International Journal of Pharmaceutical Compounding.
Int J Pharm Compd. 2024 Sep-Oct;28(5):376-381.
There are minimum levels of water activity that are required for growth of bacteria and molds. Water activity is the ratio of vapor pressure of water in a product to pure water at the same temperature. Pure distilled water has a water activity of 1. Water activity is also a measure of water in a material that is available to react with or attach itself to other material; it is also called "free" water while the unavailable water is called "bound" water. Higher water activity substances tend to support more microorganisms; bacteria usually require water activity values of at least 0.91 and fungi at least 0.6. Every microorganism has a limit of water activity below which it will not grow. To lower water activity, one can add or change the concentrations of ingredients such as sodium chloride, sucrose, alcohol, propylene glycol, or glycerin so the preparation becomes self-preserving. Also, if the preparation has too high a water activity and the addition of other substances to lower the water activity, then a preservative may be required for the preparation. Nonaqueous liquids or dry solids will not support spore germination or microbial growth due to their low water activity.
有细菌和霉菌生长所需的最低水分活度。水分活度是产品中水蒸气压力与同温度下纯水的水蒸气压力之比。纯水的水分活度为 1。水分活度也是物质中可与其他物质反应或附着于其他物质的水分的度量；它也被称为“自由”水，而不可用的水被称为“结合”水。水分活度较高的物质往往更有利于微生物的生长；细菌通常需要至少 0.91 的水分活度，真菌至少需要 0.6 的水分活度。每种微生物都有一个水分活度下限，低于该下限就不会生长。要降低水分活度，可以添加或改变盐、蔗糖、酒精、丙二醇或甘油等成分的浓度，使制剂具有自保存能力。此外，如果制剂的水分活度太高，需要添加其他物质来降低水分活度，那么可能需要在制剂中添加防腐剂。由于水分活度低，非水液体或干燥固体不会支持孢子萌发或微生物生长。
