Moisture Content Analysis

The difference between 4.6% MC and 4.8% MC translates to a difference between 0.5800 and 0.600 a_w—a huge difference to a water activity instrument with 0.003 a_w accuracy. Loss-on-drying methods can’t reliably tell the difference between 4.6% MC and 4.8% MC. A dewpoint method instrument can distinguish between 4.60% and 4.62% The magnifying effect of the dewpoint method can mean a 90% or 95% improvement in precision, a boon in some sensitive moisture content applications.

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Table 1. The relative strength of the dewpoint moisture content method to accurately predict oven loss-on-drying values as indicated by the standard error of prediction (SEP) and R2 values.

Standards Make Comparisons Easier

A customer who gets ingredient supplies from overseas perfectly describes the standards challenge: “What’s the standard for moisture content? My supplier always suggests that the shipment picked up moisture on the boat. I think that’s bull, but what can I say? There’s no independent standard for moisture.”

Whether the challenge is meeting a spec and certifying that it has been met or identifying where moisture problems happen in the supply chain, solid, reliable moisture content information is crucial.

Benchmarking moisture measurements with independent standards is a critical need. There are well-established water activity values for a range of saturated salt solutions such as potassium chloride (0.5 m, 0.984 a_w) and sodium chloride (6 m, 0.760 a_w).

Unfortunately, there are no independent standards for moisture content. But because it measures both water activity and moisture content with a single instrument, the dewpoint method can leverage water activity standards to establish reliability for moisture content as well. Users can confidently compare readings across different instruments, different locations, and different time periods.

Often the quality of moisture measurements depends on the expertise of the person making the measurement. With the dewpoint method, expertise is not as much of a factor. Water activity instruments are simple to operate. In spite of the test’s scientific precision, the actual testing process can be performed accurately with a minimal amount of training. This decreases the risk of operator error and increases testing opportunities along the supply chain.

Many manufacturers measure both water activity and moisture content. Being able to make both measurements simultaneously on a single sample would save significant time and expense.

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Table 2. Average precision values for oven loss on drying and dewpoint moisture content for all products analyzed. The values represent an average of standard deviations of triplicate moisture analyses across 10 samples for each product.

Dewpoint Moisture Content

With such obvious promise, food scientists set out to test the dewpoint moisture content method with a high-accuracy water activity meter and an isotherm generator.

Testing a moisture content method is not simple. Since there is no standard for measuring moisture content, a true accuracy can’t be calculated.1 The best way to compare moisture content methods, then, is by comparing their repeatability. The precision of the different methods was calculated as the average standard deviation of triplicate analyses across all samples for a given product.

Nine products, representing a wide variety of types, from homogeneous ingredients to complex final products, were selected for testing. The products included milk powder, flour, dry dog food, chocolate syrup, granola bars, potato flakes, solid dosage tablets, whole wheat, and beef jerky.

The moisture content in triplicate was determined for all of the products using convection oven loss on drying. Time and temperature settings for loss on drying were based on Association of Official Analytical Chemists (AOAC) recommendations when available.2 All moisture measurements are expressed as percent dry basis.

To create samples varying in moisture content, 10 sub-samples were taken for each product, five of which were wetted by exposure to 100% relative humidity in a sealed desiccator, while the other five were dried by exposure to desiccated air inside another sealed desiccator. Sub-samples were removed from the desiccators at different times to create samples varying in moisture content. As the sub-samples were removed, they were sealed in jars and set aside until all sub-samples had been removed from the desiccators.