How to turn water into wine, with raisins

Kyoto, Japan -- It's astonishing to realize how innovative our ancestors were in food and beverage production before modern science and technology. Without understanding or isolating them, ancient peoples made use of yeasts like Saccharomyces cerevisiae, the primary species behind the fermentation process that creates alcohol, though there are some non-Saccharomyces yeasts that can also produce alcohol with different characteristics.

While modern wineries typically use cultured S cerevisiae, it is thought that ancient wine production relied on the natural fermentation process of storing crushed grapes in jars. However, research has revealed that S cerevisiae rarely colonizes grape skins, casting doubt on the use of fresh grapes for alcohol fermentation.

This inspired a team of researchers from Kyoto University to investigate the humble raisin's ability to ferment into wine. In a previous study, the team had found that S cerevisiae was abundant on raisins, indicating that in ancient times they could have been used for wine production.

To test their theory, the team collected fresh grapes from an orchard and dried them for 28 days, using an incubator for some raisins, sun-drying for others, and a combination of incubator and sun-drying for the remainder. They then submerged the raisins in water and stored them in bottles at room temperature for two weeks, with three samples for each type of drying process.

The results revealed that soaking sun-dried raisins in water is a successful method of creating wine. While only one incubator-dried sample and two of the combination samples successfully fermented, all three samples of sun-dried raisins in water fermented and yielded significantly higher ethanol concentrations. In the samples that were successful, the scientists also measured an overall decline in species diversity, but a higher abundance of alcohol-fermenting yeasts. These results suggest that, long before Jesus supposedly did so, ancient peoples may have figured out how to turn water into wine using sun-dried raisins.

"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," says first author Mamoru Hio.

Though it is clear the sun-drying process facilitates S cerevisiae colonization of the raisins, how they migrate from the surrounding environment to the raisins remains unknown. Furthermore, this study used a smaller production size and was conducted outside of typical raisin-producing regions. The researchers say that future studies should focus on low-abundance yeasts that could be detected in larger samples, and that conducting a study in a drier climate would better reflect the natural conditions of ancient wine production.

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

Hashimoto also mentions a caveat: "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."

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The paper "Spontaneous fermentation of raisin water to form wine" appeared on 25 November 2025 in Scientific Reports, with doi: 10.1038/s41598-025-23715-3

About Kyoto University

Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at undergraduate and graduate levels complements several research centers, facilities, and offices around Japan and the world. For more information, please see: http://www.kyoto-u.ac.jp/en