/doi/abs/10.1021/jf00055a008 Evolution of antioxidant activity during kilning: role of insoluble bound phenolic acids of barley and malt.] ''J Agric Food Chem.'' 1995;43(7):1789–1793.</ref><ref name=mcmmad/>

It's important to understand that phenolics help to protect the wort and beer ''only as long as oxygen exposure is minimized'' during the brewing process.<ref name=kunze>Kunze W. Hendel O, ed. [[Library|''Technology Brewing & Malting.'']] 6th ed. VLB Berlin; 2019.</ref><ref name=guicur/><ref name=karhan/><ref name=frefes>Frederiksen AM, Festersen RM, Andersen ML. [https://pubs.acs.org/doi/abs/10.1021/jf801666e 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.</ref> The antioxidant capacity can easily be overwhelmed, leading to rapid [[oxidation]] of other compounds as well as the emergence of various negative aspects of oxidized phenolics.<ref name=kunze/> Therefore, it is beneficial to [[low oxygen brewing|minimize oxygen exposure during brewing]].

Some phenolic compounds provide greater antioxidant activity than others.<ref name=cuvric/> Generally the smaller phenolic compounds (e.g. flavonols, flavan-3-ols, and phenolic acids) are considered to be the most effective antioxidants in wort and beer.<ref name=wangas/><ref name=humsan/><ref name=jurhor/><ref name=Siqueira/><ref name=calcol/><ref name=Irwin/><ref name=zhache/><ref name=gouhug/> However, ferulic acid bound to [[Beta-glucans and arabinoxylans|arabinoxylans]] is a more potent antioxidant than free ferulic acid.<ref name=szwpie>Szwajgier D, Pielecki J, Targoński Z. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2005.tb00222.x The release of ferulic acid and feruloylated oligosaccharides during wort and beer production.] ''J Inst Brew.'' 2005;111(4):372–379.</ref>

'''Phenols act as antioxidants in three ways:'''
# They neutralize [[oxidation|reactive oxygen species]] (ROS)
# They act as chelating agents
# They inhibit [[lipids|lipoxygenase (LOX)]] enzymes

