Introduction

Brassica species are very popular vegetables, with cabbage, kale, broccoli, cauliflower, turnip, rapeseed, and rutabaga being widely cultivated1,2. In particular, Brassica is a critical crop in Asia, accounting for > 75% of the world’s Brassica production3. It has high nutritional value because it contains abundant water, vitamins, minerals, carbohydrates, and fiber1. Additionally, Brassica contains various bioactive compounds, such as phenolic compounds, vitamins, lutein, and glucosinolates, which benefit human health by providing antioxidant, anticancer, and anti-inflammatory activities3,4. Among these crops, cabbage (Brassica oleracea var. capitata) is widely cultivated in major regions of Asia, including the Republic of Korea, Japan, and China, and accounts for approximately 6.5% of the world’s vegetable production5. Cabbage contains various bioactive components, including vitamin C, glucosinolates, carotenoids, flavonoids, and polyphenols6,7,8.

Agricultural waste has negative impacts on the environment, economy, and society, particularly in major agricultural countries, and has become a global issue9. The amount of waste generated during food production is approximately 1.3 billion tons per year, accounting for one-third of the total food production10. The United Nations has established Sustainable Development Goals to reduce agricultural waste and devised a new approach for valuing agricultural waste and byproducts11,12. In this new approach, agricultural waste is converted into healthy functional foods, biomaterials, and biofuels (such as nutraceuticals, antioxidants, bioactives, biopolymers, biopeptides, antibiotics, industrial enzymes, bionanocomposites, single-cell proteins, polysaccharides, biodiesel, and bioethanol)9.

Cabbage is mainly consumed for its leaves which are divided into cabbage heads (CH) and outer leaves (OL). The commercially consumed part is CH, while OL and roots are removed during the production, harvesting, and distribution processes. These removed parts account for approximately 30% of the total production, generating a significant amount of waste13. To convert discarded OL into a valuable resource, the development of functional food additives14,15,16,17,18, biochar5, and nanocellulose19 has been proposed.

Given the high nutritional and bioactive potential of cabbage OL, exploring its use as a natural ingredient in skin-whitening products presents an opportunity to address both agricultural waste and the increasing demand for sustainable cosmetic materials. Skin-whitening agents, such as ascorbic acid, kojic acid, arbutin, and niacinamide, are widely utilized in cosmetic formulations due to their ability to inhibit melanin synthesis through various mechanisms20. For instance, ascorbic acid is a potent antioxidant that reduces melanin synthesis by inhibiting tyrosinase activity. However, it is highly susceptible to oxidation, particularly in aqueous solutions. Therefore, more stable derivatives are often preferred for practical applications21. Kojic acid also inhibits tyrosinase and demonstrates antibacterial properties; however, its long-term use raises concerns about potential skin irritation22,23. Similarly, arbutin and niacinamide function as tyrosinase inhibitors but have limitations in stability and penetration efficiency24. Despite their efficacy, these agents often face challenges related to stability, cost, and potential side effects, which necessitate the development of safer and more sustainable alternatives.

This study aimed to evaluate the potential use of discarded cabbage OL as health-functional food and cosmetic materials by analyzing their antioxidant, anti-inflammatory, and tyrosinase-inhibitory activities, and their contents of antioxidants, vitamins, and glucosinolates.

Materials and methods

Reagents

2,2’-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid, ABTS), 2,2-diphenyl-1-picrylhydrazy (DPPH), (3,4-dimethyl-thiazolyl-2)-2,5-diphenyl tetrazolium bromide (MTT), acetonitrile, ascorbic acid, diethylene glycol (DEG), DMSO, ethanol, Folin-Ciocalteu’s reagent, Griess reagent, L-3,4-dihydroxyphenylalanine (L-DOPA), lipopolysaccharide (LPS), methanol, menaquinone, mushroom tyrosinase, naringin, phylloquinone, potassium peroxydisulfate, progoitrin, sinigrin, sodium carbonate (Na₂CO₃), sulforaphane, tannic acid, trifluoroacetic acid, water, glucoalyssin, glucobrassicanapin, glucobrassicin, glucocochlearin, gluconapin, gluconapoleiferin, gluconasturtiin, glucoerucin, 4-hydroxyglucobrassicin, 4-methoxyglucobrassicin, and neoglucobrassicin were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). Ethyl alcohol and 1 N sodium hydroxide solution (NaOH) were purchased from Samchun Chemical Co. (Pyungtack, Korea). Dulbecco’s modified Eagle medium (DMEM), fetal bovine serum (FBS), and penicillin/streptomycin were purchased from Cytiva (Logan, UT, USA).

