Focus On Beer Flavor


by Scott Bickham - (Brewing Techniques)


The Four Basic Tastes


The beer flavor wheel


The first stop on the Beer Flavor Wheel takes us to the four basic tastes: bitter, salty, sweet, and sour. We will identify their sources, provide commercial examples, and offer tips on how to train your tongue to identify them.



The human tongue can distinguish as many as 20 distinct tastes. Of that number, four are considered the most likely to be easily identified by untrained tasters: bitter, salty, sweet, and sour/acidic. This quartet is depicted by the shaded section on the beer flavor wheel above (corresponding descriptions and reference standards are listed in Table I). This installment of Focus on Flavor describes the origins of these four tastes in the brewing process and suggests corrective actions you might consider when these flavors appear out of style. Each section includes “doctoring tips” that enable you to directly experience the flavor yourself.

Continue your home brewing journey with our beer recipes and homebrewing kits!


The Basics


Sweetness is a function of low concentrations of inorganic salts, sugars, and amino acids and is primarily perceived at the tip of the tongue. Other compounds, such as dihydrochacones, may also evoke sweet sensations at the back of the mouth.

Like sweetness, bitterness can be detected in both the front and back of the tongue, but in this case the back of the tongue is dominant. The primary bitter taste of hop alpha-acids, magnesium sulfate, and phenolics is detected on the rear of the tongue, while bitterness brought about by hydrophobic amino acids and alkaloids is perceived on the front of the tongue. The sensitivity of the taste buds on the rear of the tongue to hop bitter principles explains why beer tasters must swallow beer to properly evaluate its bitterness.

Salty flavors stem from high concentrations of inorganic ions and are perceived on the sides of the tongue, near the front. Sour and acidic flavors are evoked by Bronsted acids (hydrogen donors such as lactic and acetic acid) and are detected slightly farther back along the sides.

This road map of the tongue provides a useful tool for identifying subtle flavors in beer, but keep in mind that all of these tastes may be perceived all across the tongue and may even be coupled with astringency and other mouthfeel sensations. A detailed analysis of each flavor follows.




Let’s begin with bitterness, an unusual characteristic because it often has negative flavor connotations to the general public. Some of us might recall, for example, advertisements for a light lager that describe a “bitter beer face” as something to avoid. Indeed, studies have shown that bitter flavors have negative connotations for many people.* Many brewers still shy away from naming a beer a “bitter” for fear those uneducated in beer styles will refuse to try it. Yet some degree of bitterness is important to the flavors of many beverages, including coffee and some soda drinks (which rely on caffeine as their industries’ standard bittering agent). Hop bitterness, of course, is essential for balancing the residual sweetness in beer. Part of the psychology behind the notion of “acquiring a taste” for beer comes down to learning to overcome our inherent dislike of bitter flavors.

Sources: Most of the “good” bitterness in beer is attributable to hops, but other naturally occurring compounds such as phenolics (mostly from malt), aldehydes (a by-product of fermentation), and certain minerals can affect a beer’s flavor by adding bitterness.

*Sensory measurements involving increasing concentrations of quinidine sulfate and quinine hydrochloride in aqueous solutions demonstrated a strong correlation between bitterness perception and unpleasantness.

Hop isohumulones. Most of the bitterness in beer is provided by isomerized alpha-acids in hops, including isohumulone, iso-cohumulone, iso-adhumulone, trans-isohumulone, and cis-isohumulone. These compounds are actually phenolic compounds whose structures become chemically altered (isomerized) during the boiling process, thus releasing the bitterness.

The levels of the nonisomerized forms of these compounds in raw hops can be measured using analytical instruments available only in laboratories. These analytical methods indicate the hop’s alpha-acid value, which is typically in the 8–12% range for bittering varieties and in the 3–5% range for aroma varieties.

The level of the isomerized alpha-acids in the finished beer can also be measured in a similar manner and is expressed in International Bitterness Units (IBUs, or simply BUs), which is its concentration in milligrams per liter (mg/L) or parts per million (ppm).


