# Inline Water Activity aw Softsensor

Water activity (aw) is defined as the relative humidity of the atmosphere above a sample divided by 100, (RH/100). For convenience, this relationship can be written as equation (1) with K1 = 100.

RH = K1(aw)             (1)

In simple terms, aw is a measure of the free water available in a product that is available for microbial growth. For example, mold will not grow in dried petfood if the aw is 0.65 or below. Two inline aw sensing and control systems have been installed on petfood dryers. Both installations were installed following recall episodes.

## Problems with Traditional aw Sensing & Control

Since inline sensing and control of aw was previously not possible, samples for aw analysis were taken and analyzed by a laboratory type instrument. The time required for running the test is 3 – 5 minutes. Combining the testing time and the time between samples (hourly) introduces long gaps between samples; therefore, such control does not give assurance that an out of control condition could be detected, thus the probability of suffering a costly recall is extremely high.

## The Solution – A new Water Activity Softsensor

Since it has been determined by equation (1) that RH is directly proportional to (aw), and elsewhere it has been determined that RH is directly proportional to (∆T), by equating the two equations and solving for aw, equation (2) is produced:

RH = K2/(ΔT) = RH = Kaw

aw = K3/(ΔT)             (2)

By substituting aw for MC into the original Delta T model, a new inline aw SoftSensor and control system is produced that can provide continuous protection against producing product in which harmful microrganisms might grow and cause costly product recalls.