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Free SO2: When and How

Protecting your wine with free SO2

Keeping the required amount of free SO2 in your wine is one of the important post-fermentation care steps you’ll need to stay on top of. While some people decide not to add SO2 (which is a personal preference, though we at Musto Wine Grape definitely recommend keeping the proper amount of free SO2 in your wine at all times), others choose to keep their wine protected by using it diligently.

Why we add it

To keep the wine protected. KMS serves as an antimicrobial and antioxidant and as such is a very important part in keeping your wine out of harm’s way. Without it, your wine can go bad very quickly.

How to test free SO2

Aeration oxidation method, manual ripper method, Vinmetrica, Hanna titrator, titrets

Setup for “aerative oxidation” method of checking free SO2. Though time intensive, it yields exact and accurate reuslts.

When to add it

  • During crush (to kill off any unwanted native yeast)
  • When fermentation has completed (be sure that MLF has completed as well, if that’s the goal)
  • During aging. Example: Each month many winemakers will test SO2, make any SO2 adds if needed, and then top it off.

Investing in Vinmetrica SO2 wine analyzer makes testing easy

How to add it

You’ll need to get potassium metabisulfite, if you don’t already have it. You may prefer to use campden tablets, which is fine too. Make sure it says potassium metabisulfite rather than sodium metabisulfite. The easiest way to figure out how much free SO2 you’ll need to add is to know your pH, first of all. Then you’ll need to decide if you’re using .8 or .5/6 molecular SO2. .8 is usually used for white wines, whereas .5 is used for red wines. Measure your free SO2 if you have a way of doing so. Then use the graph below to figure out what the target free SO2 is. From there you will likely choose an easy-to-use online resource such as wineadds.com to find out exactly how many grams of potassium metabisulfite should be added. Mix it with a small amount of water to ensure it can become completely incorporated into the wine.

*Remember: just knowing your target free SO2 in ppm does NOT tell you how many grams you will be adding. You need to figure that out as a second step, with an online SO2 calculator*

Graph indicating required free SO2

Now that you have a solid understanding of why, how, and when you add meta, you may want to delve into some deeper questions, such as…

Why do I need to keep adding meta? If I add a certain amount shouldn’t it stay at that number?

This is a great question. There are a lot of variables that go into answering this in as cohesive a way possible, and this could really be a topic in and of itself. Let’s keep it simple here: there are two measurements of SO2 in wine: free and bound. The free SO2 is what is actively protecting the wine, whereas bound SO2 is what has already tacked on to other compounds within the wine and as such is no longer able to actively protect it in the same way. Both are important numbers, but if a winemaker is only checking one, it’s going to be free. Over time free SO2 can become bound to various acids, solids, sugars, bacteria, yeasts, etc., that are present in the wine, which is why this number will not necessarily stay constant, especially immediately.

Why does pH affect how much SO2 I need to add?

A rule of thumb to remember is that higher pH = higher free SO2 requirement. Lower pH = lower free SO2 requirement. The chart below demonstrates the amount of free SO2 required base don the pH of the wine. This is just one of the many reasons that checking and knowing your wine’s pH is critical.

Do I need to measure total SO2 as well?

Knowing this number is important, but not as crucial as knowing your free SO2. It’s also a little less accessible of a test to run, which is why most home winemakers don’t choose to pursue knowing the total SO2 of their wines. It is possible to overadd SO2 and have the total SO2 be higher than desirable which can be problematic, so be sure to keep a running log and closely track how much you have added from the beginning of the wine’s life onward.