by Steve Alexander (Brewing Techniques)
The dreaded precursors to chill haze and contributors of off-flavors and astringency, common phenols also play many beneficial roles in keeping beer fresh, colorful, and balanced. This article explores the simple tricks needed to control these complex compounds.
Most brewers regard, phenolic compounds with strong suspicion. Tannins are the most widely known class or phenols and are usually avoided as potential sources of bitterness, astringency, and haze. Other negative phenolic effects such as medicinal flavors and infection by-products also contribute to the fear and loathing. Yet phenolic compounds play positive roles in brewing that aren’t widely appreciated. In small concentrations, phenols can contribute subtle, but important, flavor characteristics to beer. These highly reactive compounds can also act as antioxidants that prevent other, less desirable reactions (such as staling).
Understanding these varied contributions and the means of controlling them is an important step in quality brewing. This article reviews the chemistry of phenolic compounds, describes their various contributions to finished beer, and provides guidelines for successfully managing both ingredients and brewing processes to help ensure that you get the beer you want from your efforts.
It’s easy to imagine that phenolic compounds and their chemical reactions are abstract and unimportant to everyday life. In fact, the impact of their presence is remarkably common. Phenols are organic compounds based on the common elements carbon, hydrogen, and oxygen. These simple carbon rings are called aromatics because many of the compounds they form have very strong odors. The rings are quite reactive, joining readily with other compounds, and thus they are commonly found in nature.
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Plant tissue is the major source of natural phenolic compounds and the only source relevant to brewing. These compounds play several roles in plants. For one, the phenolic compound lignin stiffens the cellulose walls in higher plants, and these cellulose walls in turn act as structural elements. Lignin is present in the woody tissues in hop bines and barley straw, for example, and to a lesser degree in barley seed husks and hop cones.
Some phenolic compounds also act as plant colorants, providing red, blue, and purple coloration. Yet other phenolic compounds have antimicrobial and antifungal activity, and cereal plants such as barley even increase their phenolic content in reaction to attack or tissue damage. Bitter phenolic compounds provide a defense mechanism in plants, rendering them inedible by herbivores. Some evidence even suggests that phenolic compounds act like animal hormones, regulating development and controlling germination and growth of the plant. Phenols aren’t all good, however; depending on what they’re bonded to, they can be carcinogenic.
Researchers have identified 67 basic phenols and several hundred phenol complexes in beer. The vast majority of these phenols can be classed as polyphenols, complex chains of simpler phenol building blocks. These large-molecule phenolics can contribute smoky, harsh, and sour flavors to a beer, but their primary effect is astringency and bitterness (and not the desirable “clean” bitterness added by the phenolic compounds in hop resins). Lignins and tannins are well-known examples of polyphenol compounds that can drastically alter the character of your beer if you’re not careful, affecting not only the beer’s flavor bur its clarity and even its shelf life.
The remaining 10–20% of phenolic compounds can be grouped as monophenols and flavanoids. Monophenols, despite their simple-sounding name, do not exist freely in plants, but instead often bind to particular sugars; ferulic acid, for example, can form compounds that give estery, clovey notes to Weizens. Monophenols contribute a larger range of flavors, which in small quantities can contribute subtly to a beer’s flavor balance.
Flavanoids, though also referred to as biphenols, are actually thought to form from a single monophenol by a series of complex reactions. Flavanols and catechins are examples of flavanoids that play a role in brewing. Like the monophenols, these molecules generally bind ro sugars, but these flavanoid–sugar bonds (ether bonds) break under very different conditions in the brewing process.
Though phenolic compounds can all contribute directly in both positive and negative ways to a beer’s flavor profile, it is their role in the brewing process, actively participating in oxidation and reduction reactions, that makes them even more interesting. To understand these processes, we need to step back for a basic chemistry lesson.
The phenol building blocks: The building block of phenolic compounds is a six-sided ring of carbon atoms with at least one attached oxygen and hydrogen molecule (called a hydroxyl group). As mentioned above, this simple six-sided ring is also known as an aromatic ring because many simple chemicals with this ring structure have strong aromas. The aromatic ring in monophenols and nearly all aromatics in live tissue come primarily from a single metabolic mechanism called the “Shikimic pathway.” Some of the simple hydrogens bonded to the carbon ring can be replaced with any of a complex set of alternative compounds to form a huge number of different phenolic compounds.
