I call them the Brimstone Demons because they cause me so much pain. But for the other 99% of America that is not sensitive, sulfur preservatives are quite marvelous creatures. When sprayed on fruit and salads, they prevent enzymatic browning keeping the foods fresh and appetizing. When added to pickled peppers and red cherries, they maintain the natural, attractive coloring of the food. When used on shrimp and seafood, they prevent black fungus spotting and other forms of spoilage. Sulfur oxides perform similar tricks on instant potatoes, french fries, grapes, raisins, wine, lemon juice, dried apricots, shredded coconut, dark breads, colas, cookies, candies of every persuasion, yogurts, syrups, cereals and the list goes on forever. You may notice I didn’t mention guacamole avocado dip. It’s just too painful for me. You see, I used to love guacamole before I learned it's preserved with sulfite and one cause of my headaches.
As preservatives, the FDA ( Food and Drug Administration ) allows only a few of the oxides of sulfur to be used in foods and they go by the following names which may appear on food labels. When you have a sulfite intolerance, food labels are a good friend and you should get used to reading them. ( Note that in many European countries, sulfite is spelled sulphite and the preservatives are often referenced by the numbers E220 through E228. )
Sodium Sulfite (Na2SO3)
Sodium Bisulfite (NaHSO3)
Sodium Metabisulfite (Na2S2O5)
Sulfur Dioxide Gas (SO2)
The first three are true sulfite salts and potassium may replace sodium in some formulations. When dissolved in water, they dissociate to free the sulfite SO3 ion. This combination of one sulfur and three oxygen atoms is a strong anti-oxidant and explains why sulfites are such good preservatives. Like other anti-oxidants, they protect food by combining with and neutralizing free radicals and oxygen. If oxygen is available, sulfite ions readily oxidize to become sulfate SO4 ions. The sulfate form is quite stable and does not preserve food. ( Sulfates in food do not cause much trouble for most people, even those with sulfite conditions. However, an unlucky few are sensitive to sodium lauryl sulfate in soap and shampoo which can cause skin rashes. If you don't get skin rashes from soap, you shouldn't worry about sulfates. ) Sulfites and sulfates should not be confused with nitrites or nitrates which are completely different chemicals commonly used to preserve meats.
The last chemical on the list above is sulfur dioxide. Strictly speaking, it is not a sulfite because it lacks the third oxygen atom. However, the human response to sulfur dioxide mimics the response to sulfites, probably because it may be oxidized to become a sulfite. As a gas, sulfur dioxide is one of those nasty air pollutants we all like to avoid. In foods, sulfur can easily combine with organic molecules taking the place of carbon and oxygen groups. In fact, there are forms of sealife near volcanic vents on the ocean floor that survive on sulfur compounds as replacements for the carbon-based nutrients you and I need. When carbon foods or tree wood is burned, carbon dioxide (CO2) is produced. When sulfur is burned, sulfur dioxide (SO2) is produced.
Sulfur dioxide gas is used to prevent discoloration when drying fruits such as apricots. In the process, SO2 is trapped and bound to the fruit and the concentration can be quite high. Unfortunately for some of us, sulfur dioxide is much more than a preservative. Since ancient times, burning sulfur has been used to temper the fermentation of wine. Most modern wine makers use the sulfite salts directly, but the result is the same. Sulfur dioxide molecules bind to the wine and all wine bottles carry the warning "Contains Sulfites". Sulfur dioxide can also be dissolved in water to have a softening effect on fibers and other organic structures. This effect is used to advantage in the corn refining process that produces corn starches and syrups. Corn refining is no backwoods industry; over the years, corn syrup has replaced sugar as the number one sweetener in the United States. Since corn refining starts with sulfur dioxide, all corn starches and syrups wind up with a sulfur content. And it doesn’t stop there. Sulfur dioxide is an important ingredient in caramel color, a colorant that is produced in much the same way as true caramel. Now, I never heard of caramel color before I became interested in foods. If it’s new to you too, read the list of ingredients on your next Coke or Pepsi. Diet colas are nearly all water and caramel color. Regular colas do add a sweetener: high fructose corn syrup, of course.
How the oxides of sulfur cause problems in humans is not understood. However, from the above description of their uses in the food industry, it is quite clear that they interfere with normal biological functions. As a side reaction, sensitive body tissues swell and cause breathing problems, headaches and other discomforts. Sometimes it appears that the SO3 ion is the culprit; sometimes it appears to be SO2 bound tightly, loosely or not at all. Whatever the form, it would be nice to simplify things. So, I will introduce the concept of biologically effective sulfur oxide, which I will call SOx. Regardless of the form of the sulfur, if you know the effective value, you know its potential for causing headaches and asthma. The SOx value for a food must be determined experimentally. In my case, I simply eat a food sample, note my headache response and equate this to a similar headache caused by a calibrated solution of sodium sulfite. The SOx weight is then defined as the weight of the SO2 atoms in the calibrated solution of sulfite.
When discussing sulfur preservatives in dried fruits, I mentioned that the concentration may be quite high. The concentration for dried apricots is typically 220 PPM. The PPM means "Parts Per Million" and refers to the weight of the preservative as a part of the total weight of fruit. For instance, if you divided the fruit into one million parts, 220 of those parts would be biologically effective sulfur oxide. You can use PPM numbers to calculate weights. Since food labels use grams for weight, we will use grams. A gram is the weight of ten drops of water.
Grams are conveniently divided into smaller parts called milligrams (mg) and micrograms (ug). The abbreviation for milligrams is "mg" which makes common sense. However, the abbreviation for micrograms is less obvious since "mg" cannot be used. So, the Greek symbol "mu" that sounds like "mew" and looks like the letter "u" was chosen, presumably by someone that drank too much Greek wine. As a result, micrograms are abbreviated "ug" or sometimes "mcg". It takes one thousand milligrams (1,000 mg) or one million micrograms (1,000,000 ug) to make one gram.
You can calculate effective sulfur oxide weight by multiplying the PPM concentration by the weight of the food. If the food weight is in grams, the answer will be in micrograms.
(Food Weight) times (PPM Concentration) = SOx
In the case of dried apricots with 220 PPM concentration of effective sulfur oxide, a 4 gram bite would be preserved with 4 x 220 = 880 ug of SOx. That would give me such a headache, I would have to bang my head against the wall for hours. That’s all the math you need to know to use food labels to predict the sulfite content of your dinner. Of course, you have to know a little bit more about food ingredients, but that’s what the rest of the book is about.