Additives perform a wide variety of useful functions including those that are often taken for granted. In salt, additives are used for two main purposes; 1) to enhance the flow properties of salt by preventing the clumping of fine crystals and; 2) to fortify salt with iodine in order to prevent iodine deficiency diseases.
Salt crystals are tiny cubes with flat surfaces that have a tendency to stick together. In addition, salt is hygroscopic, which means that the crystals absorb water vapor from the surrounding environment. When enough water vapor is absorbed, it changes into a liquid form and partially dissolves the salt surface layer, which further encourages the crystals to clump together. This prevents the free flow of salt from the shaker and clogs up the holes, thereby preventing salt from being dispensed. Anti-caking agents are added in order to ensure that salt remains free-flowing. Fine crystallized salt, such as sea salt, that does not contain these anti-caking agents is particularly prone to this problem.
The Food and Drug Administration requires food grade salt packaging to indicate all additives. Food grade salt in the U.S. must comply with the Food Chemicals Codex Sodium Chloride Monograph (2008) which specifies that salt may contain up to 2% of suitable food-grade anti-caking, free-flowing, or conditioning agents. A list of permitted additives is shown in the table below. Although the total amount of additives that can be added to salt is 2%, in reality far less is generally used. (Most table salt labels typically indicate that they are more than 99% pure sodium chloride.)
In order to prevent iodine deficiency diseases, more than half of the table salt sold in the U.S. is iodized and is available at the grocery store at the same price as plain salt. Potassium iodide is added at levels of 0.006 to 0.010% (as KI) and has proven to be exceptionally effective during the course of the last 85 years. Dextrose, when added (typically at about 0.04%), acts as a stabilizer for potassium iodide in salt, preventing it from disassociating into “free” iodine, which may be lost from the salt through simple vaporization. When combined with good packaging, these additives ensure that iodized salt retains its ability to combat iodine deficiency disorders, even while remaining in the kitchen pantry for long periods of time.
Anti-caking agents are also added to salt used for de-icing. Although de-icing salt is typically a very coarse particle size, it usually contains a small proportion of fine crystals, which cause clumping. This phenomenon is exacerbated by the salt’s continual exposure to humidity and precipitation. The most frequently used additive is sodium ferrocyanide, also known as Yellow Prussiate of Soda (YPS). Another is ferric ferrocyanide, also known as Prussian Blue. They are added in amounts of 20 to 100 ppm.
YPS is approved by the Food and Drug Administration as an anti-caking additive in table salt based on exhaustive tests demonstrating no evidence of toxicity at levels considerably higher than those used in highway deicing salts. Prussian Blue is also used in household bluing, blueprints, blue-black ink and carpenter's chalk and is also non-toxic to animal and plant life.