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WINEMAKING cops on the beat fighting crime. TA is high- er in tart wines, but low pHs mean high free acidity. In normal maturation, malic acid is burned inside the berry, furnishing energy that grapes use to concentrate sugar into the fruit and lower TA. While TA is drop- ping, pH is rising from around 3.0 to as much as 4.0. The strength of an acid is determined by the particular acid's pKa, (i.e. the pH at which half of the acid is ionized). Lac- tic acid, for example, has a pKa of 3.8. Above pH 3.8, it's mostly ionized, and below 3.8, it's mostly undissociated. Some acids have multiple pKas. Grape juice has two diprotic acids: the stronger tartaric acid (symbolized as H2 Ta) with pKas of 3.0 and 4.2, and the weaker malic acid (H2 Ma) with pKas of 3.5 and 5.0. Big difference. At wine pH (3.0-4.0), tar- taric acid is always a lot more ionized than malic acid. Malic acid is like a really good donut shop where the proton cops like to hang out instead of patrolling the wine. In geek-speak, we say that the solution is heavily buffered. A lovely characteristic of tartaric acid is that bitartrate precipitates with potassium to form crystals that reduce TA—very handy. This effect is maximized at pH 3.6, the peak of the bitartrate curve. This turns out to be a big deal, the natural great di- vide for winemaking. Here's something weird: Above 3.6, Plan A. Here's how it works. First, as the compound dissolves, it ionizes into potas- sium cations and carbonate anions: CO3 K2 CO3 => 2K+ + CO3 2- Next, the carbonate neutralizes some pro- tons, benignly turning that nasty acid taste into carbon dioxide bubbles and water: CO3 H2 2- + 2H+ => O (water) + CO2 (bubbles) KHTa (potassium bitartrate) precipitation lowers TA and raises pH, just as you'd ex- pect. However, below 3.6, TA is lowered but pH is also lowered. The acid goes down but it also goes up, resulting in softer taste, but with more stability and freshness. Not too shabby. Because of this effect, it often works to de-acidify low-pH wines by adding potas- sium carbonate (K2 ). This is always Since it removes free protons, this reac- tion raises the pH. But the K+ ions will enhance bitartrate precipitation. As long as this happens below pH 3.6, this pre- cipitation will lower both TA and pH, moving us back in the direction of the original low pH. A wine with a TA of 10 g/L and a pH of 3.1 can emerge with a TA of 7.5 g/L and a pH of 3.3 at negligible cost. Hot stuff. We are completely unprepared for the impact of our new de-acidification capabilities. Bad acid trips If we have a lot of tartness in a wine, we might expect a nice low pH. Lots of crit- ics and master sommeliers think that tart wines age longer. 'Tain't necessarily so. The wrong mix of acids can give you very tart wines with very poor shelf life. Grapes can get out of sync, so you get re- ally high pHs when you still have high TAs. A high TA means the juice has lots of Search BARRELS Online Winery Suppliers ONLINE SEARCH Cooperage Packaging Tasting Rooms Winery Equipment Winery Services Winery Suppliers WWW.WINESANDVINESBUYERSGUIDE.COM QSEE US AT UNIFIED, BOOTHS #430 AND #818 138 Wines & Vines JAnUARY 2012 Vineyard Suppliers General Suppliers