Making Sense of Reduction

Reduction is a term used by winemakers and wine critics in the context of wine production and evaluation that wine drinkers rarely understand. It is like the Big Bang theory: a mysterious dark energy that hardly any normal person apprehends. Fortunately, we have talents who can explain reduction in understandable terms. The following offers unedited discussions of reduction and its significance in wine evauation from Allen Meadows, aka Burghound, and a very geeky, scientific discourse on reduction’s role in fermentation from Kerith Overstreet, M.D., the cerebral winemaker for Bruliam Wines.

Because of the continued use, in fact growth, of reductive winemaking in Burgundy, more and more in-barrel tastings that occur after malolactic fermentations, result in notes that refer to reduction. When wines are reduced, I say so rather than substituting a wine writing euphemism. p> The reason that the term reduction is seen so often is because more and more Burgundians, for both red and white wines, are now practicing what is called reductive winemaking. Effectively, what this means is that they are increasingly working with the fine lees; while this has long been accepted practice with whites, it has now spread to the reds as well. As a result, they rack less often or not at all and when they do, it’s often with limited or no exposure to oxygen. Indeed, more and more vignerons rack only in preparation for the bottling, which means that when I and other reviewers pass by to take a look at the wines a year after they were made, they are generally in a pretty reduced state.

Without getting into a chemical analysis (see Overstreet’s discussion below), of which I am ill-equipped to guide you anyway, this practice often results in a highly reductive environment in barrel, which simply put is the absence of oxygen. Winemakers have long understood that too much oxygen, particular in whites, is not a good thing because it oxidizes them and in extreme cases, can turn both reds and whites prematurely brown. However, the reverse can be taken to an extreme as well and when this occurs, a condition known as reduction occurs. A reduced wine smells dirty and in particular of sulfur compounds. A heavily reduced wine will taste of it as well and when extreme reduction occurs, mercaptans appear. Mercaptans smell of burnt rubber, garlic, stale sweat among other descriptors and suffice it to say, that is very unpleasant and if not corrected, can ruin a wine.

Reduction is, generally speaking, easily cured simply by introducing oxygen. As a practical matter, the way that this is most often done is to rack the wine from one barrel to another, which introduces oxygen and eliminates most of the lees (though sometimes they are kept and added back). However, if reduction appears in a finished, bottled wine, it is a clear flaw and while aeration (decanting) will usually clear it up, some wines are permanently reduced and about all that can be done is to put a penny in the wine. Sounds bizarre, but it works! (Note that my wife suggests a clean penny).

The reason that reduction is significant is because it renders a wine particularly difficult to judge. Slight reduction is not something to worry about because the basic characteristics of the wine are still evident. Heavy reduction that extends to the flavors makes a wine almost impossible to accurately judge and if the condition is left untreated it can permanently mark a wine. This risk explains why traditional practice in Burgundy is to rack a wine after the malolactic fermentation is finished so as to introduce some oxygen and thus dissipate the reductive aromas. However, with ever more precise analytical tools, a winemaker can push the edge of the envelope in this regard and still not reach a condition where more radical or invasive solutions are required to fix a problem. While this practice probably does result in richer and more complex wines, it makes the review process extremely difficult while the wines are in this state. I share this with you in the interest of full disclosure as I make every effort to judge a wine as accurately as possible but when there are limitations on my ability to do so, it’s incumbent upon me to say just that......It stands to reason that the closer any wine is to its finished state the more accurate the guidance will be.

“Let’s talk about NAD+/NADH and electron swapping. It’s like wife swapping but better.

“Redox” is an abbreviation of reduction and oxidation. These are terms that describe the way organic compounds swap and exchange electrons. When you gain an electron, it’s called reduction. Your overall charge is more negative; you’re reduced from no net charge to say -1. if someone steals your electron, you’re oxidized. Since oxygen is notorious for swiping electrons from unsuspecting compounds, electron loss is called oxidation. There is a nifty pneumonic for this: “OIL/RIG.” Oxidation is loss (of electrons) and reduction is gain (of electrons).

This concept is paramount since electrons are energy. In fact, electron energy potential is stored in a compound called NADH. NADH can give away an electron to become NAD+. When this happens, NADH is itself oxidized, the electron recipient is reduced since it takes that electron. NADH’s potential energy, in the form of electron donations, is known as “reducing power.” NADH energy drives cellular functions and metabolism. It’s energy currency, but it goes both ways (like wife swapping). Say you’ve got a lot to do so you burn through all of your NADH savings. Well, you’re stuck with a stockpile of NAD+. From an energy perspective, it’s pretty useless. It’s like finding an empty Gu wrapper at mile 20 of a marathon. Its a black fly in your Chardonnay. Even so, you need some way to convert your NAD+ back to the good stuff, so you can proceed with your day. Enter fermentation.

Along the path of fermentation, grape sugars travel the trajectory from sticky, sweet grape juice to vino. The very first step of fermentation is a chemical conversion that initiates the magic, when grape sugars are transformed to pyruvate. Listen, you don’t want to down a kegger of pyruvate, but it’s a start. But in order for glucose and fructose to become pyruvate, they need to give up an electron. What you say? You mean glucose and fructose are oxidized to pyruvate? Yes, it’s true. And can you guess who sops up all of those freebie electrons? NAD+ of course. Fermentation actually helps recycle the NAD+ and renews the energy coffers. In fact, early fermentation, during the yeast exponential growth phase (characterized by an orgy of asexual budding), yeast require lots and lots of reducing power and harness the power of NADH to grow and reproduce. Luckily, our insatiable appetite for wine can make that happen. I’ve got bins of glucose and fructose at the ready.

You are aware that the final step of fermentation is the conversion of acetaldehyde to ethanol. This is the most important step since it makes wine, wine. Ethanol is the final electron acceptor. It’s a Hanukkah miracle come early. Acetaldehyde is reduced to ethanol. You know that the electron comes from NADH, the same stuff that was recycled when glucose and fructose were oxidized back at step #1. It’s a perfectly balanced cycle. The first step of fermentation generates the reducing power (NADH) that ultimately reduces acetaldehyde to ethanol. Ethanol captures that final electron, regenerating the NAD+ necessary to keep the machine churning.

Yeast stockpile a pretty limited supply of NAD+, and without its regeneration, fermentation of grape sugars would grind to a halt. To keep fermentation moving, the redox balance must remain in check. The yeast adjust to the evolving flux in sugar, temperature, and alcohol by tinkering with and maintaining the redox balance.”