Experimental materials

Cabbage seeds (Brassica oleracea var. capitata cv. Tarano) used in the experiment were provided by JOEUN SEEDS Co., Ltd. (Goesan, Korea). The seeds were sown in 50-cell trays for germination. Four-week-old cabbage seedlings were transplanted to an affiliated farm of Chungbuk National University and cultivated for 8 weeks. In July 2022, CH and OL were harvested (Fig. 1). The cultivation conditions were an average temperature of 24 ℃ (minimum 9.6 ℃ and maximum 35.4 ℃), with drip irrigation once a day. The harvested samples were freeze-dried, ground, and stored at -80 ℃ for use as experimental materials.

Fig. 1
figure 1

Images of the cabbage head (CH) and outer leaves (OL) used in this experiment. Scale bar, 10 cm.

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CH extract (CHE) and OL extract (OLE)

Approximately 1 g of dried powder from each sample was mixed with 50 mL of 70% ethanol. The mixture was sonicated for 30 min and filtered using a vacuum pump. Subsequently, the extracts were concentrated using a rotary vacuum concentrator at 37 ℃. The concentrated samples were dissolved in dimethyl sulfoxide (DMSO) to a final concentration of 100 mg mL− 1 and filtered through a 0.45-µm syringe filter. The calculation of the hydroalcoholic extract concentration excluded the solvent weight to ensure consistency with standard extraction protocols. The extraction yield (%) of each sample was calculated as follows:

$$\:\text{E}\text{x}\text{t}\text{r}\text{a}\text{c}\text{t}\text{i}\text{o}\text{n}\:\text{y}\text{i}\text{e}\text{l}\text{d}\:\left(\text{\%}\right)=\left(\frac{\text{F}\text{l}\text{a}\text{s}\text{k}\:\text{w}\text{e}\text{i}\text{g}\text{h}\text{t}\:\text{a}\text{f}\text{t}\text{e}\text{r}\:\text{c}\text{o}\text{n}\text{c}\text{e}\text{n}\text{t}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n}-\text{E}\text{m}\text{p}\text{t}\text{y}\:\text{f}\text{l}\text{a}\text{s}\text{k}\:\text{w}\text{e}\text{i}\text{g}\text{h}\text{t}}{\text{D}\text{r}\text{i}\text{e}\text{d}\:\text{p}\text{o}\text{w}\text{d}\text{e}\text{r}\:\text{w}\text{e}\text{i}\text{g}\text{h}\text{t}}\right)\times\:100$$
(1)

Antioxidant contents and activities

Total flavonoid content (TFC) was determined using the diethylene glycol (DEG) colorimetric method25. The sample (0.2 mL) was mixed with 2 mL DEG and 0.2 mL of 1 N NaOH and incubated for 1 h at 37 ℃ in a constant temperature water bath (BW3-10G; JEIO Tech. Co., Ltd., Daejeon, Republic of Korea). The absorbance of the reaction mixture at 420 nm was measured using a UV/visible spectrophotometer (Libra S22; Biochrom Ltd., Cambridge, UK). TFC was calculated from a standard curve prepared using quercetin and expressed as mg of quercetin equivalents (QE) per g dry weight. Total polyphenol content (TPC) was determined by a modified Folin–Ciocalteu method26. After mixing 2 mL of 2% Na2CO3 with 0.1 mL of the sample, 0.1 mL of 50% Folin–Ciocalteu reagent was added after 3 min, followed by reacting in the dark for 30 min. The absorbance at 750 nm was measured using a UV/visible spectrophotometer. TPC was calculated from a standard curve prepared using gallic acid and expressed as mg of gallic acid equivalents (GAE) per g dry weight.

Antioxidant activity was measured by 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assays. The ABTS assay was conducted using a modified decolorization method8. ABTS radicals were formed by reacting 2.6 mM potassium persulfate (12.31 mL) with 50 mg ABTS in the dark at room temperature for 24 h. Then, the solution was diluted to adjust the absorbance at 732 nm to 0.70 ± 0.03. Then, 950 µL diluted ABTS solution was mixed with a 50 µL sample and vortexed. The mixture was incubated in the dark at room temperature for 10 min, and the absorbance at 732 nm was measured using a spectrophotometer. The ABTS radical scavenging activity was calculated using the following formula for Electron Donating Ability (EDA, %), and the sample concentration required to reduce EDA by 50% [Reduction Concentration of 50% (RC50), mg·mL− 1] was determined.

$$\:\text{E}\text{D}\text{A}\:\left(\text{\%}\right)=(1-\frac{{OD}_{sample}}{{OD}_{blank}})\times\:100$$
(2)

The DPPH radical scavenging activity was measured as previously described27. A mixture of 0.2 mL sample and 0.8 mL of 0.2 mM DPPH solution was incubated in the dark for 30 min. The absorbance at 517 nm was then measured using a spectrophotometer. The DPPH radical scavenging activity was calculated as that described for the ABTS assay.

