When coffee liquid is concentrated, the amount of dissolved solids increases and flow properties usually change accordingly. One easily noticeable change is increased viscosity, but the increase isn't necessarily linear. Solids composition, temperature, and the structure of interactions between molecules all contribute to determining how the liquid flows. This is why solids concentration needs to be considered

How does solids concentration change flow properties?

At low concentration, the liquid can flow relatively easily with favorable heat transfer. As concentration increases, flow resistance can become greater, especially with structure-forming components or strong water interactions. If pumping equipment is designed for dilute conditions but must handle thicker liquid, flow rate can decrease and system pressure increases.

Viscosity relates to temperature and concentration

To use this relationship for equipment design, you need to move from a single measurement to a data map across concentration and temperature. This is the foundation for choosing pumps, calculating heat transfer, and determining operating limits.

Viscosity also affects heat transfer. The thicker the liquid, the more restricted convective motion becomes, reducing the ability to bring heat from the heating surface into the whole stream. This can create temperature gradients within the equipment and change the evaporation rate. In a concentration system, the final concentration is therefore usually a harder-to-operate region than the beginning of the process.

Flow properties also relate to pump and pipe selection. You need to know not just viscosity at one temperature but also its change with temperature and concentration. If the liquid shows non-Newtonian behavior in a certain condition range, the relationship between stress and shear rate must be measured rather than assuming the liquid is always Newtonian.

Flow data must be sufficient for calculating pumping and heat transfer

Solids concentration also affects the subsequent drying step. Too-dilute liquid increases the amount of water that must be removed, extending the load on the drying equipment. Too-concentrated liquid can be hard to pump, hard to spray, or create conditions for uneven structure formation. So a suitable concentration is the intersection between liquid-handling ability and powder-forming requirements.

To control this, solids concentration needs to be measured with a consistent method and recorded alongside measurement temperature. If using Brix as an indicator, remember Brix is a conventional measure for dissolved solids and doesn't always accurately reflect coffee's complex total solid composition. In process research, Brix can be combined with a dry-matter determination method

Viscosity should be measured across multiple temperatures and multiple concentration levels rather than at a single point. This data can build relationships useful for calculating pumping, heat exchange, and residence time. As equipment scale changes, this data has more value than a single parameter since it allows predicting liquid behavior under new conditions.

Concentration limits must be determined from multiple stages

Solids concentration and flow properties therefore form a pair of variables linking extraction, concentration, and drying. A change in concentration doesn't stop at changing dry-matter content; it can change pumping, heat transfer, spray ability, and the energy of the whole line. Tracking this relationship helps choose a suitable concentration point instead of pushing concentration up to

The relationship between concentration and viscosity can change strongly with temperature. A liquid concentrated at low temperature may pump more easily when heated, but if too much heat is used, the benefit of reduced viscosity can come with unwanted quality effects. So flow parameters need to be measured within the actual operating temperature zone.

For heat-exchange equipment, flow properties determine size and operating regime. If calculating for concentrated liquid based on dilute-liquid data, heat-transfer capacity can be overestimated. Viscosity and flow-rate data at multiple concentration levels help model real behavior more closely.

Flow properties also affect spray ability at the drying step. High viscosity can change droplet size, particle morphology, and drying rate. So the concentration limit before spray-drying isn't determined by pumping alone but also by the drying equipment's particle-forming requirements.

When the liquid behaves non-Newtonian, the measurement approach must change

One way to build a standard is determining a concentration range where the liquid remains pumpable, heat transfer stays stable, and the dried product meets desired properties. This range can be wider than a single target value and therefore fits raw-material's natural variation better. What matters is that the limit must be demonstrated with operating data.

If the liquid is non-Newtonian, apparent viscosity depends on shear rate. In that case, a measurement at a fixed rotation speed can't represent every position in the pipe. You need to choose a shear-rate range corresponding to the pump, valve, and nozzle.

Building an operating range instead of a single concentration value

Solids concentration can also affect droplet size during spraying. A more concentrated liquid creates different conditions for dispersion and drying compared with dilute liquid. So rheological data should be linked with spray-drying data if the goal is determining the optimal feed concentration.

In large-scale research, concentration and viscosity data should be collected on the same batches so an empirical relationship between solids, temperature, and flow properties can be built. Once this relationship is confirmed, it can support choosing a concentration point and predicting equipment load under conditions not directly tested.

References

• Telis-Romero et al. (2001), Rheological properties and fluid dynamics of coffee extract, Journal of Food Process Engineering, 24, 217–230.
• Sobolík et al. (2002), Viscosity and electrical conductivity of concentrated solutions of soluble coffee, Journal of Food Engineering, 51, 93–98.
• Effect of concentration and temperature on the physical and thermophysical properties of coffee extract (2023), Journal of Food Engineering, 340, 111304.
• European Commission JRC, Food, Drink and Milk Industries BREF, Instant coffee section.