The research team, led by Ji Young Jeong, Daun Kim, Jung Eun Yang, Chang Hee Jeong, Jong-Hee Lee, Jae Sung Kim, and In Min Hwang, working with support from the World Institute of Kimchi in Gwangju, set out to define what they call chemical stratification: the non-uniform, position-dependent partitioning of metabolites and elements along the leaf gradient of a cabbage head. Because leaves form sequentially through continuous cell division and differentiation in the shoot apical meristem, leaf position is a biologically meaningful developmental axis rather than a simple visual distinction. Enclosed inner leaves are young, shaded, and tender, while progressively outer tissues are older, more light-exposed, and structurally mature. The researchers hypothesized that this developmental and environmental heterogeneity would be expressed in a coordinated redistribution of primary metabolites, glucosinolates, inorganic nitrogen, and elemental constituents.

To test this, the team purchased market-mature kimchi cabbage heads in November, corresponding to the winter harvest season, and separated each head into four operationally defined fractions: inner leaves, the enclosed pale-yellow leaves near the central core; middle layers, the intermediate head-forming leaves; outer-layer leaves, the green leaves surrounding the edible head that could still be used for food; and discarded outer leaves, the outermost leaves commonly removed during trimming. Five independent cabbage heads served as biological replicates for each fraction. The separated tissues were rinsed with ultrapure water, frozen, freeze-dried, and homogenized into fine powder before analysis. Visibly decayed or severely damaged tissues were excluded to minimize confounding effects unrelated to leaf position.

The analytical strategy was deliberately multi-domain. Nitrate and nitrite were quantified colorimetrically using the Griess reaction and enzymatic nitrate reduction. Glucosinolates, the sulfur-containing defense metabolites characteristic of Brassica vegetables, were extracted with aqueous methanol and profiled by ultra-performance liquid chromatography coupled with quadrupole-Orbitrap mass spectrometry in negative-ion mode, covering alkyl, alkenyl, aromatic, and indole subclasses. Amino acids were quantified on the same high-resolution LC-MS platform in positive-ion mode, while sugars, including sucrose, glucose, fructose, and sorbitol, were measured by high-performance liquid chromatography with refractive index detection. For elemental analysis, freeze-dried powder was microwave-digested with nitric acid and hydrogen peroxide, and the digests were analyzed by inductively coupled plasma-optical emission spectrometry for abundant mineral elements and by inductively coupled plasma-mass spectrometry for trace and toxic elements. All quantitative methods were validated for linearity, sensitivity, precision, and spike-recovery accuracy.

Multivariate statistics revealed that the four leaf fractions occupied distinct regions in principal component analysis across every compositional domain. The first two principal components explained 81.8 percent and 10.3 percent of the variance for inorganic nitrogen species and glucosinolates, 72.9 percent and 13.0 percent for amino acids and sugars, 75.7 percent and 13.5 percent for elemental variables, and 74.6 percent and 11.0 percent for all measured variables combined. A z-scored heatmap of group means visualized coordinated positional differences: inner fractions were relatively enriched in many amino acids and sugars, whereas outer-layer and discarded outer leaves showed higher relative abundance of numerous glucosinolate variables, inorganic nitrogen species, and selected mineral and trace elements. In other words, within-head leaf position is a major axis of compositional variation, and the four fractions should not be regarded as compositionally interchangeable tissues even when they originate from the same head.

The glucosinolate data were particularly striking. Total glucosinolates increased from 1331.4 milligrams per kilogram dry weight in inner leaves to 2302.2 milligrams per kilogram dry weight in discarded outer leaves, a 1.73-fold increase. This outward accumulation was driven primarily by the alkyl, alkenyl, and aromatic subclasses. Total alkyl glucosinolates rose 2.42-fold, total alkenyl glucosinolates 2.14-fold, and total aromatic glucosinolates 1.52-fold between inner leaves and discarded outer leaves. Among individual compounds, gluconapin and glucobrassicanapin were strongly enriched in outer fractions, reaching 503.7 and 931.5 milligrams per kilogram dry weight, respectively, in discarded outer leaves. Glucoberteroin, an alkyl glucosinolate, increased 2.68-fold, while sinalbin, an aromatic compound, showed the largest individual outward shift at 3.59-fold.

Yet the stratification was subclass-specific rather than a uniform outward accumulation. Glucobrassicin, the sole quantified indole glucosinolate, followed the opposite pattern: it was highest in inner leaves at 224.0 milligrams per kilogram dry weight and declined progressively to 63.3 milligrams per kilogram dry weight in discarded outer leaves, a 0.28-fold ratio. Because indole and non-indole glucosinolates are regulated through partially distinct biosynthetic, precursor-related, and transport processes, they respond differently to leaf developmental status, light exposure, tissue function, and nitrogen allocation. Glucosinolate transport is mediated by membrane proteins with substrate specificity related to precursor origin, indicating that spatial allocation is actively regulated rather than passively distributed. Notably, members of the NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER FAMILY, including several NPF2 proteins, can transport both nitrate and glucosinolates, providing a biochemical link between inorganic nitrogen status and glucosinolate metabolism. Consistent with this framework, nitrate rose 2.15-fold and nitrite 1.48-fold from inner leaves to discarded outer leaves, mirroring the outward enrichment of most non-indole glucosinolates.

