by Scott Bickham (Brewing Techniques)
The flavors attributed to diacetyl and fatty acids are encouraged in some styles and may be undetectable to trained tasters even when in excess. Yet their instability in finished beer generally brands them flavor defects. Fresh ingredients, high FAN content, and good yeast management techniques can minimize their effect on your beer.
The previous installment of Focus on Flavors described the characteristics of sulfur-bearing compounds. The one most commonly encountered in beer is DMS, which has a flavor and aroma similar to that of cooked corn, cabbage, or celery. Low levels of DMS are desirable in most lager styles because the compound accentuates the malt character. Brewers are no strangers, however, to another group of compounds that plays a similar role in ale styles: the vicinal diketones, specifically, diacetyl and 2,3-pentanedione. These compounds contribute a pleasant smoothness and caramel flavor at low concentrations, but are often perceived as rancid at high levels. These flavors, along with other less-pleasant ones derived from fatty acids during yeast metabolism, comprise Class 6 of the beer flavor wheel and are the subject of this installment of Focus on Flavors.
The flavors in Class 6 are broken down into the four first-tier groups shown in Table I: fatty acids, diacetyl, rancid off-flavors, and oily off-flavors. The fatty-acid flavors within the first tier are further divided into the second-tier descriptors “caprylic,” “cheesy,” “isovaleric,” and “butyric.” Caprylic flavors are most often characterized by tasters as “soapy,” “fatty,” “goaty,” or “tallowy.” Isovaleric flavors are similar to cheesiness, but the term specifically refers to the flavors released by oxidized hop alpha-acids. Butyric flavors are the same as those found in rancid butter.
For the sake of convenience, we will consider diacetyl separately at a later point in the article and focus first on the other three flavors. These flavors are primarily caused by aliphatic hydrocarbons, which have a linear chain structure and are less volatile than their aromatic, ring-shaped cousins. Their flavors can still be quite strong, however, particularly as the number of carbon atoms increases. Examples of these compounds include n-octanol (an alcohol), octanoic acid (a fatty acid), octanal (an aldehyde), and n-octyl-butyrate (an ester). The first three compounds contain eight carbon atoms, whereas n-octyl-butyrate is formed from octanol and butyric acid. Oily and rancid flavors are noted only when the off-flavor has progressed to such an unpleasant extent that description demands more powerful terminology. See the box, “Aliphatic Compounds in Detail” for a closer look at the individual aliphatic compounds.
The fatty acids in wort and beer originate from three sources: oxidized hop compounds, fatty acids carried over from the malt and trub, and yeast metabolic byproducts.
Hop iso alpha-acids: Some fatty acids are introduced into wort and beer from the oxidized forms of the hop iso alpha-acids. These compounds are formed when the acyl side chains of isohumulone, isocohumulone, and isoadhumulone are cleaved to form isovaleric, isobutyric, and 2-methyl butyric acid, respectively. They are characterized by pronounced cheesy or sweaty flavors and combine with sulfur compounds to form thio esters such as S-methyl-2-methyl thiobutyrate. These esters have flavors characterized as “cabbagelike,” “sulfury,” and “soapy/fatty”. As such, they straddle the boundary between the fatty acid (Class 6) and sulfury flavors (Class 7) on the Beer Flavor Wheel.
Malt: Malt also contributes long-chain, unsaturated fatty acids to the wort that may be carried over in the trub. The most important members of this group are oleic and linoleic acids, which contain 18 carbon atoms and have one and two double bonds, respectively. The positive and negative properties of these compounds are a subject of much debate. They contribute to yeast viability and inhibit the formation of some acetate esters, but they have a negative impact on clarity and head retention (5). When they become oxidized, they produce aldehydes with unpleasant caprylic and soapy flavors, and they are also components of ethyl oleate and ethyl linoleate esters, which have goaty, vegetable oil, and tallowy flavors.
Yeast metabolism: A third group of fatty acids is formed during yeast metabolism and consists of saturated chains with no double bonds and 4–12 carbon atoms. The amount of these compounds produced depends on the yeast, with lager strains generally producing higher levels than ale strains. Once produced, these fatty acids are either passed on to the beer — in which case they impart butyric, caprylic, or soapy flavors — or they combine with alcohols to form esters that might also be described by a trained taster as caprylic or coconutlike. Though these fatty acids tend to be resistant to oxidation, their aldehydes have very low flavor thresholds and can significantly impact beer flavor. It should be noted that high levels can also result from contamination by bacteria or wild yeast.
