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.
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.
He also decoded the role of oxygen, and explained how the presence of certain bacteria after fermentation can contaminate and destroy old wine. By the time Pasteur finished the work, there was much less guesswork. It was understood that wine, once aged in its barrel, should be transferred from that permeable container to an impermeable, sterilized container such as a bottle.
“Wine mistakes” still happen – heat can change flavour; Yeast can die and stop the aging process, because even Pasteur could not answer all the questions. This process depends, after all, on the consumption and excretion patterns of the organism. Living organisms, unlike chemical and physical processes, are inherently variable.
So, well, taste this wine on Wednesday, and appreciate the mysteries it still holds. And remember its effects. Don't be like fruit flies exposed to alcohol fumes in 2013, which showed increased courtship behavior, according to a study by US researchers Anita Devigny and Ulrike Heberlein, which was published in the Annual Review of Neuroscience. They also became excited, bumped into obstacles more, moved more quickly, fell, and finally fell asleep.
Have a lovely Valentine's Day. You may only sleep where you intended.
(To contact Swetha Sivakumar with questions or comments, email [email protected])