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Monophenols: The simplest monophenol is known as benzenol (because of its benzene-like ring) or carbolic acid and has the chemical formula C6H5OH. Monophenols with a single carbon atom added to one or more of the ring carbons are commonly known as benzoic acids. These compounds are considered to have what’s called a “C6–C1” structure, and have a –COOH, or carboxyl group, attached. Benzoic acids are common in plants (including barley). C6–C2 phenols are rare, but C6–C3 phenols are common and appear in two distinct forms: Those with a straight carbon chain are called cinnamic acids, and those that form a secondary ring containing oxygen are called coumarins.
Monophenols are not free in plants; they are often bound to sugars, organic acids, or to other larger phenols such as lignin. Specific monophenols may bind to particular sugars, such as ferulic acid, which is often found attached to the sugars arabinose or xylose. The sugar molecules generally are not free either, but are part of the cellulose structure of a cell wall or part of a starch molecule. Monophenol-to-monophenol bonds are progressively broken as the pH increases above 5 or so. But the majority of monophenols in beer are bound to sugars by ester bonds, and will degrade in alkaline (pH >7) conditions rather than during the typically acidic mash.
Biphenols (flavanoids): Flavanoids appear as two C6 phenolic rings connected by three carbon atoms forming a C6–C3–C6 structure. The flavanoids are not made from two monophenol molecules; instead it is thought that a single monophenol and a complex set of reactions create the biphenolic structures. The flavanoids can be classified into many subclasses based on the structure of the central C3 unit; among these are catechins, chalcones, flavanones, leucoanthocyanidins, flavones, anthocyanidins, aurones, flavanonols, and flavanols. These classes can be further broken down to many individual chemicals. As you can see, the world of flavanoids is vast; this article, however, discusses only the classes and flavanoids important to brewing.
Most of the flavanoid classes are found attached to sugar molecules rather than in a free state. The flavanoids bind differently to sugars (ether-type bonds) than do the monophenols. The flavanoid–sugar bonds are of varying strength and can be broken under acidic (low pH) conditions or other circumstances normally encountered in the brewing process. Two important subclasses of the flavanoids, the catechins and the leucoanthocyanidins, are not bound to sugar but are instead either free or bound to other like molecules.
It’s interesting to note that one of the flavanoid subclasses, anthocyanidin, has a remarkable color changing property. It progresses from red to blue as the acidity level changes from acidic to basic. Under brewing conditions these molecules will appear red and contribute a reddish hue to beer. Leucoanthocyanidins and some combinations of leucoanthocyanidins and catechins are also known as anthocyanogens or proanthocyanid because under acidic conditions they assume the structure of anthocyanidin. These pale-colored chemicals become red or blue, depending on the pH.
Polyphenols: These repeating structures, or polymers, are formed from multiple monophenol or flavanoid molecules. The process is similar to the formation of starch from glucose or to the formation of protein from amino acids. Unlike starch and protein, however, polyphenols or phenol polymers usually become larger throughout the brewing process rather than smaller.
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Lignins and tannins. Lignins and tannins represent two broad classes of polyphenols. Lignins are polymers of monophenols that form an irregular molecular structure. Tannins are less easily defined. “Tannin” literally refers to a plant substance that has the ability to tan leather by cross-linking proteins, but the term has an extended meaning in biochemistry and brewing. Tannins, or more properly tannoids, can be defined as polyphenols that consist of 4–25 aromatic rings capable of forming bonds between protein molecules — whether they can truly tan leather or not.
The ability of tannins to bind with protein is of significant interest to brewers because of the direct implications for the brewing process. Tannins can be divided into hydrolyzable and condensed tannins. Hydrolyzable tannins are polyphenols that can be easily reduced to simple phenols such as gallic acid bound to sugars. These tannins in beer come primarily from hops, and most are removed with the break material. Condensed tannins, on the other hand, are quite resistant to degradation. These tannins are polymers of flavanoids, especially the flavanoid subclasses catechin and leucoanthocyanidin. They first form a weak bond with proteins, but can later form a stronger bond. Polyphenols increase in their protein-bonding ability as the molecule becomes larger (though beyond a certain size their solubility and ability to bond to protein starts to decrease). The tannoid tendency to bind to protein with increasing tenacity explains the presence of tannoids in both the break material and haze.
