Oxidation of Alcohols

[bio90]

I was taught that to oxidise an alcohol, you need to have an acidified solution of potassium(VI) dichromate. However, as far as I am aware, this is not present in wine. So what other catalyst is there that oxidises the ethanol present in wine?

 

Thank you.

[YT2095]

Bacteria do that in Wines, look up Acetobacter.

[bio90]

Thanks for the reply.

 

Bacteria? So it has nothing to do with metal ions like Copper and Iron? This research is proving to be quite misleading!

 

So does the acetobacter convert directly to ethanoic acid? Or is there still the intermittent stage with ethanal?

[YT2095]

Direct as far as I remember.

 

there IS a chemical way to oxidise ethanol sure, but that is unlikely to happen in Wine (as you already said in your 1`st post)

 

the Bacteria are the culprits 99.999% of the time.

[fredrik]

I agree that if you get a wine turned into plain vineagar, it's probably microorganisms and oxygen that is the problem but oxidation reactions in alcoholic beverages are complex in particular if you are talking about oxidation in the context of _flavour changes_ because subtle changes may have significant flavour impacts, but those ways are not necessarily practical to exploit as some industrial method to oxidise alcohols.

 

So I wonder in what context you ask this? Are you trying to find a method to exploit industrially or are you actually to understand flavour stability of wine?

 

Bacterias is an option, acetobacter also needs oxygen. Normal yeast can also to a certain extent oxidse ethanol, given oxygen, although these pathways are repressed during normal fermentation conditions. There are also different pathways that dominate depending on conditions. During normal fermentation ethanal is an intermediate during fermentation, that is reduce to ethanol. But part of it is oxidized further on to acetic acid which then turns into acetyl-CoA during typical fermentation conditions. This is a different pathway to produce acetyl-CoA than during clear respiration.

 

But there are other ways oxidation of alcohols take place. There are typically alot of interactiong redox pairs in beer and wine. Including metals, sulphur compounds etc. Some metals, in particular iron acts as a catalyst and can speed the negative aging, and increase the production of various aldehydes. Some of the oxdiations and reductions are part of the maturation and aging process as well, and residual biological activity is only part of the thing, also plain chemical reactions with complex chains of redox reactions is at play.

 

For example too much iron in your water is not nice if you want to make beer or wine, the flavour treshold of acetaldehyde is very low. Much lower than the treshold for acetic acid. Sometimes emergent infections are first detected as a early green acetaldehyde aroma, before the last vineagar stage comes. Acetobacter can produce alot of a acetaldehyde, so can other bacteria that occurs in beer or wine.

 

/Fredrik

[bio90]

Thank you Fredrik.

 

The reason I ask is that I am conducting an investigation into how the ethanol concentration of red wine decreases over a period of 4 days, and I know this is due to oxidation, but I am trying to research what causes this oxidation. Apart from the oxygen, of course

[fredrik]

I see. Then I'd look closer to the particular case to see what factors and conditions you have.

 

Is this primary, secondary fermentation? or is it bottled wine? And are you considering a atypical case or normal case?

 

Another complexity is esterification processes. But relatively speaking, that makes marginal modulation of the ethanol concentration, but larger relative impacts on esters and acids, but still 4 days is a short time frame.

 

Then there are also other types of alcohols besides ethanol that relatively speaking may change more during maturation.

 

I've mostly been into beer, but I'll check tomorrow if I've got some references. I almost have bookshelf with only beer and yeast articles only.

 

/Fredrik

[DrDNA]

Thank you Fredrik.

 

The reason I ask is that I am conducting an investigation into how the ethanol concentration of red wine decreases over a period of 4 days, and I know this is due to oxidation, but I am trying to research what causes this oxidation. Apart from the oxygen, of course

 

I believe that you might be confusing simple oxidation reactions (-OH ==> -COOH) with fermentation reactions that are more complicated and involve enzymes from yeast and/or bacteria.

 

Fermentation can be anaerobic or aerobic.

The fermentation that you probably desire, which converts sugar into ethanol depends on enzymes from anaerobic yeast that do this without oxygen.

 

Sugar (either glucose or fructose) + (enzymes from anaerobic yeast) → alcohol (ethanol) + carbon dioxide + energy (ATP)

 

But the reaction you usually do not want is dependent on enzymes from aerobic organisms, which are usually bacteria, that use oxygen. Aerobic fermentation is generally not desired when making wine or beer, it occurs because the enzymes from aerobic bacteria convert the ethanol into vinegar (acetic acid) in the presence of oxygen.

 

ethanol + oxygen + (enzymes from aerobic bacteria) → acetic acid

[bio90]

Is this primary, secondary fermentation? or is it bottled wine? And are you considering a atypical case or normal case?

 

Another complexity is esterification processes. But relatively speaking, that makes marginal modulation of the ethanol concentration, but larger relative impacts on esters and acids, but still 4 days is a short time frame.

