[Cupro Chlorous]

I'm trying to make acetic acid from decomposing fruit.  So far i have the fruit  in a tightly sealed bottle in a dark area. Should i do anything else to make it work faster?

[Borek]

Keep it warm and open to allow oxygen access.

[constant thinker]

Quick questions about this method.
What is yield of Acetic acid and its relative purity? High or Low
Do certain fruits work better than others?

[AWK]

This is a simple two step fermentation of fruit glucose or glucose from fruit sucrose to an alcohol, followed by oxidation  to acetic acid by aerobic bacteria. The yield will depend on glucose content and can be quite low. Vinegar contains usually 3-5 % of acetic acid

[Cupro Chlorous]

So should i leave the bottle sealed until it turns into alcohol, and then open it to turn into vinegar?

Also, i noticed alot of pressure build up in the bottle.  What gas is being produced?

[Mitch]

CO2. You shouldn't close a pressurized system. It would be better if you attached a balloon to the top of the bottle, and seal it well that way.

[billnotgatez]

In the past I have been associated with several attempts to make hard cider (fermented apple squeezings). In all cases we ended up with cider vinegar. I suppose some microbe got into the process and converted the apple sugars to acetic acid or the ethanol produce from fermentation to acetic acid. Of course we were disappointed and did not pursue what went wrong.

http://en.wikipedia.org/wiki/Vinegars
http://en.wikipedia.org/wiki/Acetic_acid

From the above sites I got the following.

-Quote-
Most vinegar today is made in submerged tank culture, first described in 1949 by Otto Hromatka and Heinrich Ebner. In this method, alcohol is fermented to vinegar in a continuously stirred tank, and oxygen is supplied by bubbling air through the solution. Using this method, vinegar of 15% acetic acid can be prepared in only 2–3 days.
-End Quote-

Based on above information, here is my suggestion to creating acetic acid at home even though I have not done it.

Liquefy the fruit as much as possible using a blender. We will assume that the skin of the fruit has the yeast and bacteria to form ethanol and acetic acid. You might want to consider getting a starter culture from a brewing supply business. Put the mash in a glass jar. You then bubble filtered air into the mash while having a device to continually stir it. It appears that this can be done a room temperature. I suppose you would want to cover the system with something to prevent dust etc. from getting in the mash. I would not seal it.

What do you think?

[mike]

I always put a "bubbler" on my fermentation tank. This is too make beer though.. this is a way to "seal" it with basically what is a continuous pressure release valve (just a plastic S with water in it works)

[constant thinker]

Maybe that's why when I tried to make just straight ethanol once I wound up with no alcohol. It was sour in taste reminding of vinegar but I never put two and two together.

[billnotgatez]

One other point I failed to make. The sugar is not necessarily the limiting factor in production. The ethanol and acetic acid tend to kill off yeast and bacteria at about 10 to 15 percent concentration. Therefore the fermentation may stop before all sugars are used up. The microbes die in their own excretions.

[mike]

Yes good point. My fermentation usually yields beer of between 4-6% ethanol after about a week of fermentation at approx. 25C. (temperature can have an influence also).

[billnotgatez]

One wonders what would be the best at home method of extracting the acetic acid after the fermentation is complete. Distillation would probably have all sorts of stuff in it like ethanol.

[lemonoman]

Question, about the products:

When you have a carboxylic acid (like acetic acid) and alcohol (like ethanol) in the same warm, acidic medium...I'd have thought that some Fischer esterification would occur...you know, carboxylic acid + alcohol --> ester + H₂O...are esters formed in this fermentation process?

If not, is it because there's a lot of water in organic foods...so the equilibrium is pushed far to the left, away from the esters?

Just wondering...

[jdurg]

I believe you need a very low pH for that esterifcation to occur as well as a lack of water.  Hence why concentrated sulfuric acid is typically added to the mixture when an ester is the desired product.