Resource

Sulfite Calculator

Introduction

The Sulfite Calculator is a simple yet very useful tool to quickly calculate the amount of sulfite needed to adequately protect a wine.

It calculates the amount of sulfite based on the difference between the current and desired free SO2 levels for any given volume of wine.

It can be used to quickly determine the amount of sulfite to be added when wine parameters, for example, pH, are not needed in the calculation. For example, it can be used to determine how much sulfite is needed to achieve 100 mg/L free SO2 to stop an active fermentation, such as when wanting to keep natural residual sugar.

When a value for pH is entered, the Sulfite Calculator uses this value along with the type of wine, alcohol content, temperature and desired molecular SO2 to calculate a recommended free SO2 level. The calculated recommended free SO2 value can then be entered as the new desired SO2 level to calculate the amount of sulfite to be added.

SO2 levels are measured in milligrams per liters, or mg/L, which is approximately equivalent 1 ppm. Both units are used interchangeably in the industry.

What’s New in Version 3.0?

Version 3.0 of the Sulfite Calculator now allows winemakers to enter the wine’s alcohol content and temperature, as well as an adjustment factor to compensate for binding, in determining the amount of free sulfur dioxide (SO2) required to protect a wine.

The different forms of SO2

When sulfite, for example, in the potassium metabisulfite salt form, is added to wine, it dissociates into mainly bisulfite ions. Bisulfite ions act primarily as anti-oxidasic (protect against oxidative enzymes) and anti-oxidant agents, while molecular SO2 also acts as anti-bacterial and anti-fungal agents.

The relative concentrations of bisulfite ions and molecular SO2 vary with pH, and to a lesser extent, with temperature and alcohol concentration. As pH increases in wine pH range, there is greater dissociation into bisulfite ions with a proportional drop in molecular SO2. Since microorganisms thrive at higher pH, greater amounts of sulfite are needed to ensure that there is sufficient molecular SO2 to protect the wine against microbial spoilage. Levels of 0.5 mg/L and 0.8 mg/L molecular SO2 are recommended for reds and whites, respectively, but should never exceed 0.8 mg/L as it can become detectable by sensitive tasters.

The sum of bisulfite ion and molecular SO2 concentrations is known as free SO2.

Bisulfite ions also have an affinity for certain binders found in wine, such as polyphenols and acetaldehyde. These binding compounds attach to bisulfite ions and render them ineffective and which no longer protect wine. Bound bisulfite ion concentration is known as bound SO2.

The sum of free SO2 and bound SO2 makes up total SO2.

Bisulfite ions are also consumed in oxidation−reduction reactions, such as in the reduction of molecular oxygen into hydrogen peroxide (H2O2), a powerful oxidant that can oxidize ethanol into acetaldehyde, which negatively impacts wine quality, or in the regeneration of brown-colored pigments into their colorless polyphenol forms. Here too bisulfite ions can bind to brown-colored pigments, and since red wines have much greater amounts of polyphenols, more binding is expected.

Bisulfite binding and reactions cause the concentration of bisulfite ions and free SO2 to decrease, and as molecular SO2 is lost to the atmosphere causing an ongoing bisulfite–molecular SO2 re-equilibrium, there is a drop in free SO2. These effects require the addition of more sulfite to maintain adequate protection.

And to compensate for binding, free SO2 is often adjusted by 10–50% or by as much as 100% (i.e. doubled) in very young, tannic wines which have received little SO2, typically when total SO2 is less than 50 mg/L. For example, if a free SO2 level of 30 mg/L is desired for a tannic red with a total SO2 above 50 mg/L, the free SO2 level can be bumped by 1/3 (33%) to 40 mg/L; the same wine but with a total SO2 below 50 mg/L is best protected with 60 mg/L, i.e. 30 mg/L + 30 mg/L (100%).

When adding sulfite to wine, it is recommended to make larger, infrequent additions as opposed to smaller, frequent additions. The latter will cause greater binding, and therefore greater drops in free SO2 over time, which will require extra additions to maintain a desired free SO level. In the above example of the tannic red wine requiring 60 mg/L of free SO2, the full dose of sulfite required to achieve 60 mg/L of free SO2 should be added, as opposed to, for example, adding 30 mg/L now and 30 mg/L in one month.

