Iodine Stability in Iodized Salt
Potassium iodide is approved by the U.S. Food and Drug Administration as the nutritional source of iodine in iodized salt. However, potassium iodide in salt is not very stable, and can easily be lost by oxidation to free iodine. The free iodine vaporizes and is lost from the salt, defeating its purpose of preventing iodine deficiency. The release of free iodine may color the salt slightly.
Potassium iodide stability in iodized salt may be adversely affected by any of the following conditions: (1) moisture in the salt, (2) humid or excessively aerated environment; (3) exposure to sunlight or ultraviolet light; (4) exposure to heat; (5) acidity in the salt; or (6) presence of impurities.
Loss of iodine in potassium iodide fortified salt can be minimized if the salt is pure (+99.5%) and dry (moisture less than 0.1%). Stabilizing agents such as sodium thiosulfate or dextrose, pH buffering agents such as sodium carbonate or sodium bicarbonate, and drying agents such as tricalcium phosphate, sodium silicoaluminate, silicon dioxide, or magnesium carbonate help prevent loss of iodine from iodized salt.
The stabilizing effect of sodium thiosulfate is the result of the following reaction:
Na2S2O3 + I2 ® Na2S4O6 + 2KI
Sodium Thiosulfate Free Iodine Sodium Tetrathionate Potassium Iodide
Dextrose provides iodide stability in a similar fashion by preventing oxidation of the potassium iodide to free iodine. pH buffering agents like sodium carbonate or sodium bicarbonate maintain the neutrality of the iodized salt. A shift toward the acid side of the pH range (less than pH 7) favors the oxidation of potassium iodide.)
Anticaking and free-flowing agents like those mentioned above, prevent the product from becoming too moist. (Moisture favors the oxidation of potassium iodide.) These agents either act to absorb moisture or to coat the iodized salt, thereby preventing moisture contact with the salt.
Iodized salt is prepared by adding 0.006-0.01 % potassium iodide (KI). Stabilizing agents dextrose and sodium thiosulfate are added at levels of 0.04% or 0.05%, respectively. Anticaking or free-flowing agents are normally held to levels of 1-1.5 %.
Different countries use different iodizing agents.
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