=== Oxygen scavenging ===
Polyphenols are the single most important group of compounds for absorbing oxygen introduced into wort and beer.<ref name=stephenson>Stephenson WH, Biawa JP, Miracle RE, Bamforth CW. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2003.tb00168.x Laboratory-scale studies of the impact of oxygen on mashing.] ''J Inst Brew.'' 2003;109(3):273–283.</ref><ref name=dadbel/><ref name=bammul>Bamforth CW, Muller RE, Walker MD. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-51-0079 Oxygen and oxygen radicals in malting and brewing: a review.] ''J Am Soc Brew Chem.'' 1993;51(3):79–88.</ref> This is because phenolic compounds readily become oxidized by [[oxidation|reactive oxygen species (ROS)]] in the wort/beer, effectively delaying the oxidation of other compounds.<ref name=aroshe/><ref name=Szwajgier>Szwajgier D. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2011.tb00505.x 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.</ref> The oxygen scavenging reactions can proceed by different mechanisms: free radical inactivation by hydrogen atom transfer (HAT), or single electron transfer (SET) reactions.<ref name=leomar>Leopoldini M, Marino T, Russo N, Toscano M. [https://pubs.acs.org/doi/abs/10.1021/jp037247d Antioxidant properties of phenolic compounds: H-atom versus electron transfer mechanism.] ''J Phys Chem A.'' 2004;108(22):4916–4922.</ref><ref name=wangas/><ref name=maisou/><ref name=lug>Lugasi A. [https://akjournals.com/view/journals/066/32/2/article-p181.xml Polyphenol content and antioxidant properties of beer.] ''Acta Aliment.'' 2003;32(2):181–192.</ref> When phenolic compounds react with free radicals by HAT or SET, the reaction product includes a phenolic radical. Due to the aromatic ring structure, the phenolic radical electron is resonance-stabilized and therefore relatively non-reactive, thus effectively terminating the radical chain reaction.<ref name=marcab/><ref name=wangas/><ref name=aroshe/><ref name=cuvric>Cuvelier ME, Richard H, Berset C. [https://scholar.archive.org/work/6ha2lihfcfgcfjn4ffi254bsjy/access/wayback/http://www.tandfonline.com/doi/pdf/10.1271/bbb.56.324?needAccess=true Comparison of the antioxidative activity of some acid-phenols: structure-activity relationship.] ''Biosci Biotechnol Biochem.'' 1992;56(2):324–325.</ref><ref name=maisou>Maillard M-N, Soum M-H, Boivin P, Berset C. [https://www.sciencedirect.com/science/article/abs/pii/S0023643896900353 Antioxidant activity of barley and malt: relationship with phenolic content.] ''LWT - Food Sci Technol.'' 1996;29(3):238–244.</ref><ref name=gouhug/> Certain phenolic radicals (e.g. semiquinones resulting from flavan-3-ol oxidation) can also bind together (polymerize), forming non-radicals and thereby preserving their radical-scavenging capacity.<ref name=aroshe/><ref name=calcol/> During mashing, phenolic compounds are also key substrates for oxidative [[enzymes]] including peroxidases, and thus enzymatically absorb oxygen after it is converted to hydrogen peroxide.<ref name=stephenson/><ref name=bsp/><ref name=adb/> Through these mechanisms, polyphenols take up much more oxygen than any other compounds in wort/beer.<ref name=dadbel/><ref name=calcol/> Oxidized phenolic compounds can be reduced through the action of other antioxidants such as [[sulfite]] or [[ascorbic acid]].<ref name=narcir>Nardini M, Cirillo E, Natella F, Mencarelli D, Comisso A, Scaccini C. [https://www.sciencedirect.com/science/article/abs/pii/S0308814602002133 Detection of bound phenolic acids: prevention by ascorbic acid and ethylenediaminetetraacetic acid of degradation of phenolic acids during alkaline hydrolysis.] ''Food Chem.'' 2002;79(1):119–124.</ref>

However, oxygen scavenging comes at a cost, particularly if oxygen exposure is not minimized during the brewing process. Here's a summary of the negative effects of phenolic compound oxidation:
* Oxidation of the [[ferulic acid]] bound to [[beta-glucans and arabinoxylans|arabinoxylans]] causes cross-linking and precipitation during mashing or lautering, which may cause a [[stuck mash]] and also results in decreased body and lower dietary fiber content.<ref name=egi>Egi A, Speers RA, Schwarz PB. [https://www.mbaa.com/publications/tq/tqPastIssues/2004/Abstracts/0803-01.htm Arabinoxylans and their behavior during malting and brewing.] ''Tech Q Master Brew Assoc Am.'' 2004;41(3):248–267.</ref>
* Malt polyphenols that become oxidized during brewing (on the hot side) are partly removed, and therefore are no longer available to help protect the beer from oxidation during packaging and storage.<ref name=bsp/><ref name=adb/>
* Oxidized phenolic compounds give darker color.
* Oxidized phenolic compounds contribute increased astringency and harsh/bitter flavors.
* Oxidation causes phenolic compounds to link with proteins, forming [[haze]].
* Oxidized polyphenols can slowly oxidize other substances (playing a pro-oxidative role).

=== Chelating activity ===
Even trace amounts of [[transition metals]] (especially [[iron]] and [[copper]]) catalyze the formation of ROS from O<sub>2</sub>, which promote rapid [[oxidation]] of various compounds. Polyphenols and phenolic acids strongly bind to these metals (mainly iron), forming stable complexes so that the metals do not readily initiate oxidative reactions.<ref name=calcol/><ref name=wangas/><ref name=Siqueira/><ref name=aroshe/><ref name=lug/> Chelation is stronger at mashing pH compared to beer pH.<ref name=merkun>https://onlinelibrary.wiley.com/doi/full/10.1002/jib.673</ref>

