Water Activity - Legend Technical Services

Water activity is not the same as water content. Water content is how much water is in something and it is determined by a simple pre-weighing of a known amount of a substance, drying the substance under known conditions, and re-weighing the known amount of substance. The weight difference is the water content generally expressed in % or parts-per-million (ppm) by weight.

Water activity is the amount of unbound water in a sample. Bound water is connected to molecules and is not available to microorganisms. Waters of hydration in a chemical is bound water. Water activity measures only the unbound water. This is the water that is available to microorganisms that can lead to spoilage. Water activity is expressed as the vapor pressure of water in a sample divided by the vapor pressure of pure water at a given temperature. Relative humidity measurement in air is similar.

Water can cause expensive problems in your products including microbial growth, caking and clumping, rancidity, and changes in composition. Measuring water activity is a cost-effective method to assess the risk of these negative impacts to your product.

Water activity meters measure the relative humidity above a known sample in sealed chamber after equilibration at a known temperature. The difference in the meters is in the sensor technology. Many sensors are affected by volatile materials in the sample other than water and can give erroneous readings if corrections are not made. These can be very complex and don’t always work. The technology LEGEND uses, tunable laser technology, is designed to measure ONLY the water vapor without interferences. The TDL’s sensor measures the relative humidity of the air in the sample chamber by emitting a finely-tuned near-infrared laser beam across the headspace. Because water vapor has a strong absorption bands in the near infrared (NIR), the sensor can measure the presence of water vapor in the headspace very precisely.