Controlling water activity—Benefits at a glance

Improve food safety

Extend shelf life

Minimize waste

Maximize profit

What is water activity?

Water activity measures the energy state of water. 

When a product’s water activity is wrong:

• Mold and bacteria grow in the product

• Moisture moves between product components

• Vitamins deteriorate more quickly, the product stales quickly or loses crispness, off-flavors occur

• Particles in powders get sticky and clump together

Everyone knows moisture is responsible for these kinds of changes. What everyone doesn’t know: microbial growth, moisture migration, degradative reactions, caking and clumping, and more are best predicted by measuring and monitoring water activity, not moisture content.

See how it works:

Create your own cereal

Your job is to create a cereal from flakes and an add-in (either raisins or almonds). Try using moisture content values to make a good match between the flake and the add-in. Then try using water activity, and see which better helps you predict what will happen on the shelf.

1

Name your cereal

2

Choose your type of flake

Bran

0.47 aW

7% mc

Multigrain

0.27 aW

3% mc

3

Choose your other ingredient

Almonds

0.47 aW

3% mc

Raisins

0.53 aW

13% mc

Pour me a bowl!

Try again!

Learn more about why and how to measure water activity.

What makes moisture move?

When packaged together, foods with different water activities will come into equilibrium. Water activity, not moisture content, predicts how water will move. It will move from the component with higher water activity to the component with lower water activity until the two components have equal water activities.

Water activity’s super power: mold and microbial growth

Moisture content can’t reliably predict mold and microbial growth. 

Water activity in action

Water activity can be used to ensure correct product texture and free-flowing properties in a powdered product like spray-dried skim milk.

Product: Dried milk powder
Process: Spray drying
Challenge: Preventing caking

Why this manufacturer measures water activity:

• To monitor and control the drying process
• To avoid caking in the finished product

Try it. Using moisture content values, see if you can predict which of the following products are safe.

Then flip the card to try the same prediction using water activity.

This table shows why it is so important for food and pharmaceutical companies alike to learn how to measure water activity.

Why can water activity predict microbial growth?

Water is essential for all living organisms, including bacteria and mold. Microbial cells need it to remain viable. Cells bring in water via passive transport. When the water activity gets low enough outside the cell, the cells can’t transport water in through the cell membrane. They starve and go dormant.

Important note: they’re not dead. They’re dormant. And if the water activity outside the cell gets high enough, they can start to grow again. Safety depends on both a kill step (like heat treatment) and processing and packaging that maintain low water activity in the product. This is why learning how to measure water activity is so valuable.

Water activity in action

Water activity can be used in water-added processes to prevent mold and microbial growth.

Product: Salad dressing
Process: Water-added mixing
Challenge: Preventing mold/microbial growth

Why this manufacturer measures water activity:

• To match water activities of different components
• To prevent moisture migration
• To guarantee shelf life without preservatives

Should I measure water activity or moisture content?

Measure both to solve complex challenges (creating mixing models, increasing yield without impacting product quality).

Water activity in action

Water activity can be used to control mold and microbial growth in a preservative-free intermediate moisture product like nutrition bars.

Product: Filled bars
Process: Convection baking
Challenge: Clean label, no preservatives

Why this manufacturer measures water activity:

• To match water activities of different components
• To prevent moisture migration
• To guarantee shelf life without preservatives

How to measure water activity

To measure water activity, you put a sample in a small measuring chamber and allow it to come to equilibrium with the air in the head space above the sample. Once it reaches equilibrium, water activity meters measure the vapor pressure in the head space by:

• Measuring the humidity by electrical capacitance or resistance
• Measuring the dew point with a chilled mirror
• Measuring the vapor pressure by optical absorption spectroscopy

All water activity meters look like they have the same speed and accuracy if you look at the manufacturer spec sheets. We wanted to know how they really compare, so we tested performance using both standards and a variety of food products.

Speed and accuracy were measured using equilibrated readings rather than “quick mode” or “fast mode” because we found that fast mode was unreliable.  We’ve summarized our results below. For the details, download our full report.

