Is 10ppm Really Zero?

Test strips make semi-quantitative (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 there is a liquid which takes up some volume whether it is a test tube, spray bottle or beaker.

Two of our test strips are also used for detecting residual levels of QAC (quat) sanitizers & sulfite preservatives on the surfaces of knives, cutting boards, potatoes, dried fruits, etc. Unfortunately, this has also incurred a misinterpretation of government safety regulations which are based on standard volumes (100mL) not surfaces millions (or more) times smaller.

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. This conforms to the standard set by the US FDA for safe levels of sulfite. 10ppm translates to 1 mg of sulfite in a 100mL (4 fluid oz.) glass of wine.

Sulfite is also a preservative for foods such as peeled potatoes, dried fruit or shellfish. This thin layer of sulfite protects the food from spoilage. The government regulated 10ppm minimum that is based on a 100mL standard volume does not take into account that the amount of sulfite in a surface layer is substantially less.

Test Strip Sulfite

Indigo sulfite test strips now come with a minimum 3 year expiration date.

Compare the sulfite in 100mL wine to the amount on the surface of a potato of the same volume.

  • A perfectly round potato with a volume of 100 cm3 (100mL), has a ~2.9 cm radius
  • Its surface area then would be ~104 cm2
  • Let’s assume the sulfite uniformly penetrates the potato to a depth of 0.01 cm (0.1mm)
  • That volume of potato would be ~1 cm3 (104cm2 x 0.01 cm thick) or 1 mL (1 cc)
  • At 10ppm this volume would contain 0.01 mg of sulfite (compared to the 1 mg in the wine).
  • The entire potato may only have 0.1ppm or 100ppb (parts per billion) or 1/100th of the safety limit.

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

Having said that, if the test strip indicates a high level on a dried fruit or potato, it may be enough to trigger an allergic response.  If this is a concern, check with a medically trained physician. It may only require rubbing a sample on your skin or tongue to evoke a reaction.

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 QAC (Quat) test strips can only measure down to 10ppm.

So, is 10ppm on a knife the same as 10ppm in 100mL of water or is it effectively zero? In other words, is it even remotely a health hazard?

Consider a knife washed after cleaning with a typical quat (e.g. Oasis 146) . Press the test strip against the knife so it absorbs some of the residue. It reads 10ppm. What does that mean?

Assume all the quat residue on the knife gets into a 100mL (100cc)  sized tomato during cutting. How much quat would you ingest if you eat the whole tomato? Let’s do the math:

  • If our knife blade is 10 x 2.5cm (4×1″ in American) 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). [1 mm is really thick so of course it’s mainly water]
  • At this volume, 10ppm works out to be 0.05 mg (10 mg/L x 0.005 L) of quat or 0.5ppm for a whole 100mL 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 (ppb).
  • On the extreme side, a superthin monolayer of pure quat would be ~1×10-8 cm thick.  This works out to 0.000000005 mg (5 picograms); that’s a million times less than ppb!

So, even if the test strip measured 100ppm on the knife’s surface, the total amount of quat in that whole tomato is still not measurable by any standard analytical method that all but the best labs could afford. Effectively, the amount of quat is “zero“. The consumer or food handler is completely safe even if they eat their weight in tomatoes.

Residual Testing of Sanitizer Quat Test Strips

Indigo quat test strips now come with a minimum 3 year expiration date.


Indigo™ Sulfite & Quat Test Strip Expiration Dates

As of early 2021, all Indigo™ test strips have a minimum expiration date of 3 years indicated on the label. For more information, see our blog: Test Strip Expiration Dates; Good Today, Dead Tomorrow?

Further, each strip comes with Stability Statement & Certificate of Analysis that can be downloaded in pdf format.

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