With that background in hand, let’s talk beer. In most beers, roughly 75% of total beer phenolics come from malt, and the remaining 25% come from hops. Of all the phenolic compounds in beer, about 10–20% are monophenols and flavanoids, while the majority are polyphenols.
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Malt: Malt husks contain lignins and some attached phenolics, while the pericarp and aleurone tissues just beneath the husk contain phenolics combined with the cellulose and pentose sugars that make up the cell walls. Together the husk and the tissue layers just beneath the husk contain the majority of all malt phenolics. The true husk of barley is water impermeable, so the extent of extraction of phenolic compounds is dependent upon the grind; excessive shredding of the husk may lead to greater phenolic extraction.
Specialty malts can add even more phenols to beer. Malts smoked with hardwood or peat fires absorb many of the burning lignin phenolics. The high temperature of the burning peat or hardwood can also chemically transform some of the malt’s simple lignin phenolics into compounds that do not normally occur in plants. Similar phenolic products are present in highly roasted malts and barley.
Hops: Hops have about 50 times the levels of total phenolics as malt, but are of course used in much smaller quantities and so contribute fewer phenolics overall. Hop flowers are rich in phenolic materials, found primarily in combination with sugar molecules. The hop resin humulone (the chemical in alpha-acids) has a phenolic structure before it becomes isomerized in the boil. A significant amount of phenolic hop material is thus released into the beer, but as we see in the discussion to follow, a corresponding amount is left behind in the hot and cold break material.
Yeast: Although minor amounts of phenolics are consumed or absorbed by yeast, the impact of yeast on the amounts of phenolics in beer is quite small. Some yeast varieties can, however, dramatically transform certain phenolic compounds.
Other ingredients: Some secondary beer ingredients can add to total beer phenolics. Spices used in some specialty beers contain phenolic compounds. The orange peel (both sweet and curaçao) used in Belgian wit beers, for example, contains considerable amounts of bitter flavanoids. Oak fermentation vessels can also leach phenolics into beer (lignins and tannins are particularly ubiquitous in oak species).
The many moods of phenolic compounds can alter the character of your beer in several ways. As aromatic compounds, phenols can add flavors (not all of which are desirable). Their coloring tendencies can alter a beer’s color. And as highly reactive compounds, phenols can cloud a beer’s clarity by bonding with proteins or can result in oxidation, affecting both flavor and shelf life.
Flavoring effects: Monophenols. Monophenols have many distinct flavors, but they can be broadly categorized. Some have aromatic or sweet aromas, like vanilla or camphor or cloves. Others have medicinal flavors and aromas, like the dreaded chlorinated phenols. Many have smoky aromas that may either be pleasant or rough and harsh. In sufficiently large quantities, monophenols may also taste bitter.
The flavors of vanillic, ferulic, and coumaric acids and the other major monophenols are too dilute in beer to be individually perceptible, but in excess their combined effect may be undesirable. Conversely, the removal of monophenols from beer has been shown to give the beer an “unbalanced” flavor profile.
In one study, a “cocktail” of nine monophenols found in lager beer was proportionately added to a 5% alcohol/water mixture. The monophenols contributed no noticeable off-flavor until levels reached 15 times their concentration normally found in beer, at which point they became noticeably bitter, with flavors of quinine, vanilla, cardboard, and salt. When a “cocktail” of monophenols in a similar concentration was added to beer, the predominant flavor addition was perceived as astringent, acidic, and bitter.
The experiment demonstrates a basic tenet of food chemistry and flavor science: Flavors are not simply additive. The normal levels of beer monophenols are not detectable in isolation, but can add a balanced quality to beer. In higher concentrations these monophenols add different flavors to beer than to an alcohol/water mixture. Polyphenolics and flavanoids likewise are important to “background” beer flavor, and beer stripped of these more complex phenols is said to taste dull and insipid. And like the monophenols in the experiment, an excess of polyphenol is judged to add bitter and astringent flavors.