Download full report (.PDF)

• Capacitance Sample read time 8-30 min. Avg: 17.0 min.
  PriceFrom $3000 AccuracyD-
• Resistive Electrolytic Sensor Sample read time 10-25 min. Avg: 14.6 min.
  PriceFrom $5000 AccuracyC+
• Tunable Diode Laser Sample read time 3-5 min. Avg: 3.5 min.
  PriceFrom $13,500 AccuracyB
• Chilled Mirror Dew Point Sensor Sample read time 2-4 min. Avg: 2.8 min.
  PriceFrom $7500 AccuracyA+

Capacitance

Speed

PriceFrom $3000 AccuracyD-

How it works

Capacitive sensors use a hygroscopic dielectric material sandwiched between a pair of electrodes forming a small capacitor. Most capacitive sensors are simply capacitors—two electrodes sandwiching a dielectric material, typically a plastic or polymer. The dielectric constant of the polymer or plastic is typically in the single digits, much lower than the dielectric constant of water vapor (80). As water vapor is absorbed by the dielectric material, the sensor capacitance increases. By relating this change in capacitance to the change in relative humidity, an instrument can calculate relative humidity from a capacitance reading.

Manufacturer

Rotronic, Meter Group (Pawkit), multiple Amazon/eBay sellers,

Download full report (.PDF)

Pros

• Simple
• No moving parts
• Cheap
• Can measure RH across the full range (0 to 100%)

Cons

• Slow
• Significantly lower accuracy than other sensors
• Unacceptably low accuracy and precision in “Fast Mode”
• Sensors degrade over time
• Not acceptable/practical for measuring most true volatiles
• Disproportionately affected by a dirty chamber
• Unbranded eBay/Amazon sensors judged unacceptable

Bottom line

These sensors seem like a good budget option, but short sensor life makes them more expensive than they look. “Fast” mode has unacceptable accuracy, especially in the presence of any contaminants, and should not be considered an option by any serious user.

Resistive Electrolytic Sensor

Speed

PriceFrom $5000 AccuracyC+

How it works

Two extremely fine glass rods are held together at each end by a bead of glass. Metal electrodes are sandwiched between the rods, and an electrolytic solution is held in the trough between them. As the sensor absorbs water vapor, ionic functional groups are dissociated, and the resulting changes in impedance are correlated to changes in relative humidity.

Manufacturer

Novasina

Download full report (.PDF)

Pros

• Solid (C+) accuracy in equilibration mode

Cons

• Slow, even in “Fast” mode
• A dirty chamber makes accuracy unacceptable for water activities above 0.7
• Sensors degrade over time
• Not acceptable/practical for measuring most true volatiles
• Lower cost models have limited accuracy and capabilities

Bottom line

Resistive electrolytic sensors are solid performers backed by a quality company. But they can’t compete with the top-performing chilled mirror sensors on speed or accuracy, and their ability to measure volatiles is overstated.

Tunable Diode Laser

Speed

PriceFrom $13,500 AccuracyB

How it works

The sensor emits a finely tuned infrared laser beam across the headspace above the sample. The beam of the laser, which is less than one nanometer wide, is specific for the commonly occurring isotope of water. Other vapor molecules, including those from alcohols, gasoline, organic solvents, and propylene glycol, do not affect the reading. Attenuation of the beam is measured by the laser receiver and the concentration of water molecules in the air is determined directly from this value. The laser sensor is the only existing sensor that can accurately measure water activity in samples containing significant concentrations of volatiles.

Manufacturer

Meter Group (TDL brand)

Download full report (.PDF)

Pros

• Can measure any sample, including alcohols, organic solvents, and propylene glycol
• Accuracy and speed were both very good, though not as good as chilled mirror
• Should not experience any degradation in performance over time

Cons

• Less accurate than chilled mirror and slightly slower
• Dirty chamber disproportionately affects this sensor

Bottom line

If you have to read the water activity of volatile samples, this is your sensor. You must run it in an environment where you can keep it clean.

Chilled Mirror Dew Point Sensor

Speed

PriceFrom $7500 AccuracyA+

How it works

Dew point is the temperature at which a sample of moist air at constant pressure reaches water vapor saturation. At this saturation temperature, further cooling results in condensation of water. Chilled mirror dew point sensors use a chilled mirror and optoelectronic mechanism to detect this condensation on the surface of a mirror. The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation, thus closely measuring the dew point temperature.

Manufacturer

Meter Group (Aqualab brand)

Download full report (.PDF)

Pros

• Nailed every standard from low to high with impressive accuracy and precision
• Measurements took three minutes or less—half to one tenth the time of the competition
• Maintains high accuracy even with a dirty chamber
• Older sensors performed as well as brand-new ones

Cons

• Can’t read certain volatiles—see manufacturer info

Bottom line

Chilled mirror sensors offer terrific accuracy, long sensor life, solid performance even when dirty, and the fastest read time of any water activity device you can buy.