Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: A.Carter on 22/06/2016 14:40:25
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I recently had a conversation with a friend on the nature of the health benefits of minimally processed salt. She claims that the reason that table salt is so white is due to bleaching, but this does not jive with what little I know of how bleaching works. I suspect that less processed salt is probably duller in color because it retains trace minerals, but I don't have any solid evidence. What say you all? Is table salt white because they bleach it?
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Bleach can oxidize organic impurities, thereby destroying the colors that are produced by organics. I don't think that the colors in salt are caused by organic compounds, and even if they were, this is probably not the most economical way of whitening the salt. Instead, I think the recrystallization of the salt is what removes the traces of colored minerals.
Sometimes salt is made whiter by the addition of titanium dioxide. This is a non-toxic compound that scatters light very effectively, causing a bright white color. It is used in white paints, and is also added to other foods (like confectioner's sugar) to make them appear whiter.
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I once visited the town of Salzburg (German for salt city), made famous as the backdrop of "The Sound of Music" film.
They mine salt there, but the local salt when it comes out of the mine is a muddy brown color from all the dirt and minerals that were washed in, only to have the water evaporate.
Himalayan rock salt is prettier, with a slight pink tinge from a low concentration of iron atoms in the crystals. But you will pick up far more iron from eating a steak (or cookies baked on a rusty pan). The supposed health benefits are grossly inflated.
Overall, sea salt has a collection of minerals in addition to Sodium Chloride, including ions of Potassium, Magnesium, and Calcium, Bromine and Iodine (although in some countries the Iodine is enriched to prevent goiter). These are all quite clear salts (or white when crushed).
Most natural sources of salt will contain a similar range of chemicals.
https://en.wikipedia.org/wiki/Seawater#Compositional_differences_from_freshwater
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Ah! This is a good start! Thank you both for your replies. I took a perfunctory look into where table salt comes from and it looks like one method involves allowing brine to sit in open evaporating pools for a time. Apparently these pools often harbor bright red algae blooms. So, if there were any color-causing organic material that would need bleaching to begin with, it could conceivably come from there. But I also imagine that separating out the algae would be a simple matter since sodium chloride is wonderfully water soluble and algae, by necessity, is not. Which leaves whatever water soluble contaminants were introduced into the brine while the algae was alive and any organic matter that may have escaped the separations process. Of course this does not affect mined salt which sounded like it wasn't processed further, but if it comes up brown, then that cannot be true. I will look into it further.
As for the cost of bleach, I would think it would be quite cost effective to use as you could convert some of the brine into bleach via electrolysis (though I may misunderstand the cost of creating an appreciable amount of bleach in saturated brine on an industrial scale). So then, perhaps removing it is problematic? Letting it decompose naturally appears to be out of the question. Of course if titanium dioxide does the job, then this is all moot. As a fun exercise then, what do you suppose we could add to a bleach-containing brine solution that would react away the bleach, leave as much Na+ and Cl- as possible, and produce a non-toxic or easily removed side-product?
Oh, and I just had another thought. Given the state of our oceans, I wonder how many heavy metals and persistent organic pollutants there are in minimally processed salt...
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In theory, citric acid or ascorbic acid (vitamin C) would react with bleach to give non-toxic products (do not try at home!).
While it wouldn't necessarily be terribly expensive to generate some low concentrations of sodium hypochlorite (bleach) electrochemically in brine solutions, it is much more expensive than letting it sit in the sun...
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First, I am tempted to try decomposing bleach with citric acid, but will do my best to control myself.
Second, If a pool of brine is opaque with algae, while sunlight might be a cheaper reagent in and of itself, would it not take much longer to get the job done?
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"Is table salt commercially processed with bleach? "
Why would it matter?