Using Dynamic Vapour Sorption to Analyse Foodstuff Moisture Content

The relationship between foodstuffs, their internal water content and any external moisture has a huge impact on food quality and shelf life. As a result, it's important for food manufacturers and distributors to understand exactly what this relationship is. Dynamic vapour sorption (DVS), a gravimetric technique that easily allows the measurement of internal water content and how it may change over time, allows foodstuff moisture to be accurately measured.

AZoM spoke to Dr. Dan Burnett, Director of Science Strategy at Surface Measurement Systems, about the relationship between moisture and foodstuffs, and the many benefits DVS delivers for the food industry.

How does the internal water content of foodstuffs influence the experience of the consumers that eat them?

The moisture content of food products is recognized as a critical factor in determining their storage, stability, processing and application performance. In particular, the initial moisture content often determines the ultimate shelf-life of food and food ingredients.

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Are different types of foodstuffs expected to have different water content levels?

Yes, food substances can have a wide range of water contents.  On the low end, dry powders like instant coffee have a very low water content, while fresh meat and fish have very high moisture contents. High moisture contents lead to increased microbial growth rates and food spoilage.

In general, the higher the water content, the lower the shelf life of food materials.  Therefore, higher water content foods require strategies to prolong shelf life (i.e. lower storage temperatures, more sophisticated packaging, and additional ingredients to prevent food spoilage).  These strategies could include diffusion resistant packaging, multi-layer films, the use of desiccants, or food formulations that help manage moisture migration.   

How does the water activity of a foodstuff affect its internal water content?

Water content is related to water activity through a non-linear relationship known as the moisture sorption isotherm curve. High water activities have a higher amount of free water which can be available for microbial growth or migration to other food ingredients.

Moisture sorption isotherms are used to aid in product stability prediction over different environmental conditions. Moisture content is a strong factor of both temperature and relative humidity. In addition, the moisture content of other food and food ingredients can affect a material’s moisture content.

Moisture sorption isotherm plots for starch at 25 °C

What features of a certain foodstuff determine what value its water activity be?

The initial moisture content of a food substance contributes to the water activity. Also, the ability for moisture transport within the food substances greatly contributes to the ultimate water activity.

High water activity foods have more free (versus bound) water, which can readily migrate to other food ingredients or support microbial growth.   

How is the water activity conventionally measured?

Water activity is typically measured using a hygrometer (capacitance, dew point, or resistive electrolytic). All of these techniques require the food material to be in thermal and vapor equilibrium with the detector. Are there any problems associated with this method?

Water activity measurements only give a single data point on how a material interacts with water vapor. Also, most water activity measurements are done at ambient conditions. However, in order to fully cover the scope of storage, transport, and environmental conditions a wide range of both humidity conditions and temperatures should be investigated.

In addition, the water activity only gives a measure of ‘free water’. The total water content is a measure of both free and bound water.

Water sorption isotherms on α-lactose monohydrate at 25 °C (red), 35 °C (blue), and 45 °C (green)

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How does dynamic vapor sorption (DVS) improve on conventional methods of determining a foodstuffs water activity?

DVS has the advantage of determining a more complete understanding of how a material interacts with water vapor.  DVS experiments can be done over a wide range of environmental conditions (i.e. temperature and humidity). In addition, DVS experiments can be done with different thermal and humidity pre-measurement conditions.

Specific to water activity determination, the DVS Payne cell method is inherently simple in concept; uses relatively small samples; independent of chilled mirror calibrations; independent of surface temperature calibrations; and relatively fast and robust.

Can DVS be used to determine other relevant properties of foodstuffs?

Yes, DVS can be used to determine a wide range of properties on food ingredients.  For instance, DVS is readily used to determine vapor-induced phase transitions, such as glass transitions, amorphous to crystalline transformations, and sample deliquescence.  Further, DVS combined with Raman spectroscopy can elucidate even more subtle moisture-induced changes in food materials.

DVS has also been used to investigate moisture-induced flavor/fragrance release. DVS can be coupled with video microscopy to study visible changes with respect to humidity and temperature.  Finally, DVS instruments can also explore how organic vapors interact with materials. Therefore, DVS can be used to directly measure flavor/fragrance interactions with foods.

Humidity-induced phase transitions for spray-dried lactose measure via DVS

What instruments do Surface Measurement Systems supply for DVS experiments on foodstuffs?

SMS offers a wide range of DVS instruments to study food materials. These products cover a wide range of application and experiment design space. To illustrate, our small-footprint DVS-Intrinsic instrument can measure water sorption properties between 20 and 40 °C. At the other end of spectrum, our DVS-Endeavor​ instrument can measure water and organic sorption properties on up to 5 samples simultaneously between 5 and 85 °C. This instrument can also be equipped with video, Raman, and pre-heating/drying options.

SMS supplies turn-key and comprehensive instruments. Our instruments are supplied with a robust and inclusive control software package that allows a wide range of experimental protocols to be used.  Additionally, our instruments come with a full suite of analysis options. Our DVS instruments are quite modular, which allow the customer to decide exactly which software and hardware options are appropriate for their research needs.

Where can our readers find out more about food analysis, DVS and Surface Measurement Systems.

Detailed information about our company and range of vapor sorption products can be found on our website.  

About Dan Burnett

Dan Burnett is the Director of Science Strategy for Surface Measurement Systems, Ltd. He is located at SMS’s North American headquarters in Allentown, PA.  He received his bachelor’s degree in Professional Chemistry from Eastern Michigan University in 1997.  He received his Ph.D. degree in Chemical Engineering from the University of Michigan in 2001.

Since joining SMS in 2001, he has continued his interests in sorption science and surface chemistry on a range of materials including: pharmaceutical powders, natural and man-made fibers, polymers, films, and food/flavor systems.  Dr. Burnett has authored or co-authored over 25 papers in peer-reviewed journals and presented at numerous national and international conferences.