=== Oxidative enzyme inhibition ===
Malt polyphenols are known to inhibit the action of lipoxygenase (LOX) [[enzymes]], helping to prevent enzymatic [[lipids|lipid]] oxidation during [[mashing]].<ref name=guicur/><ref name=aroshe/><ref name=maisou/><ref>Cos P, Bruyne TD, Hermans N, Apers S, Berghe DV, Vlietinck AJ. [https://www.ingentaconnect.com/content/ben/cmc/2004/00000011/00000010/art00009 Proanthocyanidins in health care: Current and new trends.] ''Curr Med Chem.'' 2004;11(10):1345–1359.</ref><ref>Hoult JR, Moroney MA, Payá M. [https://www.sciencedirect.com/science/article/abs/pii/007668799434115X Chapter 44: Actions of flavonoids and coumarins on lipoxygenase and cyclooxygenase.] In: Colowick SP, Abelson JN, eds. ''Methods in Enzymology.'' Vol 234. Academic Press; 1994:443–454.</ref><ref name=jasgoi>Jaskula-Goiris B, Goiris K, Syryn E, et al. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2014-0616-01 The use of hop polyphenols during brewing to improve flavor quality and stability of pilsner beer.] ''J Am Soc Brew Chem.'' 2014;72(3):175–183.</ref><ref name=gouhug/><ref name=stephenson/> High concentrations of polyphenols can also inhibit polyphenol oxidase.<ref name=quibai/>

=== Prooxidant activity ===
Phenolic compounds, when oxidized, can contribute to ROS formation and oxidative degradation of other compounds during beer aging (even when oxygen is no longer present).<ref name=savel/><ref name=dadbel/><ref name=bsp/><ref name=walhea/> This will typically occur relatively slowly due to the fairly low reactivity of oxidized phenolic compounds (i.e. radicals and quinones). Despite the overall protective effect of phenols, certain phenolic compounds (in particular the trihydroxyflavans, also called prodelphinidins) are especially prone to acting as oxidizing agents, reducing [[flavor stability]].<ref name=fix/><ref name=mikhra/><ref name=Irwin/><ref name=Siqueira/><ref name=whield>Whittle N, Eldridge H, Bartley J, Organ G. [https://onlinelibrary.wiley.com/doi/epdf/10.1002/j.2050-0416.1999.tb00011.x Identification of the polyphenols in barley and beer by HPLC/MS and HPLC/electrochemical detection.] ''J Inst Brew.'' 1999;105(2):89–99.</ref>

Prooxidant activity can also occur by another mechanism. Many substances with antioxidant activity might exert prooxidant activity when combined with transition metal ions. Phenolic compounds reduce these metal ions, which then catalyze the formation of ROS by [[Fenton reaction|Fenton]] and [[Haber-Weiss reaction]]s (a process called redox cycling).<ref name=wangas/><ref name=aroshe/><ref name=calcol/><ref name=caosof>Cao G, Sofic E, Prior RL. [https://www.sciencedirect.com/science/article/abs/pii/S0891584996003516 Antioxidant and prooxidant behavior of flavonoids: Structure-activity relationships.] ''Free Radic Biol Med.'' 1997;22(5):749–760.</ref> This effect is concentration-dependent — phenolic compounds act as prooxidants when the concentration of metals is relatively high, which may be relevant when using inadequately [[passivation|passivated]] stainless steel or other system components made of copper or aluminum such as [[wort chilling|wort chillers]] or [[sparging]] apparatus, or using brewing [[water]] with high content of metal ions. [[Ascorbic acid]] is another substance that can sometimes act as a prooxidant in this manner.<ref name=caosof/>

