Ever opened a nice bottle of white wine, forgot about it and had to throw it out? Chances are, you suffered the effects of oxidation. Ever enjoyed the nutty and earthy flavor of Sherry or Madeira wine? Broadly speaking, the same oxidative processes exercised their effects. The difference between these two situations lies mostly in intent. While ‘oxidized’ wines are a mishap stemming from neglect, ‘oxidative’ wines are primarily a product of intent. This is the reason why Sherries and their like are a welcome part of a cocktail despite being exposed to oxidation. Though what exactly is happening inside the exposed wine and how does it shape the flavor of the resulting fortified wine as well as the cocktail containing it?

Let us start off with the many ways this procedure can go wrong in standard wine. The term ‘oxidation’ in the case of wine mainly indicates the reaction with oxygen and generally describes a wine fault. There are several opportunities for a wine to be exposed to oxygen. The first opportunity presents itself when the grapes are crushed to extract the juice. In fact, this process of mechanical extraction also mixes enzymes with chemical substances in the grapes which they normally would not encounter. Thus, one major category of oxidative processes, enzymatic oxidation, is mainly happening at this stage of the wine life-cycle. Polyphenol oxidases, the enzyme family also responsible for the browning of apples, oxidize phenols (mainly hydroxycinnamates) into quinones (mainly benzoquinones) in the presence of oxygen. These hydroxycinnamate quinones then can react with catechin (one member of the flavan-3-ols, the most important flavonoids in wine) from the grape seeds or skins to result in brown pigments. This color change towards a muddy brown hue is one of the hallmarks of wine oxidation.

Enzymatic & nonezymatic oxidation processes in the sherry production process.

Once the fermented wine is maturing in wood barrels, the second category of oxidation kicks in with non-enzymatic oxidation processes. Even though at the beginning of maturation oxygen is largely absent from the wine, over time the ‘angel’s share’ is evaporating from the cask and oxygen is diffusing into the liquid through the porous wood if the wine is not topped up regularly. The first chemical reaction occurring then is the non-enzymatic oxidation of polyphenols with a catechol or galloyl group into quinones. These quinones, together with those produced by enzymatic oxidation, are a common pool of oxidized chemicals which then react further to produce the molecules imparting the wine with oxidized characteristics. Another reaction, especially for red wines, is the polymerization of anthocyanins and flavan-3-ols, leading to a brown color change. This reaction is assisted by the oxidation product of ethanol, acetaldehyde, which is also the precursor of the even more oxidized acetic acid. Additionally, some enzymatic oxidation is occurring at this stage, for instance by the enzyme tyrosinase which converts phenols into the brown pigment melanin.

Even uncontrolled, flavor products from oxidation reactions can initially increase the complexity of a wine. A compromise between oxygen-free wines and Sherries etc., micro-oxygenation of wines is used to soften harsh tannins and to stabilize its color. Specifically, California Cabernets are usually micro-oxygenated (exposed to a little bit of oxygen in a controlled manner) to reduce methoxypyrazines which would otherwise contribute a vegetal bell pepper character to the wine. However, these advantages are quickly overpowered by unwelcome flavors such as ‘boiled potato’ (methional) or ‘kerosene’ (1,1,6-trimethyl-1,2-dihydronaphthalene). Therefore, next to ensuring the absence of oxygen, sulfur dioxide (SO2) or ascorbic acid (vitamin C) is added to wines as a measure of protection against oxidation. Utilized since the 18th century, SO2 directly inhibits enzymes such as tyrosinase and both SO2 as well as vitamin C are able to reduce quinones back to their respective phenols. This is crucial as phenols make up most of the aroma of a wine, which incidentally implies that an oxidizing wine first loses its aroma and then its color.

The negative effects of oxidation notwithstanding, products such as Sherries, Madeira wines and vin jaune (literally ‘yellow wine’, from the Jura region in France) intentionally pursue an exposure to oxygen. While translucent fino Sherry has a protective layer of yeast on its surface (mostly Saccharomyces cerevisiae) which blocks the oxygen, darker Sherry types such as amontillado or oloroso are forced to interact with oxygen by the removal of this layer of so-called flor. One immediate effect of oxidation, combined with a fortification through additional ethanol, is a substantially prolonged shelf-life of Sherry and its colleagues after opening a bottle. Because how could something ‘spoil’ that has already ‘decayed’ (though in the best possible way)? In general, the rigorous prevention of oxidation in normal wines pronounces floral, herbal and fresh fruit flavors, while the controlled oxidation in Sherry & Co. results in umami, earthy and nutty flavors. Potential flavors which are characteristic of Sherries can be ‘honey-like’ (phenylacetaldehyde) or ‘nutty/spicy’ (sotolon).

Different types of oxidized wines.

Sherries have confidently found their way into the cocktail world as versatile modifiers, which can bring a complex range of flavors into drinks. The added value by their controlled decay can be clearly observed in the fact that they figure far more frequently in cocktail recipes than standard wine. Other fortified wines, such as Port wine from the Douro valley in Portugal, are also prominent in cocktail recipes. Yet, there are still underappreciated gems in the area of oxidative, fortified wines. Both Madeira wines and vin jaune are relatively well-known examples of complex, oxidative wines which could be used more in the world of drinks. In fact, during the colonial times most of the wine in the US came from Madeira, so a cocktail like the Sangaree from Derek Salerno at Shagbark in Richmond, Virginia presents a good start for the usage of Madeira wines in cocktails. Overall, I think we can consider ourselves very lucky to receive such excellent products from a process typically associated with decay and waste.