Sulfur Dioxide Measurement and Management

The following is a recap by John Daume of the talk given by Chik Brenneman, UC Davis’ Student Winery’s lead winemaker.


Sulfite (SO2) has different purposes at different wine pHs.

Above a pH of 3.6, sulfite loses almost all of its “bad bugs” protection that comes from the “molecular” form.

Consider the use of Lysozyme, at crush, if needed to kill gram-positive bacteria (not acetobacter), when working at a pH of 3.6 and up.

The “Bi-Sulfite” form still has some anti-oxidative protection at a pH of 3.6 and a bit above.

For sensory reasons, do not maintain more than 50ppm Free SO2.

After MLF, adjust acidity by taste, not by just pH or TA.

Chik reminded us that there are three forms of “SULFITE”:  “MOLECULAR” (SO2) / “BISULFITE” (HSO3-) / “SULFITE” (SO3 2-)


Only the Molecular form reduces microbial spoilage.

In your juice, SO2 levels that inhibit browning enzymes, also are sufficient for inhibition and/or a significant reduction in native flora. In general, Bacteria are more sensitive to SO2,than Yeast are. Varies with the organism.

H2O2 (hydrogen peroxide) is generated from  the oxidation of phenols (mostly tannins).

Only the Molecular form scavanges this peroxide.  Less peroxide = less acetaldehydes formed.

The Molecular form is quite volatile, being lost into the headspace, lowering free SO2


Peroxides oxidize ethanol, forming stale  smelling acetaldeydes.

Additionally, acetaldeyde is formed from microbial oxidation of ethanol under aerobic storage conditions.

It is only the BiSulfite form that binds with acetaldehydes, forming a  less noticeable hydroxy-sulfonate.

“SULFITE” (SO3 2-)

The Sulfite ion is the only form that                combines directly with oxygen (O2).

At wine pH (3.0-4.0) there is almost no             Sulfite form to act as a direct anti-oxidant.

Now that you are clear on what each of the three forms of Sulfite do, it is absolutely essential that you also know the pH of your wine at all times.

SO2 Addition Strategy

  • At Crush: Adjust acidity down to 3.6. Add 50 mg/L for whites, 30 mg/L for reds of Sulfite for minimum bio-stability. (If you are co-inoculating with M-L bugs during ferment, note that the upper limit for MLF is 30 mg/L Total SO2) (With “bio-suspicious” grapes, add 50 – 75 mg/L plus 150 mg/l Lysozyme and do M-L addition after dryness)

(  Total SO2 minus Bound SO2 = Free SO2. Your post fermentation goal is to add SO2 and                      stabilize this dynamic equilibrium between these three forms )

  • Post Fermentation: At 2 -3 weeks after dryness, adjust the acidity to taste (not to some pH/acidity value) and then test for free SO2, as the Free/Total/Bound SO2 equilibrium is being established.
  • This first SO2 addition gets bound quickly, necessitating adding 10 – 20 mg/L (ppms) higher than the chart, to establish equilibrium at the chart’s recommended value.
  • Additional SO2 additions (every 1 – 3 months. Certainly when racking) are based on the results of testing for free SO2. Check the chart. Do not go over 50 mg/L free SO2)
  • Chart free SO2 values are maintenance values. Go into your bottles with 5 – 10 mg/L SO2 above chart value to account for loss during the rigors of bottling.

In conclusion, we all know that at higher pHs (3.6 and up) our wines taste more accessible, fuller, more forward, yummy.

However –

  • At wine pH (3.0-4.0) there is only a little Molecular SO2 to provide bacterial protection. Assuming that most people can smell/taste free SO2 at 50ppm (mg/L), this is your upper limiting factor for SO2 additions (see the chart, above).
  • Unfortunately, At pH 3.6 and above, to achieve bacterial protection (.825 mg/L Molecular), you would have to exceed this 50ppm upper limit of Free SO2.
  • Fortunately, at pH 3.6 and a bit above, you do still have some anti-oxidative protection from the Bi-Sulfite form.
  • If you have crushed “bio-suspicious” grapes (other than picture perfect, mold, critter damage, over 10% raisins), consider using “Lysozyme” to dramatically reduce the population of gram-positive, spoilage bacteria like lactobacillus, etc..
  • Consult the several computer programs to figure how much Total SO2, as per your pH, to add to achieve the desired Free SO2 maintenance level.

Under all circumstances, you must constantly maintain sound, clean and consistent winery sanitation; proper SO2 maintenance relative to pH and the above free SO2 chart; bio-free barrels; topped barrels; no sweet/stuck wines that are a perfect media for spoilage bacteria.  1/29/2014