Anti-inflammatory activity

RAW 264.7 macrophages obtained from the Korea Cell Line Bank (Seoul, Republic of Korea) were cultured in Dulbecco’s Modified Eagle Medium supplemented with 10% fetal bovine serum and penicillin/streptomycin in a 5% CO2 environment at 37 ℃, and subcultured every two days. The cytotoxicity of each extract was evaluated using the MTT assay. Cells (7.5 × 105 cells/mL) were seeded in a 96-well plate, using 200 µL per well. After 6 h, cells were cotreated with lipopolysaccharide (LPS, 1 µg·mL− 1), quercetin (25 µM), and the extract (100 µg·mL− 1), and incubated for an additional 18 h. Subsequently, 10 µL of 1 mg·mL− 1 MTT reagent was added to each well and incubated for 90 min. The solution was then dissolved in DMSO, and the absorbance at 550 nm was measured using a microplate reader. Nitric oxide production was assessed by mixing 100 µL culture medium of treated cells with 100 µL Griess reagent, incubating for 10 min, and measuring the absorbance at 550 nm using a microplate reader (Epoch, Biotek, Vermont, USA). Nitric oxide content was quantified using a standard curve of sodium nitrate.

Tyrosinase inhibition activity for skin whitening

The skin-whitening activity was analyzed by mixing 0.5 mL of 0.175 M sodium phosphate buffer (pH 6.8) with 0.2 mL of 10 mM L-dopamine and 0.1 mL of 10 mg·mL− 1 extract. This was followed by the addition of 0.2 mL of 110 unit·mL− 1 mushroom tyrosinase. The resultant DOPAchrome formation was monitored by measuring the absorbance at 475 nm for 20 min at 1-min intervals using a microplate reader. Tyrosinase inhibitory activity was calculated as the percent decrease in absorbance in the treated group compared to that in the untreated group, with ascorbic acid at a concentration of 1 mg mL− 1 serving as a positive control.

High-performance liquid chromatography (HPLC) analysis of chemical components in cabbage extracts

Vitamin C analysis: Samples were sonicated for extraction with 50% acetonitrile, filtered through a 0.2-µm polyvinylidene fluoride (PVDF) membrane filter, and used as the test solution. For the standard solution, 2.0 mg vitamin C was dissolved in 50% acetonitrile, sonicated, filtered through a 0.2-µm PVDF membrane filter, and used for analysis. HPLC analysis was conducted using an Agilent 1260 Infinity II Quat Pump system (Agilent Technologies, CA, USA) equipped with a DAD WR detector and a YMC Pack-Pro C18 column (4.6 × 250 mm, 5 μm) at room temperature. The injection volume was 10 µL, and the mobile phase consisted of a gradient system with solvent A (water with 0.1% trifluoroacetic acid, TFA) and solvent B (acetonitrile) at a flow rate of 1.0 mL/min. Detection was performed at a UV wavelength of 254 nm. The gradient program was as follows: 0 min, 95% A and 5% B; 5 min, 95% A and 5% B; 10 min, 80% A and 20% B; 25 min, 80% A and 20% B; 30 min, 50% A and 50% B; and 45 min, 95% A and 5% B.

Vitamin K analysis: Samples were sonicated with ethanol, filtered through a 0.2-µm PVDF membrane filter, and used as the test solution. For the standard solution, 2.0 mg each of vitamins K1 and K2 were dissolved in ethanol, sonicated, and filtered through a 0.2-µm PVDF membrane filter. HPLC analysis was conducted using a Waters Alliance e2695 Separations Module equipped with a Waters 2489 UV/Vis Detector and an INNO C18 column (4.6 × 250 mm, 5 μm) at 30 °C. The injection volume was 10 µL, and the mobile phase consisted of an isocratic system with solvent A (water with 0.1% TFA) and solvent B (methanol) at a flow rate of 1.0 mL/min. Detection was performed at a UV wavelength of 248 nm. The gradient program was as follows: 0 min, 70% A and 30% B; 10 min, 70% A and 30% B; 20 min, 30% A and 70% B; 30 min, 30% A and 70% B; 40 min, 80% A and 20% B; and 50 min, 70% A and 30% B.

