Hold on there, tiger! If you’ve got a standard 0.44 gram Campden tablet and you’re putting it in one gallon (3.8 L) of wine, you’re blasting it with 66 mg/L sulfur dioxide, which is quite a lot if you’ve already been adding a tablet each time you rack. It’s no wonder your wine tasted a little off.
You don’t want to put too much SO2 in your wine. The American and French governments limit the amount that commercial wineries can add to 350 mg/L of total S02. At this level it’s almost certain you’d be able to pick up free SO2 in the nose and higher levels the wine can become downright unpleasant to imbibe. Also keep in mind the difference between free and total SO2. Free SO2, the portion that is not bound with aldehydes, sugars or other oxidizable substances in the wine, is the form that is available for anti-oxidant protection and anti-microbial activity. Free SO2 will disappear over time into a wine, binding with the above-mentioned substrates, causing the ability of a wine to retard oxygen and microbes to decline as well.
This is the primary reason most recipes recommend you add sulfites every time you rack because — by the time most of us get around to racking a wine (say, it’s every month) — it’s time to add some more sulfur dioxide anyway. Do you always have to do it? That answer is up to you and your wine. If the wine is throwing a ton of sediment all the time and you find yourself having to rack it every two weeks, you don’t necessarily have to add it every time. If your wine is startlingly clear and you haven’t racked in two months, your free SO2 levels might be getting dangerously low and you might want to bump your free mg/L into the 25 range again. Never add S02 without testing the wine first. There does seem to be a point in a wine’s life when the free S02 levels off and you don’t need to add it as frequently. Once a wine is off its lees and approaching bottling, be really careful about adding S02.
So how do you measure the level in your wine? Sulfite measuring kits are available ($10 U.S. for a packet of ten) under the brand name CHEMetrics Titrets. These are good at-home sulfur-assaying kits that you can use to measure the amount of sulfites in your wine. If you really want to get serious, go through a wine lab supply house like Vinquiry/Enartis (805-922-6321, www.vinquiry.com) to get a bench-top set up. It will, if you do a lot of sulfur analysis and plan to make wine for many years to come, be worth it in the long run.
With regards to pre-bottling sulfur adds: Free SO2 should always be adjusted before bottling. For white wines typical levels are 30 to 35 mg/L and for reds 25 to 30 mg/L. Most winemakers try to err on the low side as too much SO2 will be detectable in the nose. You can safely lean to the low side if the wine is clean, bright, dry and especially if it has been sterile-filtered before bottling.
Now you’re going to ask how to add it and how to calculate it out for whatever volume of wine you’ve got. Most home winemakers have access to sulfur dioxide through potassium metabisulfite, available in either powdered or solid tablet forms. As you mentioned, the solid tablets are called Campden tablets and many home winemakers see them as a mysterious “magic pill” that they know does something for their wine but they’re not quite sure what. For those of you who just want the facts:
- Potassium metabisulfite is 57% sulfur dioxide.
- Most Campden tablets weigh 0.44 g
- To figure out how many grams of potassium metabisulfite powder to add to your volume to give you the desired concentration (in mg/L or ppm) of total sulfur dioxide, use the following equation: (gallons of wine you have) (3.785) (ppm or mg/L of total SO2 you want to add)/(1000) (0.57)
Say I’ve got 5 gallons (19 L) of wine that I’m getting ready for bottling. I measured my free SO2 and have found it to be sitting at 10 free. I want to get it into the 30 mg/L free SO2 range. Since my wine is dry, the sulfur dioxide won’t get bound up as quickly so I’ll take a stab at adding 25
(30-10 + 5 for guess work) mg/L total. (5) (3.785) 25) / (1000)(0.57)= 0.83 grams
You’ll have to add 0.83 grams (or 830 mgs) of potassium metabisulfite powder to your 5 gallons of wine to get a free SO2 level of about 30 mg/L. Now, if you only have Campden tablets or don’t have a scale, keep these numbers in mind: Standard Campden tablets are 0.44g of potassium metabisulfite (though they do come in other sizes- read packages carefully!)
1 teaspoon = about 5 grams of potassium metabisulfite (though be careful measuring with teaspoons can give you a 20% error or more!) If you can afford the scale, measuring in grams is the way to go.
1 gram = 1000 mg
0.2642 gallons = 1 liter or 1 gallon = 3.785 liters.
Another alternative is to use the 10% solution method described in “Solving the Sulfite Puzzle” (Winter 2001). Just be aware that a 10 percent potassium metabisulfite solution only yields 5.7 percent sulfur dioxide??
The guess work lies in estimating how much of your sulfur dioxide will get bound up and will not be “free SO2“. Sugar, lees and aldehyde will always aid in the binding process and you’ll lose your free SO2 very quickly. Keeping this and the total amount of SO2 you’ve added over a wine’s lifetime in mind.
Do keep in mind the long-term plan for the wine. Don’t let total amounts of SO2 you’re adding get out of control. Adding too much will eventually spoil your wine. In the end, it’s best to let a free SO2 analysis and your nose be your guides – not a racking schedule. For more information refer to “Solving the Sulfite Puzzle” (Winter 2001).