About Esters

BY DAVE KAHLE

Craft beer is partially defined by the ingredients used in the brewing process. Most craft brewers follow tradition and stick to using water, malt, hops and yeast. Fortunately, there has been a great deal of experimentation with other non-adjunct ingredients in recent years. We have become used to seeing beer brewed with spices that you've never heard of before, or maybe rodent or elephant droppings, possibly butterfly tears, pies, confections, or exotic fruits.

Creativity isn't always successful, but it is fueling the Craft movement, and most of the time it's done with good intentions and not just used as a marketing gimmick. Obviously, all of these alternate ingredients are used to create the brewers target flavor profile. This is on top of a recipe usually de-signed with those four main ingredients mentioned earlier. However, the point of this article is to emphasize that with all of the ingredients used in modern craft brewing, what makes a brewer great, or their craft beer exceptional, still comes down to equipment, process, and techniques that coax or bend the physical ingredients to their will. One of the most important control points influencing beer flavor, is ester production.

An ester is a byproduct of yeast metabolism. Therefore, yeast selection is vital to recipe formulation. Yeast genetics control which metabolites can be produced and at what levels. Esters are formed by an alcohol reacting with an acid. The belief is that yeast create esters to remove the toxic effect of alcohols and acids in their environment. Esters are usually flavor active and give off notes of fruit or floral character. There are approximately 100 different esters that have been found in beer, but fewer than 10 strongly affect overall beer flavor. All beer has some level of esters in it, though ales tend to rely on them much more than lagers. When people talk about a beer being "clean", it usually refers to the beer having lower ester levels, along with off-flavors being sup-pressed. So what can a brewer do to control ester production?

It should be noted that the biochemistry of yeast esters isn't entirely agreed upon. While we might not understand ex-actly what is happening and why, a good brewer can still affect ester production with definitive techniques. Esters need a coenzyme called acetyl-CoA to act as a substrate for the enzymes to trigger ester formation. The catch is that acetyl-CoA is necessary for the synthesis of sterols that a yeast cell uses to build up cell wall material for budding and permeability. Sterols are formed by oxygen reacting with fatty acids in the presence of acetyl-CoA. Therefore, acetyl-CoA is not available for ester formation while sterol synthesis is taking place. Consequently, lower wort oxygenation levels increase ester production. Also, with higher oxygen levels, there is an increase in higher alcohols, so while this can decrease total ester production, it will produce a greater variety of esters.

The brewer can control some of the ester production by how much healthy, vital yeast is pitched. Higher pitching rates typically reduce ester formation. Pitching rate and the resulting esters is not only yeast strain dependent, and dictated by available enzymes, but linked to yeast health. Isoamyl acetate increases with a low pitching rate, while ethyl acetate decreases. Isoamyl acetate produces the banana notes that are expected in a Hefeweizen and brewers of this style often push the yeast to achieve this flavor profile. Isoamyl acetate is the second most common ester in beer, after ethyl acetate, which produces a solventy, nail polish aroma.

There will be some readers that question these theories that increase or decrease ester levels. For many years there have been conflicting studies released. This is primarily due to varying yeast strains, wort environment, and yeast health. The conflicting reports mainly concern yeast growth and pitching rates. There may not be a standard as to what a very high or very low pitching rate is, certainly when including all of the wort variables. It seems safe to say that moderately low pitching rates of healthy yeast will produce more flavor during growth, but if it is a very low pitching rate the beer will be solventy.

Wort composition is another variable. An increase in free amino nitrogen will increase ester production. High gravity wort will also lead to an increase in ester production, but a wort that was formed with high adjunct use can restrict yeast cell growth and deplete the acetyl-CoA pool, thereby reducing esters. Some scientists say there is no correlation with yeast cell growth and ester production, that it's more effected by enzyme health. However many brewers believe that an increase in cell growth means an increase in ester formation. Too much cell growth early in fermentation might cause the opposite; "The presence of high cell concen-trations early in fermentation would be predicted to lead to rapid utilization of wort amino nitrogen and oxygen for bio-synthetic reactions. In which case the acetyl-CoA pool may be depleted and ester synthesis reduced." (Boulton & Quain)

There are two other factors in ester production that aren't debated much. Fermentation temperature and fermenter shape. As, temperature increases more esters are produced, by the simple fact that there is more/faster enzyme activity as the temps increase. Higher fermentation temps also produce more fusel alcohol, which are then available as material for ester formation. At higher fermentation temperatures, yeast cell walls are more fluid and don't need to tie up as much acetyl-CoA in sterol production, leaving it available for ester formation.

Fermenter shape is a factor based on effecting the amount of CO2 surrounding the yeast during fermentation. An open fermenter will allow more CO2 to escape than a tall Cylin-dro-conical tank. The amount of pressure pushing down on the beer in a fermenter will control how much CO2 is al-lowed to blow off and out of the yeast's environment. This pressure reduces the ability of yeast to produce acetyl-CoA, alcohols, and the esters they would form.

So, if you've made it this far in the article, you probably al-ready know that the best brewers look beyond their recipes for flavor impact. Whether you adjust your yeast pitching rates, oxygenation levels, fermentation temperatures, wort composition, or fermenter shape, you will control ester pro-duction. Hopefully, armed with some knowledge of esters, you're comfortable discussing beer in terms beyond "Hoppy" and "Malty". You can punish your friends, like I do, by de-bating which factor caused the elevated ethyl acetate levels in their favorite beer.

From Issue 002 Fall 2012