I'm curious about something that maybe someone can answer from personal experience.

If I took red or concord grapes, crushed them, filtered the juice, covered it and stored it at an average temperature (say 75°F or so), what would happen to the juice? I am saying that there were no additives put in the final juice (e.g. sugar, yeast, etc.). Would the juice turn to vinegar? Wine? Spoiled juice? If wine, what would be the alcoholic content and would there be a point at which fermentation would end?

Again, just curious.

  • I blended some grapes and now it looks like something is growing inside of the bottle. When I open the bottle it smells like wine.
    – sourgrapes
    Commented Mar 26, 2017 at 21:09

3 Answers 3


Grape skins have wild yeast on them that will, in time, ferment the grape juice. Depending on the particular blend of yeast on your grapes, you may get complete fermentation, or the yeast may have low alcohol tolerance and the fermentation will halt before all the sugars have been consumed. Yeast contribute to the flavour profile of the wine, and unquantified yeast will produce unpredictable results.

From Wikipedia's article on wine fermentation with wild yeast.

The use of both "ambient" and non-Saccharomyces wild yeasts carries both potential benefits and risk. Some winemakers feel that the use of resident/indigenous yeast helps contribute to the unique expression of terroir in the wine. ... But compared to inoculated yeast, these ambient yeasts hold the risk of having a more unpredictable fermentation. Not only could this unpredictability include the presence of off-flavors/aromas and higher volatile acidity but also the potential for a stuck fermentation if the indigenous yeast strains are not vigorous enough to fully convert all the sugars.

To know the final alcohol percentage of the wine, you'll need to buy a hydrometer. Take a note of the specific gravity of the juice before fermentation, and then again after fermentation has finished. The hydrometer will have a "potential alcohol" scale. Subtracting the final reading from the initial gives you an estimate of the final alcohol percentage.

  • Thanks for your answer. So I assume by your answer that raw grape juice then naturally ferments into wine, and not into vinegar, and does not spoil. Would this be correct?
    – Adam
    Commented Apr 30, 2015 at 2:15
  • One more thing I noticed from your answer. Concerning wild yeasts, you said it will "in time" ferment the juice. Are you saying that this process would take longer than a modern winemaker's process?
    – Adam
    Commented Apr 30, 2015 at 2:17
  • Vinegar is made by various bacteria that metabolise alcohol into acetic acid, so to get vinegar you first have to make wine. Spoilage is a possibility. If there are also significant quantities of moulds and bacteria on the grape skins (entirely possible), these will be in competition with the yeast, and the winner will determine if you have wine or rotten juice. Adding wine yeast instead of letting the wild yeasts ferment the juice will be a quicker and more reliable process. Commented Apr 30, 2015 at 13:45

Tobias has a good answer, but I want to add more information.

what would happen to the juice?

Knowing that grape skin contains wild yeast, it is likely to ferment on its own, the temperature seems ideal for that (75°F or 23°C).

Would the juice turn to vinegar? Wine? Spoiled juice?

The first thing to happen is likely fermentation. After the end of fermentation (about a week or two) the wine has to be protected from air to avoid spoilage. Adam, mentionned that it is covered, but how air thight is the cover? Bottling and refrigeration would help preserve it better at that point. Acetobacter is a bacteria that converts alcohol using oxygen to acetic acid (vinegar), so this means alcohol and oxygen have to be present for this to occur (and the presence of the bateria of course). So air tight preservation is the key for protection.

If wine, what would be the alcoholic content and would there be a point at which fermentation would end?

The usual way to measure alcohol in winemaking, is to use an hydrometer to mesure the original gravity, the final gravity and calculate it from there. However, if you didn't measure the original gravity, you can use a vinometer to measure the alcohol content of a finished wine. Most yeasts will ferment the wine dry (ie converting all sugar content) which could be around 11% depending on the grapes sugar content, but without measuring the sugar content, this is only an approximation.


In this way you will find only smelled spoiled juice at the end..

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