Today, wine is mostly made from pressed grapes and added yeast. But there is another way: If you soak raisins in water, you start a natural fermentation mechanism that also produces wine containing alcohol. However, this only works with sun-dried grapes, as a study demonstrates. People may have been using this spontaneous fermentation process long before Jesus supposedly turned water into wine.
Long before modern science and today’s technologies existed, our ancestors made processed foods and beverages. They also used yeasts such as Saccharomyces cerevisiae to produce bread and alcoholic beverages such as beer and wine. Today, these microbes are specifically cultivated and deliberately added to the raw ingredients in order to ferment them. In the case of wine, S. cerevisiae is added to grape must.
Ancient wine production, on the other hand, was based on a natural fermentation process that was not understood in detail at the time. Researchers have long assumed that our ancestors stored fresh, crushed grapes in glasses or clay pots and waited until the yeasts living on them turned the grape juice into wine. However, studies show that S. cerevisiae rarely colonizes fresh grape skins. The yeast cells, however, are found much more frequently on the skins of dried grapes – on raisins. So did the first wineries produce wine from raisins soaked in water?
Wine made from sun-dried raisins
A team led by Mamoru Hio from Kyoto University has now investigated this suspicion. The researchers examined the ability of the raisin to ferment into wine without deliberately adding yeast. To do this, they collected fresh grapes from different growing regions and dried them for 28 days. They dried some grapes in an incubator, others in the sun outdoors and the rest through a combination of incubator and sun drying. They then filled the raisins obtained in this way into bottles, soaked them in water and stored them for two weeks at a room temperature of 15, 20, 25, 30 or 37 degrees Celsius. After fermentation, they measured the sugar and alcohol content of the homemade raisin wine and analyzed which yeasts were present in it.
The evaluation showed that most of the samples of sun-dried raisins produced a cloudy wine with a high alcohol content in the experiments. This was highest during fermentation at 25 and 30 degrees Celsius. After 14 days at the latest, there was nothing left of the initially contained glucose in these samples; it was completely converted into ethanol. However, of the grapes dried in the incubator and the grapes from the combination drying process, only some of the soaked batches were fermented into alcohol-containing wine. The researchers also found alcohol-fermenting yeast species such as Saccharomyces, Schizosaccharomyces and Zygosaccharomyces more frequently in the raisin wine made from sun-dried grapes than in the wine made from other raisins.
Using such microbial helpers, people could have long ago soaked sun-dried raisins and turned them into natural wine, long before Jesus supposedly did this, the team says. Archaeological studies show that people were producing wine around 8,000 years ago in many countries, especially in the Mediterranean. Since raisins last longer than fresh grapes, people could have imported raisins and made wine from them in regions where no grapes grew, such as Scandinavia.
Where did the yeast come from?
The results show that the sun-drying process facilitates the colonization of the raisins by S. cerevisiae and similar species and that these fungi do not already live on the grape skins. But from which sources in the area do the yeasts come and how do they get to the grapes? Are they brought to the grapes by insects? Hio and his colleagues now want to clarify this in follow-up studies. They then want to examine other, lesser-known types of yeast in natural wine in more detail and imitate the historical production conditions even better, in a drier climate than prevails in Japan.
“We want to uncover the molecular mechanism behind this interaction between the fruits and the microbial flora,” says senior author Wataru Hashimoto from Kyoto University. “Through natural fermentation, we also hope to develop new foods.” By the way, wine can no longer be made with commercially available raisins. “Most store-bought raisins have an oil coating that prevents fermentation,” explains Hashimoto.
Source: Mamoru Hio (Kyoto University) et al.; scientific reports, doi: 10.1038/s41598-025-23715-3