A bottle of raisin water that fermented while kept at 25 degrees Celsius. (Photo courtesy of the Kyoto University research team)
KYOTO — Ancient wine may have been made by naturally fermenting sun-dried raisins, according to recent research findings revealed by a team from Kyoto University.
Now typically made by fermenting grape juice with wine yeast, it is believed that wine was made by fermenting grapes naturally in ancient times. However, the low amount of wine yeast in grape skin has raised questions about its origins.
A Kyoto University Graduate School of Agriculture team led by Professor Wataru Hashimoto first confirmed that a significant amount of wine yeast is contained in commercial raisins. Their ability to produce alcohol was also higher than that of the wine yeast present in grapes. By soaking the raisins in water and keeping them at 25 degrees Celsius, the team apparently observed the bubbling produced by yeast in the fermentation process on the third day, and an ethanol concentration of 8% had been reached by the fourteenth day.
Next, they experimented with three drying methods to make raisins from table grapes: a drying machine, sun-drying, and a combination of the two known as semi-sun drying. The raisins were then divided into three jars, soaked in water, and observed for fermentation.
Kyoto University professor Wataru Hashimoto explains the process of creating wine using the natural fermentation of raisin water in Kyoto’s Sakyo Ward on Nov. 17, 2025. (Mainichi/Sonoko Nakamura)
While some jars with raisins from the drying machine and semi-sun drying showed no bubbling, all jars with sun-dried raisins did. New types of wine yeast that were not present before sun-drying, were detected, and the ethanol content was more than 10 times higher than that of the raisins dried using a machine.
While it cannot compare to commercial beverages, graduate student Mamoru Hio commented, “Some of it was suitable for drinking.” Following the results, the team aims to further investigate the factors that allow wine yeast to settle during the sun-drying process, with Hio saying, “It’s possible we can create unprecedented flavors using multiple yeasts or ones that haven’t been used before.”
Professor Hashimoto also expressed hope, saying, “If ethanol can be produced, we might be able to make biofuels from nonstandard products which may help with food loss reduction.”
The team’s findings were published in the online version of the British scientific journal Nature on Nov. 25.
(Japanese original by Sonoko Nakamura, Osaka Lifestyle, Science & Environment News Department)
AloJapan.com