Instant coffee's aroma isn't a single compound but a system of substances with different volatility and different degrees of interaction with the powder matrix. When the extract is dried, part of these substances can escape with water vapor, part is retained in the solid matrix, and part can transform. So the ability to retain volatile compounds is simultaneously a thermal and a structural problem.

Retaining volatile compounds doesn't mean keeping the aroma profile unchanged

Spray-drying has an advantage in throughput and drying speed. However, the extract droplet contacts hot gas and quickly forms a particle. The degree of aroma loss depends on droplet size, solids concentration, gas temperature, evaporation rate, and shell-formation ability. Gas inlet temperature alone can't represent the material's true thermal history.

Where do spray-drying, freeze-drying, and spray-freeze-drying differ?

So a valid comparison must start from the same feed liquid and track the same compound group, rather than treating two commercial products as representative of two technologies.

Freeze-drying creates the product at lower temperature during the primary water-removal stage, so it's usually chosen when the goal is preserving sensory quality. But the process is longer and uses more energy. Coffee research shows freezing and drying conditions can affect pore structure, and that structure determines how aroma is retained.

Spray-freeze-drying is a notable case because it combines droplet formation with rapid freezing before sublimation. A study on instant coffee reported higher volatile-compound retention in spray-freeze-dried coffee compared with freeze-dried and spray-dried samples in the same study, while the product also had porous structure and good reconstitution characteristics. This is evidence that

Powder structure determines how much aroma is retained and released

Assessing volatile-compound retention using gas chromatography-mass spectrometry or similar analytical techniques allows identifying which compounds change. However, the number of volatile compounds doesn't automatically predict aroma sensation. A low-content compound can still have a low odor threshold and a large effect on perception. So chemical data needs to be placed alongside sensory evaluation.

Powder structure also determines the aroma released during brewing. A particle that's too closed can retain compounds but release them slowly; a particle that's too porous can favor reconstitution but also increase exchange with the environment during storage. This is a balancing act between retention ability in packaging and release ability during use.

Designing a comparison between drying technologies

When comparing technologies, you should keep the raw material and feed liquid the same as much as possible, while recording key variables of each process. Indicators include volatile-compound content, moisture, density, particle size, structure, and sensory evaluation. If only the technology changes without controlling feed concentration or post-drying storage conditions, results can easily become confounded.

The ability to retain volatile compounds must therefore be understood as an attribute of the entire powder-forming process. Drying technology, thermal history, particle structure, and packaging all take part in determining how much aroma remains when the product reaches the user. The most reliable way to assess this is tracking chemistry, physics, and sensory evaluation simultaneously rather than choosing a single retention percentage as the criterion.

A factor often overlooked is oxygen and contact time after drying. Aroma compounds can keep changing during storage, so a sample taken right out of the dryer and one after several weeks don't represent the same state. When comparing technologies, the time from production to analysis must be locked.

Post-drying loss and the role of packaging

Particle size also affects the rate of exchange with the environment. Small particles have large surface area and can release or absorb substances faster. Large particles may retain aroma better under some conditions but have different reconstitution behavior. So the ability to retain volatile compounds should be analyzed together with particle size.

A multi-technology comparison experiment should standardize the feed liquid, solids level, and post-drying conditions. If each technology uses a different concentration, the aroma difference could stem from the liquid's composition rather than the drying mechanism. This is why designing proper controls matters just as much as the analytical equipment.

Finally, retention needs to be assessed at the level of meaningful compounds or compound groups. Total volatile-compound amount is a combined index but doesn't reveal which compound was lost. Combining chromatograms with sensory description can help determine whether a chemical change creates a difference users actually notice.

Some notes when reading the data

Packaging has a particular role for volatile compounds because material, seal integrity, and headspace in the pack can affect exchange. When comparing two drying technologies, you need to use the same packaging system and storage duration.

In a long-duration test, sampling time should be split into at least two states: right after drying and after a predetermined storage period. The difference between the two time points helps distinguish loss occurring in the dryer from change occurring afterward. This is necessary information when the goal is comparing technology rather than just describing the final product.

Packaging also determines the rate of exchange of aroma once the powder has stabilized. Materials with different gas-barrier properties, different seal integrity, or different remaining air in the pack all create different contact conditions. So assessing aroma retention over time needs to lock the packaging system, filled mass, and storage conditions.

References

• Ishwarya & Anandharamakrishnan (2015), Spray-Freeze-Drying approach for soluble coffee processing and its effect on quality characteristics, Journal of Food Engineering.
• Comparative Evaluation of Volatile Aroma Compounds Content of Freeze-Dried and Spray-Dried Instant Coffees (2024), Burapha Science Journal.
• Influence of freeze-drying parameters on the retention of flavor compounds of coffee (1973), Journal of Food Science.
• Zanin et al. (2021), Food Chemistry, 341, 128193.