What is Water Activity & pH?

Learn what Water Activity and pH is and how it Impacts to the shelf life of your product.

Water activity is defined as the ratio of the water vapor pressure of a food (P) to that of pure water (Po).

a_w = P/Po

Another way to think about it is:

the weight of water desorbed from the food into the vapor phase (P), and its ratio to the vapor pressure of pure water (Po) at the same temperature, describing the waters escaping tendency of fugacity ratio.

The a_w of pure water is 1.000.  Water activity is measured in a range from 0.030 to 1.000.  The closer a food products value is to 1.000, the more water that product has available to participate in various chemical and physical reactions.

Water activity is a better index for microbial growth that water content.

Water activity better predicts the growth of microorganisms because microorganisms can only use available water, which differs considerably depending on the solute.  On average, ions bind the most water, whereas polymers bind the least water; sugars and peptides fall into an intermediate position.  At the same molecular concentration, salt lowers the water activity more than sugar.

Because of the strong influence of a_w on the wholesomeness and safety of many foods, regulations specifying a_w levels have been established.

Measurement of Acidity/Basicity or pH

The term pH is defined as the logarithm of the reciprocal of the hydrogen ion concentration [H⁺] in solution.  It measures the effective acidity–the degree or intensity of acidity.  It is determined by measuring the difference in potential between two electrodes immersed in a sample solution.

Microorganisms generally have optimum and minimum levels of a_w and pH for growth depending on other growth factors in their environment.  Water activity and pH values can be used to predict which microorganisms will grow in a food.  Microorganisms require water of a specific energy level to grow and survive.  Additionally, the different species of microorganisms have pH requirements which must be met for growth to occur.