Key varietal characteristics of Sauvignon Blanc, including the descriptors of ‘green’ and ‘tropical fruit’, are mostly attributed to methoxypyrazines and volatile thiols, while monoterpenes, higher alcohols, esters, fatty acids, and other volatile compounds also add complexity and fruity notes to the wines. During the winemaking and ageing period, oxidation decreases the concentrations of these compounds and diminishes the flavours derived from this aromatic grape variety. Therefore, antioxidants, such as sulfur dioxide, are commonly utilized in Sauvignon Blanc wine production for better preservation of those beneficial primary aromas.
1. Introduction
Sauvignon Blanc is one of the most popular white wines around the world. Originating from the Loire Valley of France, Sauvignon Blanc was considered a wild weed before winemakers started to turn grapes into wine hence the name Sauvignon Blanc came from the meaning of ‘wild whites’ in French. Some of the best grape-growing regions for Sauvignon Blanc in the world are namely Loire Valley (France), Bordeaux (France), Marlborough (New Zealand), California (United States), and Casablanca (Chile). The flavours and styles of Sauvignon Blanc may vary by region due to different terroir and winemaking practices, but typically, Sauvignon Blanc wines are made in a dry, still, and light-bodied style with high acidity and aromatic characteristics dominated by grape-derived fruity flavours.
Sauvignon Blanc vines in New Zealand were first commercially planted in Marlborough in the mid-1970s and the wines have gained worldwide recognition since then. Until now, Sauvignon Blanc is the most widely planted grape variety in New Zealand, with over 26,000 hectares of vineyard land devoted to growing the grape. The majority of which is planted in Marlborough (23k ha), followed by Hawke’s Bay (1k ha) and Nelson (0.6k ha)
[1]. Sauvignon Blanc accounts for 72% of New Zealand’s wine production and makes up 86% of total wine exportation from New Zealand. The advantages of New Zealand’s growing environment, including cool climate, low rainfall, long sunshine hours, large diurnal temperatures, and mixed soil types, aid in reaching the freshness and crispiness of Sauvignon Blanc grapes. Reductive winemaking, which aims at minimizing oxygen exposure during vinification, is commonly adopted for Sauvignon Blanc to reduce the loss of primary aromas and limit the development of oxidative characters.
2. Antioxidants Used in Wine Production
Sulfur dioxide has been used as the main antioxidant in wine production at various winemaking stages, even though it occurs naturally in the wines as a by-product of yeast metabolism during fermentation
[2]. Wines without any SO
2 additions will still have 10−20 mg/L of total SO
2 at the end of the alcoholic fermentation originating from yeast metabolism of amino acids
[3]. The majority of SO
2 is mainly added in the form of potassium bisulfite, with typical concentration of free SO
2 ranging from 20 to 40 mg/L to protect juice or wine from oxidation, as well as, to keep the molecular SO
2 concentration below its sensory threshold of 2 mg/L.
Volatile thiols could be well preserved in red wines and model wines by the use of SO
2 under oxidative winemaking conditions in the presence of polyphenols
[4][5][6]. The maximum amounts of 3MH and 3MHA were found in the final Sauvignon Blanc wines when SO
2 was added at around 120 mg/L at harvest
[7]. However, the excessive use of SO
2 can have negative effects on human health, such as hives, swelling, headaches, stomach pain, and diarrhea
[8]. In addition, the total SO
2 in wine is regulated and the use of SO
2 should be declared on the wine label in most winemaking countries. Therefore, there is a trend that the use of SO
2 is minimized in wine production and the wine industry has always been seeking for alternatives to reduce the usage of SO
2.
Common SO2 alternatives used in wine production that have shown the efficacy of protecting juice and wine against oxidation are ascorbic acid and glutathione, to a lesser extent, glutathione-enriched inactivated dry yeast. Most of these alternatives have combined effects when used together with SO2 while some of them need to be used complementary with SO2. Various combinations of these antioxidants have been studied to find the most effective and efficient way to protect the wines while keeping the SO2 additions to a minimum level.
2.1. Ascorbic Acid
Ascorbic acid has been utilized as an antioxidant with its capability to protect wines from oxidation by preferentially reacting with oxygen before the auto-oxidation of phenolic compounds happens and by reducing
o-quinones back to the original phenolic compounds
[9][10]. The oxidation of ascorbic acid generates hydrogen peroxide and dehydroascorbic acid, which is unstable and can degrade to a wide range of products
[11]. Therefore, ascorbic acid is recommended to be used in conjunction with sulfur dioxide in wines as the presence of sulfur dioxide is essential to sufficiently remove hydrogen peroxide
[12][13] and bind the dehydroascorbic acid and its degradation products
[9].
Ascorbic acid protects white wines against oxidation and minimizes the browning of the wine under regular oxygen concentrations
[14]. The combination of ascorbic acid and SO
2 better preserves fruity aromas as well as reduces oxidative aromas of the wines than using SO
2 alone
[15]. Sauvignon Blanc wines supplemented with ascorbic acids and SO
2 were shown to contain higher levels of varietal thiols
[16], and present higher intensities of fruity, grass, and green pepper aromas compared to wines added with SO
2 alone
[17]. In addition, the presence of ascorbic acid decreases the requirement of SO
2 for a given amount of oxygen consumed in wines and thus extends the shelf-time of the wines
[18][19]. However, in wines exposed to excessive amounts of oxygen either from poor bottling practices, inadequate closures, or long-term ageing, the role of ascorbic acid might convert from anti-oxidant to pro-oxidant
[20][21]. This pro-oxidant activity relies on the rapid oxygen consumption of ascorbic acid together with SO
2, which will result in browning and a shortened lifetime compared to ascorbic acid-free wines, and can even contribute to spoilage of the wine over a longer time
[20]. Conversely, Sauvignon Blanc wines added with ascorbic acids but sealed with closures with low oxygen transmission rate were found to develop reductive characters after bottle ageing
[22]. Therefore, TPO (total package oxygen) concentrations indicating the oxidation potential should be considered when bottling
[20].
