The research, led by Jiraporn Sripinyowanich Jongyingcharoen of King Mongkut’s Institute of Technology Ladkrabang in Thailand together with Pattawee Wutthigarn, Suluh Pambudi, and Ekkapong Cheevitsopon, is the first to apply superheated steam fluidized bed drying to coconut press cake. The team compared this approach head-to-head with conventional hot air fluidized bed drying at three temperatures: 130, 145, and 160 degrees Celsius. The choice of technology matters because coconut press cake is a challenging material. Its small particle size and fibrous, lipid-rich structure make it prone to shrinkage, discoloration, and oil entrapment during drying, while its high moisture content demands aggressive moisture removal to reach a shelf-stable state. The researchers targeted a final moisture content of 0.03 grams of water per gram of dry matter, the level at which water activity drops to roughly 0.75, suitable for long-term storage.

Fluidized bed drying works by blowing a gas upward through a bed of particles fast enough that the particles become suspended and behave like a fluid. This ensures intimate contact between the drying medium and every particle, producing uniform, rapid moisture removal. The critical parameter is the minimum fluidization velocity, the gas speed at which the upward drag on the particles exactly balances their weight. In a clever twist, the team found that superheated steam actually made the cake easier to fluidize than hot air. At 130 degrees Celsius, the minimum fluidization velocity in steam was 3.70 meters per second, well below the 4.65 meters per second needed for hot air, because steam at that temperature is less dense and less viscous than air. As temperature rose, the viscosity of steam increased and its fluidization advantage narrowed, an effect the authors attribute to the competing influences of declining density and rising viscosity on the drag force.

The drying performance results were striking. Superheated steam fluidized bed drying consistently outpaced hot air, and the advantage was most dramatic at the lowest temperature tested. At 130 degrees Celsius, steam drying reached the target moisture in just 9 minutes, half the 18 minutes required with hot air, with a significantly higher average drying rate. Even at 160 degrees Celsius, the fastest condition overall, steam still edged out hot air, finishing in 5.5 minutes versus 6 minutes. The authors explain that superheated steam transfers heat more efficiently and creates an oxygen-limited environment that prevents the surface hardening and crust formation that can trap moisture inside particles. By comparison, conventional hot air oven drying of coconut press cake at 50 to 80 degrees Celsius has previously been reported to take anywhere from 70 to 565 minutes, highlighting the enormous throughput gains that high-temperature fluidization delivers.

Energy consumption told a similarly compelling story. The researchers measured the specific energy consumption, the electrical energy required per kilogram of moisture removed, using a power meter connected to the entire drying system during active drying. For both methods, energy consumption fell as temperature increased, because shorter drying times meant less cumulative energy input. But superheated steam was the clear winner at every temperature. The largest saving appeared at 130 degrees Celsius, where steam drying consumed 4.67 megajoules per kilogram of water removed compared with 8.25 megajoules per kilogram for hot air, a reduction of more than 40 percent. This advantage stems partly from the condensation of steam onto the cooler wet particles at the start of drying, which releases latent heat directly into the material and accelerates moisture transfer, and partly from the superior heat transfer coefficients of steam relative to air.

Quality attributes revealed a genuine trade-off between the two methods. Under the microscope, hot air dried samples showed a compact, dense, shrunken fibrous structure, driven by capillary forces that pull the cellulose-rich matrix together as water leaves. Steam dried samples, in contrast, retained a more open, expanded, porous morphology, apparently because rapid internal vapor generation partially counteracts structural collapse. This difference translated into color: steam dried cake was consistently and significantly whiter than hot air dried cake at every temperature, with the highest whiteness achieved at 160 degrees Celsius, where drying time was shortest. The oxygen-poor steam atmosphere limits oxidative discoloration, and reduced thermal exposure curbs non-enzymatic browning reactions such as the Maillard reaction and caramelization. Notably, steam dried samples also yielded significantly higher extractable oil content, peaking at 34.89 percent at 160 degrees Celsius, likely because their open structure allows better solvent penetration during analysis, whereas the dense hot-air-dried matrix physically traps oil inside.