Methods for Adding Sulfite

Potassium metabisulfite (KMS) is the most common and preferred sulfite salt used for sulfite additions. It can be added using one of three simple methods:

Sulfite powder

This method involves dissolving KMS powder in a little lukewarm water before adding to wine. It is most useful when large amounts of sulfite need to be added because weighing small quantities is difficult. It is however the most versatile method.

Campden tablets

This method involves crushing tablets into a powder and then dissolving it in lukewarm water. Crushing is an obvious disadvantage; however, since these come in 0.44-g tablets, no weighing is required. Their advantage is in the use of recipes with standard wine volumes. It gets trickier when only portion of a tablet is required, making measurements very imprecise.

10% dilute sulfite solution

This method is very practical, particularly for sulfiting many small batches. A 10% solution is prepared by dissolving 10 g (0.35 oz) of KMS powder in 50 mL (1.75 fl oz) of lukewarm water and then adding cool water up to the 100 mL (3.5 fl oz) level. For large batches of wine, multiply these numbers by 10 (e.g., 100 g in 1 liter) to prepare sufficient solution.

Using the Sulfite Calculator

The Sulfite Calculator can be used in one of two ways: A) to determine the amount of sulfite to add to achieve a desired free SO2 level regardless of the wine’s parameters and recommended free SO2 level; or B) to determine the amount of sulfite to add to achieve the desired/recommended free SO2 level taking the wine’s parameters in consideration.

A. Known desired SO2 level without entering a pH value

  1. Enter the total volume of wine to be treated and select the unit, liters or gallons.
  2. Enter the current level of free SO2 in mg/L, if known; otherwise, leave at 0.
  3. Enter the desired level of free SO2 in mg/L.
  4. Click on the Calculate button.
  5. The amount of sulfite to be added will be displayed in Metric and US units based on the selected wine volume unit above and preferred method of addition.
  6. Click on the Print button to print the results.
  7. Click on the Clear button to start a new calculation.

Note: When using Campden tablets, it is difficult to get the exact amount of sulfite when required to split a tablet. The calculator provides results to the closest quarter portion and then provides the actual resulting amount of free SO2 if different from the entered desired free SO2 level, depending on how close the rounding.

B. Recommend SO2 level

  1. Select your preferred method of sulfite addition.
  2. Select the type of wine—red or white—to be treated.
  3. Enter the total volume of wine to be treated and select the unit, liters or gallons.
  4. Enter the wine’s pH, % alcohol by volume (ABV), and temperature and select the unit, °F or °C.
    pH must be in the range 2.90–4.20.
    % alcohol by volume must be in the range 0.0–20.0.
    temperature must be in the range 50–80°F or 10–27°C.
  5. Enter the current level of free SO2 in mg/L, if known; otherwise, leave at 0.
  6. Enter the desired level of free SO2 in mg/L and the desired molecular SO2 in mg/L (typically 0.5 for reds and 0.8 for whites).
  7. Enter the % adjustment, a value between 0 and 100. Use the table below to determine a suggested value based on the type of wine.
    Type of wineTotal SO2
    (mg/L)
    Suggested
    %Adjustment
    Fruity whites, fruit winesX0
    Barrel-aged whites, young fruity redsX10
    Full-bodied reds< 50100
    Full-bodied reds> 5033
  8. Click on the Calculate button.
  9. The amount of sulfite to be added will be displayed in Metric or US units based on the selected wine volume unit above and preferred method of addition.
  10. Check the Notes. The calculator will recommend a free SO2 level if it is different from the desired level entered above. If a free SO2 level different from the desired SO2 is given, enter the recommended free SO2 level as the new desired level of free SO2, and click on the Calculate button.
  11. Click on the Print button to print the results.
  12. Click on the Clear button to start a new calculation.

Note: When using Campden tablets, it is difficult to get the exact amount of sulfite when required to split a tablet. The calculator provides results to the closest quarter portion and then provides the actual resulting amount of free SO2 if different from the entered desired free SO2 level, depending on how close the rounding.

Assumptions

The calculator assumes that 57% of the sulfite actually becomes free SO2 to protect wine. Most sulfite sources for home winemakers are mainly available in this form. Other forms may also be available but provide approximately 50% of free SO2. The results are therefore acceptable.

The calculator also assumes that Campden tablets have a weight of 0.44 g, again, the most common form to home winemakers.

©Daniel Pambianchi