Comparatively Speaking: Water Content vs. Water Activity

Dec 29th, 2010

Anthony J. O'Lenick, Jr., Siltech LLC

This topic of water content and water activity should be of interest to all formulators interested in product safety and stability. It is a concept associated with preservation and one often discussed by industry expert David Steinberg.

Water content refers to the sum of all the water either added to, or introduced with raw materials in, a formulation. The analytical method to assess water content is a titration method that was established in 1935 by Karl Fischer (see Karl Fischer Titration).

Water activity or a_w is different from water content in both a qualative and quantative way. First, it is a ratio of the vapor pressure of a liquid divided by that of pure water at the same temperature. Pure distilled water has a water activity of 1.00 (unitless). Second, the value in formulations is rarely 1.00. This means the ingredients in the formulation interact with the water content, altering how that water interacts with the air above the sample.

Water migrates from areas of high a_w to areas of low a_w. For example, if honey (a_w ≈ 0.6) is exposed to humid air (a_w ≈ 0.7), it will absorb water from the air.

This is of interest to the formulator since it is water activity, not water content, that is most important to preservation. Most bacteria cannot grow at an a_w below 0.9; yeasts cannot grow at an a_w below 0.85; and molds cannot grow at an a_w below 0.7. So-called dehydrated foods have a_w lower than 0.61.

For many years, researchers tried to equate bacterial growth potential with water content but found the values were not universal; rather, they were specific to each food product.

In 1953, W. J. Scott first established that it was water activity, not water content, that correlated with bacterial growth, and it is now firmly established that growth of bacteria is inhibited at specific water activity values. US Food and Drug Administration regulations for intermediate moisture foods are based on these values.

Further, water activity is related to moisture content in a non-linear relationship known as a moisture sorption isotherm curve. These isotherms are substance- and temperature-specific and can be used to help predict product stability over time and under different storage conditions.

Karl Fischer Titration*

Water determination (also called moisture determination) is one of the most frequently performed analyses in laboratories around the world. This is not really surprising, as the water content directly influences the quality, process ability, shelf life and stability of a wide range of products.

Karl Fischer titration has established itself as a reference method for general use. It is highly specific and precise, covers a wide concentration range from pap up to 100% and has short determination times. There are two types of Karl Fischer titration: volumetry and coulometry

Volumetry

This procedure titrates the absolute water contents. Volumetry also has the advantage of solid or pasty samples that can be placed directly into the titration vessel and work can be conducted with a wide range of organic solvents adapted to the sample.

Coulometry

In contrast to volumetric Karl Fischer titration, coulometry adds no titrant, but the required iodine is generated directly in the iodide-containing electrolyte by electrochemical means (electronic buret). For this reason, coulometric water determination is primarily used for the determination of small amounts of water from 10 µg to 200 mg H2O with a resolution of 0.1 µg H₂O.

*The above text is Metrohm's description of Karl Fischer titration, as illustrated on the company's website and adapted by Cosmetics & Toiletries magazine.