== Effect on color ==
[[File:Oxidation-color-example.jpg|thumb|Left: Fresh pour of a NEIPA. Right: Same beer open to air for 24 hours. Photo by Michael Tonsmeire]]
In addition to the influence of [[malt]] on beer color, there is a strong effect of [[oxidation]] and phenolic content. As oxygen exposure increases during the [[brewing]] process and/or [[beer|beer storage]], phenolic substances become [[oxidation|oxidized]] and polymerize, resulting in darker colored wort and beer.<ref name=wangas/><ref name=habkos/><ref name=bsp/><ref name=calcol/><ref name=dadbel/><ref name=chache>Chapon L, Chemardin M. [https://www.tandfonline.com/doi/abs/10.1080/00960845.1964.12006766?journalCode=ujbc19& 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.</ref><ref>Hodžić Z, Karahmetović A, Saletović M, Šestan A. [https://www.researchgate.net/profile/Zorica-Hodzic-2/publication/265575431_Relationship_between_total_polyphenols_and_colour_values_of_wort_and_beer/links/54bcdcd30cf24e50e9409d97/Relationship-between-total-polyphenols-and-colour-values-of-wort-and-beer.pdf Relationship between total polyphenols and colour values of wort and beer.] ''J Eng.'' 2007;1:1584–2665.</ref><ref name=clalar>Clarkson SP, Large SJ, Bamforth CW. [https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.1992.tb01096.x Oxygen-scavenging enzymes in barley and malt and their effects during mashing.] ''J Inst Brew.'' 1992;98(2):111–115.</ref><ref name=savel>Savel J. [http://www.themodernbrewhouse.com/wp-content/uploads/2017/04/savel_0206.pdf Negative role of oxidised polyphenols and reductones in beer.] ''BrewingScience - Monatsschrift Brauwiss.'' 2006;59(1/2):33–40.</ref><ref name=carbs>Kunz T, Brandt NO, Seewald T, Methner FJ. [https://www.researchgate.net/publication/281405909_Carbohydrates_Addition_during_Brewing_-_Effects_on_Oxidative_Processes_and_Formation_of_Specific_Ageing_Compounds Carbohydrates addition during brewing – effects on oxidative processes and formation of specific ageing compounds.] ''BrewingScience.'' 2015;68(7):78–92.</ref> In particular, the smaller flavanols are precursors for oxidative color formation. The "browning" effect can occur through various oxidation mechanisms, including autoxidation (direct reaction with reactive oxygen species), metal ion-catalyzed oxidation, or enzymatic reactions.<ref name=quibai/><ref name=quipow>Quinde-Axtell Z, Powers J. Baik BK. [https://onlinelibrary.wiley.com/doi/abs/10.1094/CC-83-0385 Retardation of discoloration in barley flour gel and dough.] ''Cereal Chem.'' 2006;83(4):385–390.</ref><ref name=chache/> Beer styles with higher levels of phenolic compounds such as IPAs have a greater potential to darken since they contain more of the phenolic precursors that can become brown when oxidized.<ref name=borsol/><ref>Chris. [https://beermaverick.com/how-to-reduce-oxidation-of-new-england-hazy-ipas/ How to reduce oxidation of New England hazy IPAs.] Beer Maverick website. July 2022. Accessed May 2023.</ref> It's important to note that higher levels of phenolic compounds do not necessarily affect beer color — it's only when they oxidize that the color changes.<ref name=jasgoi/> Brewers can produce lighter colored wort and beer by [[low oxygen brewing|minimizing oxidation]] throughout the brewing, packaging, and storage stages.<ref>Biering J. [https://www.asbcnet.org/events/archives/2017ASBCMeeting/proceedings/2017Proceedings/22_Biering.pdf Reliable scale up/scale down in process development—New possibilities to close the gap between lab, pilot brewery, and industrial scale.] Slides presented at: ''Annual meeting of American Society of Brewing Chemists.'' June 4–7, 2017; Fort Myers, FL.</ref><ref>Rabe B. [https://www.themodernbrewhouse.com/what-does-oxidation-look-like/ What does oxidation look like?] The Modern Brewhouse website. 2016. Accessed online March 2024.</ref>