Table I: Terminology for the Four Basic Tastes

First Tier

Second Tier


Comments, reference compound



T, aT





Sodium chloride



O, T




O, T

Can occur via staling, oxidized honey



O, T

May also be classified as fruity



O, T

Custard powder, vanillin



O, T




O, T

Clear (golden) syrup



O, T, M

Sickly sweet, cloying



O, T

Pungent aroma, sharp taste, mineral acid



O, T




O, T

Lactic, sour milk

†Perception key: O = Odor, T = Taste, aT = Aftertaste, M = Mouthfeel.


Style guidelines offer a suggested bitterness range that provides targets for reproducing a given style of beer. American light lagers, for example, generally have 10–15 IBUs, pale ales have 30–40 IBUs, and barleywines are sometimes brewed with 100 IBUs or more. These values compare with stimulus and difference thresholds of approximately 10 and 5 IBUs, respectively. This means that at least 10 IBUs are required to produce perceptible hop bitterness (absolute threshold) and that changes in bitterness will have to be at least 5 IBUs to be detected by most most tasters. The most common problem brewers face when it comes to alpha-acid bitterness is simply missing the mark stylistically by using a recipe that provides either too much or too little bitterness.


Definitions of the Various Perception Thresholds

Stimulus (or absolute) threshold: The lowest physical intensity at which a stimulus is perceptible.

Terminal threshold: The physical intensity of a stimulus above which changes are not perceptible.

Difference threshold: The smallest change in a stimulus’s physical intensity that is perceptible.

Recognition threshold: The lowest physical intensity at which a stimulus is correctly identified.


The challenge that brewers face is that no exact method exists for calculating the IBU level in finished beer starting with the alpha-acid level of the hops. Methods of estimating the extraction of hop bittering compounds have been widely discussed in the literature. The box, “Hop Bitterness Calculations,” reviews the basic formula used in recipe design and beer evaluation.

Keep in mind, though, that while IBUs measure the strength of the hop bitterness, they do not indicate the quality of the bitterness. Each hop variety contributes different percentages of the various iso–alpha-acids that combines to give the total bitterness, and each of the isohumulones contributes a different character. The bitterness of isihumulone and iso-adhumulone, for example, has been characterized as rounded, while iso-cohumulone has been described as harsh. So while different hop varieties can provide the same IBU value, the quality of the bitterness can differ substantially, particularly in beers with IBU levels higher than 20.

It should also be pointed out that the level of hop bitterness also decreases as a beer ages. Some of the isohumulones drop out of solution as the yeast precipitates in bottle-conditioned beers, and those that remain may be oxidized into compounds that are less bitter. In this case, the loss in bitterness may be partially compensated by the formation of aldehydes (see the paragraph on aldehydes, below).

Hops — other bittering compounds. Though the isohumulones described above provide most of the bitterness in beer, many other hop principles also contribute to a beer’s overall bitterness. These compounds include beta-acids such as lupulone, colupulone, and adlupulone, which are soft resins that, like the alpha-acids, tend to crystallize into less soluble soft resins as the hops age.

Opinions differ as to the importance of these compounds in the brewing process. The beta-acids in fresh hops are not readily soluble and are thought to be unimportant. It is generally thought that the oxidized forms (lupoxes) impart a harsh bitterness when aged hops are used, the exception being noble hops, which react to aging favorably by acquiring a pleasant hoppiness.

Phenolic compounds. Phenols and polyphenols such as tyrosol and quercetin contribute bitterness that become harsh and astringent as their concentrations increase. These compounds may be produced by enteric bacteria or, more often, by wild yeast (in the case of wild yeast, bitterness is often accompanied by medicinal flavors). These flavors typically indicate a defective beer, though the wild yeast character is desirable in some Belgian ale styles such as Saison.

Aldehydes. Aldehydes such as hexanal, trans-2-hexanal, heptanal, and octanal also have bittering properties. These staling compounds are produced from the oxidation of higher alcohols. Although the higher alcohols are usually present in very low concentrations, the corresponding aldehydes have low taste and aroma thresholds and are often perceived as having papery, vinous, or harsh flavors in addition to a bitter effect.

Mineral ions. Inorganic constituents such as magnesium and sulfate ions also increase the perception of bitterness in beer. These ions are essential for reproducing beers brewed in regions with hard local water, such as British ales from Burton-on-Trent and Dortmunder Exports. The magnesium and sulfate ions tend to impart a mineral-like dryness that accents the hop bitterness. Overhopping or adding large quantities of gypsum (hydrated magnesium sulfate) when brewing with this type of water can generate an undesirable, harsh bitterness. This is a common technical flaw in Czech Pilsener-style beers in which high hopping rates are used without first softening the brewing water.

