Other Flavor Compounds

What are hop esters?

Esters are compounds that are formed by linking an organic acid to an alcohol. Esters have fruity aromas such as banana, apple, pear, or other aromas depending on the compound. Brewers are well aware of esters as a fermentation biproduct, whereby yeast enzymes catalyze the reaction of ethanol with different organic acids to form ethyl esters that contribute to the unique flavor profile of each yeast strain.

Hops also contain esters, which comprise about 15% of the total composition of hop oils. One example is 2-methyl isobutyrate (2MIB). While hop esters receive much less attention than terpenes and thiols, they play an important role in determining the unique flavor profile of different hop varieties.

Yeast interaction with hop esters

How to optimize hop ester biotransformation 

The general trends of the isobutyric esters (examples in the table above) contributed from hops are a gradual decrease during fermentation, and a corresponding increase in ethylated versions of these esters (which are virtually not present in the initial wort). Typically around 50-60% of the initial isobutyric ester in the wort at the start of fermentation will carry over to the final beer – therefore, to increase concentration of these compounds in the finished beer, target late kettle/whirlpool/dry hop additions of hop varieties high in these esters.

If the brewers target is some degree of biotransformed ester character, use late kettle/whirlpool as your addition point for high ester hops. If the converse, consider utilising the hops mentioned in the table above as dry hop only. There is not yet very much data available for how different yeast strains interact with hop esters. 

Lactones

What are lactones?

Lactones are cyclic esters formed by the condensation of an acid and an alcohol group on the same molecule. They are typically associated with aromas of stone fruit and coconut. Lactones may also contribute to a creamy mouthfeel if present in high enough concentrations.

Sources of lactones

Lactones are present in hops and are part of the distinct aroma profile of specific varieties. One notable example is Sabro hops, which have a characteristic aroma of stone fruit and coconut.

Lactones can also be formed by the yeast during fermentation using fatty acid precursors derived from malt. Oats and corn are good sources of fatty acids due to their higher lipid content.

Lactone flavor impact and sensory threshold

Lactones are usually associated with aromas of stone fruit and coconut, but the sensory characteristics vary for each specific compound. They are highly flavor active with sensory thresholds in the ppb range. Some examples of lactones found in beer are shown in the table below.

Lactones are also interesting because they have a synergistic relationship with esters and terpenes which, when combined, can increase sensory perception of stone fruit and berry characters, as well as overall intensity¹¹.​ 

Lactone Biotransformation

Fatty acids derived from malt are oxidized in the mash into hydroxycarboxylic acids. Yeast will interact with these hydroxycarboxylic acids to form lactones. Yeast may also modify lactones by interacting with their chemical structure. One potential way to increase lactones created by yeast is to use malts or grains known to be higher in certain fatty acids, such as oats and maize/corn.

Research by Lallemand Brewing has shown that the type and amount of lactone present in a beer is yeast strain dependent. However, the mechanisms by which brewing yeast interacts with lactones is not well understood, and it is an active area of research.

References

11 Hotchko, R., and Shellhammer T.H. (2017). Influence of Ethyl Esters, Oxygenated Terpenes, and Aliphatic γ- and δ-Lactones (C9–12) on Beer Fruit Aroma. Journal of the American Society of Brewing Chemists, 75(1), 27-34.