If you've ever read past the first five ingredients on the label of a food package, you may have noticed the term "sodium bisulfite," especially on dried fruit or trail mix. It's sodium, but not salt—it's a food additive, and it is considered more sulfur than sodium. It has a variety of industrial applications, but it is also a popular preservative and bleaching agent in the manufacture of processed foods.
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Chemically, sodium bisulfite is a combination of sodium, hydrogen, sulfur and oxygen. It is also called sodium hydrogen sulfate, and is classed as a sulfite, or sulfur-based chemical. Its natural form is a white crystalline powder, but turns yellow in solution. It is acidic, considered corrosive, and is a powerful reducing agent used in water treatment, textile dye preparation and film development, but most people are more familiar with its use as a food additive.
Sodium bisulfite is used to preserve color and inhibit bacterial growth on fruits and vegetables, some seafood and in wine. It was once favored for preserving produce because it kept vitamin C from breaking down, but it not in meat because it destroys vitamin B1. It is frequently used as a bleaching agent in flour and grains, and as a dough conditioner to enhance the rise and preserve the freshness of bread. Sodium bisulfite occurs naturally in fermented products, so even wines labeled "sulfite free" still contain small amounts.
The U.S. Food and Drug Administration considers sodium bisulfite to be safe "when used in accordance with good manufacturing practice." It is not allowed to be used on fresh produce intended to be served raw, or in any meat or other food that contains vitamin B1. The only exception is raw cut potatoes -- for example, the pre-cut french fries you simply heat in an oven. Cut raw potatoes brown very quickly, and the nutrient profile is not harmed by the addition of sodium bisulfite. The potato industry claims that until a good substitute is found, banning sodium bisulfite would destroy their business.
The reason for such strict regulation is that previous liberal use of sodium bisulfite led to numerous reactions—the FDA investigated more than 500 allergic reactions and 13 deaths linked to the additive before passing the regulation. Symptoms of allergy include rapid heartbeat, hives, swelling of the face and tongue, anxiety and vomiting. Some people are not allergic, only sensitive to sulfites—but the same symptoms arise, especially in asthmatics, who can go into anaphylactic shock. The only solution for those with a sulfite allergy or sensitivity is to avoid sulfites completely.