Raisins may have triggered early wine fermentation, Kyoto University researchers find

A research group led by Graduate Student Mamoru Hio (at the time of research) and Professor Wataru Hashimoto of the Graduate School of Agriculture at Kyoto University announced that they have partially uncovered the mechanism by which wine can be produced simply by soaking raisins in water. This was discovered by analyzing changes in the microbial flora dynamics of grapes, raisins, and raisin water, as well as compositional changes through fermentation tests of raisin water. Alcohol-fermenting yeasts, which are rarely present on grapes, were detected at high frequencies in raisin water. The group also confirmed alcohol-fermenting yeasts become established by sun-drying grapes. Their findings suggest the origin of wine may have been raisins. The results were published in Scientific Reports on November 25.

Today, wine is generally produced by adding cultured alcohol-fermenting yeasts such as Saccharomyces cerevisiae to grape juice. Yet in ancient times, it is believed wine was made through spontaneous fermentation by yeasts present in the air. Multiple studies have reported that alcohol-fermenting yeasts like S. cerevisiae are rarely present on fresh grape skins, and the origins of wine remain unclear.

Raisins are dried fruit made by sun-drying grapes. When raisins are soaked in water and left at room temperature, yeasts proliferate and undergo alcohol fermentation. Raisin water has been used for bread fermentation and wine production, but it was unknown what kinds of yeasts were actually present.

The research group therefore aimed to understand the fermentation dynamics and microbial dynamics of raisin water. They also explored the theory that raisin water may have been the origin of wine. Research was conducted after obtaining a fruit wine production license.

To clarify the differences in yeast flora between grapes and raisins, the researchers conducted eukaryotic microbiome analysis of both, followed by principal component analysis.

The results exhibited significant differences between grapes and raisins. When the researchers compared the proportion of alcohol-fermenting yeasts, raisins were found to contain an abundance of S. cerevisiae, which is rarely found in grapes.

Next, to compare the alcohol fermentation capacity of grapes and raisins, the researchers compared their ethanol production capacity and found that raisins had significantly higher alcohol fermentation capacity.

To investigate the fermentation dynamics of raisin water (25% raisins), the researchers left the raisin water standing at 25℃. This produced a sparkling wine-like alcoholic beverage with a maximum ethanol concentration of approximately 7%. When yeasts were isolated from this water, 94% were S. cerevisiae.

To examine microbial dynamics, the researchers measured yeast counts, bacterial counts, glucose concentration, and ethanol concentration in raisin water over time. As glucose was consumed over time, ethanol concentration increased. Additionally, while bacterial counts remained low, yeast counts increased with time. This illustrated that yeasts are primarily responsible for alcohol fermentation.

When they examined the various eukaryotic microbiomes before fermentation (day 0), just before fermentation (day 5), and after fermentation (day 14), it was found that the pre-fermentation samples contained various eukaryotic microorganisms including the alcohol-fermenting yeast Schizosaccharomyces pombe. As fermentation progressed, the proportion of alcohol-fermenting yeasts increased. Afterwards, alcohol-fermenting yeasts, including S. pombe, accounted for 99% of eukaryotic microbiomes. It is believed that S. pombe became dominant because of its high alcohol production capacity and high alcohol tolerance.

When fermentation tests were conducted with raisin water made with raisins from various countries and production regions, sun-dried raisins fermented. On the other hand, raisins produced using mechanical dryers did not ferment.

To identify the alcohol-fermenting yeasts that naturally colonize raisins, grapes were harvested from a farm in Shiga Prefecture. Fermentation tests were conducted using raisin water made from raisins dried by mechanical dryers, partial sun drying, and full sun drying.

The results showed that the degree of fermentation was highest for sun-dried raisins and lowest for mechanically dried raisins. Ethanol production was also highest for sun-dried raisins, and the alcohol-fermenting yeasts Starmerella bacillaris and Hanseniaspora vineae, which were not present on grapes, were detected from sun-dried raisins. It became clear that these yeasts may become established during the sun-drying process and drive spontaneous fermentation.

Hio commented: "Sun-drying is believed to be important for the colonization of alcohol-fermenting yeasts on raisins. However, the factors and mechanisms by which alcohol-fermenting yeasts become established on raisins while other yeasts that were native to grapes are eliminated are still unknown. Possible factors for this colonization include selection by drying, high temperatures, and ultraviolet radiation. In the future, by elucidating these mechanisms and factors, we believe we can gain insights into controlling the complex microbial flora of fruits. We hope to apply this knowledge to fruit quality control and the creation of distinctive fermented foods."

Journal Information
Publication: Scientific Reports
Title: Spontaneous fermentation of raisin water to form wine
DOI: 10.1038/s41598-025-23715-3