Ancient winemakers used a surprising ingredient, and it wasn’t grapes

Long before chemistry kits and stainless tanks, people figured out how to coax sugars into alcohol. The usual suspect behind that alchemy is Saccharomyces cerevisiae, the yeast used in modern wineries. 

But there’s a snag in the classic origin story: surveys show S. cerevisiae is rarely found on the skins of fresh grapes.

If the yeast isn’t commonly on the fruit, how did ancient winemakers get reliable fermentations by simply crushing grapes and sealing them in jars?

A raisin-tinged hypothesis

That gap sparked a team at Kyoto University to look not at grapes, but at grapes transformed – raisins. 

Earlier work from the group had turned up abundant S. cerevisiae on dried grapes, hinting that shriveled fruit might harbor exactly the microbes needed to kickstart fermentation. 

If true, it offers a simple, elegant mechanism for early wine production: soak sun-dried raisins in water and let nature do the rest.

Raisins for wine fermentation

To test the idea, the researchers collected fresh grapes from an orchard and set up three drying regimes: 28 days in an incubator, 28 days in direct sun, and a mixed schedule of incubator plus sun. 

The team submerged each batch of raisins in plain water at room temperature, bottled the mixtures, and waited two weeks. Each drying method was run in triplicate to see how robust the process might be.

The results were strikingly uneven across the methods. All three sun-dried samples fermented successfully and produced substantially higher ethanol levels.

By contrast, only one of the incubator-dried bottles and two of the combination-dried bottles completed fermentation.

Microbial sequencing told a complementary story: in the successful jars, overall species diversity fell as fermentation progressed, while alcohol-tolerant yeasts, especially S. cerevisiae, rose to dominance.

“By clarifying the natural fermentation mechanism that various microorganisms facilitate at the molecular level, we’d like to connect our study to the creation of unique alcoholic beverages,” said study first author Mamoru Hio.

Raisin skin gains yeast

Sun-drying doesn’t just remove water; it transforms the raisin’s micro-habitat. As the skin wrinkles and sugars concentrate, the surface becomes friendlier to alcohol-producing yeasts. 

The heat and airflow of open-air drying may also ferry in microbes from dust, insects, and nearby plants, which are potential vectors that lab incubators can’t replicate.

Once rehydrated, those established yeast populations find a sugar-rich bath and begin converting it to ethanol.

Turning water into wine

The team can’t yet pinpoint how S. cerevisiae gets onto the fruit during drying. It could settle from the environment, arrive via insects, or bloom from tiny, undetectable populations already present on grapes.

What’s clear is that natural sun-drying reliably seeded the raisins with the right microbial cast for fermentation.

On its face, the protocol is almost disarmingly simple: soak sun-dried raisins in water and wait. Two weeks later, you have wine. 

That simplicity dovetails with archaeological narratives: communities without access to pure S. cerevisiae cultures could plausibly produce alcoholic beverages using dried fruit, storage jars, and time. 

The team even notes that their data support an old, folkloric notion: people might have learned to “turn water into wine” by harnessing microbe-rich raisins rather than relying solely on freshly crushed grapes.

Not all raisins make wine

There is a modern caveat. Many commercial raisins are treated with an edible oil to prevent clumping and improve shine.

That coating also blocks water and microbes from interacting with the fruit, which is bad news if your goal is spontaneous fermentation. 

“Note that this only works with naturally sun-dried raisins that are untreated. Most store-bought raisins have an oil coating which prevents fermentation from taking place,” said team leader Wataru Hashimoto.

Study limitations and next steps

The study was intentionally small and conducted outside major raisin-producing regions, so the team sees plenty of room to refine the picture. 

Larger batches would help detect low abundance yeasts that may play supporting roles. Trials in drier climates – conditions closer to ancient Near Eastern environments – could better capture the ecological dynamics of open air drying. 

“We aim to uncover the molecular mechanism behind this interaction between microbial flora and microorganisms that reside in various fruits, including grapes,” Hashimoto said. “Through natural fermentation, we also hope to develop new food products and prevent food loss.”

Solving a wine puzzle with raisins

Raisin-seeded fermentation isn’t just an archaeological thought experiment. It could inspire “wild” beverages with distinct flavor profiles born from non-Saccharomyces partners that flourish during sun-drying, adding complexity beyond a pure S. cerevisiae culture. 

The approach also hints at a circular, low waste pathway: dried fruit that might otherwise be discarded could be transformed into stable starters or novel drinks.

If the raisin hypothesis holds up across climates and scales, it helps reconcile a long-standing puzzle: how ancient people repeatedly achieved fermentation without isolating yeasts. 

Sun, time, and fruit did the selecting. Modern wineries favor precision and control. Ancient kitchens may have favored patience and process.

Either way, the microbe at the heart of wine production was there, often not on the grape at harvest, but on the grape after weeks beneath the sun.

The study is published in the journal Scientific Reports.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–