Enology: wood oxygenation.

June 27th, 2005

While continuing my investigation into current reports regarding oak and wine I was sidetracked by oxygen diffusion through barrels and microoxygenation. It is difficult to read something about oak in winemaking that does not say that one of principal benefits of oak is slow oxygen diffusion through the staves. I recall some objection to this from Boulton, and vaguely remember reading in the Ven126 reader evidence that suggest diffusion is limited if not nonexistent. If diffusion occurs, why does a vacuum form during expansion and contraction of the wine?

Here is an article from The Australian & New Zealand Grapegrower & Winemaker that is one of the best summaries I have been able to find relating to this issue. Although their citations are limited, they suggest that at most 2.5 ml/litre/month are diffused through the barrel and that this will significantly decline as the barrel becomes ‘winelogged.’ Significant decline? Does that mean after a few months the main oxygen input is from topping? How many times are people racking (I know this depends on the variety)? If diffusion significantly declines, it may become negligible in comparison to the 4ml/litre/racking. Frankly, I don’t really care if oxygen is or is not moving through the staves, as somewhat of a literalist though, I just don’t want to contniue to perpetuate a myth if that’s what it is. I hate saying something because that’s what everybody says, I want to know.

Here is an additional article on micro-oxygenation. Notice the differences (or should I say similarities)between micro-ox and barrel. I don’t have any personal experience with this, does anybody else? If the numbers are true, it is an effective way to improve wine at a lower price point, no? Lastly, the author takes for granted condensation reactions facilitated by acetaldehyde. Did not Boulton’s also reject this hypothesis?

One Response to “Enology: wood oxygenation.”

  1. Tyler Says:

    Here are some good tib bits from Boulton about these articles (which incidentally are form people trying to sell a product!)
    The major gas flow is through the bunghole, which generally shrinks as the stave in which it is dries out as evaporation takes place. The daily temperature cycle provides tha bellows effect due to expansion and contraction. Only a barrel with an air tight bung hole can develop a vacuum and this is due to pore intrusion by cappillary effects that reduces the wine volume in the barrel, hence the vacuum. This is not the case in most barels that have been used once, and those upright and loosely bunged.

    It would be possible to estimate the permeation of gas through the stave wood, but I have seen some permeability number that would allow this, but he seesm to have confused “diffusion” described by Fick’s Law and permeability described by Darcy’s Law, and uses the terms interchangebly. The calculation of gas flows in the loose bung case is almost impossible to estimate due the variation in the size and number of channels. Note that these are not objective estimates, baised ones by some bright people trying to say that their micro-ox business is just like barrel aging. It seems to me we go to barrels for flavor and mouthfeel, not oxidation, so that needs to be added into the considerations of the economics.

    You should be careful of supplier claims such as this. Note that brown polymer formation is why these wines are darker (420+520), an old trick to say that you increased color, same for the PVP index etc. Aagin, no eveidence that tannins are invl;oved even though the author claims that they are.

    The paper by Timberlake and Bridle (1976) AJEV, used about 1000 mg/L acetaldehyde in a screw capped bottle to show this reaction. We can not get to much over 10 mg/L even with micro-ox, and if not sealed, this is lost fairly quickly by flashing. At best the reaction would be 100 times slower, so that what they saw in 3 months becomes 25 years under wine conditions. The addtion with acetald. leads to Dougs SPP, not LPP which involves tannin. There is no evidence that the acetald. bridge works with the Tannin antho reaction. Note it requires that the wines do not
    have any free SO2 during this time, not usually a good practice.

    The effect of acetald. is negligible in most wines, the natural
    building of polymeric pigment from tannin and anthos, is the more important mechanism.

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