Filamentous fungi reshape kombu chemistry and aroma in solid-state fermentation study

Filamentous fungi can substantially remodel the chemical composition and volatile profile of kombu, the brown seaweed Saccharina japonica widely prized for its umami character, according to a systematic time-course comparison published in Food Chemistry: X. Researchers at the National University of Singapore fermented autoclaved, hydrated kombu with four filamentous fungi—Aspergillus niger, Aspergillus sojae, Rhizopus oligosporus, and Rhizopus oryzae—and tracked fungal colonization, surface morphology, carbohydrates, organic acids, peptides, amino acids, Folin-Ciocalteu responses, and volatile compounds over 72 hours of solid-state fermentation. The comparison revealed sharply distinct, fungus- and stage-dependent transformation patterns rather than a single uniform outcome, providing a comparative basis for selecting fungal cultures and fermentation stages in future process development.

The motivation stems from a practical constraint. Kombu is rich in dietary fiber, minerals, polysaccharides, proteins, phenolic compounds, and carotenoid-derived bioactives, and its characteristic umami taste has made it a popular flavor-enhancing ingredient. Yet its dense and recalcitrant matrix—packed with alginate, fucoidan, laminarin, cellulose-like components, proteins, polyphenols, and mineral-mediated cross-linkages—can limit access to and release of matrix-associated components during processing. Lipid-derived aldehydes, ketones, and alcohols contribute to its volatile profile, which is associated with grassy, marine, fatty, oxidized, and earthy sensory notes. Mild processing strategies that modify this matrix and its compositional and volatile profiles are therefore of considerable interest, and filamentous fungi, with their hyphal growth and extracellular hydrolytic activities, offer one such strategy under solid-state conditions.

Earlier studies had shown that fungal fermentation can modify kombu-derived substrates, but they largely focused on individual fungal cultures or selected quality attributes. Fermenting an aqueous kombu extract with Aspergillus oryzae has previously been reported to change γ-aminobutyric acid (GABA), Folin-reactive compounds, and antioxidant-related responses, and subsequent work demonstrated substantial remodeling of the volatile profile of that fermented extract, including reductions in compounds associated with characteristic seaweed odor. What remained limited, the authors note, were systematic time-course comparisons of multiple filamentous fungi during solid-state fermentation that integrate fungal colonization, surface morphology, carbohydrate and organic acid changes, peptide and amino acid profiles, Folin-reactive compounds, and volatile evolution. The new study addressed that gap by comparing four cultures chosen for their contrasting fermentation-associated characteristics: A. niger for carbohydrate-active enzymes and organic acid production, A. sojae for the proteolytic and saccharifying activities of soybean koji fermentation, R. oligosporus for its extensive mycelial development in tempeh-type fermentation, and R. oryzae for its capacity to modify substrate metabolite profiles in solid-state food fermentation.

The experimental setup was deliberately controlled. Dried kombu purchased in Singapore was milled into powder, hydrated to 65 percent moisture, sterilized at 121 °C for 30 minutes, and inoculated with approximately 5.0 log spores per gram of wet substrate. Each fermentation unit consisted of 2.5 grams of substrate spread into a roughly 2-millimeter layer in a sealed Petri dish and incubated at 30 °C for up to 72 hours without forced aeration. Uninoculated kombu substrate subjected to the same preparation and incubation served as the control. Samples were collected destructively at 0, 12, 24, 36, 48, and 72 hours from separate Petri dishes, with three fermentation units per treatment and time point. Because autoclaving may alter the native structure and chemical composition of kombu, the authors emphasize that the results represent fermentation of an autoclaved kombu substrate rather than untreated kombu.