Commercial examples: It is easy to find commercial examples that exhibit a proper bitter property. Styles characterized by high bitterness levels include IPAs (Sierra Nevada Celebration Ale), American-style barleywines (Pike Brewing’s Old Bawdy), Czech Pilseners (Pilsner Urquell), California commons (Anchor Steam), and Altbiers (Zum Uerige). Dry, foreign-style, and Imperial stouts such as those brewed by Guinness and North Coast (Old Rasputin) are also good examples of beers in which the high hop bitterness is supplemented by bitterness from roasted malts and barley.*

*The kind of bitterness/burnt flavor that comes from roasted malts falls under a separate class on the Beer Flavor Wheel, and will be addressed in an upcoming column.


The Art of Doctoring Beers

Doctoring beers — adding controlled amounts of flavor compounds to a neutral-tasting reference beer (Bud Lite, for example) — is a very practical way of becoming familiar with many of the important flavors in homebrews and craft beers. Tasting seminars and off-flavor workshops are a common feature at many home and commercial craft-brewing conferences and are an important part of any brewing education program. It is also quite easy to organize tasting seminars for your own homebrew club or brewery staff.

Beer Selection

Since compounds are perceived differently in air, water, and beer, it is essential to use beer as the base medium. The reference beer (the beer to be doctored) should be an American light lager with no noticeable defects or off-flavors. Since it is useful to prepare the sample in advance, 12- or 22-oz bottles with pry-off caps are best because they can be resealed with new caps.


The flavoring agents (or dopants) may produce some mild foaming when they are first added to the beer, which is why antifoaming agents are often used in the beverage industry for these types of tastings. Antifoamers should not be necessary, though, if the beers are well-chilled and recapped as quickly as possible after the dopants are added. I recommend recruiting an assistant to help with the task of doping the samples (both for speed of adding dopant and resealing the bottles and for calibrating the amount to be added).

The Doctoring Procedure

In many cases, the recommended amount of dopant can be obtained from recognized thresholds published in the literature. Easily measured quantities of readily available substances are also suggested in each of my columns under the subheadings, “Doctoring tips.”

The easiest method of preparing the beers is to begin with a 3-oz sample. Calibrate this small amount before adding any dopants to a larger quantity of beer. Note how much dopant you needed to add to make the off-flavor noticeable in the 3-oz sample, then add four times that amount to a fresh 12-oz bottle (or seven to eight times that amount to a 22-oz bottle), draining a small amount of beer from the bottle first, if necessary.

For dry ingredients, it is helpful first to make a solution in distilled water; in the case of spices, extract the flavors in ethanol before doctoring the beer. Commercially available extracts may be added directly if distilled. When using a spice extract, the amount added should be slightly above what you and your assistant can detect. It is better to exceed the threshold than to serve a sample with no recognizable off-flavors. It is also worth noting that while adding excessive amounts of the dopant will make the flavors obvious, it is helpful to use the threshold amounts to have a basis for comparison (and to make detection a challenge).


Provide an adequate supply of the undoctored reference beer for comparison throughout the tasting. A flight of 10 or so doctored samples should be plenty for one doctoring session. Sufficient quantities should be available to provide 2–3 oz samples to each taster. The beers should all be chilled to a temperature in the 45–50 °F (7–10 °C) range, which is ideal for the reference lager style.

The bottles should be coded in such a manner that the dopants are not identifiable. It is okay to provide the tasters with a list of off-flavors, but throw in a few extra so that the last few samples cannot be guessed by a process of elimination. Since some of the dopants affect clarity, opaque cups are recommended to minimize visual discrimination.


Excessively harsh or astringent bitterness is a flaw, so commercial examples can be hard to find. Mineral-like character can be found in many bitters and pale ales, including Marsten’s Pedigree and Sierra Nevada Celebration Ale.

Doctoring tips: It is easy to prepare a doctored bitter beer sample using isomerized hop extract, which is available from many retail and mail order homebrew supply outlets. According to the instructions, one or two drops per 12 oz is sufficient to increase the bitterness of an underhopped beer. Assuming that this addition is sufficient to make the bitterness noticeable, it should correspond to an increase of 5–10 IBUs, a difference that should be enough for most tasters to notice. The drops should be added directly to the reference beer. Additions can be made in different concentrations to provide a spectrum of bitterness.

