Introduction

I remember back to when I brewed my first batch of beer. It seems like yesterday; however, over two decades have elapsed since that faithful day. Much in the world of home brewing has improved dramatically during the last twenty years. An improvement that comes readily to mind is ingredient quality. Those of us who were participating in the hobby during the first home brewing boom can attest to having to work with hops that were often brown and malt that was past its prime. Small-scale brewers used to receive macro brewer cast-offs, and home brewers received the macro cast-offs that small-scale brewers rejected.

While poor ingredient quality and selection are a thing of the past, there are areas of home brewing that have changed very little in the last twenty years. One such area is an understanding of fermentation byproducts. We have transitioned from a hobby with an incomplete understanding of fermentation byproducts that fermented at room temperature to a hobby that uses temperature-controlled fermentation chambers to mask our incomplete understanding of fermentation byproducts. The topics covered in this blog entry are fermentation byproducts and the role that they play in beer flavor.

Fermentation Metabolites

Brewers who are relatively new to brewing often treat esters and fusel alcohols (a.k.a. fusel oils) like they are the spawn of Satan. However, beer would not taste like beer without these compounds. In fact, suppressing the production of fusel alcohols and esters can often remove much of an individual yeast strain’s character, resulting in little to no change in flavor when changing yeast strains.

Fusel Alcohols

With that said, what are fusel alcohols? Fusel is a German word that translates to “bad liquor.” Another term for fusel alcohol is “higher alcohol.” Why are fusel alcohols referred to as higher alcohols? Well, the word “higher” refers to the fact that fusel alcohols contain more than two carbon atoms. If we examine the chemical formula for ethanol, we discover that it is most commonly written as CH 2 CH 3 OH. Another formula for this chemical compound is C 2 H 6 O, which makes it clear that ethanol contains two carbon atoms. Alcohols with more than two carbon atoms have higher molecular weights and boiling points than ethanol; hence, they are higher alcohols.

One of the most commonly encountered higher alcohols in brewing is isoamyl alcohol. The chemical formula for isoamyl alcohol is (CH 3 ) 2 CHCH 2 CH 2 OH. The formula for isoamyl alcohol is often written as C 5 H 12 O. As one can clearly see, isoamyl alcohol contains more than two carbon atoms. In fact, another name for isoamyl alcohol is isopentyl alcohol due to the fact that the compound contains five carbon atoms. Another frequently encountered higher alcohol is isobutyl alcohol. The chemical formula for isobutyl alcohol is (CH 3 ) 2 CHCH 2 OH. The formula for isobutyl alcohol is often written as C 4 H 10 O. Once again, one can clearly see that this alcohol contains more than two carbon atoms.

An alcohol that is often grouped in with fusel alcohols that is not a higher alcohol is methanol. The chemical formula for methanol is CH 3 OH, which is also written as CH 4 O. Methanol has a lower molecular weight and boiling point than ethanol. One will often hear the term “heads” used to describe the first condensate that is produced during alcohol distillation. This portion of the condensate is discarded. The reason being is that the heads are mostly methanol due to the fact that methanol makes the phase change from liquid to vapor before ethanol. Methanol is generally not a problem in beer because it exists at low levels. Methanol becomes a problem when we distill beer into whiskey, which is why one should stick with beer. Due to their higher molecular weights and boiling points, the true higher alcohols appear in the condensate known as the “tails.” Higher alcohols have an oily consistency, which is why they are referred to as fusel oils.

Esters

Okay, now that we now know that higher alcohols are alcohols that contain more carbon atoms than ethanol, what is an ester? An ester is the result of a condensation reaction between an alcohol and a carboxylic acid. A condensation reaction is a reaction where two compounds combine resulting in a new compound and a water molecule.

A carboxylic acid is an acid whose formula ends in COOH. Esters are responsible for a large part of what we describe as beer flavor, especially ale flavor. A carboxylic acid that is commonly found in beer is acetic acid. Acetic acid production is integral to the yeast metabolic cycle. Every beer drinker who has tasted German-style hefeweizen has encountered an acetic acid-based ester that is available at above perception threshold levels. That ester is isoamyl acetate. Isoamyl acetate is the condensation reaction between isoamyl alcohol and acetic acid.

As mentioned above, the chemical formula for isoamyl alcohol is C 5 H 12 O. The chemical formula for acetic acid is CH 3 COOH.

Condensation reaction for isoamyl acetate

C 5 H 12 O + CH 3 COOH → C 7 H 14 O 2 + H 2 O

The reaction shown above reads one molecule of isoamyl alcohol plus one molecule of acetic acid yields one molecule of isoamyl acetate plus one molecule of water.

Two other acetic acid-based esters that are commonly encountered in beer above perception threshold levels are ethyl acetate and isobutyl acetate. As one has more than likely assumed, ethyl acetate is the result of a condensation reaction between ethanol and acetic acid. It has the sweet smell of nail polish remover. Isobutyl acetate is the result of a condensation reaction between isobutyl alcohol and acetic acid. Isobutyl acetate smells like raspberries or pears.

Other carboxylic acids that are frequently encountered in fermentation are hexanoic acid and heptanoic acid. The esters that are most commonly found in beer that are condensation reactions between these carboxylic acids and an alcohol are ethyl hexanoate and ethyl heptanoate, respectively. Ethyl hexanoate smells like red apple. Many brewers who are new to beer sensory evaluation mistake ethyl hexanoate for another yeast metabolic byproduct that smells like apple; namely, acetaldehyde. Acetaldehyde smells like tart green apple. The reason being that if we oxidize acetaldehyde, we obtain acetic acid. Ethyl heptanoate is my all-time favorite ale ester. It smells like one of those grape lollipops that were often given to children by bank tellers and medical office receptionists when I was young. Ales fermented with the Young’s Ram Brewery strain usually contain high levels of this ester when young, which is why I refer to ethyl heptanoate as the British lollipop ester.