 

 

/Fredrik

 

 

I should say first, that this is only an A Level investigation, and my chemical knowledge is a bit limited I will be using bottled wine, but I am afraid I do not know what primary or secondary fermentation is, or whether the case is normal or atypical.

 

I did wonder whether 4 days would be long enough to see a significant difference, would you recommend longer?

[DrDNA]

I should say first, that this is only an A Level investigation, and my chemical knowledge is a bit limited I will be using bottled wine, but I am afraid I do not know what primary or secondary fermentation is, or whether the case is normal or atypical.

 

I did wonder whether 4 days would be long enough to see a significant difference, would you recommend longer?

 

Most commercial bottled wine contains sulfites or similar stuff to kill the yeast and other microorganisms so they don't continue to ferment and make CO2 which will bust the sealed bottles.

 

You might be better off with "homemade" wine that has not been treated with sulfites. In any event, you will increase you chances for success by aerating the liquid as much as possible As I state above, the bacteria that convert ethanol to vinegar are aerobic so the more oxygen that you can get in there the faster it will "turn" (to vinegar).

 

Also, you might want to seed it with something that has the aerobic bacteria in it. If you had some wine that had already "turned" to vinegar that would be the best thing to seed it with, but any thing that is contaminated with bacteria (soil, dirty socks, etc) might do the trick if you aren't going to consume the vinegar.

 

I suggest you begin by googling wine vinegar and bacteria acetic acid. That will give you tons of information and tell you how to do it.

[fredrik]

I should say first, that this is only an A Level investigation, and my chemical knowledge is a bit limited I will be using bottled wine, but I am afraid I do not know what primary or secondary fermentation is, or whether the case is normal or atypical.

 

I did wonder whether 4 days would be long enough to see a significant difference, would you recommend longer?

 

I take it your task is to design an school experiment that you should carry out and explain?

 

The time required from biological processes depends on many things.

 

Usually beer and wine makers call the primary fermentation the part where the yeast converst most of the sugars into ethanol. The time depends on many things, in particular how much yeast you start with. But if you pitch industrially primary is usually 2-7 days. Peak fermentation where there is a maximum active yeast population and CO2 production usually occurs a 1-2 days after start.

 

Secondary is the finishing phase of fermentation where the last fraction of slow sugars like maltotriose are partially converted (there are always residuals) and some maturation is going on, the yeast is flocculating and so on. It's a clearning, finishing and early conditioning phase.

 

Secondary can be say another week. But sometimes it's much longer, it depends on the conditions. Some powdery strains may take up to a month or more to flocculate after finished fermentation.

 

About how long time it takes to "spoil" beer or wine it depends on how stable and contaminated it is to start with. And at least for an amateur having little sophisticated equipment, your nose is bar far your most sensitive gauge. So the question is how you intend to gauge it? Tasting, smelling or more quantitative analytical methods?

 

I often leave slants of beer and wort around in my apartment just to see what happens, and you can't get very funny things growing if you wait. But if you want something significant to happen in 4 days you need to pitch into it a culture of appropriate bacteria. Leave an aerated ethanol solution around aerated to capture acetobacter.

 

I never made vineager but I've captured acetobacter several times by mistake. If I were to make vineager properly I'lll definitely want to culture up a pure population of acetobacter to pitch with. That will make it more reproducable and much faster. Maybe the first step of your project could be to, catch wild acetobacter and keep them alive, then use them to perform your ethanol experiment?

 

/Fredrik

[bio90]

Thanks Fredrik and Dr DNA some interesting ideas there. I have decided to investigate how wine is oxidised naturally, ie just by exposing it to air, and I will be using quantitative analytical methods like density weighing, back titration and colorimetry. But those ideas will be useful in considering extension tasks. I think we have reached the conclusion though, that it is acetobacter which causes the main oxidation of ethanol in wine.

 

Thanks guys =]

[YT2095]

I`m glad there is a consensus reached

now, you May want to see if you can Isolate or obtain Drosophila (the fruit fly) you want the one with Red Eyes.

they are well known carriers of acetobacter

[Ozone]

Wine contains polyphenolic compounds (red much more so than white), many of which are o or p dihydroxyl species.

 

In the presence of a trace of iron (3+, or perhaps Cu2+) and air, a catalytic fenton-type cycle can be established (favorable kinetics occur at lower pH) whereby the "catechol" forms the Fe semiquinone which is oxidized by air to yield the benzoquinone, Fe2+ and H2O2.

 

The Fe2+ catalyzes the decomposition of the H2O2 to regenerate the Fe3+ catalyst whilst yielding -OH and .OH (which is another kettle of fish...).

 

This is just a thought on how oxidation might be done without enzymes or metabolically active whole cells. Sulfite will kill this rxn as well by scrounging your hydroxyl (and peroxyl) radicals and reducing your catalyst.

 

FYI, the practice of wine sulfitation is, IIRC, called "mutage".

 

Cheers,

 

O3