Methods for Determining Equilibrium Moisture Content in Foods

Every food product has a moisture level at which it stops gaining or losing water to its surrounding air. This point – called equilibrium moisture content (EMC) – is one of the most critical parameters in food drying, packaging, and storage. But how do food scientists actually measure it? The answer lies in two broad categories of methods: static and dynamic. Each has a distinct approach, timeline, and set of trade-offs. Knowing when and how to use each method can directly improve product quality, reduce spoilage, and streamline food processing operations.

What is equilibrium moisture content? 🔗

Before getting into the methods, it helps to be clear about what EMC actually represents. EMC is the moisture content at which a hygroscopic material neither gains nor loses moisture when exposed to air at a specific temperature and relative humidity. In practical terms, if you place a dried fruit in a room at 25 °C and 60% relative humidity, the fruit will either absorb or release water until it reaches equilibrium with that environment. The moisture level it settles at is its EMC under those conditions.

EMC is not a fixed number for any given food. It changes with temperature and humidity. For instance, safe long-term storage moisture content is approximately 12% for corn, rice, and wheat, and 11% for soybean. A food’s EMC is plotted against water activity or relative humidity to produce what is called a moisture sorption isotherm – a curve that is essential for designing drying processes, selecting packaging materials, and predicting shelf life.

Why measuring EMC matters 🔗

EMC data feeds directly into several critical food processing decisions. It tells you the target moisture level for drying operations so that the product is stable during storage. It guides packaging design by revealing how the food will interact with ambient humidity inside a sealed container. It also helps predict microbial growth risk, since moisture-related microbial growth is a major factor in food spoilage, particularly in developing countries where storage infrastructure may be limited.

Without accurate EMC values, food processors risk either over-drying (wasting energy and degrading texture) or under-drying (inviting mould and bacteria). That’s why the methods used to determine EMC must be reliable, reproducible, and appropriate for the product being tested.

Static methods for EMC determination 🔗

Static methods are the traditional approach. The basic principle is straightforward: you place a food sample of known dry mass in a controlled environment at a fixed temperature and relative humidity, then wait until the sample’s weight stops changing. At that point, it has reached equilibrium, and the moisture content is calculated.

The saturated salt solution method 🔗

The most widely used static technique involves saturated salt solutions placed inside desiccators. Different salts, when dissolved to saturation, maintain specific and predictable humidity levels inside a sealed container. For example, lithium chloride maintains roughly 11% relative humidity, while sodium chloride maintains about 75%. By placing food samples over several such salt solutions at a constant temperature, you can generate multiple EMC data points and construct a full sorption isotherm.

According to research published in the journal LWT – Food Science and Technology, samples in the static method are left to equilibrate in evacuated desiccators over these salt solutions. Once constant weight is confirmed through repeated weighings, the moisture content is calculated on a dry-weight basis.

The main advantage of this method is its simplicity. The equipment – desiccators, salt solutions, an analytical balance, and a temperature-controlled room – is relatively inexpensive and widely available. It requires no sophisticated instrumentation, making it accessible to laboratories with limited budgets.

Limitations of static methods 🔗

The biggest drawback is time. Reaching true equilibrium in a static setup can take weeks to months, especially at high water activity levels. For powdered or hygroscopic foods, constant weight may be achieved in about 48 hours, but many products – particularly dense, whole pieces – take much longer. There is also the risk of microbial contamination during extended exposure, particularly at higher humidity levels. Researchers from the Technical Faculty of Bitola have noted that this long equilibration time is the primary reason many scientists have moved toward alternative techniques.

Dynamic methods for EMC determination 🔗

Dynamic methods were developed to overcome the slow pace of static techniques. Instead of letting still air and a food sample reach equilibrium passively, dynamic methods use moving air at a controlled humidity flowing over or around the sample. This accelerated airflow speeds up moisture exchange significantly, reducing measurement time from weeks to hours or days.

Dynamic Vapor Sorption (DVS) 🔗

Dynamic Vapor Sorption is the most prominent automated dynamic method used in food science today. A DVS instrument holds a small food sample (sometimes as little as 10 mg) on an ultra-sensitive microbalance inside a temperature-controlled chamber. Humidified or desiccated air is then passed over the sample at precisely controlled humidity levels.

The instrument works by blending dry air with water-saturated air using mass flow controllers to generate any desired relative humidity from 0% to about 98%. The sample’s mass is recorded continuously. Once the mass stabilises at a given humidity step, the instrument automatically moves to the next humidity level. This cycle continues until a full sorption and desorption isotherm is generated.

The key advantages of DVS are speed and automation. As noted by METTLER TOLEDO, modern DVS systems can study up to 23 samples simultaneously, dramatically reducing total measurement time. What might take months with static desiccators can be accomplished in 3 to 5 days using DVS. The continuous gravimetric data also provides kinetic information – not just the final EMC, but the rate at which the sample approaches equilibrium – which is valuable for modelling drying processes.

Flow-through gravimetric systems 🔗

Before fully automated DVS instruments became widespread, simpler flow-through systems were used. In these setups, air of known humidity is passed over a food sample at a controlled flow rate while the sample sits on a balance. The sample is weighed at intervals until the weight stabilises. This is less automated than modern DVS but still considerably faster than static desiccator methods.

Flow rate is a critical variable in these systems. If the air moves too slowly, the method loses its speed advantage over static approaches. If it moves too fast, turbulence may disturb the sample or create uneven humidity distribution around it, compromising accuracy.

