Question 8-What esters in beer produce undesirable flavours and where in the process can their production be controlled? Erin Fleming

Erin Fleming-11209224

Assignment 1-Task 2

Question 8

Due to their high volatility and low thresholds, esters contribute significantly to the unique flavours and aromas associated with specific varieties of beer. Over 100 different esters have been identified in beer (Eblinger 2009) but most brewing esters are formed by esterification of ethanol with fatty acids and acetyl coenzyme A (Yoshioka et al. 1981).  This reaction results in the formation of ethyl acetate, an ethyl ester which is the most common ester found in beer. Although these esters make up the largest percent of all esters in beer, those which are formed by the acetates of the higher alcohols, also called “banana esters” or “acetate esters” also play a role in the flavour and aromas (Briggs et al 2004).  At high levels and in certain varieties, acetate esters may be regarded as off-flavours.  Therefore, their formation must be controlled.

Controlling the formation of acetate esters in the brewing process can be done in a number of ways.  Manipulating the yeast strain, the wort specific gravity and sugar profile, wort oxygen and lipid content, genetic modification of the yeast, as well as the fermentation temperature will impact the level of acetate esters synthesized. Fewer acetate esters results in a beer with fewer off-flavours.

One of the most important factors affecting ester production is the yeast strain selection (Verstrepen et al. 2003).  The strain of yeast used will affect both the average ester production as well as the relative proportion of each individual ester produced (Verstrepen et al. 2003).  Equally, wild yeast strains such as Hansenula and Pichia produce large quantities of ethyl acetate by aerobic fermentation (Briggs et al. 2004). Therefore, if one wishes to minimize the acetate esters produced, the yeast population should be regulated to avoid wild strains.

Another important influence on esterification is the specific gravity and sugar profile of the wort.  High-gravity operations, which are very commonly used today, result in an unbalanced flavour profile due to the overproduction of acetate esters (Verstrepen et al. 2003).  This overproduction leads to beers which are “over-fruity” or “solvent-like.” The relative amounts of various sugars contained in the wort also affects the formation of acetate esters.  In general, worts which contain higher levels of glucose and fructose produce more esters than those containing more maltose (Younis et al. 2000).  Therefore, by adding a supplemental maltose syrup, one could reduce the acetate ester concentrations.

The formation of volatile esters is diminished with an increase in wort oxygen and lipid content.  Therefore, wort aeration or oxygenation is recommended as a possible means by which to decrease unwanted esters (Verstrepen et al. 2003).  Regarding lipids, unsaturated fatty acids in the wort also decrease the synthesis of unwanted esters.  This is most likely due to the repression of the ATF gene transcription (Meilgaard 2001).  Manipulation of the wort lipid content can be achieved through changes in the filtration process (Verstrepen et al. 2003).  

Generally, a decrease in temperature during fermentation also decreases ester synthesis (Eblinger 2009).  Likewise, increased fermentation temperatures ranging from 10-25°C results in an increase in ester production (Verstrepen et al. 2003).  The exact increases are strain-dependent, but by simply decreasing the temperature at which fermentation occurs, one can minimize esterification.

Acetate ester formation in brewer’s yeast can also be controlled with the genetic modification of specific genes.  According to Verstrepen et al., it is possible to “create new yeast strains with desirable ester production characteristics.”  Additionally, the manipulation of the ATF gene specifically is one of the most important control points affecting ester synthesis (Verstrepen et al. 2003).

References:

Briggs, D; Boulton, C; Brookes, P; Stevens, R (2004). Brewing Science and Practice. New York: Woodhead Publishing. 683-687.

Eblinger, H (2009). Handbook of Brewing Processes, Technology, Markets. Online: Wiley-VCH Verlag GmbH & Co. 134.

Meilgaard, M (2001). Effects on Flavour of Innovations in Brewery Equipment and Processing: a Review. The Journal of the Institute of Brewing and Distilling.107, 271-286.

Verstrepen, K; Derdelinckx, G; Dufour, J; Winderickx, J; Thevelein, J; Pretorius, I; Delvaux, F. (2003). Flavor-Active Esters: Adding Fruitiness to Beer. Journal of Bioscience and Bioengineering. 96 (2).

Yoshioka, K. Hashimoto, N. (1981). Ester Formation by Alcohol Acetyltransferase from Brewers' Yeast. Journal of Agricultural Biology and Chemistry.45

Younis, 0; Stewart, G (2000). The Effect of Wort Maltose Content on Volatile Production and Fermentation Performance in Brewing Yeast. Brewing Yeast Fermentation Performance.1(2).