The American Journal of Enology and Viticulture recently released September’s issue laden with current information regarding phenolics in wine. Andrew Waterhouse, distinguished wine chemist at UC Davis, describes the oxidation of phenolics and a new hypotheses adding to the understanding of wine ageability. The aim was to stimulate more study and debate on the mechanisms in wine oxidation chemistry.
I believe there is little argument that oxidation plays a critical role (sometimes positive sometimes negative) in winemaking. Oxidation is essential to the sensory qualities of Madeiras and Sherries and can improve wines by reducing astringency and stabilizing color. However, all is not so rosey. Oxidation is also the chief component of wines that have past their prime or as Waterhouse technically states oxidation has been associated with sensory and/or microbiological degradation.
This technical paper reviews details of wine oxidation – essentially the oxidation of phenolics – that I will not get into. What I want to do here is focus on a few implications from this new, comprehensive scheme of wine oxidation. What the authors are NOT doing is trying to rewrite how wines age. What they ARE trying to do is add another element to how we think about the aging process that could lead to insightful research on the topic. First, an important point to remember is that oxygen has poor direct reactivity with organic molecules (of which wine is loaded) and therefore the oxidizing potential of oxygen must be achieved through the generation of reactive oxygen species. This is where Waterhouse has been able to contribute new information. Iron in wine may be a major component in creating reactive oxygen species that go on to create oxidized products that lead to browning or change other sensory properties of wine. In particular iron can react with hydrogen peroxide to generate a radical that will indiscriminately react with molecules, striking the first thing it hits. The reactive oxygen species does not react selectively with antioxidants such as phenolics, but instead reacts with all substances present in solution, nearly in proportion to their concentration This reaction mechanism leads to the supposition that many other products are also formed, in particular products of abundant components of wine such as glycerol, acids, and sugars. How the oxidation products of these compounds impacts the sensory attributes of wine is yet to be understood because until recently there was no assurance that – take glycerol for example – was even oxidized.
So what, right? Well, what I think is interesting is this: let’s say some of the major components known to impact aging – e.g. pH and phenolic levels – are exactly the same for two different wines. But what if wine A has twice as much iron (or some other heavy metal), does this mean that it will likely have less potential for long term aging? This study implies that the answer is yes, wine A will experience oxidation at a higher rate. Could differences in iron levels largely be attributed differences in the soil? Most likely. Additionally, Waterhouse comments that sulfites, and hydrogen sulfide (that stinky rotten egg smell) in particular, may serve to regenerate phenolics from their oxidized form and that this reaction is what is responsible for the disappearance of the reduced smell of ferments that are aerated. What does this mean for using Copper Sulfate to rid your wine of the stinky smell? Is this perhaps why wine such as those from Syrah and Mouvedre (Rhone and Bandol) are better after aging? These wines are infamous for being a little stinky initially but with time – i.e. with the reaction of sulfides and the oxidized phenols – their aromas come alive. Thus implying that the sulfide is acting as a preservative forestalling the formation of – or decreasing the pool of – oxidized products.
Yes its an understatement to say this is an over simplistic view of aging, but it does open the door for further study into how metals and other minute components of wine may impact the development of wines through time. Waterhouse concludes: Ultimately, control of oxidation in winemaking must address several key questions: What exactly is the role of iron and other transition metals in the rate and outcome of wine oxidation? How do sulfites intervene? It is likely that the concentration of iron, oxygen, and sulfites and the concentration and nature of the phenolics present all affect the result, as does pH. Well it will be interesting to discover how elements such as iron and copper contribute to wine aging.