Oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
and
reduction in
red wines can lead to a particularly undesirable
brick red
Maroon ( US/ UK , Australia ) is a brownish crimson color that takes its name from the French word ''marron'', or chestnut. "Marron" is also one of the French translations for "brown".
According to multiple dictionaries, there are v ...
color in red wines (or an "orangey" color in
white wines). This process is sometimes referred to as browning.
In chemical terms, this is called a
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
reaction because first the color of the wine deepens after fermentation through oxidation, and then the color begins to brown after bottling through reduction. Browning is not strictly considered a bad thing in every wine that is produced, as sometimes the oxidation process can contribute to the style of the wine in a positive and desirable way.
Generally speaking, however, browning is a sign of the wine going stale from too much exposure to oxygen. Although
wine that has gone bad is typically associated with the smell and taste of vinegar or unwanted
effervescence
Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from that release. The word effervescence is derived from the Latin verb ''fervere'' (to boil), preceded by the adverb ''ex''. It has the same li ...
, oxidation itself can actually lead to "nutty", "applesauce", and "burnt marshmallow" aromas. The sharp vinegar component comes from
acetic acid formed through
bacterial processes. A lower
pH level is typically preferred in wines because it decreases the overall risk of spoilage. The lower pH reduces the effect of browning and yields better colors, particularly in red wines.
Significance of color
![Bengt Frithiofsson evaluating wine](https://upload.wikimedia.org/wikipedia/commons/1/15/Bengt_Frithiofsson_evaluating_wine.jpg)
The color of a red wine will have many variables that influence it besides its exposure to oxygen that pertain to other
viticultural
Viticulture (from the Latin word for ''vine'') or winegrowing (wine growing) is the cultivation and harvesting of grapes. It is a branch of the science of horticulture. While the native territory of ''Vitis vinifera'', the common grape vine, ran ...
aspects such as different growing climates,
cultivar
A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture ...
s, and production methods. For example, wines made from hybrid grapes can range anywhere from light pink to purple due to their chemical composition. Dr. Leo McCloskey, best known for his contributions in developing the
Enologix software, points out that "Chemical ecology says that a wine's flavor, color and fragrance are expressions of its ecosystem."
However, when people perceive wines, the visual cues received from the color of the wine have a strong impact on their opinion of the wine. In a primitive sense, visual cues will generally have a stronger impact on humans than aromatic components since humans have evolved to identify things with eyesight, rather than through scent relative to other evolved species that rely on different sensory capacities more than they do on eyesight. This idea of a particular significance of color in the perception of wine is especially true in red wines.
Browning is therefore an important issue both for winemakers and for wine consumers, and is generally avoided. A well-known exception to this is
sherry
Sherry ( es, jerez ) is a fortified wine made from white grapes that are grown near the city of Jerez de la Frontera in Andalusia, Spain. Sherry is produced in a variety of styles made primarily from the Palomino grape, ranging from light versi ...
, which is often purposely developed with oxidation as part of the production process; however, it can be considered a white wine since it is traditionally made from
Palomino grapes.
Odor effects
![Roasted-Marshmallow](https://upload.wikimedia.org/wikipedia/commons/f/fb/Roasted-Marshmallow.jpg)
Through the aging of wine, its aroma compounds are affected as well. The
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
s in the wine that were initially created during fermentation become reduced as they break back down into
alcohol and
carboxyl groups so the floral, fruity smells that are associated with freshness in a wine decrease. Meanwhile, the nutty, cooked smells increase through
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...
degradation.
See also
*
Wine fault
A wine fault or defect is an unpleasant characteristic of a wine often resulting from poor winemaking practices or storage conditions, and leading to wine spoilage. Many of the compounds that cause wine faults are already naturally present in wine ...
References
{{portalbar, Chemistry, Wine
Wine
Chemical reactions