Here are some thoughts on wine SO2 content and how much "headspace" should be left at bottling time. These ideas are based on the following: (1) Only small quantities of gases dissolve in a liquid. Then the liquid becomes saturated, and no more gas can dissolve in the liquid. (2) Air contains about 21 percent oxygen, and when wine is exposed to air, oxygen dissolves in the wine quickly. (3) When saturated, room temperature wine contains about 7 milligrams of oxygen per liter.

Most home winemakers use a "wand" type bottle filler attached to their syphon tubing. A valve presses against the bottom of the bottle when the wand is inserted in an empty bottle, and the wine starts to flow. Wine flow automatically stops when the tube is raised. Little bubbling occurs when bottles are filled slowly, but these little fillers can generate excessive amounts of foam when bottles are filled quickly. The foam exposes a large surface area of wine to the air, and considerable oxygen dissolves into the wine. This is why wine often becomes saturated with oxygen when bottles are filled carelessly. Saturated wine contains 7 mg/l of oxygen per liter, so a 750-ml bottle of saturated wine contains about 5.2 milligrams of dissolved oxygen.

Some home winemaking shops claim these wand-type bottle fillers "leave just the right amount of head space in the bottle." I have three different fillers and each leaves almost three inches of head space in the bottles, and much of this air is compressed when the cork is driven. Since air contains 21 percent oxygen, 10 milliliters of head space contains about 2.8 milligrams of oxygen.

One of the most important reasons for adding sulfur dioxide to wine is to scavenge oxygen. But, the bottling operation adds much oxygen to the wine, so determining the correct amount of sulfur dioxide at bottling time is not easy. The molecular weight of Sulfur (S) is 32 and oxygen (O) is 16, so 4 milligrams of sulfur dioxide (32 + 16 + 16) are required to react with 1 milligram of oxygen (16). Knowing that 4 milligrams of sulfur dioxide are required for each milligram of oxygen in the wine can be used to estimate how much sulfur dioxide needed.

For example, wine splashed excessively at bottling time will be saturated, and each bottle of wine will contain about 5.2 milligrams of oxygen. If the bottle is under filled, resulting in 10 milliliters of head space, the head space contains 2.8 milligrams of oxygen. Consequently, the total oxygen content of each bottle of wine will be about 8 milligrams (5.2 + 2.8). Four milligrams of sulfur dioxide are required for each milligram of oxygen, so 32 milligrams of sulfur dioxide are needed to react with the dissolved oxygen. Since a bottle contains 750 milliliters, the wine would need to contain about 43 ppm of SO2 just to react with the dissolved oxygen. Even so, practically no SO2 would be left to protect the wine. If 15 ppm of residual SO2 is desired, a free SO2 level of 63 ppm in the wine would be needed.

Here is a second example. Wine might be about 1/3 saturated when bottles are filled with little splashing. Then, each bottle of wine will contain about 1.7 milligrams of oxygen. If the volume of the head space is reduced to 5 milliliters, the head space will contain about 1.4 milligrams of oxygen. Each bottle of wine will have a total oxygen content of 3.1 milligrams, and 12.4 milligrams of SO2 will react with the oxygen. If the wine contained 30 milligrams of SO2 per liter at bottling time, less than half (41%) of the SO2 would be consumed by reacting with the dissolved oxygen. Now, about 18 ppm of SO2 would remain in the wine to provide prolonged protection.