A special “lecture series” often occurs at many academic institutions that is open to the public and scientific community. During my Ph.D. and post-doctoral training, this was called the “Dean’s Lecture Series” where world-renowned scientists would give great talks. In a nod to my academic past (as opposed to comparing myself to Nobel laureates!), I’ve decided to start a new series of posts called the “Homebrewer’s Lecture Series”. This reflects my ongoing reading and learning since I have begun teaching a yeast class again at Keystone Homebrew Supply (expect more experiments). Each post will focus entirely on one aspect of yeast fermentation, how it can impact your beer, and what you as homebrewer can practically do to control it. As always, my target audience is not just for the hardcore beer chemist, but also for the homebrewer at home who wants to dive deeper into the science of beer.
Esters
This remarkable class of molecules is responsible for the fruity aromas and flavors found in fermented beer. Depending on the style of beer, esters play a major role. Top fermenting English ales often have more fruity esters reminiscent of apples, pears, and dark fruit. In contrast, American ales often have less esters allowing for hop flavor to shine. Bavarian wheat beers (Kristallweizen), contain large amounts of isoamyl acetate (banana/clove), while Belgian ales (Saisons) often a have complex mix of esters that make them stand out.
Reacting an organic acid with an alcohol by yeast during fermentation forms these compounds. In the example shown below, methanol is reacting with butyric acid, which smells like vomit, to produce methyl butyrate, which smells like apples or pineapples. I love chemistry.
Enzymes called ester transferases catalyze these reactions within yeast cells that are ultimately secreted from the yeast into wort. The biochemistry behind esters reflects the balance of metabolism inside the yeast at any given moment. Below is a basic diagram of yeast biochemistry during fermentation (Peddie et al., J. Inst. Brew., 1990, Vol 96, pp. 327-331).
Maltose (wort) is transported into yeast cells, converted into pyruvate and acetyl CoA. These two molecules, and acetyl CoA in particular, sit at a biochemical crux inside a yeast cell during fermentation. Any factor (temperature, wort composition, etc…) that affects the amount of acetyl CoA will also affect many other aspects of yeast biochemistry. For the discussion of esters, think of this pathway as a one-way flow. That is, anything that produces more acetyl CoA will eventually produce more esters. For example, with a higher concentration of maltose (high gravity wort) present the more acetyl CoA is produced, which feeds into ester production (hence why barleywines are often fruity).
Acetyl CoA:
The vast majority of esters in beer are produced from yeast during fermentation and are influenced by three things:
- Wort composition and aeration
- Yeast strain and health
- Fermentation conditions
Wort Composition
As I mentioned before, the higher concentration of sugars you have, the more acetyl CoA is produced ending up with an increase in ester production. However, the availability of nitrogen also plays a key role. Higher amounts of nitrogen also pulls this pathway as well, producing more esters in addition to amino acids that are required for cell growth. In contrast, wort that is low in nitrogen slows the pathway and produces less esters. This can be seen in brews that use a high percentage of nitrogen-lacking adjuncts, such as light lagers. It is no coincidence that mass market macro lagers or extremely clean and neutral in taste.
Aeration
The presence of oxygen at the beginning of fermentation is absolutely required for yeast to produce unsaturated fatty acids (UFA) and have healthy cell walls. Acetyl CoA is required for this process and therefore any oxygen will draw acetyl CoA away from ester synthesis to UFA synthesis. In contrast, not enough aeration causes a buildup of acetyl CoA and ester formation. There is also some anecdotal evidence that oxygen increases the expression of some ester producing enzymes, although there is some controversy whether this is actually true.
Yeast Strain and Health
Yeast strain is probably the biggest factor of ester production during fermentation. Genetically, certain yeast strains are designed to turn on ester producing genes that can contribute to fruity overtones. English ale strains fall within this category. American ale strains, on the other hand, have a less capacity to produce esters. Lager yeast even less so. Some strains have been cultivated to produce one specific ester, such as hefeweizen yeast ability to produce banana and clove esters.
Although yeast strains vary in their ability to make esters, all yeast will produce a lot if they are stressed, including lagers. Yeast health is absolutely paramount to avoiding strange fruity flavors in your beer. Pitching an adequate amount of fresh yeast that has been plenty of glycogen levels and oxygen is critical in avoiding unwanted off-flavors. Pitching rates also have a minor influence on esters. That is, lower pitching rates increase ester formation, while high pitching rates tend to produce less. With my experiments I have found this to be highly dependent on yeast strain.
Fermentation Conditions
The state of beer fermentation has a large impact on ester production with temperature being the biggest factor. High temperature increases metabolic activity that in turn increases acetyl CoA production as described in above. The most critical time point to controlling fermentation temperature is about 1-3 days after pitching. Allowing a fermentation to go out of a specific temperature range is a definite way to get more esters. In a previous experiment looking at different fermentation temperatures, Wyeast 1388 produced more esters compared to lower temps. By reducing fermentation temperatures one can dramatically lower the fruitiness found in beer. Although not a concern for the vast majority of homebrewers, fermentation vessels can have a big impact on ester production. Tall, narrow vessels that produce lots of pressure will cause lower esters in the resulting beer.
In summary, there are many factors that can influence ester formation and I’ve summarized it here:
Can the Homebrewer Control Ester Production?
Short answer: yes. However, a dose of common sense is needed if you are concerned about esters in your beer. Do NOT skip oxygenating your wort just because this will increase ester production. You will have many other problems to worry about besides esters in this situation. Don’t avoid high gravity beers because you hate the over ripe fruit that comes along with it. If you want your English Bitter to have an awesome fruity bouquet, do not under-pitch to the point that fermentation starts three days post-pitch. Changing the dial on esters is more of a subtle affair and I’ll list what we as homebrewers can practically do to control it (in order of importance):
- Choose the right yeast strain. The most important factor in changing ester profile. Use yeast genetics to your advantage and if you want a clean English IPA, use an American ale yeast paired with British malts and hops. If you want to brew a fruity “east coast” IPA, an English ale yeast may be your cup of tea.
- Fermentation temperature. Controlling temperature during fermentation is a great way to rein in those esters. However, if you heart is dear to American ale yeast for your IPAs, but you want some extra fruitiness, don’t be afraid to go beyond the manufacturers suggested temperature range. Likewise, if you want “lager-like” character from Wyeast 1056, don’t be afraid to ferment below 60F. Remember, increasing metabolism (yeast growth) drives ester production
- Pitching rates. This last option is really a distant third to the ones above. The reason I say this is because drastically changing pitching rates can have other effects on your beer and stress the yeast in different ways. Having said that, minor changes can have a positive change to ester production (or inhibition). An English ale that gets a slightly lower pitch rate (0.5 million cells/ml of wort/degree Plato, for example) will slightly bump esters. The same holds true for overpitching.
I hope this primer on esters is somewhat beneficial. Keep in mind that controlling esters is somewhat of a balancing act and interconnected with other molecular compounds found in beer. In next lecture series I will write about fusel alcohols, how they relate to esters, and how you as a homebrewer can reduce or increase them.