Factors Affecting Metabolic Production

I mentioned in my introduction that we have transitioned from a hobby with an incomplete understanding of fermentation byproducts that fermented at room temperature to a hobby that uses temperature-controlled fermentation chambers to mask our incomplete understanding of fermentation byproducts. Quite frankly, fermenting ales at low internal temperatures in order to avoid unwanted fermentation byproducts is treating the symptoms instead of the problem. While fermentation temperature cannot be be ignored, the role of genetics and wort composition in the production of fusel alcohols and esters are equally important.

Temperature

Fermenting at low temperatures slows yeast metabolism, and anything that slows metabolism slows growth. Most of the esters and higher alcohols are produced during the growth phase. Slowing metabolism reduces metabolite production.

Genetics

An important thing to understand is that ester formation during fermentation occurs within the cell wall with the aid of enzymes. An important ester production-related enzyme is known as alcohol o-acetyltransferase (ATTase). As I mentioned in the blog entry entitled “Carbon Credits,” an enzyme is a reaction catalyst. A reaction catalyst is a compound that increases the rate at which a reaction occurs. There are actually two AATase enzymes; namely, AATase 1 and AATase 2. Yeast cells contain two genes that are responsible for encoding these enzymes; namely, ATF1 and ATF2.

Wort Composition

Other than yeast genetics, the most important attribute in ester production is wort composition. The carbon-to-nitrogen (C:N) ratio plays a major role in ester production. As I mentioned in my blog entry entitled “Carbon Credits,” yeast cells do not consume sugar, they consume carbon, which they attempt to convert into energy. Sugar is carbon bound to water. All-malt wort has a lower C:N ratio than does wort that contains adjuncts. The amount of nitrogen that is available after dissolved oxygen is consumed determines the amount of acetyl CoA that is formed during the growth phase. Acetyl CoA is formed by combining acetic acid with coenzyme A; therefore, more acetyl CoA translates to higher acetic acid-based esters. The least desirable of is ethyl acetate.

Surprisingly, the higher C:N ratio found in adjunct wort results in lower ester levels. Macro beer is maligned beyond belief within the home and craft brewing communities; however, the German brewmasters who were responsible for creating this style were nothing short of geniuses.

American 6-row and 2-row barley have higher protein levels than continental and British barley. Higher protein levels translate to higher nitrogen levels. The addition of adjunct reduces the aggregate nitrogen level of the grist, resulting in lower nitrogen wort, which, in turn, results in lower ester levels. Protein levels also play a role in higher alcohol production. Higher alcohols are formed when amino acids are metabolized via the Ehrlich pathway.

Finally, the type of sugar being metabolized plays an important role in the creation of higher alcohols, which, in turn, plays a role in ester production. Sucrose and fructose result in increased higher alcohol production, and so does glucose to an extent. Maltose metabolism results in considerably lower higher alcohol production than does glucose and fructose.

Applying Science to Beer Production

With this blog entry almost complete, how does one put this information to work in a home brewery? Well, as Denny Conn likes to say, “Wort wants to become beer.” This statement is absolutely true. What we are attempting to do by applying science to beer production is to gain a finer level of control over the finished product. There is no one size fits all approach to brewing. There are just too many variables involved in beer production to distill the process down into a repeatable cookbook process that works in all breweries with all styles and yeast strains.

Quality Ingredients and Proven Techniques

Due to lack of access to a fully-equipped quality control laboratory, home brewers work with an incomplete knowledge of their ingredients; therefore, one should start by selecting the highest quality ingredients available and applying brewing techniques that have stood the test of time. After the basics have been mastered and a considerable amount of data has been collected (i.e., a proper brewing log is a must), a brewer can start to alter the experiment one variable at a time while taking copious notes.

Adjusting Wort Composition

We know that the disaccharide sucrose and the monosaccharides fructose and glucose tend to translate to increased higher alcohol production; therefore, one strategy to reduce to higher alcohol production would be to avoid mash rest temperatures below 150F, especially when using high protein barley such as American 2-row. A second strategy would be to dilute the protein levels found in American 2-row with a low-protein adjunct such as flaked maize at the rate of 10% of the grist. I personally prefer to use low nitrogen continental and British barleys.

Selecting for Character

While ester production is bounded by higher alcohol and carboxylic acid production, yeast genetics play a significant role because enzymes are proteins and proteins are encoded via a genetically controlled process known as transcription. We can adjust wort composition and fermentation temperature regulation to control higher alcohol and ester production, but yeast genetics play the ultimate role in the production of these compounds. I always say, “One should pick a yeast strain for the task at hand instead of attempting to trick a yeast strain into performing the task at hand.” If a yeast strain is not producing the sensory profile given by a yeast supplier when used within the given temperature range, then one needs to examine one’s wort composition and/or ensure that one’s thermometer is calibrated. Temperature measurements should be taken as close to the middle of the fermentation vessel as possible.

Closing Thoughts

In the end, brewing is a continuous learning experience. Home brewers have the luxury of being able to brew without having to maintain a profit margin; therefore, one should feel free to experiment with wort composition, temperature control, and different yeast strains while fine tuning one's brewery and brewing process.