Evaporation in an instant-coffee line has the job of removing water from the extract before drying. Looking only at the mass balance, this is a process of reducing water and increasing solids ratio. But coffee liquid isn't an ideal solution containing a single solute. Many compounds have different volatility, thermal stability, and interaction ability, so the composition of the concentrate can change in more than one way.

How does evaporation change the concentrate?

The first effect to distinguish is increased concentration due to water loss. If a substance is fully retained in the system, reducing water mass increases that substance's concentration. This is a ratio change, not creation of new matter. When assessing data, you need to convert concentration to absolute amount to know whether a substance was actually lost.

Temperature and pressure determine the liquid's thermal history

So composition change in the concentrate needs to be read together with thermal history and stream state. If you only compare initial and final concentration, the mechanism in between gets obscured.

Temperature is the second variable. Evaporation needs to supply energy for water to change phase, but the temperature the product experiences depends on pressure, equipment design, and residence time. Vacuum concentration can lower boiling point, but that doesn't mean every heat-sensitive component is fully protected. A stream at moderate temperature but with long residence time can still

Volatile compounds can follow the vapor stream

Volatile compounds are the group needing attention. As water evaporates, some aroma substances can follow the vapor stream. Without a recovery system, this is a form of quality loss. Research on coffee extraction and drying methods shows the ability to retain aroma compounds depends heavily on process conditions, so the concentration step shouldn't be overlooked when tracing aroma

Rising concentration brings changes in viscosity and heat transfer

Liquid composition also changes in physical properties. As solids increase, viscosity usually increases and heat-transfer ability can decrease. This change makes the final stage of a batch different from the early stage. If a single nominal temperature or pressure is used to describe the whole process, the data may not reflect the product's actual history.

To track composition change, you should sample at multiple points of the process. Each sample needs solids, stream mass, and target compounds like caffeine, chlorogenic acids, or aroma markers depending on the research question measured. When calculating absolute amount, you can determine how much change is due to water loss and how much is due to matter loss.

Process sampling to distinguish concentration from composition change

Evaporation design also needs to balance speed and quality. Increasing concentration level too fast can create high thermal load or make the liquid hard to pump. Under-concentrating leaves too much water for the subsequent drying step to remove. So final concentration shouldn't be chosen independently of the following drying technology.

Evaporation, then, is a composition-changing step, not just a water-reduction step. To control it, you need to read mass balance, thermal history, viscosity, and volatile-compound loss simultaneously. Only when these factors are tracked can a business know whether a change in final composition comes from concentration alone or from the operating conditions themselves.

Final concentration must suit the downstream step

An important point is that increased concentration changes the liquid's heat-transfer properties. As viscosity increases, the boundary layer near the heating surface can thicken and the heat-transfer coefficient decreases. This can extend residence time, which in turn increases the risk of composition change. These two mechanisms form a feedback loop that needs consideration when designing equipment.

If equipment operates under vacuum, pressure must be recorded alongside product temperature. Boiling point isn't a fixed value for every batch since liquid composition changes with concentration. Pressure-temperature data helps determine the true thermal history and serves as a basis for comparing batches.

Concentration can also affect subsequent steps through viscosity and particle-forming ability. Too-thick liquid can be hard to pump or spray, while too-thin liquid increases the load on the drying equipment. So evaporation's final concentration must be determined based on the requirements of the next step.

When researching, you should sample over time rather than only measuring input and output. This data series shows when solids rise fast, when viscosity starts changing sharply, and whether the target compound fluctuates at a specific stage. This is the basis for adjusting the operating profile.

Some notes when reading the data

A useful evaluation approach is building a balance between water removed, energy used, and target-compound change across each concentration interval. This helps identify the point where one additional unit of concentration starts requiring longer thermal history or creating unfavorable viscosity. At that point, the technology limit can be determined from data rather than an assumed value.

In the final stage of concentration, sampling by concentration interval can be more useful than sampling at fixed time intervals. As the liquid becomes more concentrated, the rate of property change can increase. Denser sampling around the strong-change region helps pinpoint precisely where viscosity, heat transfer, or composition starts becoming unfavorable.

References

• European Commission JRC, Food, Drink and Milk Industries BREF, Instant coffee section.
• Fryer et al. (2020), Industrial & Engineering Chemistry Research, 59, 11099–11112.
• Evaluation of the quality of coffee extracts concentrated by osmotic evaporation (2017), Journal of Food Engineering.
• Effect of concentration and temperature on the physical and thermophysical properties of coffee extract (2023), Journal of Food Engineering, 340, 111304.