Two monophenols in particular, the cinnamic acids ferulic and coumaric, play a special role in beer flavor. These C6–C3 molecules can be transformed into a C6–C2 structure by removing the –COOH, or carboxyl, molecular group. Ferulic acid becomes 4-vinyl guaiacol, the compound that lends Weizenbier its desirable spicy, clovelike aroma. Coumaric, on the other hand, is transformed into 4-vinyl phenol, which can impart a harsh, smoky aroma. Though ferulic acid is detectable only at a high concentration (about 660 mg/L), 4-vinyl guaiacol is about 2,000 times more flavorful and is detectable at only 0.3 mg/L. Similarly, 4-vinyl phenol is much more flavorful than coumaric acid.
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The carboxyl group can be removed from these cinnamic acids by heat, such as the boil, and by acidity, though the development of these flavorful phenols is more commonly traced to the action of some yeasts and beer infection organisms. Some ale yeasts — and not just Weizen yeasts — are capable of performing this decarboxylation and are designated as genetically Pof+, or “phenol off-flavor positive.” Lager yeasts, on the other hand, appear to be Pof–, or “phenol off-flavor negative”.* Wild yeasts appear to be particularly likely to decarboxylate phenols, and it’s quite likely that the complex flavors of young lambic beers fermented with wild yeasts are due in part to various decarboxylated phenolics. The 4-vinyl guaiacol and 4-vinyl phenol are unstable in beer and slowly, over a period of 14–42 months, will degrade into less flavorful guaiacol compounds.
*In a study of 59 wild and brewer’s yeasts, all 12 lager yeasts tested were Pof–, 15 of 19 (nonwheat beer) ale yeasts were Pof–, and the remaining four ale yeasts and four wheat beer yeasts were Pof+. Only one of the wild organisms was Pof–, while many were extremely efficient 4-vinyl guaiacol producers.
Biphenols and polyphenols. Biphenols (flavanoids) and polyphenols are present at much higher levels than the monophenols, but the range of flavors they present is smaller. The larger phenolics may have smoky, harsh, and sour flavors but are primarily known for theit undesirable bitterness and astringency, the dry sensation experienced when the proteins of the mouth are “tanned” by tannoid compounds.
Oxidation effects: Phenols, like many beer component chemicals, can be oxidized, generally contributing a negative impact on the flavor and shelf life of the beer. Oxidation means that a bound electron has been lost from a compound, leaving a positive charge. Reduction is the corresponding process of gaining a negative charge by picking up that electron — thus the phrase “redox” reaction. Note that despite the name, oxygen is not required for oxidation to take place, though oxygen is often involved. (The topic of oxidation and its effects on beer flavor is too complex to fully cover here; interested readers should refer to George Fix’s book, Principles of Brewing Science, for a more in-depth treatment.)
Phenols are responsible for many common examples of oxidation. For example, a fresh cup of tea is often described as “brisk” or fresh, but the same cup of tea left for a few hours will have a definite stale taste. This change is primarily due to the oxidation of tea polyphenols. Another example occurs in apples; a fresh apple browns quickly once it is cut and exposed to air. The browning may be slightly delayed by the action of natural antioxidants such as Vitamin C (ascorbic acid), but shortly the phenols oxidize and polymerize into brown compounds. The brown, oxidized apple clearly loses flavor appeal.
In beer, phenolic compounds can oxidize in the mash as the result of certain phenol–oxidase enzyme reactions (see diagram on next page). Phenolics can also oxidize in wort and beer from the presence of oxygen or other oxidized substances. This oxidized state gives many phenolics even stronger bitter and astringent qualities. Oxidized phenolics are also likely to polymerize and form larger, more astringent polyphenols with stronger tanning ability.
The oxidation of catechol shows why oxygen is often involved in redox reactions. Oxidation usually refers to the loss of hydrogen atoms, leaving the molecule with a positive charge. In this reaction, two hydrogen molecules are removed from catechol (a flavanoid) to form a water molecule with an atom of free oxygen. Though unoxidized phenols may form weak, unstable bonds with protein, only oxidized phenols can form permanent covalent bonds.
Phenols as antioxidants. Antioxidants are substances which can scavenge oxygen and reduce other compounds by accepting a more oxidized state themselves. The food additives BHA and BHT, for example, are actually powerful artificial phenolic antioxidants added to preserve freshness. Some unoxidized phenols in beer, notably flavanols and catechins, can actively protect flavor in this way. Although these oxidized phenols may themselves add astringency and contribute to haze, they are still less detrimental to beer than the soapy, goaty flavors of oxidized fatty acids, or the stale flavors of aldehydes. On the other hand, certain antioxidant phenols, such as ferulic acid, can also, unfortunately, pass their oxidized state along to other beer components and actually promote staling. So, phenols in general play an important role in beer to minimize the damage caused by oxidation — but this protection is limited.
