Understanding esters

The word esters is thrown around a fair amount in popular brewing books, but is never really fully explained. Any brewer who has researched wheat ales will be able to tell you that esters are required for the style, and could likely list several variables that affect the production of esters (yeast strain, fermentation temperature, starting gravity). However, not many books go into exactly what an ester is. Broadly speaking, when alcohol reacts with a carboxylic acid in an acidic environment, an ester and a water molecule are formed.

Types of Alcohols

There are numerous alcohols, all of which have the potential to create an ester. The most common beer esters have ethanol as the reactant alcohol since it is so abundant, however the heavier fusel alcohols can react to form esters as well. Chemically speaking, all alcohols have an OH group bound to a carbon. The figure below shows the chemical structures of some of the most basic alcohols. 

Figure 1: Structure of some common alcohols

The most basic alcohol is methanol, which only has a single carbon atom. The carbon atom has bonds to an oxygen and three hydrogen that form a tetrahedral around the atom. Methanol is the alcohol created in distilling processes that can cause blindness. It has a lower boiling point than ethanol and will be a large portion of the first liquid collected from a still.

The alcohol that we are most familiar with is ethanol, which gets you drunk. It is the alcohol that is created in the highest concentration during fermentation. If fermentation temperatures are not ideal, alcohols larger than ethanol are created. These alcohols often have from 3 to 5 carbon atoms, but sometimes more. The 5 carbon species, amyl alcohol is one of the most common fusel alcohols in beer. There are several isomers where the orientation of the carbon atoms change with reference to one another. However by definition, all the amyl alcohols have an OH group bound to a carbon atom.

Carboxylic Acid

A carboxylic acid is an organic acid that contains a carboxyl group. This class of molecule is common in nature and includes the amino acids as well as acetic acid (vinegar). Esterification of amino acids does occur, however it is rare to see the concentration of any ester reacted from an amino acid to exceed 100 parts per billion, and there is no perceivable effects until at least 10 ppm. However, acetic acid plays an important role in perceivable esters in beer.

Figure 2 - Generic Carboxylic Acid

The picture above shows the generic form of carboxylic acid. It requires a carbon to be bound to a hydroxyl branch in the same was as is required for an alcohol. However, the carbon is also double bound (it shares 4 electrons versus 2) to an oxygen. The R in the figure denotes the remainder of the molecule, which is unique for each individual acid. During esterification, the two hydroxyl groups react (among other things). The end result is that a water molecule is formed, and the two molecules become bound at the remaining oxygen.

The Reaction

Two of the more common esters are ethyl acetate and isoamyl acetate. These esters are the result of acetic acid reacting with ethanol and isoamyl alcohol respectively. Acetic acid does not exist freely in solution, instead it is bound in an enzyme called Acetyl-CoA. Through hydrolysis, it can be liberated for use as a reactant in esterification.

 Ethyl Acetate can taste like green apples in low concentrations and possess a harsh solvent property at high concentrations. Isoamyl acetate is what gives hefeweizens their characteristic banana flavor. There is actually an entire industry devoted to synthesizing the chemical for use as artificial banana flavor.

It's also interesting to note that isoamyl acetate is one of the pheromones a honey bee releases when they sting.

 All esters have a similar structure: a chain of carbon atoms with a single oxygen in the middle. The reaction requires an acidic environment. This is because the free hydrogen ions act as a catalyst, promoting the reaction. The hydrogen ions momentarily become bound to the molecules priming them for reaction. However, after the reaction has occurred, they dissociated from the ester and dissolve back into solution.

Figure 3 - Acetic Acid

Figure 3 shows two models for acetic acid. The one on the left is a 3-D rendering of the molecule. The black central atoms are carbon, the red are oxygen, and the white are hydrogen. The model to the right is a simplified version common in organic chemistry. The line segment connect to carbon atoms unless otherwise noted. In Figure 4, the isoamyl acetate shows a case where a carbon chain is broken up by an oxygen molecule.

For each carbon atom, the number of hydrogen atoms attached is equal to four minus the number of lines connected to it. This is evident in the acetic acid model, where the terminal carbon atom has 3 hydrogen and one connection, and the central carbon atom has 0 hydrogen with 4 connections (one oxygen has a double bond).

Figure 4 - Reaction producing isoamyl acetate

The above figure shows the reaction forming isoamyl acetate. Acetic acid and isoamyl alcohol are the reactants on the left and isoamyl acetate and water are the products on the right. In this example, acetic acid has already been liberated from acetyl-CoA. The colors in figure 4 show the atoms destiny through the reaction. The oxygen in the acid turns into the oxygen found in the water product, while the oxygen in the alcohol forms the bond. This image was taken from the ucdavis website (Link here).

How to promote/discourage ester formation

There are several ways to promote or discourage ester formation, all of which are done during the fermentation process. The first factor is yeast strain. Different strains of yeast will produce different levels of  different esters. The temperature also plays a role in their formation; increased temperature during fermentation leads to more fusel alcohols.

Finally, pitch rate affects the formation of esters. If a beer is inoculated with fewer yeast cells at the beginning of fermentation, the beer will have more esters. Because there is low competition for the smaller yeast culture, they reproduce at a higher rate. This generates a larger concentration of acetyl-CoA, which contains a common reactant in the esterification process.


Note: Unless otherwise noted, All images were obtained from wikipedia with some MS Paint editing.