Mechansisms Of Oxidative Browning Of Wine And Antioxidant Alternative To Sulfur Dioxide In Wine

Oxidative browning badly affects the quality of wine and is a long-standing problem in winemaking. Sulfur dioxide (SO₂) is the generally used antioxidant to control wine browning. However, concerns over its ability to induce severe allergic reactions have created a great need for its reduction or replacement in recent years. However, ascorbic acid seems unsuccessful as an alternative antioxidant in winemaking, especially in combination with SO₂. So it is critically important to study new antioxidant(s) other than SO₂ to stabilize and improve wine quality. Based on the systematically comprehensive review of numerous literatures on wine oxidative browning and the prevention of wine browning, EDTA (ethylenediaminetetraacetate) was chosen as a possible alternative antioxidant, and then antioxidant experiments in model systems and wines were done to evaluate the antioxidant activities of EDTA compared with those of SO₂, as well as the synergistic effect between EDTA and SO₂. The major results achieved were as follows:A. Results achieved in theory: iron and copper may play central roles in the process of wine non-enzymic browning.Advanced research on mechanisms of oxidative browning of wine shows that, phenols′(especially vicinal diphenols′) regenerative capacity in coupled oxidation with other compounds and their non-oxidative browning reactions constitute the major reason for wine browning; PPO is the initiator of wine enzymic oxidation, which converts some phenols into unstable active o-quinones quickly in the presence of oxygen, consequently forming brown pigments, but it happens almost in grape must; although the formation of quinones and subsequent reactions induced by them are similar to those in enzymic oxidation, non-enzymic browning predominantly prevails in wine and mainly depends on iron and copper because the formation of quinones is too slow to compare with that occurring in enzymic browning. It indicates that iron and copper may play central roles in the initiation and propagation of wine non-enzymic browning, which has important implications in studying antioxidant substitute for SO₂ in wine.B. Results achieved in experiments 1. The results obtained in model systems verified the central roles played by iron and copper in the process of wine non-enzymic browning and the preliminary efficient antioxidant properties of EDTA and SO₂ in model wines were also investigated.Considering the concentrations of the present average range found for iron and copper in typical wines, we studied their roles involved in the browning in a wine-like system treated with saturated oxygen. Results showed that Fe²⁺ markedly accelerated the browning of samples and there were significantly positive correlations between browning degrees and Fe²⁺ levels in treated wines after 42 days of incubation (R2 = 0.9838). Over a period of 42 days, browning development was shown to obey zero-order kinetics from the seventh day of the treatment, and the browning rate constants (k) varied from 10 to 40×10-5 day-1. However, with the levels used in the experiment, Cu²⁺ hardly influenced the browning of model wines and in turn had a negative effect on the browning as time went on. In addition, no synergistic effect of enhancing browning was observed between Cu²⁺ and Fe²⁺. At the end of the experiment (day 42), the results of multivariate regression analysis indicated that, a significantly positive correlation existed between consumption of dissolved oxygen (?DO), reduction of redox potential (?Eh) and enhancement of browning (?A420) (R2 = 0.8241 p < 0.01). On the other hand, the addition of hydrogen peroxide (H2O2) alone hardly appeared to oxidize (+)-catechin in model solutions, but once in combination with iron and copper, the oxidative browning of samples was significantly increased, which might be explained by the Fenton reaction. It means the production of hydroxyl radical (·OH) other than H2O2 may be the major reason for wine browning.The primary results obtained in the antioxidant experiments with EDTA (25 mg/L) and SO₂ (50 mg/L) showed that EDTA had great potentials in preventing the samples from browning.2. The results attained in wines showed that, under the experimental conditions, both EDTA and SO₂ displayed good antioxidant activities, and a powerful synergistic effect was found between them when used together, although they presented different antioxidant activities in white wines and red wines.The antioxidant properties of EDTA and SO₂ in wines were examined, employing a dry red wine (Cabernet Sauvignon) and a dry white wine (Chardonnay) and the treatments saturated with oxygen. The results showed that at low concentration (equal or lower 30 mg/L) in white wine, the antioxidant activity of EDTA was better than that of SO₂, and however, under the same conditions in red wines, SO₂ was better than EDTA. The antioxidant mechanisms were different between them: EDTA mostly seemed to inhibit dissolved oxygen (DO) consumption by wine samples while it could hardly reduce the redox potential (Eh) of them; on the other hand, SO₂ could inhibit dissolved oxygen consumption by samples to some extent, especially depending on its concentrations, whereas it greatly appeared to decrease the Eh of wine samples. However, when EDTA and SO₂ were used together at low concentration, a strong synergistic effect was observed: not only inhibiting the consumption of DO by samples, but also stabilizing the Eh, and effectively decreasing the browning degrees of wines.The data on the effects of antioxidants on individual phenolics in wine indicated that EDTA and SO₂ both protected the individual phenols in wine with a low concentration, and had few different effects on the compostion of individual phenols in red wine, but the compostion of individual phenols in white wine influenced by them were different. Compared with a single antioxidant at the same concentration, EDTA in combination with SO₂ seemed more effective to prevent the oxidation of main individual phenols in wine.3. The effect of EDTA in combination with SO₂ on the quality of wine revealed that EDTA greatly increased the antioxidant activity of SO₂ both in white wine and red wine, not only effectively decreasing the browning degrees of wine, but also having no adverse effect on wine quality.The application of EDTA in combination with SO₂ at a low concentration to a dry white wine (Chardonnay) and a dry red wine (Cabernet Sauvignon) was examined, and the physical and chemical properties and the organoleptic qualities of the treated samples were investgated after they were stored at common temperature and common pressure for three months. The results showed that EDTA significantly increased the antioxidant activity of SO₂ both in white wine and red wine, effectively decreasing the browning degrees of wine and preventing the phenols from oxidation, and furthermore, not adversely affecting the physical and chemical properties and sensory qualities of wine. A full organoleptic evaluation of the treated wine samples showed that, under the conditions used in the experiment, samples added with 20 mg/L EDTA in combination with 30 mg/L SO₂ were the best in white wines, and samples added with 20 mg/L EDTA in combination with 15 mg/L SO₂ were the best in red wines.In conclusion, it suggests that the application of EDTA in combination with SO₂ has important implications in winemaking: it can effectively prevent wine from browning without adversely affecting the quality of wine, and most of important, significantly decrease the amount of SO₂ applied in winemaking, which seems very promising in the future.