Wine provides a good example of phenolics’ antioxidant effects. Wine, particularly red wine, contains large quantities of unoxidized polyphenols extracted from grape skins — so much so that young red wines are often quite astringent. Additional phenolics are added when wine is stored in oak barrels. Once in the sealed bottle, however, the complex phenols slowly oxidize, which allows many of the wine’s other flavor compounds to remain in a reduced fresh state. These oxidized polyphenols form very large polyphenol molecules over a period of years which then drop out of solution, leaving behind fresh, nonastringent, mature wine.
Clarity: Phenolic tannoids can affect a beer’s appearance by contributing to haze. During the wort boil, much of the oxidized polyphenols from both malt and hops form complexes with the large wort proteins. These protein–phenol complexes typically remain behind as part of the break material. Some oxidized polyphenols from late hop additions can survive into the wort, but primarily the remaining wort polyphenols are unoxidized. As these phenolics later become oxidized, they form larger polyphenols with protein-bonding ability. Early in the fermentation these tannoids must compete with sugar molecules for binding sites on the protein, but as the sugars are later fermented away the proteins bind with the developing tannoids. The initial chemical bond is weak, and the resulting haze may be temporary (chill haze), but strong permanent bonds can form later to create a haze that can only be tempered by removal.
Color: Finally, although most beer color comes from carbohydrates and melanoidins in highly kilned roasted or caramel malts, certain biphenols — anthocyanidins, leucoanthocyanidins, and procyanidins — can impart some red coloration in an acidic environment such as beer. Certain polyphenols can also play a role in browning reactions and melanoidin formation in beers, contributing indirectly to beer color.
As this discussion illustrates, phenolics ate ubiquitous and unruly. They can be both desirable and abhorrent in beer, and their management can be tricky. One thing to keep in mind when deciding how to try to control them is that, with few exceptions, the levels and types of phenolics present do not change dramatically from sweet wort, to boiled wort, and even to final beer. Thus, the first line of attack is to limit the amount of phenolics extracted during the mash.
In the mash: The grind. The first line of defense is the proper milling of the grain. The phenolic compounds are contained in the grain husks, and thus overcrushing and shredding the husk can lead to increased phenol extraction. Once the husks are exposed, the phenolics may be acted upon through a number of factors. Phenolics can be freed from the plant matter by acidity, alkalinity, enzymatic activity, or by water action, areas over which brewers thankfully have some control.
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pH and temperature. Controlling both pH and temperature during the mash can help minimize phenol extraction from the malt. The bonds that hold monophenols to cellulose and other monophenols are progressively broken as pH rises above the normal mash range. The bonds from flavanoids and their polyphenols to sugar are more easily broken under acidic conditions. The pH of live plant material ranges from 3.5 to 5.5, however, which overlaps the typical mash pH range, so the bound phenolics are not likely to be dramatically freed during the mash because of pH alone. Some of the bonds that hold flavanoids to the husk may be released, however, with a combination of high temperatures and low pH. Keeping the mash in the traditional range of pH (5.1–5.6) and limiting mash-out temperature to below 170 °F (77 °C) will therefore help reduce phenolic extraction.
The same restrictions would also apply to the sparge water. Higher, more alkaline pH values will result in excessive extraction of monophenols, which are easily oxidized. Lower, more-acidic pH values will extract more flavanoids and polyphenols.
Enzymatic activity. Though monophenols typically are stable during the mash, a few enzymes that occur naturally in malt are known to release various cinnamic acids from their bonds with sugar molecules. (Note that bound sugars are not fermentable, but the amount of sugar lost to the phenolics is negligible.) One malt enzyme, for example, can release very large amounts of ferulic acid at a mash temperature of 113 °F (45 °C); some of this acid can be converted to 4-vinyl guaiacol during the boil, possibly resulting in spicy flavors even before the yeast get to it. As mentioned previously, these flavors are desirable in wheat beers but are not necessarily wanted in other styles, and controlling mash conditions can control the flavor addition.
