|
WATER ACTIVITY AND BUDs
Here is a discussion on Water Activity Considerations for BUDs that apply to Table 3 in the proposed Chapter <795>. Water activity was introduced into the proposed chapter at lines 536 through 543. A more in-depth article will be published in the July/August 2018 issue of the International Journal of Pharmaceutical Compounding. A summary/introduction is provided here.
WATER ACTIVITY CONSIDERATIONS FOR BUDs
Water activity (Aw or aw) is the ratio of vapor pressure of water in a product (P) to pure water (Po) at the same temperature.
Aw = P/Po
It is equal to 1/100 of the relative humidity (RH) generated by the product in a closed system. The relationship between aw and Equilibrium Relative Humidity (ERH) is given as:
ERH(%) = aw � 100
Another way of stating it is that water activity is the partial vapor pressure of water in a substance divided by the standard state partial vapor pressure of water. Pure distilled water has a water activity of exactly 1. As the temperature increases, aw typically increases, except in some products containing crystalline salt or sugar.
Water activity is also referred to as a measure of water in a material that is available to react with or attach itself to other material; it is "free" water, while the unavailable water is called "bound" water. This was introduced in pharmaceutics courses in college with sucrose/syrup calculations.
Sometimes the "bound" water is divided up into "absorbed" and "bound" water. Water content is different from water activity. Water content is the total amount of water in a product, both bound and free water.
Every microorganism has a limiting aw below which it will not grow.
The Water Activity Minimum that Permits Growth of Various Microorganisms:
| Organism | Approximate Minimum Water Activity |
|---|
| Clostridium botulinum, Pseudomonas fluorescens | 0.97 |
| Many common waterborne or soil organisms and non-skin pathogens (e.g., Pseudomonas aeruginasa. Clostridia, E. Coli, Vibrio cholera) | 0.95 |
| Clostridium botulinum A, B | 0.94 |
| Salmonella, Bacillus cereus | 0.93 |
| Listeria monocytogenes | 0.92 |
| Bacillus subtilis | 0.91 |
| Staphylococci and micrococci | 0.87 |
| Many yeast (e.g., Saccharomyces and Candida spp.) | 0.88 to 0.92 |
| Many fungi (e.g., Penicillum, Aspergillus, Mucor) | 0.8 to 0.9 |
| Most molds | 0.8 |
| Osmophilic yeasts (e.g., Zygosaccharomyces rouxii) | 0.65 |
| No microbial proliferation | <0.60 |
To lower water activity, the addition or changing of concentrations of ingredients such as sodium chloride, sucrose, alcohol, propylene glycol, or glycerin may lower water activity sufficiently to become self-preserving. Nonaqueous liquids or dry solids will not support spore germination or microbial growth due to their low water activity. An added preservative may be required to preserve the product for a longer BUD. Some example water activities for pharmaceutical products are shown below.
Example aw Values for Pharmaceutical Products:
| Product | Water Activity |
|---|
| Nasal inhalants | 0.99 |
| Antacids | 0.99 |
| Topical creams | 0.97 |
| Oral liquids | 0.9 |
| Oral suspensions | 0.87 |
| Topical ointments | 0.55 |
| Lip balms | 0.36 |
| Inserts and suppositories | 0.3 |
| Compressed tablets | 0.36 |
| Liquid-filled capsules | 0.3 |
Loyd V. Allen, Jr., PhD, RPh
Editor-in-Chief
International Journal of Pharmaceutical Compounding
Remington: The Science and Practice of Pharmacy Twenty-second edition
| 