The primary metabolite data told a complementary story about edible quality. L-glutamic acid, a key umami-related amino acid, declined from 18.36 to 9.82 nanomoles per gram dry weight between inner leaves and discarded outer leaves. L-proline fell 0.31-fold and L-alanine 0.42-fold, with similar inner-enriched patterns for L-serine, L-leucine, L-isoleucine, L-phenylalanine, L-tryptophan, and L-valine. Total sugars decreased from 2255.1 to 1734.8 micromoles per gram dry weight, with sucrose showing the strongest outward decline at 0.49-fold. These inner-enriched patterns align with the practical preference for inner tissues, which are associated with tender texture and mild sensory characteristics, and with previous reports that fructose and glucose preferentially accumulate in internal tissues during head formation in Chinese cabbage. However, outer leaves were not simply depleted: L-aspartic acid increased 3.40-fold toward the outside, the largest outward shift among the quantified amino acids, and L-threonine, L-arginine, and L-citrulline also accumulated outward, indicating that outer fractions carry a specific nitrogenous metabolite profile of their own.

Elemental analysis added both promise and caution. Among major minerals, calcium increased 2.42-fold from 2544 to 6151 milligrams per kilogram dry weight between inner leaves and discarded outer leaves, while magnesium, sulfur, and sodium also rose outward. Phosphorus and potassium, in contrast, were relatively inner-enriched. Among trace elements, iron, manganese, gallium, strontium, and barium were enriched in outer tissues, with manganese rising 2.07-fold and strontium 2.18-fold, whereas nickel, copper, zinc, and rubidium declined toward the outside. Toxic elements were detected at trace levels but were consistently higher in discarded outer leaves: arsenic increased 1.95-fold, lead 1.33-fold, and cadmium showed the strongest positional elevation at 4.91-fold, rising from 0.033 to 0.164 milligrams per kilogram dry weight. Applying an approximate dry matter fraction of 5 percent, the highest discarded-outer-leaf concentrations corresponded to roughly 0.0097 milligrams per kilogram fresh weight for arsenic, 0.0012 for lead, and 0.0082 for cadmium. The authors emphasize that these estimates provide practical context but are not a substitute for direct fresh-weight analysis, dietary exposure assessment, or comparison with applicable regulatory limits.

Pearson correlation analysis tied the chemical domains together into two contrasting association modules. Most glucosinolates enriched in outer fractions, including glucoerucin, glucoberteroin, progoitrin, gluconapin, glucobrassicanapin, glucoraphenin, sinalbin, glucotropaeolin, glucobarbarin, and gluconasturtiin, formed a broadly positive correlation block, varying in a coordinated manner across the leaf gradient. Glucoraphanin and glucobrassicin stood apart with predominantly negative correlations to this outer-enriched group, and selected amino acid precursors, including valine, leucine, isoleucine, phenylalanine, methionine, and tryptophan, clustered positively with the inner-associated pair. The authors stress that this correlation-based evidence demonstrates organized compositional co-variation rather than direct biosynthetic flux or causal regulation. Taken together, the findings establish chemical stratification as a practical framework linking edible quality with burden-aware valorization: discarded outer leaves are chemically enriched biomass that may be considered for glucosinolate-rich extract preparation, functional ingredient development, fermentation substrates, or appropriately screened animal feed, but any such upcycling must be preceded by contaminant screening and intended-use-specific suitability assessment, because the same outer tissues that concentrate beneficial sulfur metabolites and minerals also accumulate arsenic, lead, and cadmium.

Subject of Research: Chemical stratification across kimchi cabbage leaves links edible quality with burden-aware outer-leaf valorization

Article Title: Chemical stratification across kimchi cabbage leaves links edible quality with burden-aware outer-leaf valorization

Article References: Jeong, J. Y., Kim, D., Yang, J. E., Jeong, C. H., Lee, J.-H., Kim, J. S., & Hwang, I. M. (2026). Chemical stratification across kimchi cabbage leaves links edible quality with burden-aware outer-leaf valorization. Food Chemistry: X, Article 104652. https://doi.org/10.1016/j.fochx.2026.104652

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104652

Keywords: kimchi cabbage, chemical stratification, glucosinolates, food chemistry, leaf position, nitrate, cadmium, food waste valorization, Brassica rapa, upcycling, Chemical, stratification