Twospoons
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Oxidation of wine?
Why is it a half full bottle of red wine will turn to vinegar in a week, while a half full bottle of port will last for months? Is there some magic
threshold for alcohol content? The two are practically identical, except for the higher alcohol content of the port, and maybe a bit more sugar.
Please - no facetious remarks about " how could I let a bottle of port sit around for months" 
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Ozone
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IIRC, the higher concentration of alcohol (port is fortified to maybe, 18-22%) is bacteriastatic (acetobacter, etc.). Since the grapes used to make
port may have sugar content in the 30-40g/100g (brix) range and the fermentation process is 1 hexose for every 2 ethanols, the sugar level, whilst far
from dry, is probably insufficient to inhibit bacterial growth.
The water activity of a mixture does not become sufficiently hostile until the range (sucrose) of 50 (pushing it)-60 brix on up. In this range, the
water activity drops sharply b/c the free water is involved with solvating the sugar. I would have to look it up specifically for glucose, but it can
be calculated using the Norrish equation (and the empirical parameters that go with it).
See also:
Danilewicz, J.C. (2003). Review of Reaction Mechanisms of Oxygen and Proposed Intermediate Reduction Products in Wine. Central Role of Iron and
Copper. Am. J. Enol. Vitic. 54:2., pp. 73-85.
for some interesting chemistry involving the phenolic species (the oxidant cycles are also capable of oxidizing ethanol--see Fenton reagent,
pro-oxidant, etc.).
Cheers,
O3
Cheers,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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chemoleo
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Well from what I remember from lectures, ubiquitous organisms in the air and soil (both fungi and bacteria) are not able to grow in solutions
containing more than 15% EtOH.
This explains why fermentation processes cease even if left-over glucose is present.
Port wines are usually around 18-20%, so nothing can grow there.
Also, the more alcoholic a medium, the fewer organisms will be able to grow in it. Thus a stronger wine may take longer to acquire an 'infection' than
weak wine. Perhaps not coincidentally, white wine is kept cold (and usually has ~10%), and is meant to be consumed within a few days, while red wine
(up to 14.5%) lasts longer and can be kept at RT for weeks if you are lucky.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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Twospoons
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So what you are saying is the conversion to acetic acid is due to bacterial action? I'd have thought the SO2 (additive 220) used as a preservative
in most wines would have prevented that. I've noticed a bottle of wine in the fridge will still go off - and the bigger the air space the faster it
is. I guess they are not anaerobic bacteria then.
Curiously, I have some sweet whites (alc. ~9%) that seem to last rather longer than the dry whites (alc. ~12%).
[Edited on 30-1-2008 by Twospoons]
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Ozone
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Mutage is a finite solution. On storage, particularly in warm climates, for long periods, the sulfite becomes bound (usually as organosulfites) to
products of slow sugar decomposition (aldehydes, HMF, etc.). They are good until opened and re-innoculated.
This is an aerobic process (IIRC, the spore forming facultative anaerobes love this).
The phenolics can also oxidize in contact with air, particularly so in the presence of Fe or Cu.
In vino veritas,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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chemrox
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@Ozone I'm confused. I thought that the imperviousness of fortified wines to spoiling was due to the alcohol being fatal to the aerobes. You seemed,
at first, to be saying that the free water activity had to be at a certain level for the bugs to be happy and that additional ethanol reduced the free
water concentration to below the threshold. Is that what you meant? If so is that another factor or another way of expressing the same phenomenon?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Ozone
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The alcohol is the major bacteriostat.
Above ~60% sugar (sucrose, this differs for glucose which is saturated at ~50g/100g and fructose which is >90g/100g), the amount of free water
becomes too low to support life. This is Not likely the case, with port.
Below this (60% sugar), life does quite fine. If you plot the Norrish equation, the water activity remains more or less constant (very small downward
slope) until the 60g/100g mark. The sugar itself "consumes" free water as a consequence of solution (cage). Also keep in mind that glucose (180g/mol)
is much bigger than water (18g/mol)--a straight water fraction can be misleading.
Hope that helps,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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not_important
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Some of the phenols in wine will suffer oxidation on their own, but for the ethanol to go to acetic acid you need either bacteria or finely divided
platinum group metals.
