Beer Flavor Primer: Fruity Flavors, AKA, Esters
by George de Piro
This article will be best understood if you pour yourself a nice, fresh Hefeweizen before reading any further. Do NOT corrupt it with a lemon! If you can’t find a good Hefeweizen, you can try spiking 10-20 drops of banana flavor extract into a Coors Light. It won’t be as satisfying, but it will be educational. I’ll wait here ‘til you return…
Are you settled in with your beer? Excellent. Now we can begin:
Put your nose over the glass of Hefeweizen (or spiked Coors Light) and sample the aroma. Wipe the foam off you nose and try again, from a little further away. What do you smell? Yes, of course you smell beer, but put some more specific adjectives to it. If it is a fresh, well-crafted example of a German-style wheat beer, you should be able to discern a fruity aroma that closely resembles banana. There are probably some other potent aromas there, too, such as clove-like spiciness, vanilla and malt, but for now we are only concerned with the fruity, banana scent. How did it get there?
While some beers are fruity tasting because they contain fruit, and others are fruity because of the hops, the secret to Hefeweizen’s banana-like character is the yeast.
The yeast used to ferment a beer is critical to the flavor of the finished product. All styles of beers (pilsners, stouts, barley wines, etc.) fit into one of two major categories: ale or lager. The difference between ales and lagers is the yeast that is used to ferment them. Lager yeasts work best at cooler temperatures, about 50 ºF, and produce relatively few flavor-active compounds during fermentation. This yields a beer in which the flavors of malt and hops play center stage.
Ale yeasts work at higher temperatures, from 60-75 ºF. They tend to synthesize a host of flavor-active chemicals during fermentation. Some strains of ale yeast yield relatively neutral, lager-like beer, but others are truly prolific in their manufacture of esters and other chemicals. Esters are the compounds that give many fruits their characteristic flavors.
In the case of your Hefeweizen, the yeast produce copious amounts of the ester iso-amyl acetate, the same ester that is found in bananas. Other esters include ethyl acetate, which ranges from flowery to solvent-like as concentration increases, and ethyl caproate, which can be described as wine-like and fruity.
The astute reader will have noticed that some of the flavor descriptors in the above list do not seem all that palatable. The solvent-like ethyl acetate, for example, is unlikely to have broad appeal. Others, such as iso-amyl acetate, can define a beer style. How does the brewer control which esters are produced, and the quantities that end up in the beer?
The short answer is the careful selection of a yeast strain and fastidious control over fermentation parameters to attain the desired ester profile. The long answer will provide the brewers (and biochemists) in the audience with the foundation necessary to create the beer in their mind’s eye (um, mouth?). The rest of you may find this next bit somewhat esoteric and will want to skip to some other newsletter article.
Brewers’ yeast creates relatively large quantities of carbon dioxide and alcohol during fermentation. That is why beer is fizzy and gives you a mild feeling of euphoria. Ethyl alcohol is by far the most abundant alcohol produced by yeast. They do make some other alcohols, too, which are referred to as fusel alcohols.
All of these alcohols can be changed into esters inside the yeast cells. The chemical reaction responsible for this conversion is called esterification. The specific alcohol that is esterified determines which ester is produced. If one starts with ethyl alcohol, one ends up with ethyl acetate after the esterification reaction. Similarly, isoamyl alcohol is esterified to the banana-like isoamyl acetate. Easy, right?
This chemical reaction, like so many in living things, is controlled by a special type of protein called an enzyme. In this case, the enzyme is named Alcohol Acetyl Transferase, or AAT. Another molecule, called acetyl coenzyme A (aCoA) is also required.
To maximize ester production, as is often desirous when brewing Hefeweizen, one would select a yeast that produces copious amounts of the AAT enzyme. One would also want to maximize the alcohol and aCoA precursors. What will do this?
1. Increasing the rate of yeast growth will increase the production of fusel alcohols, which you will recall as important ester precursors. Pitching too little yeast can do this, as can high temperatures early in the fermentation. Under pitching yeast can cause other problems, however, and is not a recommended method of increasing esters. The brewer’s safest method is to pitch the yeast into relatively warm wort (68-72 °F) to encourage rapid yeast growth.
2. Increased oxygenation of the cool wort will decrease esters. Oxygen is used by yeast to produce unsaturated fatty acids, and in doing so they use up aCoA, leaving less for ester production. Oxygen also helps the yeast produce strong cell membranes, which may restrict re-entry of fusel alcohols, thus decreasing ester production by limiting a precursor’s presence in the cell.
By under-oxygenating, you will limit the amount of cell reproduction that can take place because of the limited sterol in the cells. By forcing yeast reproduction to stop well before the end of fermentation, the cells will pool aCoA rather than use it, thus increasing the amount of this ester precursor.
Like under-pitching, restricting oxygen is likely to cause more problems than the brewer wants to deal with, so it is not a recommended method of ester control for the home or small commercial brewery.
3. Fermenting under pressure, as some lager brewers do, will decrease esters by decreasing the rate of yeast growth, thus decreasing the fusel precursors. Most homebrewers and craft brewers don’t have to worry about this because they lack the equipment to ferment under pressure!
In the end, the safest way to increase esters in a beer is to choose an appropriate yeast strain and increase the rate of yeast growth by pitching and fermenting at higher temperatures. To decrease esters, as is desirable in many beer styles, choose an appropriately clean tasting yeast strain and pitch at low temperature. Always give your yeast plenty of oxygen!