Beer flavour terminology
Class 1 – Aromatic, fragrant, floral, fruity
Subclass 0130 – Estery
Esters are a class of compounds that are formed by the combination of a fusel alcohol with a fatty acid, producing many powerful but pleasant fruity aromas and tastes. It is common to most fermented products, including wine and whiskey
Synonyms for esters are vinous and winy which are very aromatic-type, positive flavours. There are at least 90 esters identified in beer. Esters are a by-product of fermentation. During the yeast growth stage, fatty acids are built up into sterols and other compounds that make up, amongst other structures, the cell wall of the yeast cell. If the wort lacks oxygen this cannot happen and the yeast cell will, instead attach these acids to alcohols, making esters. Esters also act as solvents to keep flavours in solution that are not soluble in water.
How to control ester formation
- The wort must be strongly aerated (ideally oxygenated) at the beginning of fermentation.
- The higher the gravity of the wort, the less oxygen it will hold – this is why high gravity beers always feature a disproportionate amount of esters. To hold them in check in high gravity brewing operations brewers inject pure oxygen into the wort at the start of fermentation. This unfortunately raises the diacetyl level again but the brewer can cope with this later on in fermentation.
- Ester formation is higher if the wort temperature at pitching and during fermentation is higher.
- The higher the pitching rate the lower the ester formation – because the yeast needs to reproduce less before fermentation begins. Esters and other by-products are created mainly during the growth rate and a high pitching rate will result in a large proportion tired yeast cells, especially after several repitchings. As a result, each fermentation is less vigorous than before, takes more time, and often yields a high terminal gravity. The best practice is thus to pitch a proper amount (1 litre thick slurry per hectoliter) of active, viable yeast into thoroughly oxygenated wort.
- If you get a good fermentation but unacceptable amounts of by-products, change to a “cleaner” strain of yeast. Some yeast strains are notorious for their high ester formation and are specifically chosen to brew for example a very estery Old Ale or Barley Wine.
- The tight-pot fermentation system controls ester formation. There is more ester formation in stirred or continious fermentation systems, which takes about two and a half to three days.
- If you add lipids or fatty acids, you get more ester formation. You can experiment with this by taking the squeezings from your spent grains and adding it to the wort. You will get a faster fermentation and more esters. This method unfortunately leads to oxidative flavours.
Esters predominant in beer
Ethyl Acetate – is the product of the reaction between ethanol and acetic acid (or acetyl CoA, which is called activated acetic acid). This ester has light fruity or solventlike notes (Commercial acetone has a use as nail polish remover) Ethyl Acetate is the ester with the highest concentration.
Isoamyl Acetate – is an ester made up from larger acids andIor higher alcohol’s and tend to have a powerful fruity banana or peardrop aroma and taste. Can be tasted at 2 ppm.
Ethyl Butyrate – aroma of pineapple.
Ethyl Hexanoate – applelike with notes of aniseed.
British ales yeast’s often produce noticeable amounts of fruit esters and are highly prized for this quality. A good brewer will control the amount of ester production in such a way that it does not distract from the drinking pleasure of his ales. Lager brewers on the other hand will go out of their way to eliminate esters from the flavour profile of their lager beer.
Notes compiled by:
Moritz Kallmeyer – Master Brewer of Drayman’s Brewery & Distillery