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If left unpicked after the fruit ripens, what fruits may ferment instead of just rotting? My perception is that most fruits just rot.

I know figs will ferment. I'm guessing that this is because the wasp that pollinates the fig carries yeast inside what will become the fruit.

Eating ferment figs got me thinking about how humans first discovered ethanol. I'd guess that they ate ferment fruit and fiddled around until they could reproduce the process.

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First, before talking about fruits, let's remember that fermentation is an energetic metabolic process where there is no net oxidation of the substract. Bacteria, fungi, animals like you and me and even plants can use fermentation in different occasions. Plant cells, like fungal cells (and unlike bacteria and animal cells) perform alcoholic fermentation. So, there is no need of a fungal contamination to make a plant tissue producing ethanol.

But, of course, the presence of fermenting fungi, particularly yeasts, can make alcoholic fermentation much faster and noticeable. Several fruits contain what is called "ambient yeasts" (differing from "cultured yeasts", which we add to the fruits latter) covering them, and I believe the most famous example are grapes. Grapes are covered by several genera of Ascomycota (like Candida and Pichia), which can lead to alcoholic fermentation of the fruit. These species are different from Saccharomyces cerevisiae, the most common "cultured yeast" (and the one used in winemaking), but they can produce ethanol the same way.

This "natural fermentation" of grapes is known for a very long time, and it's probably one of the first cases of natural alcoholic fermentation known by civilized man.

EDIT: references for alcoholic fermentation in plant cells:

  1. http://www.plantphysiol.org/content/149/2/1087.full

Under oxygen-limiting conditions, pyruvate can either be converted into ethanol by pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) or to lactate by lactate dehydrogenase.

  1. http://www.plantphysiol.org/content/100/1/1.full.pdf+html

During the early hours of germination [...] seeds rapidly generate high respiratory quotients and exhibit increased ADH2 activities and active alcoholic fermentation.

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    $\begingroup$ Hi Gerardo. Can you please give an example along with a suitable reference for alcoholic fermentation by plants? It is an interesting point and a reference would allow users to learn more about it. $\endgroup$ – WYSIWYG Jun 4 '16 at 9:11
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    $\begingroup$ Thanks. I was really talking about getting a relatively large amount of fermentation. So the fruit would need to be high in sugar. Grapes fit that need well. $\endgroup$ – MaxW Jun 5 '16 at 6:19

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