Tasting a new brew is like solving a mystery. Color relates to grist bill and then to flavor. Foam and bubble formation are the product of carbonation. Aroma is a function of almost everything: malt, hops, yeast, fermentation, aeration, aging, contamination and yeast strain. The time between the first whiff and the first sip is the time that counts when beer is evaluated.
Flavor is comprised of two factors, taste and aroma. Taste is what you sense with your tongue; aroma is what you sense with your nose. Aroma comprises 80 percent of the perception of flavor. This is why little kids hold their noses when eating something they don’t like.
Brewers need a good mental library of the odors classified as “off-aromas.” They also should know how to control them. It’s fairly easy to identify off-aromas and to understand why these aromas are present in beer. The real challenge is to intentionally brew beers with detectable levels of compounds that are objectionable in high concentrations but pleasant at levels right above the threshold of detection.
The aroma: Green apples, latex paint
The Origin: Acetaldehyde is an intermediate compound in the biochemical transformation of glucose into alcohol, carbon dioxide and energy (ATP). This process is called “glycolysis.” The last step in the glycolytic pathway is the conversion of acetaldehyde to ethanol (alcohol). Some acetaldehyde “leaks” from the yeast cell during fermentation but can later be absorbed and converted to ethanol during aging. This step regenerates NAD+ by adding hydrogen atoms to acetaldehyde to form ethanol. (For more information on NAD+, read on!)
A less common source of acetaldehyde is microbiological contaminants, namely Acetobacter and Zymomonas. Acetobacter is not normally found in bottled beers because it requires oxygen to grow, but it can spoil cask-conditioned beers. Zymomonas spoilage is usually associated with sugar-primed beers because Zymomonas can use glucose, fructose and sucrose for growth, but not maltose. A strong hydrogen sulfide aroma usually accompanies Zymomonas spoilage.
Acetaldehyde can also arise from oxidation of ethanol; this occurs during beer staling. Other alcohols also oxidize during beer oxidation. The aroma of this mixture of aldehydes is often described as resembling sherry.
The solution: The most common cause of elevated acetaldehyde levels is either a short aging time or insufficient yeast activity during aging. These problems are solved by pitching healthy yeast and not rushing through fermentation and aging. Acetaldehyde can be detected in some beers even after a long aging period with healthy yeast because of the qualities of that yeast strain. Coors beers have a distinct acetaldehyde aroma; it’s pleasant and subtle and is associated with their particular strain.
Zinc deficiency is another cause of elevated acetaldehyde in finished beer. Zinc, among other nutrient roles, is a co-factor for the enzyme that converts acetaldehyde to ethanol. Many commercial brewers add zinc to wort because of its importance to yeast health. The easiest way for homebrewers to add zinc to wort is by using a yeast nutrient blend and following the recommended dose.
Zinc can be added in tablet form but the desired level in wort is so low, about 0.15 mg/liter, that weighing becomes a limitation. Take note that high levels of zinc can cause severe fermentation problems.
The aroma: Butterscotch, buttermilk, artificial butter, toffee
The Origin: Diacetyl (2,3-butane dione) is related to the synthesis of the amino acid valine. During the normal coarse of fermentation, alpha-aceto-lactate (a precursor to diacetyl) is formed and excreted from the yeast cell. This precursor is then converted through an oxidation reaction to diacetyl outside of the yeast cell. Elevated temperature and exposure to oxidizing compounds, especially oxygen, accelerate the conversion from precursor to diacetyl. If the precursor is not converted to diacetyl during fermentation and aging, it can convert during racking, bottling or kegging.
Diacetyl is also associated with the lactic acid bacteria Pediococcus and Lactobacillus. Most brewers are taught to detect diacetyl at very low levels because of its association with bacterial spoilage.
The solution: Although some brewers with fancy labs and very specific knowledge about their beers can fine-tune the valine content of their worts to control diacetyl, most brewers rely on aging. The key to diacetyl control is to understand how it is formed and later reduced.
In the late stages of fermentation yeast cells have a large supply of a biochemical carrier molecule called NADH2. The counterpart to NADH2 is NAD+ and the difference between the two compounds is two hydrogen atoms. NAD+ is important in metabolism because it keeps the glycolytic pathway moving by transferring hydrogen; when NAD+ picks up two hydrogen atoms it becomes NADH2. As it so happens, diacetyl is a compound that the yeast cell can dump hydrogen atoms onto and regenerate NAD+ molecules. When hydrogen is added to diacetyl the compound 2,3-butane diol is formed. The lingo for this reaction is a “diacetyl reduction” (a reduction reaction is one where hydrogen is added to a molecule). 2,3-butane diol has virtually no detectable aroma.