Macroscopic and microscopic imaging documented distinct colonization patterns. On the substrate, visible fungal coverage increased progressively from 24 hours onward, particularly in A. sojae and R. oligosporus. A. sojae developed greenish pigmentation at the late stage, consistent with conidiation-like maturation, while R. oligosporus formed a dense aerial mycelial layer from 36 hours onward with more pronounced gray coloration at 72 hours, likely associated with sporangial development. R. oryzae showed gradual but continuous mycelial expansion with extensive surface coverage at 48 and 72 hours, whereas A. niger showed relatively limited visible colonization early on, with white mycelia appearing at 36 hours and dark pigmentation at 72 hours. Scanning electron microscopy revealed that the compact, continuous, layered surface of the unfermented control gave way after fermentation to increased roughness, pore-like features, fragmentation, and loosening, generally more pronounced at 72 hours. The authors caution, however, that enzyme activities, internal hyphal penetration, and degradation of individual cell wall components were not directly measured, so extracellular hydrolytic activity remains a plausible rather than proven contributor.

Recoverable viable propagule counts and pH trajectories diverged markedly among the fungi. Counts decreased slightly during the first 12 hours—an early decrease the authors say should not be interpreted directly as a decrease in fungal biomass or viability, because plate-count colonies may arise from conidia, viable hyphal fragments, or mycelial aggregates—then increased in all treatments. A. sojae and R. oligosporus rose relatively rapidly, reaching 8.02 and 7.90 log CFU per gram of wet substrate at 72 hours, respectively, while R. oryzae reached 7.26 and A. niger, after little change through 36 hours, jumped to 8.08. The uninoculated control held steady at pH 6.30–6.35. A. niger caused the strongest acidification, dropping the pH from 6.34 to 4.93 at 48 hours before a partial recovery to 6.05 at 72 hours. R. oryzae acidified early to pH 5.80 at 24 hours before rising to 6.64, R. oligosporus dipped mildly to 6.05 and then rose to 6.38, and A. sojae acidified only mildly before shifting to a slightly alkaline 7.13 at 72 hours. Organic acid analysis yielded limited quantitative information because most responses were not detected or below the limit of quantification; only succinic acid in A. niger at 24 hours (48.17 mg per gram dry weight) and oxalic acid in A. niger at 48 hours (1.00 mg per gram dry weight) were quantifiable. Oxalic acid may contribute to matrix changes through interactions with alginate-bound calcium, as oxalic acid treatment has been reported to promote kombu softening, though this contribution was not directly examined.

Carbohydrate measurements told a story of release followed by utilization. Total reducing sugars in the control remained stable at 29.13–30.12 mg per gram dry weight with no detectable glucose, but A. niger drove the most pronounced increase, reaching 38.64 mg per gram dry weight at 24 hours, and A. sojae reached 34.14 at the same time point. Glucose, undetectable at the start, accumulated markedly at 24 hours in A. niger and A. sojae fermentations, reaching 8.63 and 9.60 mg per gram dry weight respectively, before declining. Mannitol, an important soluble carbon-storage compound in brown seaweeds, moved in the opposite direction: the control held steady at 18.47–19.56 mg per gram dry weight, while all four fermentations progressively depleted it, most rapidly in A. sojae and R. oryzae, where it became undetectable by 36 hours. These transient sugar increases alongside mannitol depletion are consistent with carbohydrate solubilization or hydrolysis followed by fungal consumption, the authors write, although extracellular enzyme activities, residual polysaccharide composition, and carbohydrate fluxes were not directly measured.

Peptide and amino acid profiles also shifted in fungus-specific ways. In the control, peptides larger than 10 kDa accounted for the largest share of the chromatographic response at 33.78 percent. A. niger and R. oligosporus both shifted the distribution toward lower molecular weights: in R. oligosporus fermentations the >10 kDa fraction fell from 43.74 percent at 36 hours to 31.88 percent at 72 hours while the 1–3 kDa fraction rose to 30.38 percent, and in A. niger fermentations the 1–3 kDa fraction reached 33.55 percent, the highest among all groups, accompanied by a higher integrated peak area indicating the change was not merely redistribution. A. sojae and R. oryzae moved differently, with A. sojae keeping the >10 kDa fraction high and R. oryzae increasing it to 43.07 percent at 72 hours. OPA-reactive free amino groups, an overall measure of primary amino-group responses, peaked transiently in all treatments: A. sojae reached the maximum of 21.90 mg serine equivalents per gram dry weight at 48 hours, A. niger 13.50 at 48 hours, R. oryzae 13.93 at 24 hours, and R. oligosporus 12.78 at 36 hours before returning to near-control levels. A. sojae also showed the highest total free amino acid levels at both 36 and 72 hours, with concurrent increases in sweetness-related, umami-related, and bitter-associated amino acids—meaning, the authors stress, that sensory consequences cannot be inferred without direct sensory evaluation. GABA rose markedly after fermentation, with the highest level in R. oryzae at 36 hours (0.43 mg per gram dry weight), though the authors caution that the physiological functionality of the fermented kombu cannot be inferred from GABA concentration alone. Folin-Ciocalteu responses, which the authors interpret as general reducing responses rather than phenolic-specific concentrations because the reagent also reacts with non-phenolic substances, peaked in A. sojae at 3.56 mg gallic acid equivalents per gram dry weight at 48 hours, up from a control range of 1.70–1.76.

