By Scott Bickhamb (Brewing Techniques)
In the preceding installment of Focus on Flavor, we described flavors that are primarily derived from malt. We will now turn our attention to characteristics derived from hops and yeast, beginning with the flavors in Class 2 of the Beer Flavor Wheel. These flavors are described as being “resinous,” “nutty,” “green,” or “grassy.” At first glance, these flavors do not appear to be related, but they actually originate from the same groups of compounds and link the malt-based flavors in Class 3 with the hoppy and estery flavors in Class 1. These descriptors are all perceived in both aroma and flavor. Resinous flavors come from hop oils (usually during dry-hopping), whereas the most common source of the latter three flavors in this class are aliphatic alcohols and their related aldehydes.
As described in our discussion of staling compounds in beer, alcohols and aldehydes are linked by a redox reaction that removes a hydrogen atom from the active end of the alcohol chain. This reaction results in the formation of a double bond between the oxygen and carbon atoms, which is typical of carbonyl compounds. Although this interplay often causes beer staling, positive flavors may also be produced, particularly in strong ales that are extensively aged. This example illustrates one of the complex issues that are encountered in the sensory analysis of beer — namely, determining when a specific attribute is appropriate. In other words, the slight oxidation that upsets the balance of a delicate Pilsener might actually produce a pleasant nuttiness in a barleywine.
To complete the bridge to estery flavors, we will also include the closely related hoppy and floral flavors from Class 1 of the Beer Flavor Wheel. Finally, although acetaldehyde is also grouped with the aromatic flavors in Class 1, its flavor is similar to that of the aldehydes in Class 2, so we will group them with the other flavors in this article. But whereas aldehydes usually result from beer maturation, acetaldehyde is the signature of green beer.
Resinous flavors are often described as being similar to fresh sawdust, pinewood, spruce, or terpenoids (hydrocarbon compounds found in the hop essential oils). A subset of resinous flavors are woody flavors, which are likened to uncut, seasoned wood.
The most common origin of resinous flavors in beer is the extraction of high levels of the hop essential oils through the addition of large amounts of finishing or dry hops. Hop oil extracts may also be added directly to the finished beer to obtain these characteristics. In either case, the responsible compounds are terpenoids such as linalool and alpha-terpenol. Low concentrations of these compounds impart the “citrus,” “floral,” or “piney” notes found in fresh hops, but elevated levels may be perceived as resinous. Many tasters regard this flavor positively in hoppy beer styles such as IPAs, but others regard it as harsh or unrefined. Depending on the preferences of his or her customers, a brewer may wish to cut back on finishing or dry hops to control resinous flavors.
A less significant source of woody flavors is the aliphatic aldehyde 2-methyl pentanal, which is described as being “green-tasting” or like stale rice. Another possibility is acetoin, which results from the reduction of diacetyl during fermentation. Although acetoin has a high flavor threshold, as many as 0.5 flavor units have been found in some beers, so it can be a secondary flavor constituent.
The woody flavor of acetoin is invariably coupled with high early levels of diacetyl, and both can be controlled by minimizing the latter.
Commercial examples: Resinous or woody flavors are often found in IPAs, particularly if the beer is dry-hopped and unfiltered. These characteristics are less common in bottled beers, but fresh samples of Shepherd Neame Celebration Ale and Tupper’s Hop Pocket should provide benchmarks.
Doctoring tips: It is quite easy to obtain these characteristics in small-scale breweries by adding large amounts of dry hops — as much as 0.5 to 1 oz per gallon. The resinous flavor also tends to decrease with age, so for best results, use fresh hops and drink the beer immediately after kegging or bottling. Similar flavors may be reproduced in a reference beer by doctoring with 2–3 drops of hop oil extract or essence of spruce, but both will be monodimensional compared with the raw flavor derived from fresh hops.
The second group of flavors in Class 2 consist of those generally perceived as “nutty.” The main group of nutty flavors is compared to Brazil nuts, hazelnuts, or sherry, and second-tier descriptors are “walnutty,” “coconutlike,” “beany,” and “almondlike” flavors. The most common origin of the nutty or sherrylike flavors are the aliphatic alcohols 2-pentanol and 3-pentanol. Like all fusel alcohols, these can be formed through the metabolism of wort amino acids or the synthesis of additional amino acids by the yeast. In beer maturation, an equally important pathway is the condensation of short-chain aldehydes to form longer-chain ones (such as pentanal), which may subsequently be oxidized to alcohols (such as 2- and 3-pentanol).
