Ester

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Ester
Ester Aroma/Flavor chart


What is an Ester?

From Webster: es·ter Pronunciation: 'es-t&r Function: noun. Etymology: German, from Essigäther ethyl acetate, from Essig vinegar + Äther ether. Any of a class of often fragrant compounds that can be represented by the formula RCOOR´ and that are usually formed by the reaction between an acid and an alcohol with elimination of water.

More easily stated, an ester is a molecule of alcohol "fused" with a Carboxylic acid.

Esters are flavor and aromatic compounds that contribute to the flavor profile of whiskeys, rums, and brandies. Esters can create complex flavor profiles and can be detected at very low levels - a few PPM (parts per million). Many esters have distinctive fruit-like odors. They can also introduce flavors of constituent materials that are not actually present (bananas, cherries, cloves, etc). Many occur naturally in the essential oils of plants. This has also led to their commonplace use in artificial flavorings and fragrances when those odors aim to be mimicked. Their production can be encouraged or discouraged by changing the conditions and materials used in the fermenting, distilling, and aging processes.

Ester Formation

In brewing esters are formed through enzymatic activity during fermentation or at any time through Fischer–Speier esterification. Typical preference in distilling is for longer chain fatty acid esters. Those are more likely to be formed by enzymatic action during fermentation. Shorter chain esters are more typically formed during distillation and aging by Fischer esterification. The heat of the distillation process appears to increase the speed of the Fischer reaction.

Ester formation in Fermentation

During the growth cycle yeast will produce fatty acid esters in order to build cell walls. These esters are formed by enzymes. The enzyme ester formation reaction is reversible, meaning that the enzyme will also return esters to an alcohol and acid. These esters are bound inside the yeast cell and cannot be used as long as the cell wall has not been ruptured. During autolysis (when the yeast dies) the cell walls are ruptured by protease enzymes. After the cell walls are ruptured the enzymes will both create and destroy esters. For ferments the level of ester production increases until day 10-12 depending on wash make up, at that point ester levels equalize.

Ester formation in the still and during aging

Esters can form anytime an alcohol molecule is in the presence of an carboxylic acid. This is mainly attributed to Fischer esterification. Fischer esterification increases as temperatures increase and with lower [pH] levels. This leads to a significant increase in ester formation during distillation. It may also be a contributing factor to the "Slow Distillation" movement to create a better product. In aging the ester formation process continues but at a slower pace as product is normally stored at room temperature. If stored in a wooden barrel tannic acid from the wood is absorbed into the product at it ages and it lowers the pH slowly. This acidity increases ester production. This leads to a more complex flavor profile.

Increasing/Decreasing Esters

There are several methods of varying ester formation:

  • Yeast Selection: Yeast that expresses extra acetyl coenzyme A(aCoA) or acetyltransferases (ATF) will increase ester production. Yeasts with these characteristics - Safale US-05, US-04, or other English Ale yeasts. Lager yeasts are considered to low ester producers.
  • Pitching rate: Esters are formed during yeast growth. A low pitching rate will encourage ester production, high pitching rates will decrease it.
  • Fermentation Temperature: Warmer temperatures increase yeast growth and ester production. Ferment at the top end of your yeast's range for more esters.
  • Top pressure: increased pressure in the fermenting vessel decreases both ester and fusel alcohol formation.
  • Under oxygenating wash: During reproduction, yeast use oxygen to produce unsaturated fatty acids which also uses up aCoA, a precursor for ester production. Therefore by reducing the amount of oxygen available for the yeast, you will effectively be increasing the available ester precursor acetyl coenzyme A, making it available to produce more of the fruity esters you want.
  • Doping with a catalytic acid: Doping the wash or distillate with a catalytic acid will increase ester formation. Sulfuric acid is very strong acid that increases Fischer esterification.
  • Doping with a precursor: Muck Pits or Dunder Pits are used to dope ferments with Carboxylic acids to make extra available for ester formation. One can also target specific esters by doping the distillate with certain carboxylic acids.
  • Doping with tails: Tails contain fatty acids and fusel alcohols. By doping a ferment with tails it is similar to using muck. By going deeper into the tails during distillation will allow for more ester formation during maturation.

Expression in Distillates

Esters, regardless of type, tend to express in the foreshots, heads and early hearts. There are few esters found in the later hearts and none in the tails. Tails do contain fatty acids and other alcohols that can form esters.

External Links

Forum Posts

Ester Research

Some of these links are easy to read, some require a better than basic understanding of chemistry. Once you have started reading the easier ones the tougher ones start to become understandable. Don't be afraid, keep reading and it will come with time.

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