In order to effectively brew a beer with little to no diacetyl it is critical to allow the diacetyl precursor to convert to diacetyl. The so-called “diacetyl rest,” when beer is held at fermentation temperatures for a period following fermentation, provides enough time for the precursor to convert to diacetyl and for yeast to reduce diacetyl. Ales do well with a 2- to 4-day diacetyl rest at 70° F following fermentation. Lagers need between 7 to 14 days at about 50° F.
If you want a bit of diacetyl in your beer, then a rapid fermentation followed by quick cooling to stop yeast activity is a sure-fire way to get a healthy dose of diacetyl. Certain yeast strains — for example, many English ale strains — are also known to leave detectable levels behind. A detectable but controlled level of diacetyl can add complexity to styles like stout, porter and English-style bitters.
The aroma: Fruity, floral, solventy
The Origin: Esters are a wide range of compounds that are formed in fermentation when an alcohol combines with a biochemical intermediate called an acyl-CoA (carbon chains carried by coenzyme-A). Acyl-CoA compounds and oxygen are involved in the synthesis of unsaturated fatty acids essential for membrane development during yeast growth. The most common alcohol in fermentation is ethanol and the most common acyl-CoA compound is acetyl-CoA. When these two molecules are combined the resulting ester is ethyl acetate.
Esters can also be formed in reactions between organic acids and alcohols that are not catalyzed by enzymes. These reactions take a very long time to occur and are not a significant source of ester compounds in finished beer.the solution: Ester levels increase whenever the concentration of alcohols or acyl-CoA compounds increase. Alcohol levels increase with wort specific gravity. This means that high-gravity beers, such as bocks, barleywines and Belgian strong ales, have more esters than their lower gravity counterparts. Acyl-CoA compounds are related to wort amino acid levels and cell growth. Anything that increases wort amino-acid levels (such as high gravity and all-malt worts) or restricts cell growth (such as low wort aeration) will increase the level of acyl-CoA compounds.
Wort aeration and the presence of oxygen in solution during fermentation is key to ester control. Normal-gravity worts hold more oxygen than high-gravity worts, but high-gravity worts usually have a higher amino-acid content. This means the yeast cell has more acyl-CoA compounds to convert to unsaturated fatty acids but are limited by oxygen content.
High temperature (above 72° F for ales, above 55° F for lagers) and under-pitched fermentations also increase ester levels. Thorough wort aeration, proper temperature and a healthy pitching rate keep ester levels in check. A smack pack stepped up to one quart is a good pitching rate for a five-gallon batch.
Moderate ester levels do not equate to a defect; brewing higher-gravity beers and fermenting them at elevated temperatures with certain yeast strains (like Belgian strains) is one way to create more aromatic or fruity beer.
The aroma: Spicy, floral, vinous, alcoholic
The Origin: Higher alcohol production is directly related to amino acid metabolism. Yeast cells transport amino acids into the cell, remove the amino group and form compounds known as oxo-acids. Some oxo-acids are used to synthesize amino acids and others are reduced to an alcohol and excreted from the cell. The most common oxo-acid in metabolism is pyruvate and its corresponding alcohol is ethanol. Ethanol has two carbon atoms; higher alcohols are so named because they are heavier than ethanol and have more than two carbon atoms.
The solution: Higher alcohol production is affected by many of the factors influencing ester levels. Elevated amino-acid levels (in high-gravity and all-malt worts), low wort aeration, high fermentation temperatures and large amounts of yeast growth (due to under-pitching) are all conditions that increase the level of alcohols in beer.
Although higher alcohols lend complexity to beer aroma, they are well-known for their ability to give consumers skull-splitting headaches. Traditional distilled spirits such as scotch, cognac and armagnac owe much of their rich aroma profile to higher alcohols. This is why one too many of these tipples usually results in a very unpleasant hangover. Most brewers try to minimize higher alcohol levels, with the exception of strong beers in which they’re significant to the aroma profile, such as Eldridge Pope’s Thomas Hardy Ale or Sierra Nevada’s Bigfoot Barleywine.
