New findings regarding ascorbic acid in wine

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New findings regarding ascorbic acid in wine

Wessel du Toit

Wessel du Toit
Viticulture and Oenology Department
Stellenbosch University
Ascorbic acid (AA) or vitamin C has long been used in the wine industry as an anti-oxidant, the reason being the ability of AA to rapidly remove molecular oxygen (O₂) from juice or wine. This reaction between AA and O₂ takes place almost 1700 times more rapidly than between sulphur dioxide (SO₂) and O₂. SO₂ does not react directly with O₂, but instead it reacts mainly with hydrogen peroxide (H₂O₂), which is formed through the oxidation of a phenolic component. During this process mainly cinnamic acid derivatives, such as caffeic and caftaric acid, are enzymatically oxidised to their corresponding quinones, which results in the browning of juice exposed to O₂. In wine the same reaction is possible, but here chemical oxidation takes place, which is a much slower process, as a result of the inhibiting effect of alcohol on oxidation enzymes. AA occurs in grapes at low concentrations and it is rapidly oxidised during racking and crushing of grapes, but may be added to must or wine by the winemaker. AA may reduce the oxidised quinone back again to a phenol, but some of the byproducts of this reaction are dehydro-ascorbic acid and H₂O₂ (Fig. 1). The formation of H₂O₂, a fairly strong oxidant, means that sufficient free SO₂ has to occur in the wine to react with it so as to prevent further oxidation. The addition of AA is controversial at present. Some researchers have found additions to result in fruitier wines, while others have found it to increase the potential for oxidation. It seems as though AA initially prevents oxidation and then promotes it under certain conditions. In artificial wine media it has been found that the oxidation process by means of AA goes through two phases. The first is a total oxidation of AA which is absorbed in the visible light spectrum (with a yellow colour especially), while the second step generates species with little or no colour. In a wine medium with catechin, AA additions resulted in a more pronounced yellow brown colour. Initially the yellow brown colour is reduced when AA is added, compared to the control which has no AA addition. During the second phase, however, when the AA concentrations started to diminish, it resulted in a more pronounced yellow brown colour. Higher dosages of AA also caused the first phase to take place more slowly, but eventually resulted in a more pronounced yellow colour. It seems therefore that AA goes through two phases, initially acting as an anti-oxidant, and later, when the AA concentrations are reduced, as a pro-oxidant.
The addition of SO₂ to the wine medium prolonged the initial phase. A molar ratio of 0.8:1 AA to SO₂ prolonged the initial phase to four days, but the SO₂ concentrations were greatly reduced, a 100 % reduction compared to 43 % when no AA was added. The 43 % reduction in free SO₂ was mainly due to the H₂O₂ formation by the oxidation of the catechin. If the SO₂ concentration was increased to 200 mg/L, however, this crossover did not take place after two weeks. Chemically speaking 1 mol SO₂ should react with 1 mol AA, but it has been found that the ratio is actually 1.7 to 1. SO₂ is unable, contrary to popular belief, to reduce dehydro-ascorbic acid back to AA. If 100 mg/L AA is added to wine and it reacts completely with O₂, 62 mg/L SO₂ is theoretically required to react with the AA oxidation products in a 1:1.7 ratio, instead of 36 mg/L in a 1:1 ratio. AA reacts to O₂ in a 1:1 ratio. This means, for example, that if a wine picks up 3.5 mg/L O₂ let's say during bottling, it will react to 20 mg/L AA and reduce the latter's oxidation products' SO₂ levels by 12 mg/L. These ratios were achieved in an artifical wine medium, however, and in a wine with a higher phenolic concentration the O₂ will also react with some of these components, but it may serve as an indication in low phenol white wines for example.

Fig.1: Reactions of ascorbic acid in juice and wine.

It was also found that the oxidation of AA only produced 21 % of the expected amount of H₂O₂, which raises the question whether the oxidation of AA also forms other components, which may react with the SO₂ or influence the wine. Other oxidation products of AA included diketo-gluconeic acid, oxalic acid, furfural etc with dihydro-ascorbic acid as one of the initial oxidation products. It therefore seems as though initially the oxidation products of AA do not oxidise sufficiently to assist oxidation in the presence of AA, but this may occur once the AA concentration becomes low. One of the reactions that are involved is the formation of glyoxylic acid, which is created by the oxidation of tartaric acid. This product then forms a bridge between two catechin molecules in the presence of Fe3+ or Cu2+. Dehydration and oxidation of the products forms a xantilium salt which has a yellow colour. Tartaric acid may be added to wine in SA, which may perhaps influence the colour of white wine if it contains any of these products, but further research in this regard is necessary. The initial anti-oxidant activity of AA therefore lies in its ability to prevent the formation of these yellow products. The formation of brown colour was found to be less in Chardonnay and Riesling wines that underwent 5 years' bottle maturation with AA. The yellow colour of these wines was more prominent in the Riesling wines to which AA had been added.
It is therefore apparent that AA in low concentrations may accelerate oxidation under certain conditions. The addition of AA to bottling should therefore be reconsidered, especially since small quantities of O₂ may be added to the wine during bottling and certain winemakers tend towards lower SO₂ levels. If AA is therefore added to wine, it makes sense to add even more SO₂ to the wine than what may previously have been considered necessary. Additional research is necessary, however, to confirm whether the crossover of AA from an anti- to a pro-oxidant as observed in an artificial wine medium also occurs in white wine.
Summary
Ascorbic acid is commonly used as an anti-oxidant in grape must and wine. Recent research showed, however, that ascorbic acid seems at first to act as an anti-oxidant, but when its concentrations are too low, enhanced oxidation occurs in an artificial wine medium. Byproducts other than dehydro-ascorbic acid and H₂O₂ have been identified and could lead to ascorbic acid enhancing the yellow colour of white wine. Ascorbic acid also seems to react at a 1:1.7 ratio with SO₂ instead of 1:1. This should prompt winemakers to re-assess the use of ascorbic acid, as well as SO₂ levels in wine.
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