Taste evaluation tip. Like most tastes, the sensation of bitterness is temporal. Studies have shown that when a mouthful of beer is swallowed, the perceived bitterness increases to a maximum, then dissipates over a 30–60 second interval, depending on the level of IBUs in the beer. In a separate study, repeated ingestions of beer resulted in either an increased or decreased perception of bitterness, depending on the individual taster and the composition of the beer. The decline in bitterness was more common when the beer was initially perceived to be more bitter, which suggests that the taste buds can become desensitized to the bitter compounds. By the same token, the increased perception of bitterness in beers with low IBU levels could be due to the sensitization of the taste buds. In either case, judges should be alert to this phenomenon when comparing beer samples across a flight.




Sweetness in beer is primarily the result of unfermented sugars. Table II shows the relative sweetness of some of the more important sugars. Most strains of brewing yeast will fully ferment mono-and disaccharides such as glucose, fructose, maltose, and maltotriose, but differ in their ability to ferment trisaccharides and trace sugars. In fact, this provides one recognized method of distinguishing ale yeast strains from lager strains. Most ale yeast will not ferment melibiose and only partially ferment raffinose, which consists of a sucrose molecule joined to melibiose. Lager yeast, on the other hand, will metabolize both sugars. Some wild yeast and superattenuating yeast strains will ferment the more complex sugars, resulting in beers with less residual sweetness.


Hop Bitterness Calculations

The basic formula for estimating the hop bitterness in finished beer has the following form:

IBU = Utilization x Hop Weight (mg) x Alpha Acid (%) / Volume (L)

After converting the hop weight and volume in the above formula into ounces and gallons and converting the utilization decimal to a percentage, we obtain the more common expression:

IBU = 0.7489 x Utilization x Hop Weight (oz) x Alpha Acid (%) / Volume (gallon)

In these formulas, the volume of the beer and weight of the hops are the only exact quantities. The alpha-acid level of the hops is usually provided by the packager, but this value is accurate only at the time of measurement. As the hops age, the soft resins crystallize into hard resins and the alpha-acid level decreases. This process can be minimized by storing hops in evacuated or nitrogen-barrier pouches in the freezer, but some deterioration will occur with any form of storage.

The utilization factor you use in the formula depends on variables such as boil time, type of hop (whole leaf, pellet, or plug), and the specific gravity of the wort, just to name a few. It also often incorporates a factor that takes into account the age and stability of the hops under storage.

Many attempts have been made to predict utilization, and a good analysis of the various formulas can be found in Hall’s article (see “Further Reading”). Most popular brewing books contain tables with estimates of utilization based on boil times, form of hops, and specific gravity of the wort. These formulas and tables offer some good guidelines to use, but ultimately the only way to really know is to have your beer analyzed.

Sample calculations: For a 5-gallon batch, 1 oz of 5% alpha hops, and a utilization of 25%, the calculation yields approximately (0.75 x 25 x 1 x 5) ÷ 5 = 18.8 IBUs. This utilization is typical of what is obtained with whole hops during a 60-minute boil, but the actual value could be higher or lower.

For purposes of recipe design, you want to work the equation backwards to determine the weight of the hops you need to add during the boil. Let’s assume you want to brew a pale ale that calls for 35 IBUs and that you want to use a Cascade 6% alpha hop. For a 5-gallon batch, assuming 25% utilization, the equation becomes 35 = (0.75 x 25 x hop weight (oz) x 6) ÷ 5, which simplifies to 35 x 5 = 112.5 x hop weight (oz), leading to a calculated hop weight of 1.6 oz.

Final note: It is important not to confuse IBUs with Homebrew Bitterness Units (HBUs). HBU is simply the product of the hop weight (in ounces) and the alpha-acid percentage and has no direct relationship to the resulting hop bitterness.