These alcohols are often found at near-threshold levels and are a positive feature in aged strong ales such as barleywines, English old ales, and Russian imperial stouts, but indicate staleness or poor flavor stability in other styles. Although most discussions of beer aging focus on aliphatic aldehydes, these related alcohols are also major participants in beer staling. The heterocyclic ketone 2-acetyl furan and the aldehyde 2-propenal also contribute nutty flavors, but these compounds are not nearly as significant because typical concentrations are orders of magnitude below the flavor threshold.
Whereas nutty flavors are primarily caused by aliphatic alcohols with five carbon atoms, coconut and walnut flavors are associated with longer-chained alcohols such as n-hexanol and n-heptanol. These compounds are usually tertiary or background flavor constituents, but they may combine synergistically to produce perceptible flavors. Some long-chain esters and heterocyclic acids are also recognized as having coconutlike flavors, whereas walnut flavors are attributed to the aliphatic monoketones 2-heptanone and 2-octanone. These compounds are only found at very low levels, however, and are therefore not as important as the aliphatic alcohols to this flavor class. Beany and almondlike flavors (also characterized as “bean soup” or “marzipan,” respectively) probably originate from the same group of compounds, but none of the alcohols listed in my sources were given these descriptors. Other possible contributors to this group of flavors include the aldehyde 2-butanal, which possesses “almond,” “green leaf,” and “apple” flavors, and benzaldehyde and benzyl alcohol, which are both compared to bitter almonds.
Commercial examples: As mentioned above, nutty or sherrylike flavors are positive characteristics in most strong ales as long as they are in balance with the other flavors. Commercial examples that exhibit this quality include Anchor’s Old Foghorn, Rogue Ales’ Old Crustacean, and Hardy Brewery’s Thomas Hardy Ale. The nutty flavors that are found in Samuel Smith’s Nut Brown Ale and other English brown ales are derived from toasted malt, not alcohols, but there are similarities. Some brewers have even experimented with adding hazelnuts or piñon nuts to produce distinctive flavors in brown ales.
Doctoring tips: Coconut and nutty flavors are easy to reproduce by adding essences of coconut, almond, or walnut to the reference beer. These additives can be found in the baking section of most supermarkets. Approximately 0.1 mL, or one to two drops, of each is adequate for doctoring a 12-oz reference beer. An excellent way of reproducing sherry flavors is to add 4 mL (or approximately ¾ tsp) of cream sherry to a 12-oz reference beer.
The final group of flavors in Class 2 is characterized as “green” or “grassy,” with “freshly cut grass” and “strawlike” as second-tier descriptors. Synonyms for the former include “leafy” and “alfalfalike,” whereas “haylike” is a synonym for the latter. One might expect these flavors to derive instead from malt, because barley is a member of the grass family, but grassy flavors are instead generally attributed to aliphatic alcohols, which is why they are placed in this class of the Beer Flavor Wheel. Note that they are located adjacent to the similar malt-derived flavors in Class 3, however, which are usually described as “husky” or “grainy.” (The difference between grassy and grainy flavors is in the degree of greenness. Grassy flavors are compared to freshly cut grass or leaves, whereas grainy flavors have more in common with dried vegetation.) Of the aliphatic alcohols, n-hexanol and trans-2-hexanol have green leaf flavors, while freshly cut grass flavors are attributed to 1-hepten-2-ol and 1-octen-3-ol. These compounds are usually tertiary flavor constituents, but may combine to produce a perceptible grassiness in the flavor.
These grassy flavors are commonly found in lighter ales and lagers that are past their prime, but minimization of oxygen uptake while racking and bottling can prevent, or at least delay, their appearance.
Aldehydes: Several aldehydes, primarily acetaldehyde, pentanal, and 2-methyl butanal, also contribute green or grassy flavors. The latter two compounds have five carbon atoms and are usually formed during the maturation or oxidation of beer. Acetaldehyde, by contrast, has two carbon atoms and is actually an indicator of green beer. It is often difficult to distinguish between these compounds at low concentrations, but acetaldehyde has an unmistakable green appleskin flavor when it is a secondary or primary flavor constituent. In a normal anaerobic fermentation, acetaldehyde is reduced to ethanol, but may be oxidized to acetic acid upon exposure to air or contamination by Gram-negative bacteria. Because of the grassy flavors it shares with some of the higher aldehydes and because of its possible reappearance after fermentation, acetaldehyde is discussed here rather than with the ester and alcohol flavors with which it is grouped in Class 1 of the Beer Flavor Wheel.
