Moisture Content Analysis

The time in the wet and dry desiccators for the sub-samples of each product was adjusted based on the diffusion properties of the product. All sub-samples for a product were then analyzed in triplicate for moisture content and water activity.

The isotherm testing results were characterized using the Guggenheim-Anderson-Deboer, or double log polynomial, model. Dewpoint moisture content values were compared to average moisture content values from oven loss on drying. Standard error of prediction (SEP), interpreted as the 95% confidence interval for the predicted value around the actual value (a smaller value is better), and R2 value, interpreted as the goodness of fit of the correlation (closer to one is better) were used for comparisons between different isotherm curve/model combinations.

The relative strength of a secondary method is measured by how well it matches the reference method. For this study, the SEP value can be considered a measure of the ability of the dewpoint moisture content method to correctly match reference data.

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Table 3. Commonly reported precision (also reported as accuracy) values for most frequently used moisture content determination methods.

A Good Method

Dewpoint moisture content values agreed well with oven loss-on-drying values for all products, as evidenced by the low SEP and high R2 values (see Table 1, above). Most secondary methods for moisture content consider an SEP of 0.60% or lower to be acceptable. All SEP values except for chocolate syrup and jerky were close to that range, indicating that dewpoint moisture content can be considered a viable secondary method for lower- and intermediate-moisture products. It may be possible to develop better predictions with larger data sets for high-moisture products like chocolate syrup and jerky, but that was not attempted.

Table 2 (see p. 43) shows a comparison between the precision of the oven loss-on-drying method and dewpoint moisture content method. For every product investigated, dewpoint moisture content gave better precision, even though loss on drying is considered the reference method. Table 3 (see above) shows that, in comparison to reported precision values for other methods, dewpoint moisture content has the highest level of precision.

Dewpoint moisture content is an excellent moisture content measuring option and is especially attractive when both water content and water activity measurements are needed on the same sample. A product-specific isotherm, which can be obtained manually or using an isotherm generator, is needed. The precision of this method is the best of any of the secondary methods and exceeds that for loss on drying. It does an acceptable job of matching reference values, but the accuracy cannot be assessed because there is, to date, no absolute method for measuring moisture content. ■

Mumford is a science editor at Decagon Devices Inc. Reach her at [email protected] or (509) 332-2756.

References

1. Isengard HD. Water content, one of the most important properties of food. Food Control. 2001;12(7):395-400.
2. Mulvaney TR. In: Cunniff P, ed. Official Methods of Analysis of AOAC International. 16th ed. Arlington, Va.; 1995:42-1-42-2.