A brief look at the organisms that give us beer
By Jack Kenny
Yeast is complicated and sciency, and it rarely comes up when you’re selling beer. Even when it does, the smarter-than-average customer is usually satisfied to learn that yeast converts the sugars in beer into alcohol and carbon dioxide and some aroma components. Lately, however, folks have been asking questions that call for more detailed answers, so it’s time to focus a bit on this mysterious little fungus.
Mankind lived in blissful ignorance of the nature of yeast from the dawn of time until the 18th and 19th centuries, when its nature was revealed. “The effect of alcohol on the human mind is profound enough that every ancient society that stumbled on alcohol associated it with the divine. Little wonder that the foamy evidence of yeast in action was known to brewers in the Dark Ages as ‘Godisgood.’ They couldn’t see where it came from, they couldn’t explain it, but they knew it turned mundane ingredients into something inspirational,” wrote Julie Johnson Bradford in All About Beer magazine back in 2003.
In the 17th century, Antoni van Leeuwenhoek observed yeast through his high powered lenses and described it as globules floating in a liquid, probably parts of the grain matter. Nobody at the time thought it was an organic entity. Over the ensuing decades several natural philosophers (the word “scientist” was unknown at the time) made steps toward understanding fermentation, but the nature of yeast eluded them.
Only in 1857, when Louis Pasteur published his findings on alcohol fermentation, did we learn that brewers yeast is a living organism that consumes sugars and converts them into a range of byproducts. In 1907, Eduard Buchner won the Nobel Prize in chemistry for his discovery of zymase, the collection of enzymes inside yeast cells that carry out the actual fermentation.
Ale yeast ferments beer at higher temperatures, roughly 55° to 75° F, and settles at the top. Lager yeast requires cooler fermentation, in the 45° to 60° range, and settles to the bottom as it finishes. Ale yeast is high in esters, which give characteristic aroma and flavor components to various styles. Ester content in lagers is low, by preference, meaning that malt and hop flavors will dominate.
Esters are tricky. Some are desirable in one style of beer yet offend the senses in other styles. Some are present in all beers, yet do not make themselves known until the conditions (age, temperature, exposure to light or air) change. The most common ester found in beer is ethyl acetate, which has the aroma of nail polish. Fortunately, this particular ester has a high odor threshold, so it rarely gets noticed by our olfactory apparatus. Altogether there can be up to 60 esters in beer, but only 10% or so make their impact on flavor and aroma.
Here’s a list of the most common esters (from Beer Advocate’s Yeast Guide):
• acetaldehyde: associated with a green apple aroma
• diacetyl: a buttery taste or aroma, butterscotch
• dimethyl sulfide: taste or aroma of sweet corn, cooked vegetables
• fruity/estery: bananas, strawberries, apples, other fruit
• medicinal: chemical or phenolic character
• phenolic: plastic, Band-Aids, smoke, or cloves
• solvent: reminiscent of acetone or lacquer thinner
• sulfur: rotten eggs or burnt matches
One can get obnoxious, if one prefers, by dropping some rich ester names at parties. Isoamyl acetate is common in wheat beers, smelling of artificial banana or circus peanuts. Ethyl butyrate gives us tropical fruit, pineapple and Juicy Fruit gum. Ethyl hexanoate smells like red apple or anise.
In an essay titled “The Poetry of Esters” at craftbeer.com, Julia Herz makes a powerful statement about yeast and the studied enjoyment of beer: “Not being aware of beer’s yeast-derived offerings, such as esters, is like hearing a song in the background at a shopping mall compared to sitting in your favorite chair listening to the same song with headphones and picking out prominent instruments and vocals. As with music, it’s possible to decipher how each element in a craft beer adds layer upon layer of influence to the whole gorgeous production.”
Jack Kenny has been writing The Beer Column for The Connecticut Beverage Journal since 1995. Write to him: firstname.lastname@example.org