Water activity (Aw) and coffee bean moisture: How do these two indicators differ?

In green coffee storage, moisture content and water activity are often discussed together. However, these two parameters do not measure the same property. Moisture content indicates the amount of water present in the beans, while water activity (symbolized as aw) reflects the thermodynamic state of water through its equilibrium vapor pressure relative to the sample.

Correctly distinguishing between these two indicators helps producers, processing units, roasters, and exporters assess raw materials more accurately. A standalone moisture measurement cannot replace a water activity measurement, and vice versa.

1. What does coffee bean moisture measure?

Coffee bean moisture content is the ratio of the amount of water in a sample to either the total sample mass or the dry matter mass, depending on the reporting method. In the green coffee trade, moisture content is typically expressed as a percentage by mass.

When results are reported on a wet basis, the formula is:

Where:

• m_water is the mass of water in the sample.
• m_sample is the total mass of the sample, including water and dry matter.
• M_wb is the moisture content calculated on a wet basis.

Calculation example: If a 100 g coffee sample contains 11 g of water, the wet basis moisture of the sample is 11%. This is an illustrative arithmetic example, not a measurement result from an actual coffee lot.

Moisture content indicates the amount of water in the raw material but does not directly reveal how the water interacts with the bean structure, nor does it fully indicate the availability of water for chemical reactions or microbial activity.

2. What does Water Activity (Aw) represent?

Water activity is a dimensionless quantity defined as the ratio of the partial equilibrium vapor pressure of water above the sample to the saturation vapor pressure of pure water at the same temperature.

Where:

• p is the equilibrium vapor pressure above the sample.
• p₀ is the saturation vapor pressure of pure water at the same temperature.
• aw is the water activity, which ranges from 0 to 1 within the standard definition.

At equilibrium, water activity is related to the equilibrium relative humidity of the air in a sealed chamber:

For example, aw = 0.60 corresponds to an equilibrium relative humidity of 60% under equilibrium measurement conditions. This does not mean the sample contains 60% water. Water activity is also not the percentage of free water determined directly by weighing.

Aw reflects the energy state of water in the material system. Water interacts with components of the bean, such as polysaccharides and proteins, and may have varying levels of availability for chemical and biological processes. Therefore, water activity is useful when assessing the stability of food and agricultural products during storage.

3. Comparison of moisture content and water activity

This difference is documented in the scientific literature on water activity: two foods with the same moisture content can still have different water activities due to differences in composition and how water interacts with the material matrix.

4. Illustrating the two metrics in an easy-to-understand way

The figure below is a conceptual diagram developed by SAVINA to explain the two measurements. This is not experimental data and cannot be used to infer a quantitative relationship between moisture content and Aw of coffee.

5. Why can two coffee samples with the same moisture content still have different Aw values?

Water does not exist merely as a homogeneous liquid quantity within the bean. Water interacts with chemical groups in the material, pore structure, and water-holding components. Therefore, total water content does not uniquely determine water activity.

In research and storage control, the relationship between moisture content and Aw is often investigated through moisture adsorption or desorption isotherms. This curve describes the relationship between water content and water activity at a defined temperature.

A universal formula should not be used to directly convert moisture content to Aw for all types of coffee. To determine this relationship, experimental data appropriate to the sample type, processing state, temperature, and measurement method are required.

6. How is water activity related to the storage of green coffee beans?

Green coffee beans can absorb and release moisture according to environmental conditions. When the ambient temperature and relative humidity change, the equilibrium moisture state of the beans may also change. Therefore, measuring moisture content only at the time of warehouse entry is insufficient to fully assess the behavior of the raw material during storage.

Studies on the storage of green and roasted coffee have examined changes in water activity and quality attributes under different storage conditions. The results indicate that environmental conditions, packaging, moisture content, and Aw must be considered simultaneously when evaluating the ability to maintain quality.

Aw is relevant to the risk of microbial growth, but it is not a direct test to determine whether a sample is contaminated with mold or mycotoxins. Aw results must be interpreted alongside processing history, storage duration, temperature, humidity, packaging condition, and other necessary tests.

For green coffee, a single Aw threshold should not be adopted as a storage standard for all varieties, processing methods, and storage conditions without specific justification. General food safety thresholds do not automatically become quality standards for coffee.

7. How to correctly measure moisture content and Aw?

Moisture content measurement

Moisture content can be determined by oven-drying using an appropriate procedure or by calibrated measuring instruments. When comparing results between entities, the test method, calculation basis, and sample preparation conditions must be verified. Rapid measurement devices also require periodic checks to limit errors.

Water activity measurement

According to ISO 18787:2017, principles for measuring Aw include dew point measurement or determining changes in the electrical properties of a sensor. Measurements must be performed according to the device instructions, with temperature control and allowing the sample to reach equilibrium as required by the method.

Aw should not be inferred solely from the results of a conventional moisture meter, unless the device has a dedicated Aw measurement function and that method has been validated as appropriate.

8. Practical control procedure for coffee warehouses

1. Representative sampling: Sample according to a procedure appropriate for the batch, avoiding selection of only a few beans from the surface of the bag.
2. Moisture content measurement: Clearly record the result, unit, method, and batch code.
3. Measure Aw as needed: Use specialized equipment, control temperature, and follow standard sample preparation procedures.
4. Record storage conditions: Monitor temperature, ambient relative humidity, packaging condition, and signs of condensation.
5. Evaluate over time: Compare data from the same lot to detect trends in change rather than relying on a single measurement.
6. Inspect when abnormalities are detected: If mold or spoilage is suspected, conduct appropriate assessments and tests; do not draw conclusions based solely on Aw.

Storing data by batch code helps cross-reference raw material quality across different time points and supports traceability. You can refer to additional professional content in the SAVINA Coffee Knowledge Library to build a raw material quality control documentation system.

References

1. International Organization for Standardization (ISO). ISO 18787:2017, Foodstuffs — Determination of water activity. This standard specifies the principles and method requirements for determining water activity in food. View ISO standard.
2. U.S. Food and Drug Administration (FDA). Water Activity (aw) in Foods. This document explains the definition of water activity, equilibrium relative humidity, sorption properties, and the role of Aw in food stability. Read FDA document.
3. U.S. Department of Agriculture, Agricultural Research Service (USDA ARS). Water Activity in Food. A technical document on the difference between water activity and moisture content, and its application in assessing microbial growth risks. Read USDA ARS document.
4. Effect of green and roasted coffee storage conditions on selected characteristic quality parameters. A study on the impact of storage conditions on water activity and quality parameters of green and roasted coffee. Read full study.
5. Anokye-Bempah, L. et al. (2023). The use of desiccants for proper moisture preservation in green coffee during storage and transportation. Journal of Agriculture and Food Research, Vol. 11, Article 100478. A study on fluctuations in moisture, water activity, and green coffee quality during storage and transportation. DOI: 10.1016/j.jafr.2022.100478.