Beer: It’s the Yeast that Counts

March 13, 2015

By Medical Discovery News

Humans have been drinking fermented beverages such as beer for thousands of years. Long before microbrews were invented, men relied on wild yeast to produce their ale. But we aren’t the only ones who found something so appealing in yeast – fruit flies do as well.

A beer’s aroma and flavor are due to it ingredients, which usually include malted grains, commonly barley and wheat, then hops for bitterness. However, a key determinant of a beer’s flavor is the yeast. During fermentation, yeast introduces a variety of aromatic molecules called acetate esters. The question of why yeast produces these aromatic compounds has been a mystery, until now.

Fifteen years ago, a scientist in Belgium named Kevin Verstrepen rushed out of the lab to get to a bar one Friday evening, leaving three flasks of yeast sitting open on his lab bench. Upon returning on Monday, he discovered that fruit flies from a neighboring genetics lab had invaded his own. Fifty flies were floating in the flask of yeast that had been engineered to make 100 times more of the aromatic molecules that a standard strain. Two flies were in the flask of normal yeast, and none were in the flask of yeast that did not produce those aromatic molecules. He deduced that yeast produces aromatic molecules with aromas similar to ripening fruit to attract flies, but wondered why.

Years later, Verstrepen teamed up with other scientists to repeat the experiment, and once again the amount of aromatic molecules produced by a strain of yeast correlated with the number of flies attracted to it. To be sure the aroma was responsible for attracting the flies, they synthesized the aromatic molecules and added them to a flask of yeast that did not produce them. Sure enough, the flies returned. They then used flies whose brains had been modified to express a fluorescent protein, which would light up when there was neural activity. They discovered that a compound called isoamyl nitrate, which has a smell similar to overripe bananas, causes intense brain activity in flies. Fruit flies apparently can use such aromatic molecules to find yeast, an important part of their diet.

They then wondered whether the yeast that attract flies have an advantage by spreading further into their environment. Using normal yeast and scentless yeast that had been colored differently showed that the aromatic yeast spread four times more into the environment. Basically, yeast emit signals in the form of molecules that flies are attracted to as a food source, but the yeast benefits by being spread by the flies. Yeasts do not have any means of propulsion, so hitching a ride to a new food source on the tiny hairs of a fruit fly’s legs solves their travel problems.

While not reported in a scientific publication, they collected fruit flies from their homes, crushed them up, and analyzed them for yeast. Sure enough, the flies harbored yeast that produced the aromatic compounds. For the final experiment, Verstrepen cultured the yeast and guess what he did with them?  Made beer, of course.  It was reportedly delicious.

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