Chemical Test Kits for Sulfites

Sulfite Test Techniques

Have you ever wished for a test that could tell you beforehand when a food was sulfited? Boy, I sure have. And I have experimented with a few candidates. The problems with such a dream test revolve around four difficulties: sulfites and sulfur dioxide are different chemicals, food is very complex, human digestion is involved and the body can be ultra sensitive. The most popular industrial test is called the Modified Monier-Williams Procedure. This test requires a full chemistry lab and many hours to conduct. Not exactly something you want to bring to a restaurant. The test food must be boiled in concentrated sulfuric acid for more than an hour and the released sulfur dioxide gas captured in a glass column. Monier-Williams is an accepted standard and gives reasonably reliable results for the total reducing sulfur in a food from both sulfites and bound sulfur dioxide. But it is not perfect. It is cumbersome and of limited value to lay persons. Boiling sulfuric acid is not equivalent to human digestion and the measurements often overstate the potential for allergic harm. Finally, the sensitivity is limited to 10 ppm which is crude by the standards of sulfite sensitivity. For instance, a can of cola contains 173 ug of sulfur oxide by my headache standard which represents a concentration of ½ ppm. My body has no problem detecting this concentration yet it is well below the threshold of Monier-Williams. On the other hand, if the coloring in the cola were examined in its concentrated form, the Monier-Williams test would produce a result that is four times higher than the effect in people because caramel coloring is poorly digested in a real tummy. So you can see how complicated this business of testing really is.

The test that I use to calibrate my solutions of pure sodium sulfite is called the SU-5 by Hach and is capable of measuring concentrations down to 1/2 ppm and amounts of sulfite as low as 50 ug. Unfortunately, it is based on the Ripper Procedure used by wine makers and actually measures all antioxidants in the test sample. So, anything with vitamin C or reduced iron will generate a false positive reading. Well, with that limitation, it's not much of a food tester and it only works well for wine, vinegar and water. It is also limited by the state of the sulfur oxide. If the sulfite is bound into the food, you get no reaction and it ignores sulfur dioxide in any form.

Merck and Prolab Test Strips

The test strips manufactured by Merck (EM Quant item EMD-10013-1) or Prolab Scientific (item P-1002-003) may be of some value to persons with a quick allergic response to sulfites. Quick allergic reactions are often characterized by a selective sensitivity to sulfites at fairly high levels. In other words, a glass of wine and instant potatoes drive you wild, but corn syrup, food colors and Jello present no problem. If that is the case, Merck and Prolab test strips may be helpful. You just dip the padded end of the strip in a test solution and wait a few seconds for a pink color change. But here are the limitations. The strips only measure free sulfite. They do not react to bound forms of sulfur that require digestion for release. The strips are only sensitive to 10 ppm. And finally, be prepared for some false positive readings. In my limited experience with these strips, I encountered false positive readings to potatoes and tuna fish, although there was no reaction to vitamin C. What does all of this mean? You are only likely to detect sulfites with these strips, if they are present at levels above 1000 ug for typical servings. You will not detect bound forms of sulfur oxide at all. And, you will be scared away from perfectly good food that generates false readings. But if your reaction is swift and dangerous, such limitations may be insignificant compared to the benefit.

The Merck strips (100 for $54) are more expensive than those made by Prolab (50 for $13). The main difference seems to be the color of the target patches on the strips. The Merck patches are pure white while the Prolab patches are slightly cream colored. This difference makes it a bit easier to detect low level pink color changes using the Merck strips. So when using Prolab, I suggest you cut the strips down the middle to create a control and a test strip. Dip the control strip in plain water. Dip the test strip in your sulfite sample. Compare the two to judge if a pink color change has actually occurred. Note that if you cut the strips in half, Merck will test 200 samples while Prolab will test just 50 because it needs a color control. This makes the cost per test the same for the two products.

Test Strip Procedure

The Merck and Prolab test strips are thin, white plastic pieces with a fuzzy pad on one end. The pad contains chemicals that turn pink when wetted with a solution containing 10 ppm sulfite or higher. They are quite specific for sulfite and do not give false readings for vitamin C. Unfortunately, they are only sensitive down to 10 ppm and do not detect chemically bound forms of sulfite or sulfur dioxide. With these limitations, the strips don't offer universal help. But if you are less sensitive and can tolerate a little sip of wine, then these strips may be useful.

1. The fuzzy pad on the strips must be wetted with a test solution to work. If you are testing a liquid, just dip the padded end into the liquid. Remove and wait for 30 seconds, noting any pink color changes. If you are testing a solid, wet the solid with a few drops of water and press the padded end of the strip against the wet surface. Again, remove and wait for 30 seconds. (For Prolab strips, you may wish to cut the strips down the middle to create a control and a test strip. Dip the control in plain water and use it to judge small changes in color.)

2. The sulfite test strips will not work in a strong acid. If the fuzzy pad stays white, your test solution may be sulfite free or just too acidic. For instance, lemon juice concentrate at 220 ppm sulfite may stay white or turn only slightly pink because it is highly acidic. If you have pH paper or a swimming pool water test kit, you can test your sample for pH. Ideally, the pH of the sample should be 7 or higher. If not, add baking soda (not baking powder) until your test solution turns basic, pH reading above 7.

3. If the fuzzy pad turns even the slightest bit pink, you have detected sulfite. The darker the color, the higher the sulfite concentration. Below is the color key:

Pure white: less than 10 ppm
Hint of pink: 10 ppm
Regular pink: 40 to 80 ppm
Rusty strong pink: 180 to 400 ppm

Since this is a color test, there are some obvious limitations. If your food is colored red or even brown, you can't use the strips because pink will be masked by the food color. And if your strip stays pristine white, it doesn't mean there are no sulfites. It just means the concentration is below 10 ppm. For instance, if you are testing an 8 oz serving (240 ml) of a beverage, a white strip means that the total amount of free sulfite is below 240 x 10 or 2400 micrograms of sulfite. That's quite a bit. The beverage could also contain an unhealthy dose of sulfur dioxide. The strips will not respond to chemically bound sulfur oxide. As an example, if you dissolve a sulfa drug in a little pool of water, the strips stay white as snow. I have tried this.

Finally, the strips are subject to false positives. They can turn pink even when no sulfites are present. The manufacturers claim that this is not true. They list a whole range of chemicals that do not cause false readings but these are mostly simple inorganic compounds. In my tests, fresh or canned tuna and raw or fried potatoes turned the strips pink. I ate the tuna and potatoes without a problem. Sorry, but that's what happens. To work around this limitation, confirm your results with 3% hydrogen peroxide from your medicine cabinet. Hydrogen peroxide is a very strong oxidant. When mixed with a sulfite, it converts the sulfite to sulfate. Sulfate does not cause much trouble in most tummies, nor does it trigger the strips. So, we add another step.

4. If the test strip turns pink and you suspect a false positive, blot the strip dry and place a drop of hydrogen peroxide on the pink test pad. The hydrogen peroxide will guarantee that no sulfite remains on the pad. If the pad stays pink, it is reading a false positive from something other than sulfite. If the pad turns white, the previous pink reaction was due to sulfites and can be believed.

By the way, make sure the samples are not hot or the test may give inaccurate results. Please don't lick the test strips; they contain sodium nitroprussside, potassium hexacyanoferrate and zinc sulfate. And store the strips in a cool, dry place so they do not degrade.