The two main factors involved with the stability of freeze-dried probiotic preparations during the manufacturing and storage process is water activity and temperature.  

What is the water activity test?

Water activity defines the free water in the product, which is available for micro-organisms' metabolic purposes. It is the ratio of vapour pressure of H20 in product (P) to vapour pressure of pure H20 (Po) at the same temperature. It is numerically equal to 1/100 of the relative humidity (RH) generated by the product in a closed system. RH can be calculated from direct measurements of partial vapour pressure or dew point or indirect measurement by sensors whose physical or electric characteristics are altered by the RH to which they are exposed.

Why is it important?

Collecting water activity data during the formulation, manufacturing and storage process is a great way to increase confidence that sufficient levels of viable cells are available throughout its shelf life.  While direct enumeration tells you how many cells are left viable at the time of analysis, water activity measurement can assist in predicting the rate of death of viable cells over a period of time. It also highlights if any issues occurred during the manufacturing process that were not picked up by the enumeration testing. 

Principle of the test

A specialised electrolytic moisture sensor is required for water activity determination which quantifies the air humidity directly over a sample in a closed chamber after reaching the humidity equilibrium between free water in the sample and water in the surrounding air. 

Sample requirements

3-5g is required. 

Turnaround time

2-3 days.

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