Sources: Many people incorrectly believe that dextrins, which are typically unfermentable sugars, influence sweetness. Although these compounds do improve a beer’s body and head retention, they are, for the most part, flavorless. Rather, sweetness is determined by the amount of reducing sugars in the beer; that is, fermentable sugars that remain in the beer after the fermentation is complete. The level of these reducing sugars is determined not only by the raw ingredients and the brewing process, but also by the yeast strain and the fermentation conditions. Reducing sugars are desirable in low concentrations, but can be perceived as cloying at higher levels. This characteristic may be desirable in a barley wine or old ale, but is inappropriate in highly attenuated beers such as German Pilseners.


Table II: Relative Sweetness of Selected Compounds*


Relative Sweetness











Invert sugar








*For purposes of comparison, sucrose (table sugar) has been assigned an arbitrary rating of 100.


Troubleshooting the Four Basic Flavors




Excessive bitterness

Too much hops in recipe

Double-check calculations; scale back hop schedule.

Insufficient bitterness

Not enough hops in recipe

Double-check calculations; scale up hop schedule.

Harsh bitterness with astringency

Hop has too much iso-cohumulone; hops are oxidized

Use lower alpha hops.

Get fresh hops.

Harsh bitterness with medicinal flavors

Phenolic compounds produced by enteric bacteria/wild yeast

Check sanitization procedures; check yeast supply for contaminants.

Papery, vinous, or harsh flavors

Aldehydes (staling compounds) from oxidation of higher alcohols

Minimize introduction of air while racking or bottling.

Harsh bitterness with mineral-like dryness

Mineral ions

Reduce/eliminate gypsum and epsom salts. If high hopping rate with hard water, soften water.

Cloying sweetness

Abbreviated or stuck fermentation; nonattenuating yeast strain

Ensure wort has sufficient FAN and is properly aerated; increase fermentation temperature; repitch fresh, healthy yeast.

Thin body/dry

Mix of fermentable sugars favors highly fermentable forms

Use crystal or dextrin malts; adjust mash schedule to include high-temperature alpha-amylase rest; use less attenuative yeast strain.

Fruity sweetness


Change yeast strain; use lower fermentation temperature.

Honeylike flavors

Aldehydes and ketones

Avoid oxidation of hot wort or fermented beer; change water source.


Errors in water treatment program

Check water treatment methods.

Sourness (like sour milk)

Lactic acid bacteria

Check sanitization procedures; check yeast supply for contaminants.

Vinegarlike acidity

Acetic acid

Check sanitization procedures; check yeast supply for contaminants.


Adjuncts such as rice and flaked maize yield a greater percentage of fermentable sugars than malted barley and are often used to lighten body and increase alcohol. High statch conversion temperatures during the mash promote higher concentrations of oligosaccharides and dextrins, whereas lower temperatutes allow beta-amylase enzymes to work, producing a more fermentable wort. Additional minor changes in the composition of the wort take place during the boil, but the amounts of the various sugars are primarily determined by the ingredients (particularly specialty and caramel malts) and the mashing procedure.

A yeast’s ability to metabolize these sugars is not only strain-dependent, but is also influenced by the viability and alcohol tolerance of the strain. The yeast and the sugars must have sufficient contact time at an appropriate temperature for the fermentation to proceed to completion.

Many other compounds may contribute to the overall sensation of sweetness in beer, including ethanol and glycerol, which also improves mouthfeel. Esters produced during the fermentation process such as ethyl acetate, isoamyl acetate, and ethyl hexanoate are usually perceived as fruity in low concentrations, which means they also have a sweet component to their flavors. Aldehydes and ketones (also by-products of fermentation) such as 2-propenol and 3-pentanone, respectively, are often perceived as honeylike, but they have more distinct, often unpleasant, flavors at highet concentrations. (Levels are influenced by how long they’re in contact with the trub.) Low levels of sodium and chloride ions are yet another source of sweetness (these ions are discussed in more detail in the section on saltiness, below).

Commercial examples: Sweetness is a desirable characteristic of sweet stouts such as Mackeson’s XXX Stout and Watney’s Cream Stout, which either have lactose added or are sweetened with sucrose and then pasteurized. A certain amount of sweetness is also appropriate in malt-accented beers such as barley-wines (Thomas Hardy), Scotch ales (McEwen’s), and Doppelbocks (Ayinger Celebrator); but undesirable in fully-fermented styles such as American light lagers, Berliner Weissbiers, and lambics. Lagers generally have slightly lower levels of residual sugars than ales because of the higher attenuation of lager yeast and longer conditioning times.