{| class=wikitable
|+Color of Some Phenolics and Oxidation Products<ref name=dadbel/>
! Phenolic Compound
! Color
! Color When Oxidized
|-
|Ferulic acid || White || Reddish brown
|-
|p-Coumaric acid || Yellow || Pale brown
|-
|Vanillic acid || White || Pale brown
|-
|4-Hydroxybenzoic acid || White || Pale brown 
|-
|(+)-Catechin || Cream || Reddish brown
|-
|Quercetin || Yellow || Pale brown
|-
|Kaempferol || Yellow || Coffee brown
|}

== Effect on flavor and mouthfeel ==
Phenolic compounds can indirectly affect flavor through their action as antioxidants (see [[Oxidation]] and [[Flavor stability]]), but they can also have a direct impact on flavor and mouthfeel. The direct effect on flavor occurs through two different mechanisms. First, the phenolics naturally present in beer can cause unwanted harsh flavors and astringency when they become oxidized — a widespread problem since oxidation is often not properly avoided. Second, the phenolic acids can be modified during the brewing or fermentation steps in order to produce a wide variety of "phenolic" flavors.<ref name=habkos/> This second mechanism usually only occurs in specialty beers where particular brewing strains or sometimes wild [[yeast]]s are used for [[fermentation]] in order to create flavorful compounds using phenolic acids as precursors.<ref name=len/> Phenolic acids are also modified by heat, potentially resulting in phenolic flavors under certain conditions. Finally, chlorine compounds in tap water or residual halogen-based (e.g. chlorine or iodine) [[sanitation|sanitizing]] agents can modify phenolics to create nasty off-flavors even at very low levels (chlorophenols; see [[Remove chlorine from tap water]]).<ref name=habkos/><ref name=fix/><ref name=pre/>

It is generally recognized that polyphenols contribute to the overall mouthfeel of beer, in particular to fullness.<ref name=jasgoi/> This aspect of phenolic compounds is not well-studied in beer.

One interesting note is that gallic acid (a phenolic acid) is known to induce a distinct and long-lasting sweet aftertaste, providing a pleasant tasting experience.<ref>https://patents.google.com/patent/US20020068123A1/en</ref> Some studies show that gallic acid is the predominant phenolic acid in beer, whereas other studies show it to be entirely absent. These polarized results could mean that gallic acid is rather fragile (susceptible to destruction by oxidation),<ref name=kikhis>Kikuzaki H, Hisamoto M, Hirose K, Akiyama K, Taniguchi H. [https://pubs.acs.org/doi/abs/10.1021/jf011348w Antioxidant properties of ferulic acid and its related compounds.] ''J Agric Food Chem.'' 2002;50(7):2161–2168.</ref> and its presence may be part of the reason why unoxidized beer tastes so fresh and pleasant.

