In a concentrated-coffee line, a liquid can shift from easy-flowing to hard-to-transport as concentration changes. So rheology needs to be studied as a property that varies with conditions, not as a fixed number.

What rheology is and why coffee liquid needs its own measurement

Rheology describes how a liquid responds when subjected to a force making it flow. For concentrated coffee, this isn't a secondary product characteristic but data directly relevant to transport, heat exchange, and equipment selection. As water decreases, solute concentration rises and interactions between components change.

An important point is that viscosity isn't always a constant. In a region with plenty of water and high temperature, the liquid can behave close to Newtonian; as water decreases or temperature drops, power-law behavior may fit better. So the question to ask isn't just “how much is the viscosity” but under what conditions it was measured.

Concentration changes how the flow responds to force

As the liquid gradually concentrates, solute amount per unit volume increases and viscosity usually increases. The increase may not be linear, especially in the high-concentration region. Studies on the physical properties of coffee extract have measured across multiple concentration and temperature levels to build predictive relationships. This shows a single measurement isn't enough.

The basis for expressing concentration also needs to be consistent. Brix, dry-matter percentage, and water content aren't fully equivalent ways of recording in every context. When combining data from multiple experiments, you need to clearly determine what basis each value was calculated on. Otherwise, part of the difference can come from how

Temperature affects both viscosity and the process simultaneously

Increasing temperature usually makes the liquid flow more easily, but in production heat doesn't only affect viscosity. It also changes heat transfer, residence time, and the rate of chemical reactions. So using heat to reduce flow resistance must be weighed against the thermal history the product receives.

Coffee-liquid data shows thermophysical properties depend simultaneously on concentration and temperature. So a data table serving design should cover both variables rather than measuring only at room temperature. A model can be correct at 25 °C but insufficient to predict behavior when the liquid passes through heating equipment.

From flow curve to pressure drop

When liquid travels through a pipe, viscosity and flow characteristics partly determine pressure drop. Research on coffee-extract fluid dynamics has combined rheological parameters with pressure-drop data to calculate friction factors. This is an important bridge between lab measurement and pipe design.

If you only report a single viscosity value without recording shear rate, the data is hard to convert into equipment parameters. Two samples can have the same apparent viscosity at one condition but differ when shear rate changes. So measurement should produce a curve wide enough to cover the actual operating region.

When a non-Newtonian model is needed

A non-Newtonian model needs consideration when apparent viscosity changes with shear rate in the region where equipment operates. This is especially notable at pumps, valves, and narrow gaps, where deformation rate can be much higher than in a static measurement.

You shouldn't choose a model just because it gives a high correlation coefficient. The model needs testing under conditions similar to the equipment and must predict a meaningful quantity, such as pressure drop. If it's only correct within the measured region but wrong when extrapolated, the model creates risk for design.

Designing measurements so data can be compared

A rheological measurement should clearly record concentration, temperature, equilibration time, instrument geometry, and shear-rate program. For liquids that can change over time, the time from sampling to measurement also needs controlling. These details determine whether two datasets are genuinely comparable.

Instead of measuring only one concentration, you can build data across a range of concentrations and temperatures. This approach allows recognizing the region where viscosity rises quickly and supports determining operating limits. When the data is linked to flow rate and pressure drop, the measurement's value increases markedly.

In line design, the most useful rheological data range is the one covering exactly the concentration, temperature, and shear rate at which the equipment actually operates.

References

• Telis-Romero et al., Rheological Properties and Fluid Dynamics of Coffee Extract, Journal of Food Process Engineering.
• Silva et al., Effect of concentration and temperature on the physical and thermophysical properties of coffee extract, Journal of Food Engineering, 2023.