from [wikipedia][1]:

> **No-additive salts for canning and pickling** 
> 
> In contrast to table salt, which often has iodide as well as
> anticaking ingredients,
> special canning and pickling salt is made for producing the brine to
> be used in pickling vegetables and other food-stuffs. This salt has no
> iodine added because the iodide can be oxidised by the foods and
> darken them—a harmless but aesthetically undesirable effect.


I have been looking for a concentration, but I suspect it is below the threshold that would harm microflora, as it typically has to be visibly yellow (~6ppm if i remember correctly) to be effective as a contact antimicrobial agent...
  [1]: http://en.wikipedia.org/wiki/Iodised_salt