The volatile remodeling was among the most striking findings. Forty-one volatile compounds were detected, and principal component analysis clearly separated the control from the fermented samples and differentiated the two fermentation stages, with the first two components explaining 43.8 percent and 15.1 percent of the variance. The control profile was aldehyde-dominated—aldehydes accounted for 62.90 percent of the total semi-quantitative volatile response—but that share collapsed to 16.67 percent in A. sojae fermentations at 36 hours and 3.17 percent at 72 hours, with reductions in lipid oxidation-derived aldehydes including hexanal, octanal, nonanal, and several unsaturated C7–C10 aldehydes associated with green, fatty, oxidized, or marine-related odor descriptors. In their place, alcohols rose to 45.82, 88.28, 68.37, and 53.40 percent of the total response in the 72-hour A. niger, A. sojae, R. oligosporus, and R. oryzae samples respectively, driven largely by 1-octen-3-ol, a characteristic fungal C8 volatile with mushroom-like aroma. Higher alcohols such as isoamyl alcohol and 2-phenylethyl alcohol also increased in selected samples. The authors emphasize that these are semi-quantitative chromatographic responses, not absolute concentrations, and that sensory significance cannot be established without GC-olfactometry or sensory evaluation.

The study stops well short of prescribing a recipe. Volatile compounds were evaluated only at 36 and 72 hours, sensory responses were not measured, and the data do not establish an optimal fermentation endpoint. Extended fermentation was associated with lower responses of several lipid oxidation-associated aldehydes but greater contributions from selected fungal C8 volatiles and higher alcohols, and the authors suggest that approximately 36 hours may represent a candidate intermediate stage for subsequent process optimization rather than a validated optimum, since substantial colonization and compositional changes were already evident by then while pigmentation and apparent sporulation became more pronounced thereafter. Food-safety considerations also require separate evaluation, because the food-fermentation history of the selected fungi does not establish the safety of the specific strains under the present conditions; strain-level assessment of toxigenic potential, microbiological control, and targeted evaluation of undesirable metabolites would be required before food application. Future work under milder or non-sterile processing conditions, together with quantitative structural characterization and evaluation of digestibility, bioaccessibility, functional and sensory properties, and process scalability, would help establish the potential of fungal-fermented kombu as a food ingredient.

Subject of Research: Comparing how four filamentous fungal cultures remodel the chemical composition and volatile profile of kombu during 72 hours of solid-state fermentation

Article Title: Solid-state fermentation with selected filamentous fungi remodels the chemical composition and volatile profile of kombu ( Saccharina japonica )

Article References: Geng, C., Liu, S. Q., & Lu, Y. (2026). Solid-state fermentation with selected filamentous fungi remodels the chemical composition and volatile profile of kombu (Saccharina japonica). Food Chemistry: X, 40, Article 104684. https://doi.org/10.1016/j.fochx.2026.104684

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104684

Keywords: kombu, Saccharina japonica, solid-state fermentation, filamentous fungi, Aspergillus niger, Aspergillus sojae, Rhizopus oligosporus, Rhizopus oryzae, volatile compounds, peptides, GABA, food chemistry