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CL ARK SMITH Postmodern Winemaking Sulfur Dioxide basics Revisited ventional SO2 management. A table I T hirty-two years ago my first published piece, printed in the inaugural issues of the Uni- versity of California, Davis, Extension's Enology Briefs,1,2 concerned the basics of con- worked out with pencil and paper in a Shields Library basement can still be found tacked up on winery lab walls throughout the United States. There are omissions I have since regretted, and it is high time for a rewrite. Sulfur dioxide is an added preservative with anti-microbial and anti-oxidative effects, and it occurs in wine as a variety of sulfites. These have almost nothing to do with the stinky sulfides that occur as artifacts of reduction and are never add- ed. Added to wine, sulfur dioxide, when added to wine, will bind with whatever aldehydes are present as well as with anthocyanin pigments; whatever is left over is called "free" or "FSO2," existing simultaneously in three forms distribut- ed according to the wine's pH: SO2(molecular) + H2 (bisulfite) <=> 2H+ pKa1 = 1.8 pKa2 = 6.9 O <=> H+ + SO3 + HSO3 2- (sulfite) Each of these species has very different properties. (See table "Distribution of Free SO2 does not attract a gel of polar water mol- ecules the way the ionized forms do. This allows it to volatilize into the headspace, where it exhibits the stinging odor of a freshly struck match. Its small, apolar na- at Various PHs on page 58.) Because it is not charged, molecular SO2 56 Wines & Vines MARCH 2012 2- Highlights • The author updates a paper he wrote in 1980, which became a common refer- ence on sulfur dioxide management for winemakers. • His most important omission was about high-pH wines. For wines above pH 3.6, winemakers should either correct the pH or employ a high-pH winemaking strategy. • Sulfur dioxide as a method of preserva- tion is not harmful to health, but going without it opens the door to interesting new wine styles. ture also allows it to penetrate cell mem- branes and carry toxic acidity into cells, destroying their functionality.3 In my 1983 thesis, I examined the hy- pothesis that sulfur dioxide is not funda- mentally reactive at all, but owes its toxic properties (like acetic acid and chlorine bleach) to the membrane permeability of its low-pH molecular form. The reason this is so important is that it explains why sulfur dioxide is toxic to microbes without similar effects for hu- mans. Compared to a glass of wine, mi- crobes are tiny and we are huge. A low-pH environment assures conditions in which a microbial cell in equilibrium is inevita- bly pickled. The cells of a human body, by contrast, can remain at neutral pH while ingesting a glass of wine or a dill pickle. This is the same reason salt is a good pre- servative but is non-toxic to people. The percentage of free SO2 that occurs as molecular SO2 varies tenfold over the pH range of wine (see table.) A cider study at Long Ashton Research Station was used by Roger Boulton4 at UC tenance level to suppress microbial activ- ity during cellaring. Once bottled, a target level of 0.5 mg/L has been advocated by Lisa Van de Water of The Wine Lab, which is closer to the sensory threshold. In anticipating residual molecular Davis as the basis for establishing 0.8 mg/L molecular SO2 SO2, the winemaker is obliged to esti- brought about by the reaction with oxygen picked up during bottling. mate the decline in free SO2 ½ O2 + SO2 + H2 O => H2 SO4 etry to this reaction. For every 1 ppm of dissolved oxygen (DO) picked up during bottling, 4 ppm of SO2 Since oxygen weighs 16 daltons, and SO2 64 daltons, there is a 1:4 stoichiom- will be reacted, usually within a few months. This should be subtracted from the anticipated FSO2, along with an additional 6 ppm if a cork is used (they dispense compressed air), be- fore calculating the residual molecular SO2 in the bottle, which can vary widely based on pH. Bisulfite is the predominant species within wine's pH range, comprising 94% at pH 3.0 and 99% at pH 4.0. Its actions in wine are not pH-dependent. In juice, as with lettuce in salad bars, potatoes, apples and apricots, bisulfite inhibits the browning enzyme polyphenol oxidase (PPO). In addition, bisulfite inhibits non- oxidative browning by tying up oxida- tive intermediates called quinones before they get a chance to polymerize. Counter-intuitively, browning in juice does not result in brown wine but rather precipitates browning precursors so the re- sulting wine is pale and lower in phenols. Winemakers divide themselves into two as the recommended main-