Doctoring tips: Sucrose (table sugar) can be used to increase the sweetness of a reference beer. Its threshold value is 2,600 ppm, which is roughly equivalent to ¼ tsp of ordinary table sugar to 12 oz of beer. The easiest way to make this addition is to dissolve the above amount in ½ tsp of distilled water and add the solution to the beer.




Saltiness is a flavor that every taster should recognize, though it is rarely encountered in beers except perhaps as a background flavor in some styles. The flavor becomes noticeable with high concentrations of sodium and chloride ions.

Sources: Most of the salt ions in beer come from the brewing water. A comparison of mineral levels in the waters of different brewing centers puts Edinburgh and Dortmund at the top of the list with 55–66 ppm of each ion, followed by Button-on-Trent with 25–35 ppm. These concentrations are well below the threshold and tend to contribute a softness to the mouthfeel. But higher concentrations, such as might be found in homebrew if water adjustments are made haphazardly, will likely result in a salty flavor instead. To put these numbers in perspective, ¼ tsp of ordinary table salt contributes 28 ppm of sodium and 42 ppm of chloride to 5 gallons of brewing liquor. So while brewing salts may be used to adjust the character of your local water, they can easily be overdone. An excellent description of water adjustments can be found in Noonan’s New Brewing Lager Beer.

Commercial examples: Excessive levels are rare in commercial beers.

Doctoring tips: Salty flavors may be produced by adding table salt (iodized or noniodized) to a reference beer. The easiest method is to dissolve ⅛ tsp in 1 oz of distilled water. Each ⅛ tsp of this solution will add approximately 15 and 23 ppm of sodium and chloride ions, respectively, to a 12-oz sample of beer. In a trial run, I found that ½ tsp of the solution gave a slight saltiness to a homebrewed Weizen. This contributes a total of approximately 150 sodium and chloride ions, which is well below the individual thresholds of 560 ppm, but enough to be detected. When doctoring your reference beer, add the solution in ⅛-tsp increments to a 3-oz calibration sample until the salty flavor is recognizable and then add four times the total amount to a fresh 12-oz bottle.




Unlike the other three flavors already discussed, sourness and acidity are perceived in the aroma as well as in the taste. Sourness and acidity are caused by various organic acids that, when present in low concentrations, can lend a background acidity that is desirable in most beer styles, particularly ales.

A survey of the data in Wahl and Henius’ American Handy-Book of the Brewing, Malting and Auxiliary Trades, for example, shows that the levels of lactic acid in lagers, pale ales, Berliner Weissbier, and lambics were on the order of 0.1, 0.2, 0.4, and 1.0, respectively. The units were not given, but the trend should be evident from the relative magnitudes. Ales generally have higher levels of acids than lagers, and particularly high levels are expected in some specialty Belgian and German ales. In the more traditional styles, perceptible levels are usually the result of accidental bacterial contamination.

Sources: Lactic acid. The most common type of sourness is attributable to lactic acid. In beet it is normally present at concentrations of 20–60 ppm, compared with the threshold of 400 ppm. Higher levels are associated with contamination from the Gram-positive bacteria Pediococcus and Lactobacillus. These organisms thrive under anaerobic conditions and are prevalent in dust, soil, and saliva, making them nearly impossible to eliminate completely from the home brewery. They have even been found in commercially available yeast samples, so brewers may not always be at fault. Good sanitization and yeast handling procedures, however, should minimize the effect of these organisms.

In some cases, a brewer may intentionally introduce lactic acid into the brewing process as a method of pH adjustment. In soft or very alkaline water, it is sometimes difficult to reduce the mash pH to a level appropriate for proteolytic or diastatic activity. Some German maltsters make a special malt called Sauermalz, or sour malt, which is traditionally made by inoculating the steep water with Lactobacillus bacteria. The modern production method entails adding lactic acid itself directly to the steep water, where it is absorbed by the malt. Either way, adding a small quantity of Sauermalz to the grist helps acidify the mash without violating Reinheitsgebot.

Home brewers (and even the occasional commercial brewery, such as Guinness) have been known to use a different technique of controlled lactic acid production called sour mashing. After doughing in, the mash is tightly covered and allowed to rest overnight at temperatures in the 100–120 °F (38–49 °C) range. This procedure provides ideal growing conditions for Lactobacillus and other bacteria residing on the malt. Although these organisms may also introduce potentially undesirable flavors, some brewers prefer this technique over the riskier one of adding bacteria during fermentation.

