Estimating the sodium in food using Chloride test strips.

Part of www.findlowsaltfood.info

Disclaimer:- This is the method I use and it may be wrong in some way. I have no qualifications in this area.
Your comments are welcome  peter@findlowsaltfood.info

Introduction.

When adding new products to my low sodium website I occasionally come across a few that I think may have the sodium understated.  A laboratory sodium assay is the gold standard but costs me $32.45 (2011). I looked around for a rough screening test and came across chloride test strips. These can give an indication of the likely sodium level and are accurate enough to highlight possible problems that should be considered in the light of a proper laboratory test.The sort of problem that a $2.50 chloride test can uncover is a bread labelled sodium 100mg/100g that is actually 400mg/100g. It is difficult to declare a labelled sodium level wrong. Determining the average sodium level that should go on a nutrition panel is not a simple process. In most cases there are production tolerances and seasonal variations that have to be taken into account so multiple samples need to be taken.

There are a great many products where measuring the chloride can give a good estimation of the sodium as most of the chloride is associated with added salt (sodium chloride)(NaCl). There are some processed foods however that have excess sodium or chloride from food additives and the sodium in them can not be deduced from the chloride. One example is self raising flour which has a high sodium level from the raising agents used as does anything made with it. Some foods naturally have a, usually low, background level of sodium or chloride and if this is known from previous tests or from a food database, corrections can be made for it.

Methodology.

In many products like cheese the main difference in chloride levels between samples relates to the amount of salt incorporated. As a first approximation one could determine the chloride, assume that it all came from salt and calculate a value for the associated sodium. The sodium and chloride in salt are in a fixed proportion which are near enough to 393 parts sodium to 607 parts chloride. This method ignores the possibility that some sodium or chloride may not be associated with salt. By using a food database or prior knowledge it may be possible to correct for this source of error. Instead of assuming that all the chloride is associated with salt, assume that the difference in chloride from the reference foods level is associated with salt, take that difference,  calculate the associated sodium and apply the difference to the sodium reference level.

For example. The average roast chicken (cofids database)  had sodium 80mg/100g and chloride 75mg/100g. If a chloride test gave 75mg/100g the sodium estimate assuming all chloride was associated with salt would give 48mg/100g. The method outlined above would give 80mg/100g. A chloride level of 10mg/100g would give 6mg/100g by the first method and 38mg/100g by the second.  The improvement or not in the sodium estimate that the outlined method makes, relies on the similarity of the sample product to the reference product. 

The first step is to prepare a solution from the sample that the chloride test strips can work with and to get all of the chloride into solution.  The strips have a chloride range of about 30 to 600mg/100g i.e. a salt equivalent sodium of 20 to 400mg/100g. The aim is to make a solution dilute enough so the liquid can easily wick up the test strip but not so dilute or concentrated that the reading is off the scale. You will soon get the hang of it. Perhaps the first time you test a bread you will find that the bread crumbs have absorbed all your water and not a drop remains to be filtered. You may also find problems with oily samples that do not filter. I usually use around 10g of  finely divided sample with around 30g of water and allow it to soak for 30 minutes with stirring at the beginning, middle and end.
Here is an example form that I use when making a test Work sheet for Quantab test strips pdf.  it contains some explanations and a worked example in the notes. See below for a work sheet .doc to customise. In order to make some adjustment for the sodium or chloride that may not be caused by salt in the product there is mention on the form to reference values of chloride and sodium. The only database I have been able to find on line that shows chloride is in the form of a large Excel spreadsheet cofids.xls maintained by the UK Food Standards Agency. Not having Microsoft Office I use the free Open Office Calc to read it. Googleling for cofids.xls should find it and the 2 explanatory pdf documents.

For another example of using Quantab test strips Google for  "rapid and convenient salt measurement for italian cheese"

Equipment.

Equipment


The test strips are available from Arrowscientific.com.au

12.5cm filter papers, 75mm poly filter funnels and 50ml poly beakers etc. available from  wiltronics.com.au

I use Pureau pure water from the supermarket in a food squirt bottle.

Funnel holder was home made.  

The taper of the medicine glass means that the minimum amount of filtered liquid is needed.

The scales weigh to 100g in .01g increments.

Work sheet in .doc format 




If you fold the filter paper in quarters it will fit exactly in a 60 degree filter funnel. The only area that the liquid can then pass through is the small area at the tip of the filter where it does not press against the sides of the funnel. By making the second fold as shown and using the larger section to form the cone the filter paper will only touch the funnel at the top and the whole paper can pass fluid.



Results.




Results obtained when approximately 10g of minced rotisserie roast chicken from 3 shops was steeped in 30g water,
Sodium estimation 462, 285, 31mg/100g from chloride levels of 666, 393, 0 mg/100g using reference Cl 75mg/100g, Na 80mg/100g
The results confirmed the taste.