=== Bitterness, harshness, sourness, and astringency ===
Phenolic compounds (especially flavonol glycosides, flavanols, and phenolic acid ethyl esters) bind to proteins in the mouth, imparting an [[astringency|astringent]] sensation and flavors of bitterness, harshness, bitter-sweetness, and sourness.<ref name=wangas/><ref name=kunze/><ref name=bsp/><ref name=habkos>Habschied K, Košir IJ, Krstanović V, Kumrić G, Mastanjević K. [https://www.mdpi.com/2306-5710/7/2/38 Beer polyphenols—bitterness, astringency, and off-flavors.] ''Beverages.'' 2021;7(2):38.</ref><ref name=aroshe/><ref name=dadbel/><ref name=mikhra/> [[oxidation|Oxidized]] polyphenols in beer generally have lower flavor thresholds as compared to their precursors, making them more potent.<ref name=dadbel/> In particular, harshness, bitterness, and astringency are generally more pronounced in oxidized samples, and oxidation may produce other undesirable off-flavors as well, contributing to aged character (see table below).<ref name=dadbel/><ref name=fix/><ref name=habkos/><ref name=calcol/><ref name=pre>Prechtl C. [https://www.mbaa.com/publications/tq/tqPastIssues/1967/Abstracts/tq67ab16.htm Some practical observations concerning grain bitterness in beers and its amelioration.] ''Tech Q Master Brew Assoc Am.'' 1967;4(1):98–103.</ref><ref name=mcmmad>McMurrough I, Madigan D, Kelly RJ, Smyth MR. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-54-0141 The role of flavanoid polyphenols in beer stability.] ''J Am Soc Brew Chem.'' 1996;54(3):141–148.</ref> On the other hand, non-oxidized (reduced) polyphenols are associated with beer freshness.<ref name=dadbel>Dadic M, Belleau G. [https://www.tandfonline.com/doi/abs/10.1080/00960845.1973.12006011?journalCode=ujbc19 Polyphenols and beer flavor.] ''Proceedings of the Annual meeting of the American Society of Brewing Chemists.'' 1973;31(1):107–114.</ref> In malt-forward beers, the term ''herbstoffe'' is sometimes used to describe the harsh grain bitterness present when phenolic compounds become oxidized, which is a different flavor than hop α-acid bitterness.<ref name=fix/><ref name=pre/> Heavily dry hopped beers can cause a more extreme form of this sensation, commonly referred to as ''hop burn''.<ref name=nag>Naglich M. [https://vinepair.com/articles/hop-burn-homebrewing-hazy-ipas/ How to avoid ‘hop burn’ when homebrewing hazy IPAs.] VinePair website. 2020. Accessed May 6, 2022.</ref> It is characterized by a harsh astringency upon swallowing the beer, due to the high amount of phenolic compounds in suspended particles,<ref name=cargui/> and it is frequently exacerbated by dry hopping methods that promote oxidation. The occurrence of all these off-flavors from phenolic compounds is avoidable by holistic [[low oxygen brewing|brewing practices designed to minimize oxidation]].

While it's clear that off-flavors and astringency resulting from oxidized phenolic compounds are problematic and are very likely responsible for the "harsh" notes that many conventional high-oxygen brewers detect in their young beer, not all hope is lost for those beers. Oxidation increases the polymerization of certain polyphenols, and the resulting large compounds will eventually precipitate.<ref name=calcol/><ref name=wangas/> Consequently, the harsh flavors resulting from oxidation generally tend to improve while aging over a period of days to months, giving the offending compounds time to precipitate and settle or degrade.<ref name=mikhra/><ref name=nag/><ref name=wangas/><ref name=cheyni/> However, aging the beer to remove these off-flavors is not ideal, especially since other types of [[flavor stability|flavor deterioration]] will occur during that time. Therefore, [[low oxygen brewing]] can be a solution for brewers looking to produce beer without this harsh, bitter, astringent character, and/or produce beer that is drinkable much more quickly.

{| class=wikitable
|+ Potential flavors of phenolic compounds<ref name=dadbel/>
! Major flavors
! colspan=2 | Minor flavors
|- style="vertical-align:top;"
|
* Bitter †
* Harsh †
* Astringent †
* Sour-acidic *
* Bitter-sweet
* Dry †
* Aromatic *
* Vanilla *
|
* Malty
* Winey
* Sweet
* Medicinal
* Cinnamon
* Hoppy *
* Hot *
* Green *
* Fruity *
* Mellow *
|
* Metallic †
* Papery †
* Earthy †
* Vinegar †
* Diacetyl †
* Perfume †
* Peppery †
* Woody †
|}
&ast; decreases when oxidized<br />
† increases when oxidized

