[Furanone]

Dear colleagues,

I am having some trouble with an analysis, and am a bit baffled and know that some of the pure chemists on this site might be able to help me with possible causes of this reaction.

Some background: We got a customer complaint about an ingredient we supplied to a customer that when iodized salt (in form of potassium iodide) was added to our product (a powder) in water, it turned bluish-purple (the typical colour change of a starch with amylose (non-waxy variety)). This was difficult to believe, but sure enough when I tested their product adding iodized salt, it turned blue. Adding non-iodized salt, there was no colour reaction and it remained a white/off-white colour. It turns out our supplier of one of the ingredients that went into our customer's blend had added starch (which they had not declared) and it was only that one batch as all their previous and post shipments were shown to not contain starch (based on iodine reaction test, FTIR analysis and a wet chemistry fractionation).

So potassium iodide should not react with starch alone to turn it blue (or pink for waxy starches with amylopectin only). It needs to be in an iodine (I2) form which then forms tri-iodide in water and this forms the colour complex of the polyiodide chain complexed within the helices of the amylose (blue) or amylopectin (pink). When putting the iodized salt with another pure starch sample showed that that starch did not change colour.

Therefore I am baffled as to how this starch-iodine colour change reaction happened, and am thinking is it possible that the ingredient we purchased from our supplier had some sort of chemical catalyst that oxidized the iodide to the iodine form? I know strong acids could affect this but pH of their product was not very low, and also chlorine could also do this, but this was not detected either.

I am at a loss. Any ideas? Or even any suggestions as how to perform further testing to narrow things down? thank you all.

[Corribus]

Don't some enzymes have the ability to catalyze oxidization of iodide to iodine? If you're working in food matrices, this could be responsible. ?

[Furanone]

Thanks for response Corribus -- no enzymes are ruled out as one of the tests I performed was heating at 95 C for 30 min followed by multiple washes of collected starch followed by drying at 110 C for 3 hours, and starch still turned blue colour with addition of the iodized salt. Also ruled out, would be hydrogen peroxide and other volatile oxidizers.

I have confirmed that hydrogen peroxide added to the iodized salt turns it to a yellow colour (I2) then adding any type of starch will turn bluish-purple (or pinkish for waxy starch). So definitely something in the ingredient that will not wash out and is heat stable will oxidize the potassium iodide in the iodized starch to iodine that will turn the starch in the ingredient blue.

Are there any metal catalysts that could be in a food powder ingredient and are strong enough to oxidize iodide to iodine such as zinc/aluminum/etc? We have sent out sample for ICP-OES testing but not sure what to look for other than big differences compared to past tests.

thanks again

[Corribus]

Ferric ions can do it.

http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Elements_Organized_by_Block/2_p-Block_Elements/Group_17%3A_The_Halogens/1Group_17%3A_General_Reactions/More_Reactions_of_Halogens

[Arkcon]

A basis of the peroxide value for fats is the reduction of iodides to free iodine, as visualized with starch.  A large peroxide value, for a fat, means it is becoming rancid, or is oxidizing across its double bonds. A large enough peroxide value may even give a noticeably rancid taste to the fat. Could that be what's happening in your case:  https://en.wikipedia.org/wiki/Peroxide_value

[Furanone]

Thanks Arkcon -- we thought at first it might be peroxide but with my fractionation of the starch from the whole powder, I washed it several times in 80% methanol/20% water, and then oven dried it at 110 C for 3 hours. I would think if it was hydrogen peroxide, it would have been washed out by then, yet when the iodized salt was added to the dried/fractionated starch in water, it still turned blue colour suggesting that whatever is oxidizing the iodide from salt to iodine and then turning starch bluish-purple, is very heat stable and remaining with the starch (not soluble in the 80% MeOH wash).

[billnotgatez]

It makes you wonder if your supplier did more than add starch.

[Furanone]

Hydrogen peroxide is commonly added as both a disinfectant to kill bacteria but also to bleach yellowish/off-white powders to get a more white "pure"-looking powder. That is why peroxide was first suspected.

I will report back anything in future that we find out such as our ICP-OES results.