Water Activity

Novasina - Water Activity

The water activity is given as the aw – value and ranges between 0 (absolute dryness) and 1 (condensed humidity). Only this component takes an active part in the exchange with the ambient humidity and can possibly form the ideal medium for microbiological growth on the surface which influences the microbiological stability. Water activity also has an important effect on the chemical reactions in food.

The relative humidity is measured after reaching the equilibrium humidity over a sample (partial water vapor pressure) The relative humidity is measured in % RH and relates to the aw–value as follows: aw = ERH/100

The influence of water activity in foods

The humidity balance value of a product, which is ascertained through its partial the pressure of water vapor on the surface depends, on the following factors:

  • chemical compound
  • temperature
  • water content
  • storage conditions (T / RH)
  • absolute pressure
  • packing

“Free” water in products is jointly responsible for the growth of undesirable organisms such as bacteria or fungi, which produce “toxins” or other harmful substances. But also chemical/biochemical reactions (e.g. the Maillard reaction) increasingly take place and possibly change the following factors of a product:

  • Microbiological stability (growth)
  • Chemical stability
  • Content of proteins and vitamins
  • Colour, taste, and nutritional value
  • Stability of the compound and durability
  • Storage and packing
  • Solubility and texture


USP <922> Water Activity

Introducing USP <922>, the first officially published method for determination of water activity in pharmaceuticals. It includes a brief theoretical background explanation, outlines the best practices for water activity measurement, and provides an overview of potential applications for water activity in pharmaceutical products.

In terms of suggested uses for water activity, USP <922> extends beyond the usage suggestions of USP <1112> to include:

  • Selecting ingredient isolation and product manufacturing processes conditions in terms of maintaining aw below the critical threshold to obtain thermodynamic control of the desired solid form (e.g., hydrate versus anhydrate)
  • Selecting excipients for which aw may impact their material flow, compression characteristics, hardness, and performance characteristics (e.g., disintegration and dissolution) of dosage forms
  • Optimizing fluidized bed drying processes
  • Reducing the degradation of active ingredients within product formulations (e.g., those susceptible to chemical hydrolysis)
  • Establishing the level of protection to product formulations to moisture by primary packaging materials during their shelf life
  • Optimizing the shelf-life stability of probiotics
  • Providing a complementary method for monitoring changes in water content
  • Controlling and monitoring physical, chemical, and microbial product stability
  • Optimizing formulations to improve the antimicrobial effectiveness of preservative systems
  • Reducing the susceptibility of formulations to microbial contamination
  • Providing a tool to justify the reduction of microbial testing of nonsterile drug and dietary supplements formulations (see Application of Water Activity Determination to Nonsterile Pharmaceutical Products 1112)


Bakery Products
Cosmetics & Personal Care
Dairy Products
Meat & Fish
Powder & Spices


Water Activity Buyers Guide – Verification
Water Activity Buyers Guide – Maintenance
Water Activity Buyers Guide – Accuracy
The new MASTERPIECE of Novasina AG – LabMaster-aw neo
USP 922 Water Activity
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