After the drying step, instant coffee powder still contains a certain amount of water. This number is usually used as a control indicator, but moisture percentage alone doesn't fully describe the state of water within the powder. The same total water amount can create different levels of mobility depending on composition and material structure. So assessing stability needs to look beyond a single percentage.

What does post-drying moisture tell you?

In dry food material, part of the water is bound relatively tightly to polar components, while more mobile water can take part in physical and chemical processes. When powder absorbs moisture from the environment, water can change the state of the amorphous matrix, increase molecular mobility, and promote unwanted changes.

Post-drying moisture also relates to particle structure. Highly porous particles can exchange moisture faster than dense particles. If the powder has large surface area, packaging and environmental conditions become part of the control system. A study on vacuum-belt-dried coffee showed instant coffee's sorption characteristics can be described using the GAB sorption isotherm.

From water in the particle to caking and change risk

So the same moisture number can lead to different behavior if particle structure and the state of water within the powder matrix differ.

As moisture rises, powder can lose flowability, cake, or change reconstitution time. Some chemical changes can also be accelerated if water increases the mobility of reactive species. For coffee, the issue also relates to aroma decline and sensory change. So a moisture limit shouldn't be chosen just because it's a commonly used number in the industry.

To set a reasonable limit, you need to track samples at multiple moisture levels and temperature/relative-humidity conditions. Indicators can include moisture, water activity, flowability, caking level, reconstitution time, color, volatile compounds, or sensory evaluation. When change curves are placed side by side, you can identify the region where the product starts changing rapidly instead of relying on a fixed number.

Establishing a moisture limit with stability data

Moisture also depends on the measurement method. Results from oven drying, infrared devices, or other methods may not be fully equivalent if measurement conditions differ. So quality control needs to standardize method, temperature, time, and sample preparation. If a method changes without checking the correlation, a change in the reading could be misinterpreted as a change in the product.

Packaging is the next part of the problem. Dry powder can absorb moisture if the packaging material's vapor barrier is inadequate. In that case, reducing moisture at the end of the line only solves the problem at one point in time. Stability data must be used to determine the rate of moisture increase over time and the packaging's protective ability under intended conditions.

So post-drying moisture should be seen as an input variable of the stability problem, not a final conclusion. It has the most meaning when linked to water activity, powder structure, packaging, and storage time. This approach helps distinguish a powder with low moisture but prone to absorbing moisture from one with more stable water state under the same environmental conditions.

Packaging and warehouse temperature can change the outcome

The sorption isotherm is a useful tool for describing how powder exchanges moisture with air. Knowing the relationship between equilibrium moisture and relative humidity lets you predict whether the product will tend to absorb or release water under storage conditions. This helps choose packaging based on data rather than relying only on initial moisture.

Another issue is warehouse temperature. The same packaging and same initial moisture but different temperature can change the rate of chemical reactions and the rate of water movement. So stability testing should have controlled temperature and humidity conditions, with sample history clearly recorded.

Moisture also relates to the strength of particle structure after packaging. If water turns the amorphous matrix more mobile, particles can soften and stick together. At that point, the issue isn't just appearance but also the ability to dose, dissolve, and distribute the product within the package.

A good limit needs to be tied to the attribute being protected

A trustworthy moisture limit needs to originate from the point where the attribute being protected starts changing beyond an acceptable level. If the product stays stable across a wide range, there's no need to set an overly tight limit, since that would increase drying cost without a corresponding benefit.

An accelerated moisture-sorption test can be used to determine what level of vapor barrier the packaging needs. Samples are placed at controlled relative humidity and mass is tracked over time. From the rate of moisture increase, you can estimate the level of change over the intended storage period.

You also need to distinguish powder stability from whole-package stability. A powder that's stable when left open in the lab can still decline quickly if the packaging has poor seal integrity. So final-product testing should also simulate packaging conditions.

References

• Impact of Environmental Humidity on Instant Coffee Stability: Defining Moisture Thresholds for Quality Degradation and Shelf Life Prediction (2025), Foods, 14, 1826.
• Instant Coffee, ScienceDirect Topics, Storage and Stability section.
• Peleg (2020), Humidity Caking and Its Prevention, Water Activity in Foods, Wiley.
• Alternative flexible plastic packaging for instant coffees (2023), Food Research International.