ReviewMechanisms of oxidative browning of wine
Introduction
Considerable amount of data obtained in vitro and in vivo experiments have shown that phenolic compounds in wine have great antioxidant activities, such as scavenging of reactive oxygen species (ROS), relaxing blood vessels, anticarcinogen, anti-inflammation, and antivirus, among others (Fernández-Pachón et al., 2004, Giugliano, 2000, Lodovici et al., 2001, Makris et al., 2006). However, many constituents of wine, including phenolic compounds, certain metals, tyrosine and aldehydes, are susceptible to oxidation during the winemaking process and lead to browning. Browning is an oxidative process involving sugars, lipids, amino acids or phenols in food. It is one of the main problems encountered during the vinification of wine as it on one hand, adversely affects the sensory properties of wine (loss of colour, flavour and aroma, and increase of astringency) (Escudero et al., 2002, Ferreira et al., 1997, Schneider, 2001, Silva Ferreira et al., 2002) and on the other, results in the loss of nutritional value of wine (Bonilla et al., 2001, Sioumis et al., 2005).
Sulphur dioxide, often abbreviated to sulphite or SO2, is the most important and widely used chemical to prevent wine from browning. Besides antioxidant activities, SO2 also has antimicrobial properties and other important functions, which has been successfully used for years. However, the addition of SO2 to wines raises health-related objections due to serious allergic reactions incurred by sulphite-sensitive individuals, and concerns over sulphites have resulted in regulatory restrictions set by World Health Organization (WHO) and International Organization of Vine and Wine (O.I.V.) (Li et al., 2005, Ribéreau-Gayon et al., 2006, Warner et al., 2000, Yang and Purchase, 1985, Zhou et al., 2004). Furthermore, winemakers are also concerned about SO2 addition to juice and wine for quality reasons: its excessive use can drastically compromise the quality of wine and excessive quantities of SO2 can actually give the wine unpleasing flavours and aromas or may favour the wine to turn cloudy during its keeping (Li et al., 2005). Therefore, a growing awareness concerning the use of SO2 in wine has created the need for its reduction or replacement in recent years (Comuzzo and Tat, 2003, Divol et al., 2005, Ribéreau-Gayon et al., 2006, Threlfall and Morris, 2002). To date, a single replacement product performing the same roles without the disadvantages of SO2 is unsuccessful, though a lot of work has been done (Barbe et al., 2000, Comuzzo and Tat, 2003, Main, 1992, Ribéreau-Gayon et al., 2006). However, it is perfectly operable to study partial replacement that has antioxidant properties as or better than SO2 does in winemaking.
Ascorbic acid (vitamin C) and its optical isomer, erythorbic acid, probably the alternative to sulphites, have been widely used as antioxidants in winemaking, especially white wine production, primarily because of their chemical properties (including reducing character and oxygen-scavenging capacity) and their approval for wine use (Bradshaw et al., 2001, Marks, 1990). However, more and more extensive studies have shown that ascorbic acid may have a pro-oxidant role rather than antioxidant depending on its levels in wine under some conditions (Bradshaw et al., 2003, Bradshaw et al., 2001, Main, 1992, Scollary, 2002). Many recent research results in model systems and white wines indicate that if ascorbic acid is used in combination with SO2 then it can cause accelerated consumption of SO2 and accelerate the production of yellow pigments (browning) (Bradshaw et al., 2004, Oliveira et al., 2002). Presently, the use of ascorbic acid is not widespread in winemaking and not authorized in France (Ribéreau-Gayon, Dubourdieu, et al., 2006).
Therefore, it is essential to find an appropriate antioxidant(s) other than SO2, and then a comprehensive understanding of the mechanisms of wine oxidative browning should be considered. However, the mechanistic chemistry of the process in its entirety is not outlined, though some advanced research on wine oxidative browning has been achieved in recent years (Danilewicz, 2003, Waterhouse and Laurie, 2006). In this paper, an overview of wine oxidative browning including enzymic and non-enzymic browning is offered, and this is important to fully understand the chemistry of wine oxidation, thus to study alternative antioxidant for wine protection and eventually produce wine with high quality, stability and safety.
Section snippets
Phenolic compounds
Some metal ions, SO2, organic acids, ethanol, and phenolic compounds present in grape must and wine, are susceptible to oxidation, and among them phenolic compounds are considered the major substances to cause browning (Junquera et al., 1992, Macheix et al., 1991, Toit et al., 2006, Vivas et al., 2001). However, the oxidation of phenols is very complicated, largely depending on phenolic compositions and their levels, and that o-diphenols are the most oxidizable ones (Boulton et al., 2001,
Mechanisms of wine oxidative browning
As described above, phenols especially o-diphenols are responsible for the oxidative browning of wine. The browning of wine can be classified into enzymic browning and non-enzymic browning in terms of the initiative mechanism: the former almost entirely occurs in grape must; the latter can happen both in grape must and wine, while as a result of fermentation and some operations it prevails in wine (Es-Safi et al., 2003b, Main, 1992, Spagna et al., 2000, Sullivan, 2002).
Activation of oxygen in wine
A redox reaction is a process involving electron transfer from reducing species to oxidizing species, and whether the redox reaction can happen, or carry out completely, both depend on the difficulty of the electron transfer or on the potentials (EH) of them. EH is considered as an important criterion to evaluate a redox reaction occurring, reaction succession and performance level (Pi, 1987). Traditionally, standard potentials are described as potentials when pH value is 0 and solute molarity
Conclusions
The advanced research on wine oxidative browning reveals 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. Enzymic browning almost entirely occurs in grape must, which has been thorough studied and can be controlled at present; non-enzymic browning prevails in fermented wine, essentially depending on the presence of iron and copper. It indicates
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