Dry-cured grass carp, a staple of Chinese aquatic processing, owes its prized flavor to a dynamic web of biochemical reactions. The fish flesh is rich in unsaturated fatty acids, which undergo autoxidation, enzymatic breakdown, and photo-oxidation during curing. These processes generate a diverse array of volatile compounds that collectively define the product’s sensory identity. While salting begins the transformation, the drying stage is recognized as the critical window during which flavor accumulates and quality stabilizes, because the intensity of biochemical reactions peaks as moisture leaves the muscle.

Traditional sun-drying delivers the characteristic aroma, but natural sunlight can exceed 100,000 lux under direct outdoor exposure and remains hostage to geography, season, and weather, leading to long production cycles and inconsistent quality. Modern industrial methods such as cold-air, heat pump, and infrared drying are efficient, yet their products often lack the nuanced sun-cured notes consumers expect. The research team, led by Wangli Dai of Zhejiang Shuren University, set out to bridge this gap by testing controllable artificial light intensities of 0, 5,000, 10,000, and 15,000 lux on salted grass carp fillets dried for 24 hours at 50 degrees Celsius and 50 percent relative humidity under LED light spanning 380 to 800 nanometers.

The chemistry at play is elegant. Photo-oxidation proceeds through two pathways: a Type I mechanism in which light directly initiates free-radical chain reactions, and a Type II mechanism in which excited photosensitizers convert ordinary triplet oxygen into highly reactive singlet oxygen. Singlet oxygen attacks the double bonds of fatty acids directly, bypassing the slow induction period of autoxidation and dramatically accelerating the formation of flavor precursors. Myoglobin, the heme protein that gives fish flesh its color, is a particularly efficient photosensitizer. When illuminated, the porphyrin ring of its heme group undergoes conformational changes that convert oxymyoglobin to metmyoglobin, releasing reactive heme iron and reactive oxygen species that lower the activation energy for lipid peroxidation.

To test this synergy, the researchers salted fillets with 9 percent food-grade salt for five days, then supplemented minced samples with either ultrapure water or a 1.6 milligram per milliliter myoglobin solution before drying. They tracked myoglobin concentration and its redox state, measured thiobarbituric-acid-reactive substances (TBARS) as a marker of secondary lipid oxidation, quantified the reactive aldehydes 4-hydroxyhexenal and 4-hydroxynonenal by stable-isotope-dilution gas chromatography-mass spectrometry, and profiled total lipids, triglycerides, phospholipids, and free fatty acids. Volatile compounds were mapped using gas chromatography-ion mobility spectrometry, with each compound’s odor contribution estimated through relative odor activity values.

The results were striking. As light intensity climbed from 0 to 15,000 lux, myoglobin content fell significantly in both groups, from 3.81 to 3.06 milligrams per gram in controls and from 4.60 to 3.68 milligrams per gram in supplemented samples, reflecting light-induced oxidative degradation. Meanwhile, the proportion of oxymyoglobin dropped while metmyoglobin rose, from 23.11 to 26.53 percent in controls and from 24.72 to 28.03 percent in the myoglobin-treated fish. Because metmyoglobin and the heme and ferric iron it releases are potent pro-oxidants, this redox shift primes the muscle for accelerated lipid breakdown. Superoxide radicals generated during the conversion dismutate into hydrogen peroxide, which reacts with metmyoglobin to form a highly reactive complex that aggressively promotes lipid oxidation.

Oxidation markers told a consistent story. TBARS values rose by 29.65 percent in controls and 40.64 percent in myoglobin-supplemented samples across the light gradient, and at 15,000 lux the treated samples reached 2.63 milligrams of malondialdehyde equivalents per kilogram versus 2.23 in controls, the largest gap observed. Two-way analysis of variance confirmed a significant interaction between light and myoglobin, meaning the two factors amplify each other. The aldehydes 4-hydroxyhexenal and 4-hydroxynonenal, derived from omega-3 and omega-6 polyunsaturated fatty acids respectively, climbed by 20.04 and 27.13 percent in the supplemented group under increasing light, while total lipids, triglycerides, and phospholipids all declined more steeply with myoglobin present. Triglycerides proved especially sensitive, falling 35.69 percent in treated samples at the highest light intensity, and free fatty acids rose steadily to 0.47 grams per 100 grams, evidence of extensive lipid cleavage.

Crucially, this oxidation translated into flavor. The researchers identified 42 volatile peaks corresponding to 26 compounds, dominated by aldehydes, alcohols, and ketones, which together exceeded 90 percent of the profile. Total volatile signal intensity increased 2.47-fold in controls and 3.21-fold in myoglobin-supplemented samples as light intensified. Hexanal, octanal, and nonanal, products of unsaturated fatty acid degradation responsible for grassy and fatty aromas, rose sharply, with hexanal in treated samples at 15,000 lux reaching 1.41 times the control level. Branched-chain aldehydes from the Strecker reaction added malt-like and fruity notes, while esters such as ethyl acetate contributed fruity, wine-like scents. Although off-flavor compounds like heptanal and 1-octen-3-ol also increased, their relative odor activity values remained far below those of the desirable aroma compounds, suggesting the overall flavor balance shifted favorably.

Statistical correlation analysis tied the whole mechanism together. Mantel and Pearson tests revealed that metmyoglobin negatively correlated with total lipids, triglycerides, phospholipids, and polyunsaturated fatty acids, while positively correlating with free fatty acids, TBARS, and 4-hydroxynonenal. Volatile compound classes correlated strongly with myoglobin and lipid oxidation markers, confirming that the extent of oxidation directly shapes flavor intensity. The authors conclude that modulating light intensity and myoglobin-mediated photo-oxidation offers a practical strategy for regulating flavor formation, potentially allowing producers to dial in the sun-cured character of dry-cured grass carp with precision lighting, standardizing quality while preserving the taste that centuries of tradition created.

Subject of Research: Effects of light intensity and myoglobin supplementation on lipid oxidation and flavor formation in dry-cured grass carp

Article Title: Impact of light intensity and myoglobin supplementation on lipid oxidation and flavor development in dry-cured grass carp

Article References: Dai, W., Wang, Y., Cui, M., Bao, R., Zhou, X., Ding, Y., & Gu, S. (2026). Impact of light intensity and myoglobin supplementation on lipid oxidation and flavor development in dry-cured grass carp. Journal of Agriculture and Food Research, 31, Article 103319. https://doi.org/10.1016/j.jafr.2026.103319

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103319

Keywords: dry-cured fish, grass carp, myoglobin, lipid oxidation, photo-oxidation, singlet oxygen, volatile flavor compounds, TBARS, GC-IMS, food flavor chemistry, drying technology, aquatic products

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