Grape Forecast: Swetha Sivakumar on the science of wine

As Valentine's Day approaches, with its promise of sugar and spice, let's drift past the hearts and balloons and look over a glass of wine.

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The process ultimately depends on the consumption and secretion patterns of the organism – the yeast – over a long period. As a result, radical differences can occur, in ways that remain unexplained, even today. (stock struggle)

From a scientific point of view, this is a wonderful liquid. The recipe seems simple enough. However, what unfolds in the barrels is a process so complex that it was only deciphered by Louis Pasteur in the 19th century.

Most wines are made by pouring grape juice into large vats and adding yeast (which is technically yeast). A type of fungus).

Now, grapes consist largely of the simple sugars glucose and fructose (about 18% to 30%), and the rest of their volume is made up of water and small amounts of acids, proteins, and flavonoids. Simple sugars immediately attract the attention of any microbes present. Thus, filamentous fungi, yeast and bacteria that were trapped in the air and grape juice, or that were present in the barrels themselves, now compete to consume this sugar.

Yeast races ahead of the rest, because of the trick it has up its sleeve. The standard metabolism of sugars allows them to divide and grow quickly. But at the same time, and crucially, it triggers the so-called Crabtree effect (named after biochemist Herbert Crabtree), where it also slowly breaks down sugars into ethanol, a type of alcohol.

This process produces about 20 times less energy than a normal decomposition cycle. But it raises the alcohol levels in the grape juice, and quickly makes the environment in the barrel too toxic for most microbes. In this way, yeast completely eliminates its competitors.

However, like many victors, yeast doesn't know when to stop. If left to themselves in the anaerobic space, they will continue to metabolize sugar into ethanol, even at their own expense. Once alcohol levels reach 10% to 15%, most of the yeast dies itself, becomes dormant or clumps and sinks to the bottom. This works to the advantage of the oenophile. What comes out of the keg is a clear, spiked drink that bears little resemblance to the cloudy, opaque juice that went in.

Of course, things don't always go so smoothly. Sometimes there is sudden tension and breakdown in the process. In fact, fermentation was once so unpredictable that the Sumerians, c. 4000 BC, the differences are attributed to the whim of the gods. They called her Ninkasi (or Lady Who Fills the Mouth), in a gesture of appeasement.

Until the 19th century, batches of wine had to be discarded after they became sour, bitter, or flavorless. In France, the world leader in production, this has caused huge losses.

Finally, in 1863, Napoleon III asked the famous chemist Louis Pasteur to help cure the “diseases” of wine, by discovering exactly what was happening in the barrels, and how it could be better controlled. It is easy now to downplay the importance of Pasteur's findings, but in just three years he was able to erase eons of ignorance.

Humans have long known what yeast can accomplish, but they have never understood how. Pasteur used observation and testing to build a biological model and clearly show that the fermentation process involved living organisms such as yeast and bacteria. Making fermentation is not just a chemical process, but a biochemical process.