Just like with nutrient additions, adding a cookie-cutter dosage of Potassium Metabisulfite (K-Meta) per gallon is not going to render you effective or repeatable results. Before adding sulfite to your mead, you should understand the difference between Free SO2, Bound SO2, Total SO2, and Molecular SO2. It's a lot easier than it seems. Let's break it down as simply as I can.
Any time a sulfite addition is made, some of the sulfite will bind with various chemical compounds and solids naturally present in your mead, essentially taking up some of the sulfite addition's effectiveness. This "taken up" sulfite is considered Bound SO2.
This is what we are really looking for when dealing with and measuring sulfite in order to figure out how much additional sulfite is needed. Free SO2 is SO2 that simply hasn't been bound and is available for effective protection against oxidation and microbial contamination. That's the whole idea here, isn't it? There can be naturally occurring Free SO2 produced during the fermentation process, much more so when fermenting fruit, so knowing exactly how much Free SO2 you are starting with will assist you with knowing how much more is needed to properly protect your mead.
The total of Free and Bound SO2. Contrary to the belief of some, Total and Free SO2 are not the same thing. Every time you add sulfites, whether the sulfite is bound or not, you are adding to the total amount SO2 present in your mead.
The one that most people have either never heard of or never fully understood, which is ironic since calculating sulfite additions based on Molecular SO2 is the most effective and consistent method of protecting your mead. Molecular SO2 is essentially the heart of Free SO2.
Free SO2 consists of three different parts:
- HSO3 (bisulfite)
- SO3 (sulfite)
- Molecular SO2 (the actual active component protecting against oxidation and contamination)
In the wine world, a target Molecular SO2 level for red wine would be 0.5 ppm, and for whites 0.8 ppm. However, with sweeter wine, or your high end semi-sweet or sweet meads, a target Molecular SO2 level of 1.5 ppm should be considered. Figuring out your K-Meta additions to hit your desired Molecular SO2 is going to be dependent on the final pH of your mead.
VinoEnology's website provides a table reflecting how much Free SO2 is needed to reach desired Molecular SO2 levels dependent on the pH level of your mead, as well as a series of calculators catering to whatever SO2 related need you may have.
If you have read through all of the above, you might better understand why I always emphasize the importance of having high quality and accurate methods of calculating both pH levels and Free SO2. If you invest some money into any testing equipment, these are the first two pieces of equipment you should consider if your intentions are to protect your mead via the use of sulfite.
Any method of measuring Free SO2 is not cheap unfortunately. As a high and low end example, you can take a look at the Vinmetrica SO2 Kit and the Hanna HI84500 Mini-Titrator. Should either be over your budget, you can consider going the route of assuming you are starting with zero Free SO2. Although not an ideal route to take, you will still be adding more than enough protection to your mead.
Other Resources and Online Calculators
VinoEnology Sulfite Calculators (sometimes quirky when using Safari)
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