Acetaldehyde has a flavor threshold of 10 or 25 ppm, depending on the medium and what type of threshold (difference or recognition) is being measured. At typical concentrations of 2–3 ppm, it is a tertiary flavor constituent, but the 7–9 ppm found in American Budweiser makes it a secondary flavor element. At this level, it lends a crisp snappiness in the finish of the beer as opposed to the less desirable flavors it contributes at higher levels.
There are three principal sources of acetaldehyde in beer:
The first two factors are often accompanied by acetic acid or “cidery” flavors, which makes them discernible from the green apple flavors of pure acetaldehyde. In a normal fermentation, the maximum acetaldehyde concentration occurs during the peak of the logarithmic growth phase of the yeast, usually in the first four days of the fermentation. The concentration gradually decreases during the secondary and lagering stages, as it is reduced to ethanol. Spillover levels of acetaldehyde result if the yeast is removed from the beer before this reduction is complete. Kräusening may also contribute to elevated levels of acetaldehyde because the yeast has less contact time with the beer than it would during a full fermentation.
Pitching rate has also been found to affect the acetaldehyde level, but the results are not conclusive. Some studies have concluded that the pitching rate has no influence, whereas others obtained an increase in acetaldehyde with the amount of yeast. Geiger and Piendl observed little dependence with nonflocculent yeast, but under- and overpitching both resulted in higher levels with flocculent yeast.
In practice, perceptible levels of acetaldehyde are usually found only in light styles such as Kölsch and American light lagers because of a combination of a neutral background and short maturation times.
The yeast strain, fermentation temperature, wort composition, aeration, and vitamin deficiencies all affect the level of acetaldehyde production. For a given yeast strain, the level is minimized with a low pitching temperature, a cold primary fermentation, and a warm secondary fermentation. In addition, nonflocculent yeast generally yield lower levels than flocculent strains because they have more time to reduce acetaldehyde to ethanol.
Commercial examples: As noted above, the most popular example of a beer with acetaldehyde flavors is Budweiser. Experienced tasters can usually distinguish it from other Anheuser-Busch products and similar beers from other breweries.
Doctoring tips: As mentioned previously, grassy flavors are generally undesirable, but their characteristics should be familiar to anyone who has mowed a lawn. These flavors are sometimes compared to alfalfa, so it might be possible to make a grass or alfalfa tea and add that to a reference beer, but I have not tried this.
Acetaldehyde in beer can be reproduced by doctoring a 12-oz beer with ¼ tsp of thawed apple juice concentrate made from Granny Smith apples. This will produce a snappy apple flavor that is distinguishable from the sweet apple character of the ethyl hexanoate ester.
The last two groups of flavors to be covered here are “floral” and “hoppy” flavors that, although they are lumped with the aromatic flavors in Class 1, do not come from esters. These floral notes are primarily derived from aromatic alcohols and hops, whereas “hoppy” describes the generic aroma and flavor of fresh hops. The esters that do have floral notes, notably methyl formate and ethyl butyrate, will be covered with the other aromatics in the next Focus on Flavors.
Floral flavors: One second-tier descriptor for floral flavors is 2-phenyl ethanol, which is an alicyclic alcohol with “roselike” or “perfumy” characteristics. At typical levels, it is a tertiary or secondary flavor constituent and can add complexity to the fruitiness desired in most ale styles. Related compounds are 2-phenyl acetate, an ester with sweet, roselike flavors, and phenylacetaldehyde, an aldehyde that is compared to hyacinths and lilacs.
This flavor should be distinguished from the floral or citrus characteristics of geraniol, which is also common enough to warrant its own second-tier descriptor. Geraniol flavors are also sometimes compared with roses, but are also often likened to limes and hyacinths. These flavors are an important part of the profile of most American hop varieties, particularly Cascade, Willamette, and Liberty.
In addition to the specific compounds described above, the “perfumy” descriptor is also used for floral flavors derived from other sources. Those other compounds include linalool, which is an important component of the citrus, floral, or aniseed aroma of Cascade hops.
