Expectations of the Grape: Swetha Sivakumar on the Science of Wine

Expectations of the Grape: Swetha Sivakumar on the Science of Wine

As Valentine’s Day approaches, with its promises of sugar and spice, let’s move past the hearts and balloons and look into a glass of wine.

The process depends, ultimately, on the consumption and secretion patterns of one living thing – the yeast – over an extended period. As a result, dramatic changes can occur, in ways that remain unexplained, even today. (Shutterstock)

From a scientific point of view, this is an attractive liquid. The recipe seems quite simple. What unfolds in the barrel is such a complicated process, however, that it was only decoded by Louis Pasteur in the 1800s.

Most wine is made by pouring the grape juice into large containers and adding yeast (which technically is a type of mushroom).

Now, grapes consist mainly of the simple sugars glucose and fructose (about 18% to 30%), while the rest of their volume consists of water and small amounts of acids, proteins and flavonoids. Simple sugars immediately attract the attention of any microbes present. And so, filamentous fungi, yeast and bacteria that were trapped with air and grape juice, or that were present in the barrel itself, now compete to consume this sugar.

Maya gets ahead of the others, because of a trick up her sleeve. Its standard metabolism of sugars allows it to divide and grow rapidly. But at the same time, and crucially, it causes something called the Crabtree effect (named after biochemist Herbert Crabtree), in which it also slowly breaks down sugars into ethanol, a type of alcohol.

This process yields nearly 20 times less energy than a regular decomposition cycle. But it increases the alcohol levels in the grape juice and quickly makes the environment in the barrel too toxic for most microbes. In this way the yeast eliminates its competition, once and for all.

However, like many winners, yeast does not know when to stop. Left to itself in an anaerobic space, it will continue to metabolize sugar into ethanol, even to its detriment. After alcohol levels reach 10% to 15%, most of the yeast itself dies, becomes dormant, or collects and sinks to the bottom. This works for the oenophile. What emerges from the cask is a clear, elevated brew that bears little resemblance to the cloudy, dark liquid that entered.

Of course, things don’t always go so well. Sometimes there is a sudden drying and a breakdown of the process. Fermentation was once so unpredictable, in fact, that the Sumerians, c. 4000 BC, set the variations at the whim of a goddess. They called her Ninkasi (or, the mouth-filling lady), in a gesture of appeasement.

Until the 19th century, wine barrels would have to be thrown away after they turned sour, bitter or tasteless. In France, already a world leader in production, this caused devastating losses.

Finally, in 1863, Napoleon III asked renowned chemist Louis Pasteur to help treat wine “diseases” by discovering exactly what was going on in the barrels and how it could be better controlled. Now it’s easy to discount Pasteur’s findings, but in just three years, he managed to wipe out ages of ignorance.

People have known for a long time what yeast could achieve, but never understood how. Pasteur used observation and testing to build a biological model and clearly demonstrate that fermentation involved living beings such as yeast and bacteria. Making fermentation not just a chemical process, but a biochemical process.

He also deciphered the role of oxygen and showed how the presence of certain bacteria after fermentation can contaminate and destroy aged wine. By the time Pasteur finished, there was much less guesswork involved. It was understood that the wine, aged in the barrel, had to be transferred from that permeable vessel to an impermeable and sterile one, such as the bottle.

“Summer mistakes” still happen – heat can change the taste; yeast can be extinguished and stop aging – because even Pasteur could not answer all the questions. This process depends, ultimately, on the consumption and excretion patterns of an organism. And living things, unlike chemical and physical processes, are by nature variable.

So enjoy that wine on Wednesday, appreciate the mysteries it still holds. And keep in mind its effects. Don’t be like fruit flies exposed to alcohol vapors in 2013. They showed increased courtship behavior, noted a paper by American researchers Anita Devineni and Ulrike Heberlein, published in the Annual Review of Neuroscience. They also got excited, bumped into obstacles more, moved faster, fell, and finally fell asleep.

Have a nice Valentine’s Day. You can only fall asleep where you thought.

(To contact Swetha Sivakumar with questions or comments, email [email protected])

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