Acetic acid. Another common sour flavor, acetic acid is also characterized as vinegarlike (vinegar is simply a 5% solution of acetic acid). Acetic acid may be produced by Gram-negative bacteria such as Acetobacter (carried by the common fruit fly) or Hafnia proteus, also known as the Obesumbacterium. Both of these organisms prefer an aerobic or microaerobic environment, which makes them most problematic in beer dispense lines.

Acetic acid in bottled beer is more likely to be produced by Brettanomyces, a common wild yeast that produces acetic acid in addition to other compounds normally found only in lambic-style beers. Another Gram-negative bacterium that produces acetic acid (as well as propionic acid) is Pectinatus. This bacterium is a strict anaerobe, which makes it a dangerous contaminant in conditioning tanks and bottled beer. Acetic acid may also be produced by heterofermentative Lactobacilli, which also generate lactic acid, ethanol, and carbon dioxide.

Other sources. Lactic and acetic acids are the most commonly encountered flavors in this class, but pyruvic, malic, and citric acids are also found in most beers at subthreshold levels. Pyruvic acid is an important intermediate product in the fermentation of glucose by way of the glycolytic pathway and is usually transformed to acetaldehyde and then ethanol. It is also a branching point for the formation of other acids such as oxaloacetic and citric (finished beer usually contains residual amounts of these compounds). British ales generally have higher levels of these compounds, which give the beers a pleasant background acidity that complements the fruity esters.

Commercial examples: The most intensely sour beers are traditional lambics such as those produced by the Boon and Cantillon breweries in Belgium’s Senne Valley. The best examples of this style have a moderate-to-intense sourness with a balance of acetic and lactic acids. Also worthy of note are the sour brown and red ales from Flanders. Liefmans Goudenband, the classic example of the brown ale (Oud Bruin) style, is fermented with a mixed culture that contains lactic acid bacteria. Its red counterparts include Rodenbach, which is aged in oak casks that are host to both lactic and acetic acid bacteria. Both of these styles have a more subtle sourness than traditional lambics.

Belgium and Germany also produce wheat beers that are traditionally soured with lactic acid bacteria to create thirst-quenching summer beverages. Sour German wheat beers are generally associated with Berlin, but a similar style is also brewed in Bremen. The best commercial examples are Schultheiss and Kindl; both have an intense sourness that results from fermentation with a mixed culture that includes Lactobacillus delbrückii.

Belgium’s wheat beer, known as white beer or witbier, is typically spiced with coriander and orange peels. Commercial examples include Blanche de Bruges and Dentergems as well as the Celis White brewed in Austin, Texas. One method that has been used to produce the sourness in commercial beers is to pitch Lactobacillus directly in the primary. When a target pH is reached, the brewer arrests the lactic fermentation by filtering. (Tip: Home brewers looking to emulate this style have had success refermenting their home-brewed witbiers in the bottle with a culture containing lactic acid bacteria.)

Doctoring tips: It is easy to prepare your own sour beers by adding lactic and acetic acid to a reference beer. Taste tests have shown that the appropriate level of lactic acid is about 0.4 mL per 12 oz of beer. To add this amount, first prepare a solution of ⅜ tsp distilled water and ⅛ tsp food grade (USP) lactic acid (88% strength, the concentration at which it is normally sold). Then blend 1/3 tsp of that solution to the 12-oz sample. Be sure to use rubber gloves, safety goggles, and plastic or glass measuring equipment when working with lactic acid.

To prepare the acetic acid sample, adding 1 tsp (approximately 5 mL) of household vinegar per 12 oz of reference beer gives a concentration that can be detected by most tasters.


Taste Will Tell


The four basic flavors — bitterness, sweetness, sourness, and saltiness — are all important components of any beer, but high levels are generally found only in defective beers or selected specialty beer styles. (An obvious example is the lactic sourness that characterizes several Belgian and German ales, but which would be symptomatic of bacterial contamination in other beer styles.) As with any other flavors, participating in a beer tasting session can help identify the levels that are appropriate for the beers that you want to brew.

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