Molecular Weight ：176.13

Melting Point：192 ºC

Density：1.65 g/cm3 (solid)

Acidity di-protic, pKa1 4.2, pKa2 11.6

Allowable Additive： Yes

Legal Limits： Yes*

EU： 300 mg/l

Norway： 150 mg/l

Switzerland： 150mg/l

Typical quantity in wine：  10-150 ppm

Analytical Methods： DCIP or Iodine titration HPLC reverse phase

• Ascorbic acid (AA) has been used as an antioxidant in wine for many years (as well as in other food stuffs). It can be added as its naturally occurring form L-ascorbic acid (vitamin C) or as the optical isomer erythorbic acid. Its main roles are to prevent oxidative browning and pinking and to add freshness to a wines profile. In recent times however it efficacy in white wines has been called into question with some studies showing that in certain circumstances it can act as an oxidant and accelerate the browning. Other recent studies however have shown that wines stored long term (5 years) with AA, reasonable levels of SO2 and good DO control appear fresher and younger than wines stored without AA. This just tends to highlight the complicated chemistry involved and that ascorbic acid must be used with a degree of caution.
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• Ascorbic acid is an effective oxygen scavenger reacting with O2 (which would otherwise react with phenolics to produce browning) around 1700 times more quickly than SO2. However the products of the sacrificial oxidation of AA (among them H2O2) further react to produce compounds which, it would appear can accelerate oxidative effects once the levels of AA have dropped low enough that it is no long effective as an antioxidant. SO2 here seems to play a role in scavenging these side products delaying the onset of browning. However, since SO2 is not as effective an anti oxidant as AA, the by-product catalysed browning proceeds in a competitive manner with the SO2 at most playing a delaying role. To add to the complexity of the situation, any excess free copper or iron also seems to catalyse the browning caused by these compounds. Despite this most authors agree if wine is packaged with sufficient SO2 and good DO control (including ensuring that O2 transmission through the closure is limited) that the AA levels will not decline to the point where these secondary adducts will play a role in the reasonably expected lifetime of a white wine.
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• As a rough guide each 1 ppm of dissolved oxygen will require 6 ppm of AA which in turn will need 4 ppm of SO2 to scavenge reaction products. However it needs to be remembered that the levels and effectiveness of both the SO2 and ascorbic acid will be influenced by the phenolics in the wine. Typical levels of 100ppm ascorbic and >30ppm free SO2 would appear to offer beneficial effects, especially in wines that have been properly managed for DO, have secure seals (screw caps) and are destined for long term storage.
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• References

1. Wessel du Toit, www.newworldwinemaker.com G.R. Scollary, Factors Determining Wine Oxidation Final report to WDC, 31 May 2002
2. G. Skouroumonis et al., Australian Wine Research Institute, pers. Com Godden et al., AWRI Technical Review, no. 129 December 2000