Commercial examples. Because of the interplay between floral flavors derived from 2-phenyl ethanol and those due to hops, it is often difficult to distinguish the two. Strong, fruity beers such as Hair of the Dog’s Adam are good candidates for the ethanol-derived floral flavors, but the many other flavors will make it difficult to distinguish the floral notes. Floral hop notes should be evident in any fresh American pale ale or in Czech Pilseners brewed with Saaz hops.
Doctoring tips. Varietal hop extracts are available in an increasing number of flavors from most hop suppliers. A few drops in a 12-oz reference beer should be sufficient for most tasters to distinguish between major hop varieties such as Cascades, Saaz, and East Kent Goldings.
Hoppy flavors: Secondary flavor descriptors for “hoppy” are “kettle hop,” “dry hop, and “hop oil,” which refer to different methods of obtaining hoppiness in beer. As described below, the delineation between these flavors is not as clear-cut as when the Beer Flavor Wheel was developed, but they are still useful descriptors.
Kettle hops. The means by which the hop oils are processed en route to the finished beer accounts for the difference in flavors. Kettle hops are added late in the boil, which allows some of the essential oils and other volatiles to evaporate and oxidize as they are extracted by heat. For example, most of the myrcene is transformed into linalool, geraniol, geranyl acetate, and geranyl isobutyrate, all of which have floral characteristics. These oxygen-bearing compounds are important constituents of the profiles of noble and other fine aroma hops, whereas the pungent aroma of myrcene itself is less desirable. So, ironically, the oxidation of myrcene by exposure to heat and oxygen can actually be beneficial to hop flavor, in contrast to the oxidation of humulene, which produces grassy or cheesy notes. The intensity of the flavor, however, will be reduced by evaporation and purging by carbon dioxide during the primary fermentation.
Dry hops. Dry hops, on the other hand, are added during the secondary fermentation, where hop oils are extracted by alcohol rather than heat. Because fewer compounds are broken down, alcohol extraction has the advantage of retaining a greater percentage of the hops’ native oils. Some unwanted volatiles are also extracted, though, which often leaves some harsh notes in the finished beer. The fact that some positive degradation products might be lost in the process can be offset by the fact that some degradation of hop oils does occur as hops age, so unless the hops are fresh-picked, you can expect that a spectrum of hop oils and their derivatives will be extracted into the beer by either method.
Hop oil. The last second-tier descriptor for hoppy flavors is “hop oil,” which refers to the addition of oil that has been extracted by heat or solvents. Neither method, however, is commonly used today in the production of modern hop extracts; rather, gentler methods such as extraction by supercritical fluid CO2 are used to retain more of the fresh hop flavor and aroma. These extracts have found favor with many of the larger breweries and are accepted as adhering to the Reinheitsgebot by most German brewers.
Commercial examples. Hop extract is now listed as an ingredient of a surprising number of popular German beers. In the United States, breweries are not required to state whether hop extracts are used in the production of their beers, but many brewpubs offer IPAs or bitters that have been dry-hopped. These smaller breweries will also be more willing to divulge information about the production process — for example, which hop varieties are used and how long the dry or kettle hops are in contact with the beer or wort.
Doctoring tips. Hoppy flavors are best reproduced by making hop teas from whole and pelletized hops of different varieties. I generally put ¼ oz of hops in a muslin bag and steep in a cup of hot water, although with pellets, it is easier to use a coffee maker with a filter. These hop teas are educational and tasty substitutes for the herbal teas sold in supermarkets. The hop character can be adjusted when brewing these teas, with shorter steep times being more akin to dry-hopping and longer times to kettle hopping. Unfortunately, although sniffing and sipping the teas is instructive, the teas fall a little short in reproducing the complex aroma of beer. A better solution may be to add a few drops of hop oil extracts to a reference beer. Extracts are available from most commercial and retail hop outlets for an increasing number of varieties. In addition to training the palate to recognize floral flavors, these experiments are valuable for teaching the subtle differences between similar hop varieties.
In this Focus on Flavors, I introduced the basic flavors originating from aliphatic alcohols, hops, and acetaldehyde. The alcohols are the end products of beer aging and have flavors that range from grassy to nutty. Hoppy flavors may be resinous or floral, depending on the variety, type, and method of extraction. This leaves us with a small, but complex, group of aromatic flavors to complete Class 1 of the Beer Flavor Wheel. These flavors are primarily derived from esters and fusel alcohols, and their characteristics and origins will be discussed in the next Focus on Flavor, along with methods for controlling their levels.
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