Spray-drying turns coffee liquid into small droplets, then dries them quickly in a hot air stream. Each droplet's thermal exposure time is short, but the inlet air temperature can be high, and drying creates a sharp change in surface solute concentration. The aroma profile of the resulting powder is therefore the result of a balance between heat transfer, mass transfer, and the particle structure that forms.

From liquid droplet to powder particle: where does aroma change?

Aroma compounds don't behave the same way during drying. Some highly volatile compounds can escape the droplet along with water vapor, while others get retained within the solid matrix. As the droplet quickly forms an outer dry layer, diffusion of volatiles from inside to outside continues, but the rate changes with the particle's state.

Droplet size is an important variable. Small droplets have a short diffusion path and large surface area, so they dry fast. But large surface area also increases the opportunity for exchange with the gas phase. Solids composition and the rate of shell formation also affect whether aroma compounds stay trapped in the particle or escape. This is why process design needs to consider all these factors together.

Droplet size and drying conditions govern aroma retention

Because each liquid droplet is its own micro-drying environment, differences in droplet size and shell-formation rate can change the path aroma compounds take before the powder particle forms.

A study comparing spray-drying, freeze-drying, and spray-freeze-drying for instant coffee found clearly different levels of volatile-compound retention between methods. Spray-freeze-drying in that study retained a higher proportion of volatiles than the control samples. This result doesn't mean one method is always best for every product; it shows particle structure

Aroma profile also shouldn't be assessed by total volatile amount alone. Two samples can have a similar total but different ratios between odor-forming compounds, leading to different perception. So gas chromatography-mass spectrometry or similar analytical methods can provide a chemical map, while sensory analysis helps determine whether those changes are meaningful.

Aroma profile must be read by composition, not just total amount

Besides aroma, spray-drying also has to meet requirements for moisture, density, flowability, stability, and solubility. A condition that reduces aroma loss but creates sticky or hard-to-package powder isn't necessarily a good production condition. So process design should use multiple responses rather than optimizing a single indicator in isolation.

Optimizing spray-drying is a multi-attribute problem

When building process data, you should record inlet and outlet air temperature, feed rate, solids concentration, air flow rate, droplet size if measurable, powder moisture, and aroma indicators. If possible, sample at different points to determine which compounds change during the process. Data like this helps distinguish loss due to evaporation from change due to thermal reaction.

Spray-drying, then, shouldn't be described simply as a fast drying method. For instant coffee, it's a process that simultaneously forms particles, removes water, and redistributes volatile components. To control the aroma profile, you need to look at droplet kinetics and product structure alongside chemical and sensory data. This is the basis for choosing drying conditions matched to the goal.

The feed liquid's solids concentration also affects particle formation. A dilute liquid has more water to remove, while a more concentrated liquid can form a shell quickly and change the moisture-diffusion path. So the same drying-air regime but a different liquid concentration can create particles with different structure and aroma profile.

Post-drying control and scale-up

Post-drying conditions also need to be separated from equipment effects. Powder can lose more aroma during holding, transport, or sitting in a hopper before packaging. If samples are only taken right after the equipment and compared with stored commercial product, conclusions about spray-drying's effectiveness won't be fair. Sampling timing needs to be standardized.

A deeper direction of analysis is linking marker compounds to perceived odor regions. When a compound group decreases, you need to determine whether that change exceeds the perception threshold. This approach helps avoid treating every chromatographic change as a quality problem of equal severity.

In practice, the optimal condition is usually an operating region rather than a single point. You can define a range of temperature, feed rate, and liquid concentration within which moisture, throughput, and aroma profile all fall within acceptable limits. This approach suits production better since raw material and equipment always carry a certain level of variation.

Powder moisture and product temperature also need to be tracked alongside the aroma profile. As water decreases toward the end of the process, product temperature can rise faster, changing the actual thermal history. So inlet air temperature alone can't fully represent the level of impact on aroma compounds.

Some notes when reading the data

At industrial scale, heat and residence-time distribution aren't perfectly uniform. Part of the product may pass through different conditions than the rest. Representative sampling and checking repeatability between batches are therefore important when assessing aroma profile.

References

• Ishwarya & Anandharamakrishnan (2015), Spray-Freeze-Drying approach for soluble coffee processing and its effect on quality characteristics, Journal of Food Engineering, 159.
• Influence of drying techniques on sensory profile and chlorogenic acid content of instant coffee powders (2022), Measurement: Food, 6, 100030. DOI: 10.1016/j.meafoo.2022.100030.
• Zanin et al. (2021), Modulation of aroma release of instant coffees through microparticles of roasted coffee oil, Food Chemistry, 341, 128193.
• Volatile retention in the formation and dehydration of food powders, Loughborough University doctoral research.