Moisture content vs water activity: Choosing the right measurement

Moisture content 101: What it is moisture content and how it’s used?

Water is everywhere. In the food we eat, the air we breathe, and the objects around us – and it significantly influences the physical characteristics of all of the above. It’s no wonder that workers from carpenters to petroleum scientists want to know: How much water is in this material?

It’s a simple question. It seems like it should have a simple solution. Unfortunately, it’s very difficult to get a clear and precise answer.

The primary challenge (though there are many) has to do with measuring only moisture while ignoring the rest of the material. Once water makes its way into a material, it often doesn’t come out as easily – which makes it difficult to remove and measure it. Other methods, like measuring moisture without removing it from its host material, present a host of other challenges.

Which method of measurement you use comes down to two factors: how precise you need to be and what tradeoffs you’re willing to make.

• If you’re a petroleum scientist studying infinitesimal amounts of water in oil or plastic, you may be willing to spend the hours (and dollars) necessary for titration.
• If you’re a carpenter or contractor on-site trying to decide whether a piece of wood is suitable, you may be willing to trade lab-grade precision for the ease of a quick handheld device that approximates moisture levels.
• If you work in food or cannabis production, you might want something in between, like loss-on drying – a relatively high precision measurement within a reasonable amount of time.

The examples above are far from all the uses for moisture content measurements, but the tradeoffs are similar no matter the industry. Whichever method you choose, beware the many misconceptions about what moisture content can and can’t tell you. A simple rule to remember is that moisture content is a “quantity” measurement.

For example, moisture content is a valuable measure of yield. Many products are sold by weight, so since water is such an inexpensive (and heavy) ingredient, measuring and managing moisture content can have an outsized impact on profits.

Moisture content also provides information about texture, since increasing levels of moisture provide mobility and lower the glass transition temperature.

What won’t moisture content tell you? Much at all about “quality” metrics like consistency, microbial safety, moisture migration, and nutrient deterioration. Those key factors are better left to another measurement – water activity.

The simple amount of water in a substance doesn’t give a complete picture of the impact that water will have on the product. That’s where water activity’s strengths lie.

Viewed too closely, water activity can appear to be a very complex concept. Fortunately, you don’t need to know the thermodynamics and obscure equations that prove how it works. The following high-level principles are enough for most people’s purposes.

1. Water activity measures how energetic and available water is. Some substances hold a great deal of water, but the water is locked in place with molecular bonds. That makes it less able to seep out or interact with the world around it – we’d call that low water activity, even though there could be a great deal of water present. The inverse can happen, too: Some materials hold very little water that is also very accessible.
2. Water wants equilibrium. If you leave a soft cookie outside in a dry climate, the low water activity atmosphere draws the moisture from the high water activity cookie – trying to create equilibrium. On the other hand, two materials with the same water activity level – i.e. a cake and the frosting that coats it – won’t exchange moisture with one another. In essence, water activity can tell you which direction water wants to move.
3. Water’s energy levels factor into many valuable predictions. Like which microorganisms will grow on a substance, how its texture will change over time, when it will decompose, how it will react with other substances, and plenty more.

Which one should you use?

It depends on what you’re trying to accomplish. Neither measurement is simply better than the other. They measure very different concepts, and each offers big benefits in different environments.

If you’re trying to nail the right product weight to maximize profit, moisture content should be your go-to. If your goal is to keep a snack food crispy or crunchy for months on the store shelf, or how to put jelly inside a snack cake without the cake getting soggy, choose water activity.

For a deeper dive into the science behind these two measurements, explore our knowledge base —>

Since moisture content does affect texture, some may tell you that it’s the only measurement needed to track both quantity and quality. However, those who try that approach soon learn that it’s inefficient and nearly impossible to manage microbial growth and other quality-related issues using only moisture content.

Moisture sorption isotherms: How moisture content and water activity work together

Water activity and moisture content are valuable separately, but linking the two can reveal exactly how to solve a long list of moisture-related mysteries.

Sometimes called “moisture maps,” moisture sorption isotherms graph how moisture content and water activity levels change as moisture is adsorbed and desorbed from a material held at a constant temperature.

Stated more simply: an isotherm shows changes in both moisture content and water activity on the same graph.