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Grape expectations: Swetha Sivakumar on the science of wine

BySwetha Sivakumar
Feb 10, 2024 10:51 PM IST

The recipe for wine is simple enough. But what unfolds in the barrel is a process so intricate that it was only decoded, by Louis Pasteur, in the 1800s.

As Valentine’s Day nears, with its promises of sugar and spice, let’s drift past the hearts and balloons and peer into a glass of wine.

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

From a scientific perspective, this is a fascinating liquid. The recipe seems simple enough. What unfolds in the barrels is a process so intricate, however, that it was only decoded, by Louis Pasteur, in the 1800s.

Most wine is made by pouring the juice of grapes into large vessels, and adding yeast (which is technically a type of fungi).

Now, grapes are made up largely of the simple sugars glucose and fructose (about 18% to 30%), with the rest of their volume consisting of water and small amounts of acids, proteins and flavonoids. The simple sugars instantly draw the attention of any microbes present. And so, the filamentous fungi, yeast and bacteria that were locked in with the air and the grape juice, or that were present in the barrels themselves, now compete to consume this sugar.

Yeast races ahead of the rest, because of a trick it has up its sleeve. Its standard metabolisation of the sugars allows it to divide and grow rapidly. But simultaneously, and crucially, it triggers something called the Crabtree effect (named after biochemist Herbert Crabtree), in which it slowly also breaks sugars down into ethanol, a type of alcohol.

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

Like so many victors, though, the yeast doesn’t know when to stop. Left to itself in an anaerobic space, it will continue to metabolise sugar into ethanol, even to its own detriment. Once alcohol levels reach 10% to 15%, most of the yeast itself dies, becomes dormant or clumps up and sinks to the bottom. This works for the oenophile. What emerges from the barrel is a clear, elevated drink that bears little resemblance to the cloudy, opaque juice that went in.

Of course, things don’t always go this smoothly. There is sometimes a sudden souring and a breakdown of the process. Fermentation was once so unpredictable, in fact, that the Sumerians, c. 4000 BCE, put the variations down to the whims of a goddess. They named her Ninkasi (or, lady who fills the mouth), in a gesture of appeasement.

Down to the 19th century, batches of wine would have to be disposed of after they turned sour, bitter or flavourless. In France, already a world leader in production, this caused devastating losses.

Finally, in 1863, Napoleon III asked the renowned chemist Louis Pasteur to help treat the “maladies” of wine, by finding out exactly what went on in the barrels, and how it could be better controlled. It is easy to now devalue Pasteur’s findings, but in just three years, he managed to sweep away aeons of ignorance.

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

He decoded the role of oxygen too, and laid out how the presence of certain bacteria post-fermentation could contaminate and ruin aged wine. By the time Pasteur was done, there was far less guesswork involved. It was understood that wine, once aged in its barrel, had to be transferred from that permeable container to an impermeable and sterile one such as a bottle.

“Wine faults” still occur — heat can alter flavour; yeast can die off and halt aging — because even Pasteur couldn’t answer all the questions. This process depends, after all, on the consumption and excretion patterns of a living thing. And living things, unlike chemical and physical processes, are by nature variable.

So, well, savour that wine on Wednesday, appreciate the mysteries it still holds. And do keep in mind its effects. Be not like the fruit flies exposed to alcohol vapours in 2013. They showed increased courtship behaviour, noted a paper by American researchers Anita Devineni and Ulrike Heberlein, published in Annual Review of Neuroscience. They also became excited, bumped into obstacles more, moved more swiftly, fell over, and finally fell asleep.

Have a lovely Valentine’s Day. May you only fall asleep where you intended.

(To reach Swetha Sivakumar with questions or feedback, email upgrademyfood@gmail.com)

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