Other sources: In addition to the sources identified above, flavors similar to those derived from fatty acids can originate from the oxidized forms of certain malt compounds. For example, hydroxymethyl furfural (HMF), 2,5-dimethyl pyrazine, and tetramethyl pyrazine have oily, cheesy, and soapy flavors, respectively. These compounds are generally not as significant to beer flavor as the aliphatic hydrocarbons, but they are still a possible source of off-flavors, particularly if they are accompanied by stale or musty flavors.
Several steps can be taken to eliminate or reduce these unpleasant fatty acid flavors. Using fresh hops and minimizing oxidation will eliminate the cheesy, sweaty flavors associated with isovaleric acid and old hops. Using fresh, properly stored malt (it is especially important to properly store caramel or roasted malts because these malts have more melanoidins) will reduce the pyrazine off-flavors. Caprylic and related flavors can be reduced by separating the wort from the trub more efficiently, minimizing the contact time with the trub, and preventing oxidation. Since the production of the saturated fatty acids is a property of yeast, selecting a different yeast strain is also an option. Most commercially-available strains should produce subthreshold levels of these compounds. Finally, because bacteria and wild yeast also produce fatty acids and related compounds, careful sanitization should be practiced.
Food-grade sources of these compounds ate not readily available, so no beer doctoring guidelines will be given here. Safety is also a question because many of these compounds are toxic when concentrated. All the fatty acid flavors have common descriptors, however, so they should be recognizable and added to the vocabulary of a trained sensory analyst. Untrained tasters may benefit from using very old hops and well-aged European cheeses as sensory guideposts.
Controlling the level of diacetyl in beer has long been a concern of brewers. Diacetyl has a low flavor threshold of 0.1 ppm and a flavor that is described as buttery or similar to butterscotch or toffee. At moderate levels, it also imparts a silkiness to the palate that improves the mouthfeel and accents the malt character. These characteristics are often mistaken for caramel malts and are regarded as attractive by many consumers, especially in British-style ales. But while the flavors imparted by caramel malt are relatively stable, diacetyl tends to become increasingly unpleasant as the beer ages. In fresh beer, though, these flavors are often difficult to differentiate, particularly since 20% of the population cannot detect even high concentrations of diacetyl.
Definition: Diacetyl is a member of a group of compounds called vicinal diketones (VDKs), which have a structure consisting of two ketone groups localized on adjacent sites in the molecule. The more formal name for diacetyl is 2,3-butanedione. Several other VDKs have been identified in beer, but 2,3-pentanedione is the only other VDK that occurs at levels comparable to its flavor threshold. This compound has a flavor that is more honeylike than buttery and is reported to be an important constituent of some Belgian ales. It shares a similar fate with diacetyl in the brewing process, and they are almost always grouped together as the total VDK content of beer in commercial laboratory analyses.
Origins: The dominant pathway in fermentation involves the conversion of pyruvate to acetaldehyde, which is then reduced to ethanol. A small amount of pyruvate is also used for biosynthesis, and this is the pathway that leads to diacetyl formation. In this process, acetaldehyde is enzymatically converted to alpha-acetolactic acid, which is excreted by the yeast into the beer and nonenzymatically converted to diacetyl in a redox reaction. So while yeast produce the precursors of diacetyl, they are not directly involved in the final step of its formation.
The influence of FAN content. An important variable in the formation of diacetyl is the free amino nitrogen (FAN) content of the wort, particularly the level of the amino acid valine. Studies have shown that the assimilation of valine inhibits the conversion of alpha-acetolactic acid to diacetyl. Most diacetyl is produced during the first few days of fermentation, and the total amount is dependent on the presence of valine. Valine assimilation results in two typical patterns of VDK formation. In a wort with high FAN content, the diacetyl level reaches a plateau in the early part of the fermentation. This plateau is also evident in the fermentation of a low FAN wort, but is followed by an intense second peak. Under normal conditions, the diacetyl levels of both worts will then begin to decrease, but the low FAN wort will have a higher residual level at the end of the fermentation. In practical terms, this means that because diacetyl production is inhibited by valine assimilation, low FAN worts produced from a high percentage of nonmalt adjuncts will not supply enough amino acids to suppress the second diacetyl peak. The valine supply is never depleted in a high FAN wort; therefore only a limited amount of diacetyl is produced.
Yeast growth factors. Other factors that influence the production of VDKs in the finished beer are the early fermentation temperature, the amount of trub, the availability of oxygen during the lag phase, and the pitching rate. Generally, any factor that increases yeast growth will also increase the amount of diacetyl produced, since this growth process depletes the pool of essential amino acids in the wort. For example, higher fermentation temperatures increase the rate of yeast growth, thus producing more diacetyl in the early part of the fermentation.
