Water Activity vs. Moisture Content: What Each Measurement Tells a Food Producer

Water Activity vs. Moisture Content: What Each Measurement Tells a Food Producer

If you make food, you’ve probably heard both terms used as though they mean the same thing. They don’t, and mixing them up can lead to a product that passes its moisture spec and still spoils on the shelf.

Moisture content: how much water is in the product

Moisture content is the total amount of water in a sample, expressed as a percentage of its weight. It’s usually measured by drying the sample and weighing what’s lost (oven drying or a halogen moisture analyzer), or by Karl Fischer titration for very low-moisture products.

It answers the question: how much water is there?

That matters for:

• Yield and cost (you’re selling water by the pound, whether you meant to or not)
• Texture and consistency
• Meeting specifications for ingredients like flour, grain, powders and dairy solids
• Process control in drying, baking and blending

Water activity: how available that water is

Water activity (aw) measures the energy status of the water in a product, on a scale from 0 to 1.0. Pure water is 1.0. It tells you how much of the water is free to support microbial growth, chemical reactions and texture change, as opposed to being tied up by salt, sugar, proteins and other solutes.

It answers a different question: how much of that water can actually cause problems?

Water activity is what drives:

• Microbial growth. Most spoilage bacteria are held back below about 0.91, most yeasts below about 0.88, and most moulds below about 0.80. Below 0.60, microbial growth essentially stops.
• Shelf life and storage stability
• Lipid oxidation, enzymatic activity and non-enzymatic browning
• Caking, clumping and texture in dry and semi-moist products

Why you can’t swap one for the other

Two products with the same moisture content can behave completely differently.

Honey is roughly 17 to 18% moisture, yet its water activity is low enough that it resists microbial growth almost indefinitely. A fresh, unsalted food at the same moisture percentage, with little bound water, would spoil quickly. The water is the same amount, but in honey it’s tied up by sugar.

The reverse is also true. A product can sit comfortably within its moisture spec and still have a water activity high enough to grow mould, because the formulation doesn’t bind water well.

Using them together

For most producers, the two measurements do different jobs:

• Moisture content is your process and quality measurement. It tells you whether drying, baking or formulation is consistent from batch to batch.
• Water activity is your stability and safety measurement. It tells you whether the product will stay safe and shelf-stable, and it’s a useful control point in a food safety plan and for shelf-life validation.

A few practical tips:

• Measure water activity at a controlled temperature, since aw shifts with temperature.
• Build a simple relationship between your own moisture and aw results for each product (a sorption curve). Once you have it, routine moisture checks can tell you a lot about where aw is sitting.
• Re-test after any change in formulation, salt, sugar, humectants or packaging.

Where we come in

At Cambridge Environmental Products, we work with food producers across Canada who need reliable numbers for both. We supply A&D moisture analyzers and Steroglass aWLife water activity meters, and we’re happy to talk through which measurement fits your product, your process and your food safety program, even if the answer is that you don’t need new equipment yet.

If you’re sorting out water activity or moisture testing for your plant or lab, reach out. We’d be glad to help you figure out what makes sense.