Limitations of dynamic methods 🔗

Dynamic methods generally require more expensive equipment. A high-quality DVS instrument represents a significant capital investment, and maintenance and calibration demand trained personnel. There is also a subtle technical concern: because dynamic methods push the sample toward equilibrium faster, there is a risk that the instrument records a mass plateau that is not true thermodynamic equilibrium but rather a kinetic steady state. Careful validation against static reference data is therefore important.

The isotenoscope: measuring EMC through vapor pressure 🔗

Among the specialised instruments used for dynamic EMC assessment, the isotenoscope deserves particular attention. An isotenoscope is a device that measures the vapor pressure of a substance using a built-in manometer connected to a pressure measurement system and a vacuum pump.

In the context of food science, the isotenoscope works by measuring the vapor pressure of water within a food sample and comparing it to the vapor pressure of pure water at the same temperature. This ratio gives the water activity (a_w) of the food, which is directly related to EMC. If you know the water activity at a given temperature, you can locate the corresponding EMC on the moisture sorption isotherm.

The U.S. EPA’s guidelines for vapor pressure measurement describe the isotenoscope procedure as placing a liquid sample in a thermostated bulb connected to a manometer and vacuum pump. Dissolved gases are first removed through degassing at reduced pressure. The vapor pressure is then determined by balancing the sample’s vapor against a known inert gas pressure. While this protocol was developed primarily for chemical compounds, the same principle applies when assessing water vapor pressure in food systems.

Advantages of the isotenoscope approach 🔗

The isotenoscope provides a direct measurement of vapor pressure rather than relying solely on weight change. This makes it particularly useful for validating gravimetric EMC results and for situations where understanding the thermodynamic relationship between food moisture and its environment is important. It can also provide continuous, real-time data on how vapor pressure changes as the sample equilibrates.

Practical considerations 🔗

Isotenoscope measurements require careful sample preparation, thorough degassing, and precise temperature control. The equipment is more complex than a simple desiccator setup and demands operator expertise. For routine EMC determination in food processing, DVS instruments have largely taken over this role. However, the isotenoscope remains a valuable research tool where direct vapor pressure data is needed.

Choosing the right method 🔗

The choice between static and dynamic methods depends on several practical factors.

Budget and equipment access: Static methods using saturated salt solutions require minimal investment. If a laboratory is setting up for the first time or operates on a tight budget, the desiccator method is a sensible starting point. Dynamic methods like DVS require a substantial initial purchase but pay off in throughput and data richness over time.

Time constraints: If you need EMC data quickly – for instance, during new product development where many formulations must be screened – dynamic methods are the clear choice. A DVS instrument can produce a complete sorption isotherm in 3 to 5 days, compared to weeks or months with static methods.

Data requirements: If you only need EMC at a few specific humidity levels, a static method may be sufficient. If you need a full isotherm with kinetic data, diffusion coefficients, or hysteresis curves, DVS is far more informative.

Sample characteristics: Sample size and form matter. Grinding a food sample increases surface area and speeds equilibration in both static and dynamic methods, but it can also alter the material’s structure in ways that affect moisture behaviour. Thicker or denser samples will always take longer to equilibrate, regardless of the method used.

The role of EMC data in food processing 🔗

Accurate EMC determination directly influences several stages of food processing. In drying operations, EMC tells the processor when to stop – drying below the EMC for the intended storage conditions wastes energy without improving stability. In packaging, knowing the EMC helps engineers select barrier materials that maintain the desired internal humidity. In storage, EMC data combined with ambient climate information predicts whether a stored product will gain or lose moisture over time.

The concept of hysteresis – the phenomenon where adsorption and desorption isotherms do not overlap – also comes from EMC measurements. A food sample that reaches a certain EMC by losing moisture (desorption) may have a different EMC than the same food reaching the same humidity level by gaining moisture (adsorption). This difference has real implications for predicting product stability, particularly when foods move between different storage environments.

Modern advances in EMC measurement 🔗

Research continues to improve both the speed and accuracy of EMC determination. One promising approach involves using mathematical modelling to estimate EMC from short-duration experimental data, potentially reducing even static method timelines. The technique requires only an initial moisture measurement and two subsequent data points, then uses an empirical sorption model to predict the final equilibrium value.

Other researchers are exploring infrared spectroscopy and equilibrium relative humidity sensors as faster, field-deployable alternatives – particularly relevant for small-scale farmers in developing countries who need quick, affordable moisture assessment tools.

Neural network models have also been developed to predict EMC across different food products based on temperature and humidity inputs, reducing the need for extensive experimental work once the model is trained on sufficient data from diverse food types.

What do you think? If you were designing a food drying operation for a product with strict texture requirements, which EMC determination method would you prioritise – and how might the choice between static and dynamic methods affect both your timeline and final product quality?

References

1. https://en.wikipedia.org/wiki/Equilibrium_moisture_content
2. https://www.sciencedirect.com/science/article/pii/S0924224418304898
3. https://www.sciencedirect.com/science/article/abs/pii/S0023643820308215
4. https://www.researchgate.net/publication/317573048_Different_methods_of_equilibrium_moisture_content_determination
5. https://en.wikipedia.org/wiki/Dynamic_vapor_sorption
6. https://www.mt.com/us/en/home/perm-lp/market-organizations/usa/DynamicVaporSorption.html
7. https://en.wikipedia.org/wiki/Isoteniscope
8. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-R/part-796/subpart-B/section-796.1950
9. https://particletechlabs.com/analytical-testing/dynamic-vapor-sorption/
10. https://www.tandfonline.com/doi/full/10.1080/10942912.2011.582974