Bacteria. High levels of diacetyl may also be produced by lactic acid bacteria, especially the Pediococcus species. These organisms were identified by Pasteur as the source of Sarcina sickness in beer, but it was only in 1939 that diacetyl was recognized as the source of the buttery flavor. The diacetyl produced by lactic acid bacteria usually becomes increasingly pronounced during the first month of contamination and then diminishes as it is converted to lactic acid. Lactic acid bacteria are notorious contaminants in pitching yeast and are also present in saliva and dust.
Reduction factors: Of equal importance to the factors that affect the production of diacetyl are those that lead to its reduction. To a certain extent, yeast take care of cleaning up their own diacetyl, converting it to compounds that typically do not influence beer flavor. Yeast import the diacetyl into the cell and enzymatically convert it first to acetoin and then to butanediol. Acetoin is a staling compound with a musty, moldy flavor, but rarely exists at levels above its flavor threshold. Butanediol has a neutral flavor at the levels typically found in beer.
The most important factors in this reduction process are the yeast strain, temperature, and availability of oxygen. Yeast differ in their abilities to reduce diacetyl, and some strains also have a tendency to mutate into respiratory deficient cells that have lost their reduction capacity. By the same token, if the beer is prematurely removed from the yeast by filtration, the yeast will not have the opportunity to reduce diacetyl to an acceptable level. This frequently occurs in smaller breweries that must accelerate the production process to meet consumer demand. While the beer may ferment out in a week or less, an additional week of aging allows the yeast to reduce diacetyl and stabilize the beer flavor.
Higher temperatures in the latter part of the fermentation also favor the reduction of diacetyl by increasing the activity level of the yeast. For this reason, lager breweries often incorporate a period of warm conditioning called “ruh storage” or diacetyl rest. In this procedure, temperatures are raised to 57–61 °F (14–16 °C) for a few days at the end of the primary fermentation to encourage the complete conversion of alpha-acetolactic acid to diacetyl, and its subsequent reduction to butanediol. This period of increased yeast activity also helps scrub out and reduce any residual dissolved oxygen and purges unwanted volatiles from the beer. It should be noted that when fermentation is carried out at a uniform temperature, higher temperatures have a greater effect on the ability of the yeast to reduce diacetyl than it has on its formation. Warmer fermentation temperatures in general therefore tend to yield lower diacetyl levels.
A final consideration is that the reduction of diacetyl to butanediol cannot take place during aerobic conditions, therefore the introduction of oxygen after the lag phase may lead to elevated levels of diacetyl in the finished beer. This can occur when using open fermentors or when aerating the yeast, which is a feature of the Burton Union and Yorkshire Square Stone fermentation systems. A similar reaction can occur in the finished beer if it contains any residual levels of alpha-acetolactic acid. In this case, oxygen in the headspace or melanoidins and tannins that underwent hot-side aeration can act as oxidizing agents and convert the alpha-acetolactic acid to diacetyl.
Commercial examples: In spite of the efforts of many American brewers to eradicate diacetyl, it is still an accepted, and even desired, component of many styles. Dry and foreign-style stouts can have levels as high as 0.6 ppm, which is six times the flavor threshold. Lower concentrations can also accent the caramel maltiness of Scotch ales, porters, and other British ales. Perceptible levels of diacetyl are generally undesirable in American lager styles, but are part of the character of Czech Pilseners and Viennastyle lagers. Commercial beers in which diacetyl can usually be tasted include Guinness Extra Stout, Pilsner Urquell, Budweiser Budvar, Samuel Smith’s Pale Ale and Old Taddy Porter, Pete’s Wicked Ale, and Samuel Adams Boston Lager and Bohemian Pilsener.
Doctoring tips: Diacetyl is the main ingredient in the artificial butter flavor added to movie theater popcorn and is available in the spice section of most grocery stores as imitation butter flavor. A product from Shilling, for example, contains “butyric acids and other organic acids, diacetyl and other ketones, ethyl propionate and other esters …” dissolved in a solution of water and propylene glycol. The concentrated aroma has buttery, solvent and vanilla notes, but is recognizable as diacetyl in beer. Doctored beers may be prepared to taste by adding 4–5 drops of this substance to a reference beer. Since flavor thresholds can vary by an order of magnitude, it is useful to prepare samples with different concentrations. This will help pinpoint high and low sensitivities to diacetyl and help judges calibrate themselves for evaluating beers in competitions.
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