=== Phenolic flavors (volatile phenols) ===
Phenolic acids (such as [[ferulic acid]]) are always present in beer, but they typically have no direct flavor impact since their concentrations are well below flavor threshold.<ref name=wangas/><ref name=vanvan/><ref name=len/><ref name=habkos/><ref name=calcol/> However, they act as flavor precursors because they can be modified during [[brewing]] and/or [[fermentation]] to create much more potent flavor compounds, called volatile phenols.<ref name=wangas/><ref name=vanvan/><ref name=len/><ref name=vandel/><ref name=szwpie/> The aroma and flavor profile contributed by volatile phenols in a beer is influenced by not just by the concentrations of individual phenolic compounds, but also the relative amounts, the total concentration of phenolics, their interactions with other compounds, and variation among individuals for sensitivity to these compounds.<ref name=len/> The resulting flavors may be desirable, or they may be considered off-flavors depending on the beer style. Styles that intentionally feature volatile phenolic flavors include [[weissbier]], [[rauchbier]], [[saison]], and various Belgian beers such as [[witbier]], [[belgian tripel|tripel]], and [[traditional sour beer]]s.<ref name=vanvan/><ref name=len/><ref name=habkos/> In these styles, volatile phenols often impart pleasant spicy, clove-like, sweet, and vanilla-like flavor notes, although, at higher concentrations they may be also add unpleasant medicinal flavor notes.<ref name=wangas/><ref name=len/><ref name=habkos/><ref name=coljer/> The "black pepper" character often present in certain styles is considered a phenolic flavor, however the compounds responsible have not been identified.<ref name=len/> Depending on the yeast(s), other volatile phenols can be produced that contribute "funky" flavors such as barnyard, horsey, leathery, and smoky.<ref name=len/> ''[[Brettanomyces]]'' yeast is notable for creating these funky flavors.<ref name=coljer/> Since volatile phenols are mainly formed during fermentation, the choice of an appropriate [[yeast]] strain is the primary way to control the final volatile phenol character in beer.<ref name=len/><ref name=vanvan/><ref name=habkos/> Among yeast varieties, the so-called "phenolic off-flavor"-positive (POF+) strains are responsible for creating the volatile phenols.<ref name=len/><ref name=langra>Langos D, Granvogl M. [https://pubs.acs.org/doi/abs/10.1021/acs.jafc.5b05606 Studies on the simultaneous formation of aroma-active and toxicologically relevant vinyl aromatics from free phenolic acids during wheat beer brewing.] ''J Agric Food Chem.'' 2016;64(11):2325–2332.</ref> A "ferulic acid rest" during [[mashing]] may be useful to increase the level of the precursor if a high clove flavor (4-vinylguaiacol) level is desired.<ref name=len/> Apart from the desired phenolic aroma-active substances, the toxic compound [[styrene]] is formed via the same pathway.<ref name=wangas/><ref name=schwarz/>

Phenolic flavors can become undesirable if the level becomes too high, or even if the character could be considered pleasant (spice, clove). This could occur due to inappropriate yeast strain choice (inadvertently choosing a POF+ strain instead of a typical brewing strain), or fermentation at a temperature above the recommended range.<ref name=len/> In some cases, strong and offensive medicinal, plastic strip (Band-Aid®), goaty, burnt, or creosote (tar) character can be present, among others.<ref name=habkos/> For beers that are dominated by offensive or unexpected phenolic character, the most likely source of volatile phenols is contaminating [[Contamination|spoilage organisms]].<ref name=len/>

Aside from microbial/fermentation issues, phenolic compounds are transformed during beer storage/aging and the products are potential contributors to stale flavor — even in beers that did not start with significant phenolic flavors.<ref name=calcol/><ref name=len/> In particular, volatile phenols (4-vinylsyringol) may be at least partly responsible for the characteristic "old beer" aroma.<ref name=coljer/><ref name=caldas>Callemien D, Dasnoy S, Collin S. [https://pubs.acs.org/doi/abs/10.1021/jf051772n Identification of a stale-beer-like odorant in extracts of naturally aged beer.] ''J Agric Food Chem.'' 2006;54(4):1409–1413.</ref> Lastly, beers with a desirable clove/spice (4-vinylguaiacol) flavor may lose this character over time in favor of a sweeter taste (vanillin and/or apocynol).<ref name=len/><ref name=vansai/><ref name=habkos/><ref name=coljer/> This degradation occurs faster with higher storage temperature, with the presence of oxygen, and at lower pH (under 4.6).<ref name=vansai/><ref name=walhea/><ref name=wangas/>

