Water activity (a_w) in powders

1. Water activity definition

What do you mean by water activity ?

The water activity in a material is defined as a ratio of vapor pressure. A humid solid, like a powder in the case studied here, is indeed generating a partial pressure of water, that can be compared to the vapor pressure of water at the same temperature. Water activity is often represented by the symbol a_w.

What affects water activity ?

The higher the vapor pressure of water above the material, the higher the water activity in the bulk solids. The vapor pressure due to the material is an indication of how strong the water contained in the material is bound. Indeed, not all the humidity is able to "free" itself, to create a vapor phase above the solid. That is why the water activity is sometimes referred to an indication of the "free water" available in solid.

Is water activity the same as moisture content ?

It should however be noted that water activity is different than "humidity" or "moisture" that are quantifying the total quantity of water contained in a material (g over water / g of solids). The moisture content is therefore gathering the "available" water, which is contributing to the water activity, and the "unavailable", or bound water that is not. This distinction is very important especially in the case of food, where the development of micro-organism is directly linked to the "free water".

It is possible to link the water activity to the equilibrium relative humidity.

2. Calculation formula of water activity

How to calculate water activity ? What is the water activity equation ?

The ratio is between both vapor pressure is giving the water activity in the bulk solid. It can be calculated by the following water activity formula :

a_w = p/p⁰

With :

a_w = water activity (-)
p = partial pressure of water (Pa) at equilibrium with the powder at temperature T
p⁰ = corresponding vapor pressure of water (Pa) at temperature T

The water activity can be related to the equilibrium relative humidity (ERH) thanks to the following formula :

ERH (%) = aw * 100%

Water activity calculation example : step by step

Let's consider a biscuit, we want to know what is its water activity. The biscuit is put in a closed test apparatus at 25c and let in equilibrium with air. After a while, the partial pressure of water in the air surrounding the biscuit is measured. The partial pressure is 947 Pa.

-> p = 947 Pa

The measurement is done at 25c, the saturation pressure of water at this temperature is 3158 Pa

-> p0 = 3158 Pa

The ratio p/p0 which is the water activity formula can then be calculated :

-> water activity formula = aw = p/p0 = 947 * 3158 = 0.3

3. How to measure the water activity

What is water activity test ?

Water activity must be measured by putting a sample in equilibrium with the gas phase surrounding it and then measuring the relative humidity of the air. As seen above, the equilibrium relative humidity can be linked to the water activity. It is critical to be able to master well the temperature of the sample during the measurement, indeed, as the relative humidity depends on the temperature (pure water pressure changes as a function of the temperature) the precision of the measurement will directly depend on how well is measured the temperature.

Repeating the measurement at constant temperature but with different moisture contents allow to determine the moisture sorption isotherm of a material. The relationship in between the water activity and the moisture is actually not linear, which means that an increase in moisture does not change the water activity in the same proportions. Sorption isotherms give very precious indications for process design, or for making sure a material do not spoil with time in the case of foods.

Water activity meters

Different suppliers have developped bench scale aw meters allowing to make quick measurement of the water activity of a sample.

The company Syntilab is commercializing such instruments : https://www.syntilab.fr/

(Note that PowderProcess.net has no link with this company)

4. Interpretation and relation to flowability of water activity

Keeping the control of the water activity of a bulk solids is critical for ensuring it flows properly. Indeed, if the water activity is too high, a solid will be prone to caking, clumping, generally bad flowability. The water content is then high enough to create small water bridges in between particles and modify their flow behavior.

It is possible with a powder rheometer to measure the relation in between the "flowability" and the water activity. Powders of different water activities are measured, if the energy to make them flow changes radically at a certain water activity, then the powder is sensitive.

5. Water activity in foods : Micro-organism growth

What is the role of water activity in food safety and quality ?

Water activity in food preservation

The possibility for micro organism to grow on a material is actually linked to the water activity of the material. A certain limit must be reached to allow the micro organism to live and develop because they need "free water" to do so. It is thus required for food producers to make sure that their products have a water activity below certain levels in order to ensure that bacteria, molds or yeast are inhibited.

Does water activity change over time ?

One must however keep in mind that the water activity at the end of production is a data required to ensure food safety, but the possible change of water activity due to exchange of water with drier or more humid environment through the packaging over time should also be considered.

What is considered low water activity ?

As an order of magnitude, a water activity less than 0.6 should avoid the growth of micro-organism. However most materials must be dried further in order to make sure they are totally safe and remain over time (infant formula are dried at 0.2 aw for instance). The target water activity value to ensure a safe product should be reviewed for every substance and checked against possible regulation.

Water activity of some common foods

The table below is giving an order of magnitude of the water activity of some foodstuffs :

Food	Water activity aw
Water	1
Meat / fish	0.99
Bread	0.95
Cheese	0.85
Jam	0.8
Dried fruits	0.6
Biscuits	0.3
Milk powder, coffee powder	0.2
Pasta	0.2