Flavor Esters: An Introduction

Figure 1. Active flavor esters (adapted from Verstrepren et al., 2003)

Many of the fruity flavors (aromas) we perceive in a beer are in fact the products of yeast metabolism and not from the addition of fruits (Figure 1). Some of the esters produced by yeast are considered “off” or undesirable flavors, while others are appropriate at low levels. Some esters even give certain beers (e.g. hefeweizens) their characteristic taste.

To be perceived, the concentration of each ester must meet a specific threshold level; therefore, low levels of ester production will not impart any noticeable aroma. However, esters can have a synergistic effect on individual flavors, affecting overall beer flavor at concentrations well below their threshold⁽¹⁾.

Esters found in beer are synthesized from Acyl-CoA (a long chain fatty acid with Coenzyme A attached, which breaks down into Acetyl-CoA), and fusel alcohols by ester synthase enzymes (e.g. Alcohol acetyl transferase) found in yeast (Figure 2)⁽¹⁾. Beer is not the only place you will find esters; they can also be found naturally occurring in fruits and artificially in many of the candies we eat. In fact, the industry of flavor ester production is in high demand, producing esters for all of those artificially flavored foods and drinks we love to consume in addition to the cosmetics and pharmaceutical industries⁽²⁾.

Figure 2. Enzymatic ester synthesis (Verstrepren et al., 2003)

Unlike ester synthesis in beer, the majority of flavor esters for these industries are produced in labs by chemically synthesizing each compound (Figure 3)⁽²⁾. Recently, the growing trend for “natural foods” has generated a push against the consumption and use of products containing artificial ingredients. These industries have responded by turning to the noble yeast, albeit a different genus and species from those used for brewing, to enzymatically synthesize esters. Esters produced by this process can be labeled as “natural” quelling the fears of the chemically conscientious consumers⁽¹⁾.

Figure 3. Chemical ester synthesis

But I digress...Back to the important things in life, brewing! Temperature, specific-gravity, oxygen, fusel (German for “hooch”) alcohols, and fatty acids all have effects on the production of esters but they may vary depending on the yeast strain. There is a direct relationship between temperature, specific gravity, fusel alcohols, and activated fatty acids (Acyl-CoA, and Acetyl-CoA) with the production of esters⁽¹⁾. The manipulation of these factors is important when considering the style of beer you plan to brew. In general, ales contain higher levels of esters than lagers in part because of the yeast species used (read more) as well as the higher temperatures used during ale fermentation ( above 58°F) compared to those used to "lager" (32°F-56°F). In the next esters post I will talk about each about each factor in greater depth and the mechanism by which they modulate ester production. Until then, happy brewing!

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1. Verstrepren, K.J., Derdelinckx, G., Dufour, J.P., Winderickx, J., Thevelein, J.M., Pretorius, I.S., Delvaux, F.R. 2003. Review: Flavor-Active Esters: Adding Fruitiness to Beer. J Bioscience and Bioengineering 96(2): 110-118 
2. Larios, A., Garcia, H.A., Oliart, R.M., Valerio-Alfaro, G. 2004. Synthesis of Flavor and Fragrance esters using Candida antartica lipase. Appl Microbiol Biotechnol 65:373-376