Moisture content vs water activity: Choosing the right measurement
Moisture content (MC) and water activity (Aw) both have to do with water, but the distinction between the two isn’t always clear. Is one better than the other?
It doesn’t have to be complicated. These are the facts.
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?
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 moisture determination 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.
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Water activity 101: What it is water activity (Aw) and how is it used?
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.
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.
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.
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.
Water activity can be measured in a few different ways, with the two most popular being capacitance sensors and chilled mirror instruments. Capacitance sensors are often inexpensive, though at the cost of accuracy and precision. Chilled mirror devices are more accurate and are generally considered the food industry standard. Other more niche methods include resistive electrolytic sensors and tunable diode lasers.
Unlike moisture content, water activity is most helpful for managing the quality and safety of a substance or product, not quantity. As such, it has been widely adopted in the food, pharmaceutical and cannabis industries.
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.
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.
Every substance has a different isotherm, and the relationship between moisture content and water activity isn’t linear and can be complex and unpredictable – until you see it mapped on an isotherm, that is.
In the past, isotherms weren’t particularly practical, due to the month or more of lab work needed to create a single graph – the process involved putting samples into desiccators then weighing them repeatedly for days or weeks to get a single data point. Newer technologies have automated the process and helped make isotherms more accessible and practical.
Today, they’re frequently used in the food industry to pinpoint the exact levels where changes like caking, clumping, and loss of texture occur, predict how a product will respond to formulation changes, estimate shelf life, and plenty more. They’re also frequently used to analyze wood, building materials and textiles.
Drawing your own conclusions
Moisture content and water activity have different strengths. Visualizing them together as an isotherm reveals more valuable insight than either one provides alone.
In many cases, neither is a one-size-fits all measurement – it all depends on the application. It’s up to the user to decide.