The aroma: Vinegar, sour, cheesy, soapy, goaty and rancid
the Origin: The two main sources of volatile acids — such as acetic, lactic, valeric, butyric and propionic acid — are microbiological spoilage and oxidized or “cheesy” hops. Bacteria that convert ethanol to acetic acid are collectively called acetic acid bacteria. The species most notable in beer spoilage is Acetobacter. All acetic acid bacteria require oxygen for growth. Acetic acid is associated with beer exposed to air, for example, cask beers and beer taps.
Beer spoilage from lactic acid bacteria can also lead to a sour smell in beer. Although the term sour strictly refers to a taste sensation, soured products, such as sauerkraut, have a distinctive aroma tipping us off about the pending taste. Lactic spoilage of beer is almost always a severe defect, except for intentionally soured beers like Berliner weisse and lambics.
Oxidized hops also give rise to volatile acidity in beer. Old hops have a particularly foul aroma. When an alpha-acid oxidizes, a fatty acid splits off the alpha-acid molecule. The aroma of fatty acids can be best described as cheesy, rancid or goaty. The characteristic smell of goats and goat cheese come from caprylic acid; parmesan cheese gets it distinctive aroma from propionic acid. These aromas are almost never desirable in beer. Lambics are one style that typically and “normally” contain such aromas.
the solution: Volatile acids can be controlled through good sanitation practices, keeping kegged beer under a carbon dioxide blanket and by evaluating hops for freshness prior to use. Hop evaluation is best conducted by rubbing pellets or cones to warm and release the compounds contained in the yellow lupulin glands. This method makes it fairly easy to detect the signs of hop oxidation. Most brewers avoid using hops with any cheesy, rancid or goaty aromas.
The aroma: Clove, medicinal, musty, pitch, smoky
The Origin: Phenolic aromas are usually due to wild yeast contamination, certain brewing yeast (such as weizen and certain Belgian ale strains), wort spoilage bacteria and chlorinated water, although some phenolic aromas come from roasted and smoked malts. Certain wild and brewing yeast strains are able to convert phenolic acids, like ferrulic acid, from malt to phenols. The aroma typically associated with phenols from yeast is clove. Brettanomyces is a yeast used in the production of lambics and produces a phenolic aroma described as horsy or wet-blanket.
Homebrewers don’t have many problems with wort spoilage bacteria, but these bugs can cause problems in commercial breweries because they can quickly grow in wort lines, heat exchangers and lauter tuns.
Chlorophenols are formed when phenols react with chlorine. Chlorophenols have a much lower threshold of detection than phenols and can be detected at only a few parts per billion, or about one thousand times lower than the threshold of phenols such as 4-vinyl-guaicol (the clove aroma in weizen). Trichloranisol (TCA) is a chlorophenol formed when corks are exposed to chlorine and has a distinctive musty aroma. TCA causes the “corked” defect usually associated with wine, but can also show up in beer bottles sealed with a cork.
the solution: Many phenolic compounds in beer can be avoided with good sanitation practices. Sanitation is the best way to keep contaminating organisms out of wort and beer. This is a double-edged sword since chlorine sanitizers intensify phenolic aromas. Chlorophenols can be avoided by never exposing beer to chlorine. Phenols from yeast can be easily controlled by yeast strain selection and by buying a good quality brewing yeast (poor quality yeast can contain wild yeast neatly stored in the packet).
For example, weizen and certain Belgian wheat and Trappist strains produce phenolic aromas. Most suppliers include this in the yeast-strain description. Avoid the temptation to use baking yeast in beer! Wild yeast poses no problem in baking, and baking yeast frequently carries “wild” yeast strains.
Some beer styles, like weizen, should have a pronounced phenolic aroma. 4-vinyl-guaicol is formed by the action of weizen yeast on ferrulic acid. Wort ferrulic acid levels can be augmented by using a higher portion of wheat malt (wheat malt contains more ferrulic acid than barley malt) and using a low temperature mash rest at about 115°F to boost the release of ferrulic acid into wort. If you are brewing lambic and want an authentic aroma profile, use a lambic yeast that contains Brettanomyces and let the horses run wild!
Ashton Lewis is BYO’s technical editor and Master Brewer at Paul Mueller Co. in Springfield, Missouri.