Water solubility. Water alone is a relatively poor solvent for many phenolic compounds. When compared with organic laboratory solvents such as methanol and acetone, water often leeches only 10–20% of free phenolics from the malt grist. It has been demonstrated, however, that water-to-grist ratios above 3:1 (that is, above 1.5 qt/lb or 3 L/kg) can result in a greater amount of extracted phenolics, so limiting the total quantity of mash water also reduces phenolic extraction.
Length of sparge. Oxidized polyphenols (tannoids) are extracted from the mash first, and the less soluble oxidizable polyphenols (haze precursors) are extracted later. Stopping the sparge early can thus reduce the amount of haze-forming polyphenols extracted. (The other tannoids will likely drop out in the boil.)
Removing phenols at mash time. A number of haze reduction methods are based on creating oxidized polyphenols (tannoids) from unoxidized polyphenols during the mash so that they can be removed from the wort boil as hot break. Many of these methods require the introduction of oxidizing agents such as air or hydrogen peroxide in the mash. Based on current knowledge of the negative effects of oxidized compounds in beer flavor, however, these methods are not recommended.
In the boil: Because the majority of tannoids are complexed with protein and will flocculate out as hot and cold break, a full-length rolling boil is recommended as is, obviously, the separation of the break material from the wort.
Phenol extraction from hops. The extraction of hop phenolics is largely out of the control of the brewer, except that the later hop additions may not boil long enough to form tannoid–protein break matter and these tannoids may then pass into the beer and result in haze. One might speculate that using the freshest possible hops for these late additions might be helpful, but hops are relatively rich in oxidized polyphenols so this may be wasted effort. Various hop extract products are entirely free of normal phenolics, but the quality of their flavor and aroma makes these a questionable choice as a late hop addition replacement.
During fermentation: After the boil and break removal, the majority of polyphenolics remaining are unoxidized, and the yeast do a nice job of removing free oxygen quickly and generally act to reduce rather than oxidize the wort. The phenolics should then be relatively stable. Once fermentation ceases, however, unoxidized phenolics can proceed to oxidize, polymerize, and form tannoids if oxidizing agents are present or added to the beer. These oxidizing agents include melanoidins and fatty acids oxidized during the boil, hop components oxidized by improper hop storage, and oxygen derived from air trapped inside the keg or bottle. These potential oxidizing agents can transfer their oxidized state to the phenolics, which can then develop into more astringent and haze-causing tannoids.
The only way to prevent this problem is to avoid adding oxidizing matter to beer in the first place by using fresh ingredients (that is, fresh hops and freshly crushed malt) and reducing the exposure of wort to oxygen whenever possible, particularly at high temperatures that can promote oxidation.
Removal by fining. Prevention is usually the best medicine, but when all else fails, various fining methods that effectively remove tannoids are readily available to home and professional brewers. The fining agent Polyclar AT (standard form of povidone, formerly known as polyvinylpyrrolidone [PVPP]) acts as an artificial protein, at least to the point of allowing weak and temporary bonds with tannoids. These materials provide an extremely effective means of removing large tannoid molecules that are haze precursors and have astringent flavors. Unfortunately, they will also remove a fraction of the smaller phenolics that could never tan (or cross-link) proteins or cause haze and that might be desirable for balanced flavor. Thus, these techniques should be considered corrective only. Likewise, filtration is an effective cure for haze, but represents a problem cured rather than a problem prevented. Filtration, if carried too far, will have subtle negative flavor effects too. Ironically, hazy beer can also be cleared by adding tannins, which promote additional protein binding and haze material sedimentation.
Phenolic compounds are as numerous and widespread in brewing as they are in every aspect of life. They can have positive, if subtle, flavor effects on beer and freshness-preserving antioxidant qualities as well. Phenolic compounds are also responsible for several of the high-profile beer flavors in a few beer styles such as Weizenbier, wit, and Rauchbier. The damaging effects of phenolic compounds, including astringency and haze, can usually be mitigated by paying attention to the malt crush, purchasing fresh ingredients, avoiding unnecessary aeration, controlling the pH and temperature of the mash and sparge, moderating the total water volume, and by treating the wort to an extensive rolling boil. Polyclar AT is effective at removing astringent flavors from beer, but may also remove antioxidants and phenolics that contribute positive flavor traits. Learning to manage the many positive and negative aspects is the challenge phenolics present to brewing well-balanced beer.
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