{| class=wikitable
|+Flavors of common volatile phenols<ref name=len/><ref name=calcol/>
! Volatile phenol
! Flavor description
! Notes
|-
| 4-vinylguaiacol (4VG) || Low level: pleasant clove/spice flavor<br /> High level: pharmaceutical off-flavor || Formed during boiling or fermentation. Derivative of ferulic acid.
|-
| 4-vinylphenol || Phenolic, medicinal, bitter || Always considered to be an off-flavor. Derivative of p-coumaric acid.
|-
| Guaiacol || Smoky || Formed during beer aging by degradation of 4VG. Major contributor to the "smoke" character in smoked malt.
|-
| 4-vinylsyringol || "Old-beer-like" || Very low taste threshold. It is released during aging. Derivative of sinapic acid.
|-
| Vanillin || Sweet, vanilla || Oxidation product of 4VG, which may change the flavor profiles of phenolic beers during aging.
|-
| 4-ethylguaiacol || Clove, phenolic, spice, woody, smoky, vanilla || Degradation product of 4VG during beer aging or yeast metabolism.
|}

Besides beer, volatile phenols have been reported to contribute to the aroma of wine, sherry, and whisky as well as nonalcoholic beverages such as fruit juices and coffee.<ref name=vanvan>Vanbeneden N, Van Roey T, Willems F, Delvaux F, Delvaux FR. [https://www.sciencedirect.com/science/article/abs/pii/S0308814608003348 Release of phenolic flavor precursors during wort production: Influence of process parameters and grist composition on ferulic acid release during brewing.] ''Food Chem.'' 2008;111(1):83–91.</ref><ref name=vansai>Vanbeneden N, Saison D, Delvaux F, Delvaux FR. [https://pubs.acs.org/doi/abs/10.1021/jf8019453 Decrease of 4-vinylguaiacol during beer aging and formation of apocynol and vanillin in beer.] ''J Agric Food Chem.'' 2008;56(24):11983–11988.</ref> In wine, both vinylphenols (4VP and 4VG) and their ethyl analogues (4-ethylphenol and 4-ethylguaiacol) have been detected and are important contributors to aroma.<ref name=vansai/>

== Effect on haze ==
Phenolic compounds are directly involved in the formation of beer [[haze]], typically by binding to [[protein]]s as a result of [[oxidation]]. See [[Haze]] for more information.

== Effect on foam ==
Some studies have found a correlation between total phenolic content and [[foam]] stability in beer.<ref name=wangas/><ref>Jurić A, Ćorić N, Odak A, Herceg Z, Tišma M. [https://onlinelibrary.wiley.com/doi/full/10.1002/jib.254 Analysis of total polyphenols, bitterness and haze in pale and dark lager beers produced under different mashing and boiling conditions.] ''J Inst Brew.'' 2015;121(4):541–547.</ref><ref name=karhan/> However, it appears that this is not a direct result of these compounds creating or stabilizing foam, but rather some other mechanism.<ref name=mikhra/><ref name=defbra/><ref name=hablon>Habschied K, Lončarić A, Mastanjević K. [https://www.mdpi.com/2304-8158/9/2/238 Screening of polyphenols and antioxidative activity in industrial beers.] ''Foods.'' 2020;9(2):238.</ref><ref name=jasgoi/> One possibility is that phenolic compounds help indirectly by preventing oxidation of [[lipids]] (oxidized lipids are potent foam-negative compounds). See [[Foam]] for more information.

== Effect on human health ==
Phenolic compounds show a wide spectrum of beneficial health-related properties. See [[Health and safety]].

== See also ==
*[[Ferulic acid]]
*[[Tannin additives]]
*[[Haze]]
*[[Oxidation]]

==References==
Note: When looking at studies, keep in mind that the Folin-Ciocalteu method measures constituents other than phenolics, and its specificity is poor.<ref name=ziekoz/><ref>Khoddami A, Wilkes MA, Roberts TH. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6270361/ Techniques for analysis of plant phenolic compounds.] ''Molecules.'' 2013;18(2):2328–2375.</ref><ref name=gankub/><ref name=cheyni/>