Wine in Dr Stone: alcoholic fermentation explained | Behind the Lore

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At the very start of the pilot episode, Senku undertakes the first real chemical production of his “kingdom of science”: making wine. He takes wild grapes that Taiju happened to gather, crushes them by hand, pours everything into a primitive jar, stirs gently each day, waits three weeks, and tastes. The resulting wine is, in his own words, “ten billion times worse than a commercial wine”, yet drinkable.

A seemingly trivial gesture, but in fact the first narrative domino of the whole season: without wine, no ethanol; without ethanol, no nital (a mix of ethanol and nitric acid); without nital, no de-petrification.

The subject in depth

The must, a sugary medium just waiting for a microbe

A ripe grape is a concentrate of sugars. Its juice, called the must once pressed, typically contains 170 to 220 g of sugars per litre, split roughly equally between glucose and fructose ^[Livre] . To this are added water (about 75%), organic acids (malic, tartaric), nitrogen compounds, minerals and polyphenols. An ideal substrate for a microbe able to extract energy from it.

And that microbe is not found just anywhere: it already lives on the grape skin. The bloom, that whitish veil covering the berries, hosts a dense microbial flora, between 10⁴ and 10⁶ cells per square centimetre, dominated at more than 90% by Saccharomyces cerevisiae ^[Web] . It is the same species that leavens bread and ferments beer. The genius of primitive winemaking, Senku’s as much as that of the Neolithic Georgians, was to grasp empirically that it is enough to crush the grapes and leave them alone for the magic to happen.

From glucose to ethanol, the molecular mechanism

Alcoholic fermentation proceeds in two stages. First glycolysis : a chain of ten enzymatic reactions splits one six-carbon glucose molecule (C₆H₁₂O₆) into two three-carbon pyruvate molecules, producing along the way two ATP molecules (the usable energy) and two NADH molecules (a reduced electron carrier). This glycolysis is the common trunk of all living things: bacteria, yeasts, plants, animals.

It is then that the yeast takes a particular route. Under anaerobiosis (absence of oxygen), it cannot send its pyruvate into the Krebs cycle to extract much more energy. It instead directs it toward an alternative pathway: pyruvate decarboxylase removes CO₂ from the pyruvate to yield acetaldehyde, then alcohol dehydrogenase reduces that acetaldehyde into ethanol, consuming the NADH produced during glycolysis ^[Wikipedia] .

The overall balance is famous:

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

One glucose molecule yields two ethanol molecules and two carbon dioxide molecules. Energetically, the yeast extracts only 2 ATP per glucose molecule, against 30 to 32 ATP in aerobic respiration. Fermentation is therefore, for the cell, a low-yield energy plan B that works without oxygen. It is precisely this metabolic inefficiency that produces the ethanol we drink: what the yeast cannot use as energy, it discards into the medium.

Why a closed vessel

If the must is left in the open air, two unpleasant things happen. First, the yeasts find oxygen again and switch to aerobic respiration: they burn the sugars for their own growth without producing ethanol, and you get a flat, lukewarm juice. Second, another bacterium shows up, Acetobacter aceti, which oxidises ethanol into acetic acid. This is exactly the vinegar process. The wine then turns to plonk within a few days.

Senku knows this implicitly: his jar is closed. Not airtight (the CO₂ produced must be able to escape, on pain of bursting the vessel), but just enough to exclude oxygen continuously and keep Acetobacter at bay.

The natural 16% limit

A question always comes up in chemistry class: why does a wine never naturally exceed 15 to 16% alcohol? The answer is elegant: the yeasts poison themselves. Ethanol is toxic to Saccharomyces cerevisiae beyond a certain concentration. The cell membrane becomes fluid, the enzymes denature, the metabolism stops. The yeast dies in the very product it has just made.

This limit (around 14 to 16% for the most resistant strains, sometimes 18% for modern selected strains) explains why all the “strong” spirits in human history are obtained by distillation (heating to separate ethanol from water) and not by prolonged fermentation. Right after this scene, Senku announces that he will distil his wine to make brandy.

The fact-checker’s eye

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Going further

• Alcoholic fermentation: cross-cutting concept entry
• Discipline: Chemistry: all the site’s chemistry entries
• Coming soon: Distillation: the physical principle of separation by change of state
• Coming soon: Brandy: a strong spirit obtained by distilling a wine
• Coming soon: Nitric acid: the chemical pivot of season 1

Glossary

Glycolysis ↗

A metabolic pathway shared by almost all living things that splits one glucose molecule into two pyruvate molecules, producing two ATP along the way.

Anaerobiosis ↗

A way of life or a metabolism that takes place in the absence of oxygen. The opposite of aerobiosis.

ATP ↗

Adenosine triphosphate. The molecule that serves as the unit of chemical energy in all living cells.

Pyruvate ↗

A three-carbon organic compound, the end product of glycolysis. It can then be oxidised (with oxygen) or fermented (without oxygen).

Sources

1. Wine, Wikipedia EN ^[Wikipedia]
2. Ethanol fermentation, Wikipedia EN ^[Wikipedia]
3. Patrick E. McGovern, Ancient Wine: The Search for the Origins of Viniculture, Princeton University Press, 2003 ^[Livre]
4. Émile Peynaud & Jacques Blouin, The Taste of Wine, Dunod, 2013 ^[Livre]
5. INRAE, Saccharomyces cerevisiae: the yeast of wine and bread ^[Web]