TB wrote:
> Luk wrote:
>
>>Michael Pronay ha scritto:
>>
>>>Luk > wrote:
>>>
>>>
>>>
>>>>>>The refrigerator, as Mark will confirm, will increase the
>>>>>>oxidation risk.
>>>
>>>
>>>>>Sorry Mike, but this is contrary to common wisdom, my
>>>>>experience, and chemistry. Cool temperatures slows down
>>>>>oxidation.
>>>
>>>
>>>>It results that at 5 °C the oxigen content into water (or wine)
>>>>is twice than at 20°C
>>>
>>>
>>>So what? Oxidation is the reaction of wine with oxigen, not
>>>the content in itself. Reaction *is* much slower at cooler
>>>temperatures.
>>>
>>>M.
>>
>>IMHO it is disputable if oxidation occurs at the wine surface because of
>>enviromental oxygen or inside the wine when it absorbs oxigen. You are
>>right, but it is also true that cooling wine it absorbs oxigen.
>>
>>Luk
>
>
> Pardon me Luk, but this does not hold water. Solubility of gases in a
> liquid does decrease with temperature (all other things being
> constant), but the rate of oxidation of an organic molecule increases
> with temperature (all other things being constant). One does not offset
> the other - the oxidation rate increases in spite of higher solubility.
>
> And what we are discussing is oxidation of, not dissolution of oxygen
> in, the wine. Cooling wine (or any organic chemical) will make it less
> likely to oxidise - whatever be the solubility of oxygen in it.
>
> Cooling will also make the mixture less volatile and certain "light"
> hydrocarbons in the mixture will not evaporate so easily. Depending on
> the wine in question, this may be a good or a bad thing of course, but
> is usually a good thing.
>
> Also, the difference of solubility at 20 % degree Centigrade is
> probably 3 mg for half a bottle of wine. This small quantity is not
> going to oxidise the wine so very appreciably. I do not have a model
> which estimate oxidation rates of any organic chemical at various
> temperatures, but the impact oxidation has on the wine (reflected by
> change in odour, taste and sedimentation) is more in the deci- or whole
> grams order of magnitude.
>
> Difficulty in oxidising hydrocarbons at low temperature is why one
> needs all manners of efforts to ignite car engines on cold morning.

Wow, an actual chemistry discussion here. Let's see what I can add:

1. Yes, oxygen does have a higher solubility in cold water than in room
temp water; that's why coffee should be made from cold water brought to
near-boiling (it has the highest amount of dissolved oxygen).

2. The rate of oxidation of any substance in solution will depend on
tepmerature, its concentration and the concentration of oxygen in
solution, hence the relevance of solubility data. For the science wonks
in the audience, the functional form is:

rate = k[organic][O2]

where k is a so-called rate constant specific to the reaction in
question, and the brackets indicate concentration

3. rate also depends on the inverse of temperature, the functional form
being:

rate = Ae^(-ˆ†G€¡/kT)

where A is a constant, ˆ†G€¡ is the energy barrier (reaction-specific), k
is Boltzmann's constant and T is of course temperature

4. The issue of oxygen solubility becomes virtually moot if one makes
any effort to exclude oxygen from the bottle, either by evacuation or
filling to the top, or blanketing with nitrogen, etc.

5. My own experience is that cooling the wine down does hlpe preserve
it, but I usually take pains to exclude oxygen.

HTH
Mark Lipton