Sulfur Dioxide

What forms of sulfur dioxide can be used to make wine from organically grown grapes? In particular, can potassium metabisulfite be used?

By Brian Baker

Sulfur dioxide (SO₂) is a gas at standard temperature and pressure. It appears on the National List at §205.605(b), which permits its use so long as it is limited to the production of wine labeled “made with organic grapes,” provided that the total sulfite concentration does not exceed 100 parts per million (ppm). SO₂ can be delivered as a gas or in liquid form in aqueous suspension. 

Potassium metabisulfite (KMS) is a synthetic substance that is chemically, functionally and structurally different from SO₂. It does not appear on the National List at §205.605(b) and is therefore prohibited for use in or on organic food. Sulfites are a serious food safety issue for those allergic or sensitive to these compounds. For both that reason and the fact that sulfites are chemical preservatives, the Organic Foods Production Act specifically limits the use of added sulfites to wine production and the National List of Allowed and Prohibited Substances further restricts the use of added sulfites solely to wine labeled “made with organic grapes.”

When first considering the allowance for sulfur dioxide, the Technical Advisory Panel (TAP) reviews specifically mention KMS as unnecessary, given the availability of SO₂. Wine makers who petitioned SO₂ didn't believe KMS was necessary and preferred SO₂.

There were multiple reasons given by winemakers who wanted SO₂ and did not want KMS included. Firstly, SO₂ occurs in nature; KMS does not. While KMS needs to undergo a chemical reaction to be activated, SO₂ is the pure functional agent. Additionally, there is evidence that SO₂ leaves lower residual free sulfites than the various salts, including KMS. That evidence was backed by statistically more sulfite-related deaths being linked to KMS and not SO₂. While these deaths were with salad in restaurants and not associated with use in wine, there was concern about the link. Finally, winemakers who petitioned SO₂ for use in making wine from organically grown grapes perceived KMS as producing inferior quality wine, often not much better and sometimes worse than unsulfited wine. 

The NOP published Policy Memo 10-2 in 2010, stipulating that grape wines containing flavorings from other organic fruit are subject to the same sulfite concentration restrictions annotated at §205.605(b), and can be labeled “made with organically grown grapes.” Sulfited non-grape fruit wines, however, are prohibited from displaying organic claims on their labels. Also in 2010, the NOSB was petitioned to remove the limitation to “made with organic” labeling of sulfited wine, but voted to make no change to the National List annotation.

This article originally appeared in the summer 2009 edition of the OMRI Materials Review newsletter, and was revised in May 2022 by Senior Technical Coordinator Jarod Rhoades.