2.2. Glutathione
Glutathione (GSH) is a tripeptide consisting of L-glutamate, L-cysteine, and glycine. The unique reducing and nucleophilic properties of glutathione that allow protein thiolation and modification of protein structure and function are ascribed to its free sulfhydryl moiety of the cysteine residue
[23]. When acting as an antioxidant, glutathione can be oxidised enzymatically to glutathione disulfide (GSSG)
[24]. Under unstressed conditions, GSSG can then be reduced back to glutathione by glutathione reductase, resulting in over 90% of glutathione existing in a reduced form
[25][26].
Glutathione is naturally present in grapes and accumulates during grape maturation
[27]. The concentration varies due to different grape varieties and is affected by both viticultural and oenological practices
[28][29][30]. It can also be supplemented into musts or wine during the vinification process. A study revealed that glutathione added to Sauvignon Blanc juice shortly after crushing can provide antioxidant protection to the juice, reduce the use of SO
2, and produce more volatile thiols in wines
[31]. However, the amount of glutathione supplementation is regulated by the Organisation Internationale de la Vigne et du Vin (OIV) with a limited dose of no more than 20 mg/L in the must
[32]. The antioxidant properties of glutathione rely on the capability of its high affinity for oxygen which allows it preferentially oxidise its thiol group into a disulfuric group and form grape reaction products (2-S-glutathionyl caftaric acid), which terminates the oxidation process and thus protect other molecules from the attack of reactive oxygen species
[33].
The effect of glutathione on the stability of wine flavour and wine colour has been demonstrated. It limits the formation of browning pigments by trapping
o-quinones in a colourless form
[34] and by limiting the production of xanthylium cation pigment precursors and
o-quinone-derived phenolic compounds
[35]. Additionally, glutathione protects varietal thiols from oxidation during bottle ageing
[36], while its cysteinyl residue can be used as a source of sulfur to increase the concentration of polyfunctional mercaptans by reacting with trans-2-hexenal to form Glut-3MHal
[37]. In addition, glutathione decreases the degradation of polyfunctional mercaptans during storage. Glutathione inhibits the decrease of several aromatic esters and terpene alcohols, such as isoamyl acetate, ethyl hexanoate, linalool, and α-terpineol
[38], as well as limits the accumulation of acetaldehyde
[39] during storage. Furthermore, it suppresses the formation of stolon and 2-aminoacetophenone (2-AAP), which release unpleasant odours in wines and contribute to atypical wine ageing defects
[40]. However, glutathione is found to favour the accumulation of hydrogen sulfide and methyl mercaptan, especially in the presence of copper under low oxygen conditions
[41].
The use of glutathione in conjunction with SO
2 has a combined effect on protecting volatile thiols against oxidation
[42], also, the addition of SO
2 slows down the enzymatic reduction of trans-2-hexenal and inhibits the enzymatic oxidative loss of glutathione
[37]. The combination of glutathione and ascorbic acids strengthens the antioxidant capabilities and protects the phenolic compounds from oxidation
[42], in addition, glutathione delays the loss of ascorbic acids and inhibits the reaction of ascorbic acids degradation products and (+)-catechin
[35].
2.3. Glutathione-Enriched Inactivated Dry Yeast
Despite glutathione has good antioxidant property, the application of glutathione to winemaking is still limited as the amount of glutathione supplementation is regulated
[43]. The recommended approach of adding glutathione to juice or wine is through the addition of inactivated dry yeast (IDY) with guaranteed glutathione levels
[44]. The preparations of glutathione-enriched inactive dry yeast (GSH-IDY) are manufactured from the thermal inactivation of
Saccharomyces cerevisiae, which is cultivated under specific conditions (highly concentrated sugar medium) to stimulate the intracellular accumulation of glutathione. Commercial GSH-IDY were claimed to boost glutathione content either by liberating glutathione into the wine or by allowing the yeast to assimilate glutathione precursors during alcoholic fermentation for increased glutathione production
[45]. Sauvignon Blanc wines supplemented with GSH-IDY preparations are shown to increase the concentration of certain volatile compounds, including thiols, higher alcohols, fatty acids, esters, and monoterpenes, and lead to higher intensities of aromas associated with riper tropical fruit than GSH-added wines
[46]. The reasons that influenced the aroma profile of the wine are ascribed to the release of compounds other than glutathione by yeast products
[42][46]. Previous studies that supplemented GSH-IDYs into the wines had some promising findings (
Table 1), most of them affirming the antioxidant capability provided by glutathione released into the wine. More research is needed to better understand the full potential of this prospective antioxidant in white wine production, e.g., optimization of glutathione accumulation process, dosage rate, and timing of addition.
Table 1. Summary of previous studies on glutathione-enriched inactivated dry yeast (GSH-IDY).