What is an Ester?
From Webster: 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.
Most common Esters
Ester produce the most common aromas and flavors associated with spirits. Acetic acid esters have the glue/pear smell familiar with Bourbon, Rum and Grappa. Lactic acid esters smell fruity (soft, mild, creamy) and are in Malt Whiskies like Scotch and Bourbon. Butyric acid esters smell fruity (tropical fruits, bubblegum) to create the essential Rum flavors.
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 10-18 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 and a catalytic 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. Doping ferments and low wines with small amount of strong acids, like sulfuric, increases the speed of this reaction. It is not recommended for the final product.
Some common bacterial infections create carboxylic acids that can become esters. Lactic acid bacteria like Lactobacillus produce lactic acid, Acetic acid bacterias like Acetobacter produce acetic acid, Butyric acid bacteria like Clostridium produce butyric acid. Lactobacillus is found on grains and fruit. Acetobacter is spread by fruit flys and other insects. Clostridium occurs in food preparation areas. In distilleries infections aren't considered significant unless they occur early and heavy in the fermentation process. The bacteria will compete for sugars with the yeast and will reduce alcohol yield. Once yeast establishes itself it will out compete the bacteria. Once the yeast has used up the simple sugars the bacteria can use the remaining more complex sugars without detriment to yield. Brewers will boil their wort to kill any infections. Most distillers will not boil their wort as to not damage Alpha-amylase or Beta-amylase since infections are seen as having either neutral or a positive effect on flavor. This helps increase conversion and alcohol yield. It is considered wasteful to expend the money and time on boiling the wort to kill infections. The longer the ferment the more carboxylic acids lower the pH, increasing the reaction.
There are several methods of varying ester formation:
- Yeast Selection: Yeast that expresses excess acetyl coenzyme A(aCoA), an acetyltransferase (ATF) enzyme, will increase ester production. Yeasts with these characteristics - Safale US-05, US-04, or other English Ale yeasts. Lager yeasts are considered to be 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. For practical purposes this means do not oxygenate or aerate the wort. You can further reduce oyxgen by boiling mashing water before use. Boiling reduces dissolved gases.
- Open Fermenting: In direct contradiction to the previous item, open fermenting appears to increase ester formation. Most of the highest ester spirits are typically openly fermented (Scotch, Rum).
- Doping with precursors: Muck Pits, Dunder Pits and Sour Mashing 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. These methods are also used to inoculate the wash with "good" bacteria.
- 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.
- Clarified Wort: In the book Whisky Technology, Production and Marketing it is stated that Yamazaki and Fuji-Gotenba clarify their wort. It does not explain if it is to target specific esters or if it increases esters.
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 tails. Tails do contain fatty acids and other alcohols that can form esters.
- High ester yeast for whiskey
- Pure acids and ester production
- Esters in whiskey mash - good/bad/indifferent??
- Does a dunder pit create esters or a feedstock for them?
- Table of esters and their smells
- Dr. Patrick Heist Questions and Answers
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.
- http://sourbeerblog.com/understanding-esterification/ <- Contains chart of acids and esters they form
- https://ucanr.edu/repositoryfiles/Oberholster_Ester_taints-92483.pdf <- Wine based, but show a chart of Ester formation over time
- http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0101-20612011000200012 <- Esters are in the highest concentrations in the heads, much less in the hearts, none in the tails.
- https://sciencebrewer.wordpress.com/2013/04/ <- Chart showing how to enhance or decrease ester formation
- https://www.khanacademy.org/science/organic-chemistry/carboxylic-acids-derivatives - Video series on esterification.