Wine chemistry from a dummy
There are two gaps in my education that still impact my life. The first is that I studied French instead of Spanish. I have worked with a mostly Hispanic cellar crew for over 20 years and still understand little and speak less. My French is better, though I have little opportunity to use it. The other clear gap in my education was my avoidance of chemistry. If wine making is a blend of art and science, chemistry is the science. And a lot of the art is just common sense and good housekeeping.
So as a non-chemist wine maker I am in a good position to explain what I understand of wine chemistry to fellow non-chemists. Obviously wine starts out as grape juice, and it is the sugar in it, with the help of yeast, that is converted to alcohol. As grapes ripen, the sugar levels rise and the acidity falls away. In fact, it is the unique climate in the North Bay with warm days and night time temperatures falling into the 50s that keep the acids high enough to produce world class wines. If left in an open container, the alcohol in wine will oxidize to sherry, then vinegar and will further degrade to smell like nail polish remover. Our job is to preserve the liquid as wine. Since the transformation of wine to vinegar is an aerobic process (needing contact with oxygen), the first rule is to keep wine containers topped and sealed. If only it was that simple. Even with the best efforts to protect wine from contact with air, topping alone is not enough. In our system, white wines age from six months to nine months and our red wines age for up to three years before release. The primary preservative of wine is its natural acidity. Wines that are more tart have a better chance of lasting. We can help by boosting the acidity. There are many acids in wine including ascorbic, citric and malic, but the most prevalent and our choice for an acid addition at harvest is tartaric. Unfortunately, wine’s acidity alone is not enough to protect it from the oxidative process.
While some wine makers depend on natural acidity and good housekeeping to preserve their wines, we feel that to deliver wine without oxidized characteristics we need more. This brings us to the subject of sulfur dioxide. So let me take this opportunity to make a case for sulfites in wine. First, sulfites are created naturally during the fermentation process. Sulfur is an antioxidant and literally protects wine from oxidation. Our bodies create sulfites and they may do the same for us. Sulfites are a good thing, unless you are allergic to them in which case they are a bad thing; however this condition is relatively rare. It affects certain asthmatic individuals.
So our goal is to preserve wine by first establishing a healthy acidity and then augmenting that with just enough sulfur dioxide (SO2) to do the job. On the acid-base PH scale of 0 to 14, with 0 to 6.9 being acidic, 7 neutral, and 7.1 to 14 being basic, wine sits between three and four. Three is often too tart to drink and four is so unstable that no amount of sulfur can preserve it. In fact, to our way of thinking a wine with a PH of 3.81 needs double the sulfur of a wine with a PH of 3.50 to protect it. Sulfur dioxide occurs in wine in two forms; bound and free. Together they add up to total SO2. Over time free sulfur moves to bound sulfur. The free SO2 is the part that is doing the work of preserving wine by protecting it from oxidation. By starting with a healthy acidity (PH) we need to add less SO2. Also it moves to bound more slowly and lessens the need for further additions, thus we end up with a lower total SO2 at bottling. We measure SO2 in wine as parts per million and we like our total SO2 at bottling to be under 100 parts per million. By comparison, raisins have near 500 parts per million total SO2.
This meek understanding of chemistry helps me through my day in the lab. I just wish I had a better understanding of the Spanish language to help me through my day in the cellar.