Water Activity

Mar 1, 2007 | Contact Author | By: David C. Steinberg, Steinberg & Associates
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Title: Water Activity
preservationx water activityx glycolsx HACCPx CCPx
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Keywords: preservation | water activity | glycols | HACCP | CCP

Abstract: Water activity is a key consideration in preserving cosmetics. As this article discusses, there is an important difference between the amount of water present in a product and availability of this water for microorganism growth.

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All microorganisms need sufficient water and nutrients to grow. The concept of controlling water activity to prevent this growth is as old as time, and the practice of using water activity to prevent spoilage began with honey. Honey would appear to be the ideal growth medium for microorganisms—it contains water and sugar—but it is in fact an excellent example of a self-preservating product because the sugar ties up the water molecules, making the water unavailable for microorganism growth. This is a simple example of the very important difference between water content and available water, also known as water activity. Water activity is written as aw and is defined as the ratio of the water vapor pressure of the product compared to pure water at the same temperature, where P is the vapor pressure of the product, Po is the vapor pressure of pure water, n1 is the number of moles of solute, and n2 is the number of moles of water:

aw = P/Po = (n2/(n1 + n2))
Eq. 1

Pure water has an activity of 1.00 while something considered “bone dry” is 0.00. It is numerically equivalent to 1/100 of the relative humidity (RH) generated by the product in a closed system. RH can be calculated from the direct measurements of partial vapor pressure or dew point, or indirect measurements by sensors that are altered by being exposed to RH. Thus, the relationship between water activity and the equilibrium relative humidity (ERH) is:

ERH (100%) = aw x 100
Eq. 2

Although it is possible to calculate water activity, measurements are significantly more accurate when taken instrumentally. The food industry has used water activity for many years to determine the need for preservatives.

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Table 1. The approximate aw needed for growth of various microorganisms

The approximate a<sub>w</sub> needed for growth of various microorganisms

The approximate water activity needed for growth of various microorganisms has been published and these amounts are generally recognized as the minimums needed for growth.

Table 2. The specific aw needed for growth of the five common test organisms

Table 2. The specific a<sub>w</sub> needed for growth of the five common test organisms

Some molds can grow with water activities as low as 0.7; therefore, it is a good rule of thumb that water activity be kept below 0.7.

Table 3. Typical water activities of various personal care products2

Measuring water activity is especially useful for formulations that are atypical, such as: w/o emulsions, low-water content products, anhydrous products such as lipsticks and powders, and water extracts where the addition of high levels of glycols, even as extracting solvents, can be used to lower the water activity below 0.7 to avoid the need for additional preservatives.

Basic Principles of HACCP

The seven basic principles of HACCP are as follows:

1. Identify microbial hazards and preventive measures.
2. Determine CCPs as related to the identifi ed hazards.
3. Establish the critical limits that must be met at each CCP.
4. Establish procedures to monitor the critical limits.
5. Establish corrective active plans to be implemented when critical limits are exceeded.
6. Establish recordkeeping systems that document the HACCP plan.
7. Establish procedures for verifi cation that HACCP system is working correctly with documentation.

For more information on HACCP, see Borovian’s3 and Easter’s4 papers as well as the International Life Sciences Institute’s Web site5 and Technical Consultancy Services inhouse training software.6

Footnotes [Steinberg 122(3)]

a Aqua Lab CX-2 is a device from Decagon Devices; A2101 is available from Rontronic .

b The Osmicide line is a product of Croda Inc.

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