Spice and sulfur: Recent research news from Australia

Wine Australia’s “research, development, and extension” arm periodically releases briefings on research they’re funding. This month, two of those briefings promise particular interest for folks outside Oz.

Spice (Rotundone)

Short: How rotundone (and the peppery flavors it yields) develops in grapes still lacks good scientific explanations, but scientists are working on it and suspect that it’s a multi-factorial process involving sunlight and air exposure as well as enzymes.

Longer: Not necessarily positive for wine quality, but stellar for talking about how chemistry produces flavor. Rotundone is the “impact compound” behind the peppery flavors prominent in some Australian shiraz (and some other red wines), and rarely does a single compound correlate so clearly with a single and very easily identifiable wine sensory note. The sensory correlation seems simple (caveat emptor: expect it to become more complicated as scientists spend time studying it). How rotundone forms is anything but. A lot of scientific activity (and not just in Australia) over the past year or so has been working out both the chemical pathway (enzymes and intermediates) responsible for rotundone and the viticultural parameters describing where it forms.

A major rotundone conundrum has been why its concentrations seem highest in cooler sites – within the plant, the vineyard, and a region – and yet rotundone characterizes ripe Australian grapes. As is so often the case in scientific conundrums, the confusion may stem from trying to pin causality on the wrong set of variables. Rotundone is formed by an oxidation reaction. Recent work says that that oxidation may be motivated by enzymes, sunlight, or oxygen. Or all three. And so, even if rotundone = peppery flavors is a simple equation, solving X + Y + Z = rotundone is shaping up to be a good deal more complicated.

Expect big rotundone news from Australia in upcoming years. Sussing out a viticultural recipe for maximizing (or minimizing) pepper flavors in shiraz could set up Australian shiraz to do what the Marlborough sauvignon blanc industry has done, creating an international brand around a distinctive flavor profile, fueled by scientific research into how to make those flavors ever more obvious. What that research means for smaller producers who aren’t aiming for those sorts of flavor profiles is a different, and interesting question. 

Sulfur (and copper)

Short: Adding copper to finished wines to remove or prevent sulfur aromas may not work the way everyone hopes it does.

Longer: Wines made with very little oxygen exposure and bottled under screw cap don’t have much chance to blow off smelly sulfur compounds produced via this sort of reductive winemaking. (Why sulfurous aromas are a problem in reductive winemaking involves some complex microbiology that’s summarized well here.) A standard prophylactic against eau de cabbage or rotten egg in your freshly unscrewed bottle is adding some copper before bottling; copper binds to the smelly sulfur compounds and acts as a heavy anchor of sorts, keeping the malodorous molecules from volatilizing, entering sniffable air space, and registering as an undesirable aroma. Adding a copper penny to a sulfurous wine glass is a common parlor trick for confirming that particular wine fault; if you’re really smelling sulfur, in theory, the penny should mitigate the problem.

A nice, simple solution to nasty sulfur aromas would seem to be adding copper to bind to and “lock away” sulfur compounds, then counting on pre-bottling filtering to remove the copper-sulfur compounds.

Problem #1: It seems that filtering doesn’t reliably remove the copper.

Problem #2: The copper-sulfur binding isn’t always stable or permanent, so the copper may go off and do other (undesirable) stuff no one was counting on.

It seems likely that copper additions are useful under some if not all circumstances. The future work – of researchers and winemakers working together, one hopes – is defining “some if not all circumstances” more precisely.

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