Wine Diamonds Are A Girl's Best Friend
“By the way, what is this stuff?” my dad asked. “What stuff?” I looked up long enough to avoid being impaled by a ginormous jig saw dangling off of a work bench. I was gingerly weaving my way through his wood workshop, tiptoeing over half-completed projects.
“The guys and I were afraid to taste it,” he continued, pressing on to the back of the building.
I caught up to him as he was rubbing white sandy-looking crystals from one of the wine barrel staves he’d deconstructed. A veteran police officer in the narcotics division, I figured he thought it looked like crack or crystal meth. Although I’d never actually seen either one except in the movies, I assumed it might look like the tartrate salts he was scratching off with his thumb.
“Oh, that stuff is just tartrate. It’s from the grape acids. It precipitates out as a salt. Buffer system. pH dependent.” Luckily my step-dad also has an engineering degree; he’s used to science talk.
Tartaric acid is one of the two primary acids that live in grapes. The concentration of tartaric acid in any given wine depends on the grape varietal, growing conditions, climate, and maturity. It’s a weak acid that likes to give away either one or two protons (positively charged H+). If tartaric acid dumps a proton into the surrounding grape juice, it’s left net negative. A negatively charged unit (in this case a bitartrate ion) wants to hook up with anything positive, to squelch that uncomfortable, nagging negative feeling. When a negative bitartrate sticks to a positive potassium ion, you get “wine diamonds.”
“Wine diamonds,” “wine stones,” “shards of glass,” “residual sugar,” and “chemical residue” are all colorful names for potassium bitartrate salts, which can precipitate out of wine when their concentration exceeds a solubility threshold. As strongly attracted as they are to one another, hooking up in amorous rendez vous, bitartrate (negatively charged) and potassium ion (positively charged) may also coexist separately depending on the pH of the wine. But even when bound together, their coupling stays soluble as long as their relative concentration is below the saturation threshold. Above this magical number, the adjoined compounds form the white, chalky salty stuff my dad found stuck to the barrel staves. Obviously it’s not a problem if residual goop sticks inside a wine barrel that the drinking public never sees. It’s uglier if it happens inside a wine bottle, shocking your dinner companions as “shards of glass” tumble into their Reidel stemware as you top off an aperitif. The tartrate salts are harmless. They don’t affect taste; they are safe to consume. But they are an aesthetic detriment. They look ugly.
So what pushes these guys from unattached wine components, Rated-G to XXX conjugated material? A number of factors up the ante from saturated to supersaturated. The most common reasons that potassium bitartrate precipitates out of solution are cold temperatures and lots of alcohol, same reasons drunken coeds hook up at the Dartmouth Winter Carnival. Pre-fermentation the alcohol concentration is low enough to hold bitartrate in solution. Stick your wine in cold cellar storage after fermentation, and you’ve got trouble. It’s worse for white wines. In reds, the additional tannins stick to bitartrates and hold them in solution. Without the added tannins, white wines are simply prone to tartrate instability. The cheapest and most common way to rectify tartrate instability is a process known as cold stabilization. White wine are chilled down to cold temperatures and “seeded” with a fine potassium bitartrate powder to increase the surface area. Over time, the unstable potassium bitartrate precipitates out as a solid. You stir and agitate the wine to maximize solid salt formation. Finally, you filter your blush or white wine while its still cold to prevent salts from re-dissolving when it’s warmed back up. These are the fundamental steps that keep white and rose wines stable and clear.
In case you were wondering, my dad is using recycled wine barrels to build me some Adirondack chairs. Hopefully he’ll power wash the tartrate salts off before they’re completed. I don’t want that gunk staining my pants.