Focus On Beer Flavor

by Scott Bickham - (Brewing Techniques)

The Four Basic Tastes

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.

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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.

Bitterness

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.

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).

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.

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.*

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

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.

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.

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