Can 10ppm of Sulfite & Quat Really Mean Zero?

Can 10ppm Really Be Zero?

Test strips make fairly accurate measurements of pH levels, concentrations of sanitizers such as chlorine & hydrogen peroxide or the amount of Vitamin C in juice. The common factor in all these is that the measurement is made on a bulk solution. In other words, the liquid in question occupies some volume of a test tube, bottle or bucket.

Several of these strips have been repurposed for detecting residual levels of quat sanitizers & sulfite preservatives on the surfaces of things. Unfortunately, this has lead a misinterpretation of government regulations by so called experts who are either training or inspecting commercial food operations.

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Sulfite Strips for Allergies & Asthma

Sulfite is used as a sterilizing agent in wine making & may cause allergic reactions or trigger asthma attacks in some people. The lower detection limit of a sulfite test strip is 10ppm which conforms to the standard set by the US FDA for safe levels of sulfite. 10ppm translates to 1mg of sulfite in a 100ml (4 fluid oz.) glass of wine.

Sulfite is also used as a preservative on foods such as peeled potatoes, dried fruit or shellfish. The 10ppm standard also applies but  the total amount of sulfite in this instance is substantially less.

Let’s compare the amount of sulfite in 100ml wine with what might be found on the surface of a potato of the same volume.

  • A perfectly round potato with volume of 100 cm3 (100ml), has a 2.88cm radius
  • Its surface area would be ~104cm2
  • Let’s assume the sulfite uniformly penetrates the potato to a depth of 0.01cm
  • The volume of potato with sulfite would be ~1cm3 (104cm2x 0.01 cm thick) or 1ml
  • At 10ppm this volume would contain 0.01mg of sulfite.
  • If this sulfite were distributed throughout the whole potato, it works out to a concentration of 0.1ppm or 100ppb (parts per billion).

You would have to eat 100 potatoes to get the same amount of sulfite as that 100ml (4oz) glass of wine. 10kg (22 pounds) is a lot of French Fries to eat at one sitting!

Residual Testing of Sanitizer-Sulfite Preservative Test Strips

  Quat Strips for Organic Certification

Quat (QAC or quaternary ammonium chloride) sanitizers are used in hospitals & restaurants as a general cleaning agent. They are also popular in the organic food industry for use in cleaning knives & cutting surfaces.

The current standard for organic certification is a 0ppm residue on these utensils & surfaces but Quat test strips are only reliable down to 10ppm. For this reason, concerns have been raised whether these strips should be used in the certification process.

Consider a typical quat (Oasis 146) is found on the surface of a knife. The concentration of the residue can be measured by lightly wetting the test strip & pressing it against the surface of the knife. So, what if reads 10ppm? Let’s say that if all the quat residue on the knife were transferred to a 100ml (100cc) tomato cut by this knife, how much quat would you be ingesting?

  • Our knife blade is 10 x 2.5cm so the surface area of both sides would be 50 cm2
  • A quat residue 0.1 cm (1 mm) thick would have a volume of 5 cm3 (5 mL or 0.005L)
  • This volume 10ppm works out to be 0.05 mg (10 mg/L x 0.005 L) of quat or 0.5ppm for the whole tomato
  • A more realistic residue, say 0.1 mm, would result in 0.005 mg in the tomato or 0.05ppm or 50 parts per billion.
  • On the extreme side, a monolayer of quat would probably be somewhere around 1×10-8 cm thick.  This would equate to 0.000000005mg going into the tomato or 0.005 nanograms.

So, even if the test strip were to measure 100ppm on the knife’s surface, the total amount of quat in that pureed tomato is not measurable by any standard analytical method even the best labs could afford. Effectively, the amount of quat  is “zero”.

Residual Testing of Sanitizer Quat Test Strips
The color chart for this item has been revised to show 10-100ppm.
Article Name
Testing Residual Chemicals-When 10ppm Really Mean Zero
Low level Sulfite & Quat (QAC) test strips can measure residual levels of these chemicals. They are potential useful in organic food certification but there is considerable confusion as to what constitutes a safe level.
Indigo Instruments

About Stephan Logan

VP Sales & Marketing at Indigo Instruments. Previously sold hi tech science instruments to researchers. B.Sc. & M.Sc. from McGill University.

2 thoughts on “Can 10ppm of Sulfite & Quat Really Mean Zero?

  1. Great information and as explained makes sense in the real world situation, not in a theoretical world. Thanks for being ‘realistic’ and not like our organic certifier who says we “must test” even when we have to test the actual material after it is processed on the equipment after we cleaned with QA. So if there is residue levels that are ‘problematic’ in the actual product, we will know it.

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