Sunday, March 19, 2017

Walking the tightrope between oxidation and reduction in white wine production: Oxidation

In this series I will be examining the winemaker's challenge in navigating between the twin evils of reduction and oxidation, both faults but both having potentially desirable characteristics close to the center of the continuum. I begin with this post on oxidation.

According to Lukacs' research (Inventing Wine), modern wine did not arise until the advent of the relevant scientific and technological advances of the Enlightenment. Prior to that period, wine drinkers consumed oxidized, sour wines which were "fortified" with all manner of additives designed to either slow its decay or make it more "palatable." Lukacs points out that winemaking in the first half of the 20th century was a reprise of thousands of years past -- "a process of letting nature run its course."

When Emile Peynaud (famed Bourdeaux enologist) began his work in the early 1950s, growers were harvesting early and, as a result, the wines were "excessively green or vegetal." He observed that there was a further striking uniformity about the wines: they were all oxidized.

Wine oxidizes when exposed to air via two primary mechanisms: enzymic and non-enzymic oxidation.  The effects of oxidation on white wine are browning, loss of fruity aromas, and aldehydic aromas. Because of these characteristics, oxidization is widely viewed as a wine fault.

Enzymic Oxidation
Enzymic oxidation (which primarily afflicts wine must) requires the presence of the enzyme Tyrosinase* (or Laccase**, in the case of botrytized must), phenolic compounds (hydroxycinnamic acids, with the main player being caftaric acid but others — including coumaroyl, tartaric acid, and catechin — as alternates) to perform the role of substrate, oxygen, and metallic co-factors (iron, copper, etc.). These enzymes interact with the substrates to form caftaric acid quinone which, in turn, reacts with glutathione (normally a powerful anti-oxidant) in the must to form Grape Reduction Product (GRP). The conversion of the oxidized quinone to GRP limits the browning of the juice to some extent (duToit and Kritzinger). Once the glutathione is depleted, the remaining caftaric acid quinone reacts with other must constituents to form caftaric acid and begins the oxidation process anew. Browning occurs when the flavanols oxidized by caftaric acid quinones polymerize and precipitate out. Unlike the case of wines, these brown pigments are insoluble in must.

Because tyrosinase is associated with grape solids, its enzymic activity is significantly diminished once the solids have been removed from the equation. Laccase is difficult to eliminate from grape juice.

The activity of these enzymes will be impacted by (Boulton, et al, Principles and Practices of Winemaking):
  • The concentration of major phenols
  • Competition between substrates for binding and reaction
  • The caftaric and glutathione content of cultivar (the state at which glutathione is depleted will determine the level of potential browning)
  • The ascorbic acid content
  • Temperature
  •  Wine pH.
Both tyrosinase and laccase use catechin, anthocyanin, flavanols, and flavanone as substrates but, as indicated in Table 2.1 of Boulton et al., laccase acts on a far wider range of substrates than does tyrosinase. UCDavis pegs the added scope of laccase as encompassing anthocyanin pigments and ascorbic acid, the latter of which is itself used as an antioxidant

Non-Enzymic Oxidation
Non-enzymic oxidation, also known as chemical oxidation, occurs in fermented wine. In this case, oxygen does not react directly with phenolic compounds. Rather, it functions through a reaction catalyzed by Cu+ or Fe+ that converts oxygen into a highly reactive radical capable of oxidizing organic compounds.

Non-enzymic oxidation in white wines can result in premature aging, browning, and pinking, all resulting in wine deterioration and loss of quality. Strategies for combating this fault include removing metals -- oxidation catalysts -- and reducing the concentration of phenolic compounds -- oxidation precursors --in wines.

©Wine -- Mise en abyme

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