Interestingly, hot air drying claimed one quality victory of its own. Cake dried with hot air at 160 degrees Celsius exhibited the highest swelling capacity, at 12.37 milliliters per gram of dry matter, and the highest water holding capacity, at 9.74 grams of water per gram of dry matter. The authors suggest that intense thermal exposure under hot air may modify the protein matrix surrounding starch granules and disrupt cell walls, enhancing the material’s ability to absorb and retain water. Water retention capacity, however, showed no significant differences across any treatment, indicating that neither the drying medium nor temperature substantially altered the capillary retention mechanisms within the fibrous matrix. Overall, the hydration properties of the dried press cake matched or exceeded those of commercial dietary fiber sources such as apple, oat, wheat, and pea bran in several respects, underscoring its potential as a functional fiber ingredient.

The microbiological findings may be the most consequential for food safety. Fresh coconut press cake carried an aerobic plate count of 123,000 colony-forming units per gram, along with measurable coliforms, E. coli, and yeasts and molds. Both drying methods dramatically reduced microbial loads, but steam was consistently more lethal at equivalent temperatures. At 160 degrees Celsius, steam drying brought the aerobic plate count down to 2,950 CFU per gram, compared with 10,500 for hot air, and reduced yeast and mold counts to below the detection limit of 25 CFU per gram. Critically, only steam fluidized bed drying at 160 degrees Celsius satisfied every microbiological criterion of the Philippine National Standard for coconut flour, the benchmark the team selected because it explicitly covers coconut residue. E. coli and Salmonella were not detected in any dried sample, though the authors caution that without inoculated challenge tests these results cannot be read as validated pathogen elimination. Prior research lends mechanistic support: microbial death rates in superheated steam can be dramatically faster than in hot air, with documented D-values for E. coli of under 0.10 minutes in steam at 300 degrees Celsius versus 1.12 minutes in air.

To tie the whole dataset together, the researchers ran an exploratory principal component analysis across fourteen process, quality, and safety variables. The analysis revealed that longer drying times and higher energy consumption clustered with higher residual microbial counts, while the steam treatments grouped toward lower microbial loads, greater whiteness, and higher extractable oil content. Hot air drying at 160 degrees Celsius stood apart, aligned with the superior hydration properties. The authors are careful to acknowledge limitations, including the absence of sensors to measure oxygen and steam concentrations in the drying chamber and the lack of direct lipid oxidation measurements, and they frame the multivariate results as descriptive rather than causal. Still, the practical conclusion is clear. For producers seeking maximum water binding in the dried cake, hot air at 160 degrees Celsius remains a defensible choice. But for a combination of speed, energy efficiency, whiteness, oil availability, and regulatory-grade microbial safety, superheated steam fluidized bed drying at 160 degrees Celsius emerged as the standout process, offering tropical coconut industries a credible route to transform a perishable waste stream into a safe, high-value functional food ingredient within minutes rather than hours.

Subject of Research: Comparative evaluation of superheated steam versus hot air fluidized bed drying of coconut press cake for energy efficiency, quality, and microbial safety

Article Title: Integrated evaluation of superheated steam and hot air fluidized bed drying of coconut press cake: Impacts on energy efficiency, product quality, and microbial safety

Article References: Jongyingcharoen, J. S., Wutthigarn, P., Pambudi, S., & Cheevitsopon, E. (2026). Integrated evaluation of superheated steam and hot air fluidized bed drying of coconut press cake: Impacts on energy efficiency, product quality, and microbial safety. Journal of Agriculture and Food Research, 31, Article 103307. https://doi.org/10.1016/j.jafr.2026.103307

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103307

Keywords: coconut press cake, superheated steam drying, fluidized bed drying, food safety, energy efficiency, functional foods, dietary fiber, microbial inactivation, food processing, agro-industrial waste, drying technology, sustainable food