Ethanol is bacteriostatic and bactericidal, concentrations in the 40 to 80 percent range being the most effective at killing. Concentrations above
that actually tend to suspend cell activity without quickly killing the cells. Spores can survive alcohol treatment, but spores in an alcoholic
mixture will either be kept in the spore state or killed or suppressed if they convert to the growing state. Alcohol kills partially through
upsetting osmotic balance, partially through disruption of the cell membrane, and partially through denaturing of cell proteins; it is effective
against both aerobic an anaerobic bacteria. The denaturing effect can destroy some virii, but is not particularly effective against all of them.
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WizardX
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This should cover it.
Attachment: WINEMAKING CHEMISTRY.zip (19kB)
This file has been downloaded 759 times
Albert Einstein - \"Great ideas often receive violent opposition from mediocre minds.\"
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chemrox
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I got an error on Wizard's zip file. Could be my program or the file....
@O3- is the bacteriostatic quality of port or sweet sherry due to the EtOH or both sugar content and alcohol combined?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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chemrox
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Related question- is it one bug or a whole genus that converts EtOH to EtO2H?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Ozone
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I cannot get that file to behave, either. Upon extraction it says "no files". :^/
I am pretty sure that the effect is primarily due to the ethanol rather than the sugar.
Production of acetate is popular with bugs, and is more widespread than the genus. Remember, all life is driven by reductive power.
Cheers,
O3
[Edited on 30-1-2008 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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sparkgap
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The ZIP file had an HTML file inside. Try this one.
sparky (~_~)
Attachment: WINEMAKING CHEMISTRY.htm (63kB)
This file has been downloaded 3160 times
"What's UTFSE? I keep hearing about it, but I can't be arsed to search for the answer..."
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DerAlte
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As a wine-drinker (hardly a night passes without a glass or two!) I know that any light wine rapidly becomes spoiled to undrinkable once uncorked, if
stored at room temperature. The life can be prolonged to about a week in a refrigerator at 7C or 45F. The life of fortified wines or spirits at
>16% alcohol is indefinite. The maximum strength of a no-fortified wine produced by fermantation is about 14.5%, the yeasts being apparently killed
off at around that level. (I once had a superb late-harvest Zinfandel @14,5%)
The action of spoilage is not completely due to re-fermentation, however. The quality of a wine suffers from exposure to oxygen, especially with red
wines. A well aged red will have a phenolic bottle bouquet when freshly opened. It may aso, if ~13.5%+, deposit tannins and tartrates. If the
temperature is raised, these tarttrates can redissolve, which is undesirable as it increases the acidity of the wine. With such a red, I cool it first
in a refrigerator and decant cold, even filtering with a coffee filter, and then allow it to get to RT.
Reds must be drunk at about RT. This releases the complex congeners which make the character of a red. These oxidise rapidly, too, so it's best to
keep an opened red in the refrigerator. In other words, although the alcohol may not turn to acetic acid, the character is rapidly destroyed by
oxidation if kept too warm.
Whites show their best character cool. They are less complex and although they must suffer oxidation of congeners in the same way, can be kept longer
in a refrigerator without obvious deterioration.
Wines low in alcohol will not keep long even when sealed. Chemical changes occur sowly, and in a poor wine, bacterial degradation as well. Any wine
below about 9% acohol should be drunk quickly, if it is palatable at all!
There is one exception to this. As noted above, high sugar content also delays bacterial deterioration, and perhaps delays oxidation by offering an
alternative reduction. The German rhine wines are a case in point.
I have had a superb trockenbeerenauslese over 20 years old with only ~9% alcohol. Pure nectar. Similarly French Sauternes which usually achieve 12-13%
are magnificent when decades old because of the high suger content. As are Hungarian Tokays. All these wines make use of a special fungus on te
grapes , the edelfaule or noble rot, which gives a unique flavor to these very special wines.
Any wine that is even vaguely acetic is undrinkable!
Der Alte
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Twospoons
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Actually, the upper limit for alc% by fermentation is a lot higher than 14.5%, though commercial wines are rarely higher than 14.5%. There are yeast
varieties that can work to 18% for wine (I used one unknowingly, and produced a magnificent and rather dangerous peach wine once). There are also
specialist yeasts for spirit production that are capable of more than 20%, with care and feeding. http://stillspirits.com/wa.asp?idWebPage=6161&idDetails=...
I agree that acetic wine is horrible, but rather than tip it down the drain you should let it ferment into wine vinegar, and make salad dressing with
it! Very tasty.
Helicopter: "helico" -> spiral, "pter" -> with wings
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