Iodine

Iodine by itself is a very good sanitizer, but it stains almost everything and is irritating to skin and other tissues. So lutions of iodine complexed with a high molecular weight carrier are more commonly used today and are called iodophors. The high molecular weight carrier is typically a polymer which is simply a molecule made up of a large number of atoms with a repeating structure. The complexing of the iodine with the polymeric carrier serves three basic functions. First, the solubility of the iodine is increased. Elemental iodine has limited solubility and combining it with a polymeric molecule greatly improves this. Second, the iodine-carrier complex provides a sustained-release reservoir of iodine because the iodine stays bound to the carrier until the free iodine concentration in solution falls below an equilibrium level. And finally, the equilibrium between the free form and the complexed form keeps the amount of free iodine low, yet at a level that kills microorganisms. Thus, the otherwise highly toxic iodine can be used safely in food and beverage applications.

Iodine can enter a microorganism fairly easily. Once it does, it kills the cell via a number of possible mechanisms. It is generally accepted that the most significant reaction involves the oxidation of the sulfur-hydrogen groups in the amino acid cysteine. Once this occurs the microorganism can no longer synthesize proteins and it dies. Other mechanisms for the disinfectant properties of iodine have been proposed but need not be discussed here (6). Suffice it to say that iodine is a very effective sanitizing agent. Data indicate a 10-minute exposure at 15 parts per million (ppm) will kill 99.999 percent of the microorganisms that cause contamination in the homebrewing environment (6).

All iodophors are produced by what is called a "cold process," meaning it uses no external heating. This process occurs in an acidic environment and the final complex in pure form has a pH of about 3, depending on the carrier used. Some formulations contain added phosphoric acid. These are primarily made for the dairy industry where the additional acid helps dissolve calcium deposits on surfaces from milk. The formulations made for the food and beverage industry, and what you are most likely to find in your homebrew shop, do not contain any added acid. This is desirable because they are safer to handle than the acid-containing formulations. You may encounter the formulation made with acid if you purchase iodophor at a dairy industry supply store. Iodophors are sold as a concentrate that is diluted to a working concentration in water. The label gives directions on how to dilute the iodophor to achieve an available iodine concentration of 12.5 ppm. Soaking equipment for 10 minutes in a solution of 12.5 ppm of available iodine is all that is needed to kill the majority of microorganisms that occur in the brewing environment. At 12.5 ppm the solution has a faint brown color that you can use to monitor the solution's viability. If the solution loses its color it no longer contains enough free iodine to kill microorganisms.

When iodophor is diluted in cold water an equilibrium is reached between the free (measurable) and bound forms. The chemistry of this equilibrium is quite complex and is not relevant to our discussion. Those of you who are interested in more details should consult reference (6). What the chemistry boils down to is this: as iodophor is added to a water solution, the free iodine in the solution reaches a maximum amount and then actually begins to drop off. WestAgro Inc. of Kansas City, MO, the manufacturer of the iodophor complex used in several commercial iodophor products, says the maximum amount of free iodine (that which kills microorganisms) that can be achieved in a water solution is 75 ppm. There is no advantage to using more than the specified amount. In addition to wasting the product, you risk exposing yourself and your beer to excessive amounts of iodine. In this case, more is not better. Another important point is the action of iodophor is inhibited if the pH is outside the range of 3 - 6. Achieving this range is not usually a problem because of the acidic nature of the iodophor complex. If you live in an area with high pH water (greater than 9) you should check the pH of your diluted iodophor and make adjustments with citric or phosphoric acid. Acidify your water below a pH of 9 then add the appropriate amount of iodophor. One iodophor manufacturer we spoke to recalled only one instance where an industrial user had this problem, so it should not be a major issue for homebrewers.

Make only as much iodophor sanitizing solution as you need for each use. Iodine is volatile and will outgas from the solution with time, losing its sanitizing ability. You may have noticed that an iodine solution left in an open glass jar will lose its brown color. If you do have leftover solution, store it in a tightly sealed glass jar or a PET plastic soda bottle. Solution stored this way is stable for about a week. Do not store the solution in other types of plastics because they will either absorb the iodine fairly quickly or allow it to volatize because of their gas permeability, again causing a loss of sanitizing ability. Iodophors, like other sanitizers, are most effective when used on clean surfaces. Proteins and other organic substances will bind the iodine making it unavailable for sanitizing purposes. Sulfur-containing compounds in particular are efficient iodophor inactivators.