Sulforaphane analysis: Sample (4 g) was sonicated with 50% acetonitrile and filtered through a 0.45-µm PVDF membrane filter to prepare the test sample. Sulforaphane (2.0 mg) was dissolved in 50% acetonitrile, sonicated, and filtered through a 0.45-µm PVDF membrane filter to prepare the standard solution. HPLC analysis was conducted using an Agilent 1260 Infinity II Quat Pump system equipped with a DAD WR detector and an INNO C18 column (4.6 × 250 mm, 5 μm). The column temperature was maintained at room temperature, and the injection volume was 10 µL. The mobile phase consisted of a gradient system with solvent A (water with 0.1% TFA) and solvent B (acetonitrile), with a flow rate of 1.0 mL/min. The detection wavelength was set to 254 nm. The gradient program was as follows: 0–5 min, 80% A and 20% B; 5–10 min, 80% A and 20% B; 10–15 min, 80% A and 20% B; 15–20 min, 80% A and 20% B; 20–25 min, 60% A and 40% B; 25–30 min, 60% A and 40% B; 30–35 min, 40% A and 60% B; 35–40 min, 40% A and 60% B; 40–45 min, 20% A and 80% B; and 45 min, 80% A and 20% B.

Glucosinolate analysis: Freeze-dried samples (4 g) were extracted with 50% acetonitrile and filtered through a 0.45-µm PVDF membrane filter. For DEAE-Sephadex A-25 activation, 30 g resin was swollen in distilled water, washed with 1.5 L water, and activated with 0.5 M sodium acetate before final washing with 300 mL distilled water. The resin was stored with a 2–3 cm water layer. Disposable polypropylene columns (Thermo Fisher Scientific, Waltham, MA, USA) were packed with activated DEAE-Sephadex A-25. Crude glucosinolates were extracted from a 50 mg sample using 5 mL of 70% methanol, sonicated, and filtered through a 0.2-µm PVDF membrane. The extract was applied to the column, washed with 2.5 mL distilled water, and eluted with 200 µL arylsulfatase solution (115 mg/5 mL). The eluate was incubated overnight and filtered through a 0.2-µm membrane. For the standard solution, sinigrin (1.0 mg) was dissolved in 10 mL water, and a 0.5-mL aliquot was similarly processed. HPLC analysis was performed using an Agilent 1260 Infinity II Quat Pump with a DAD WR detector and an Inertsil ODS-3 column (3.0 × 150 mm I.D., 3 μm) at room temperature, with a 10 µL injection volume, and a gradient mobile phase of water (A) and acetonitrile (B) at a flow rate of 0.4 mL/min, with the detection wavelength set to 227 nm. The gradient program was 100% A for 5 min, 69% A/31% B for 10 min, and 100% B for 27 min.

Data collection and statistical analysis

Antioxidant and whitening activities were performed in triplicate, while anti-inflammatory activities were conducted in six replicates. The mean and standard error of all collected data were calculated. To assess differences between treatments, an analysis of variance (ANOVA) was performed using Statistical Analysis Software version 9.4 (SAS; SAS Institute Inc., Cary, NC, USA), followed by a post-hoc analysis. The significance of each treatment interval in antioxidant activity was evaluated using a t-test (P < 0.05), while the significance of whitening and anti-inflammatory activities was assessed using Tukey’s honest significant difference (HSD) test (P < 0.05).

Results

Antioxidant contents and activities in CHE and OLE

The differences in antioxidant contents (TPC and TFC) between CHE and OLE were investigated (Fig. 2a). TPCs of CHE and OLE were 6.8 and 9.4 mg GAE g− 1, respectively. Similarly, TFCs of CHE and OLE were 2.2 and 10.1 mg QE g− 1, respectively. Antioxidant activities showed a trend similar to the antioxidant contents in CHE and OLE (Fig. 2b). The ABTS radical scavenging activities (RC50) of CHE and OLE were 6.0 and 4.2 mg mL− 1, respectively, while the DPPH radical scavenging activities were 13.3 and 12.3 mg mL− 1, respectively.

Fig. 2
figure 2

TPC and TFC contents (a) and DPPH and ABTS scavenging activities (b) of cabbage head extract (CHE) and outer leaves extract (OLE). Different letters indicate a significant difference (t-test, P < 0.05). Bars represent standard error (n = 3).

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Anti-inflammatory activities of CHE and OLE

RAW 264.7 cells showed excellent viability after treatment with the extracts (Fig. 3a). The cell viability after CHE (94.3%) and OLE (101.3%) treatments were higher than that after LPS treatment (86.0%). NO production decreased by treatment with quercetin, CHE, and OLE (Fig. 3b). In particular, CHE and OLE treatments led to significant reduction in NO contents to 19.7 and 18.2 µM, respectively, compared to that by LPS (22.5 µM).

Fig. 3
figure 3

Effects of cabbage head extract (CHE) and outer leaves extract (OLE) on cell viability (a) and nitric oxide production (b) of RAW 264.7 macrophages. QCT, quercetin of 25 µM. Different lowercase letters indicate a significant difference (Tukey’s HSD test, P < 0.05). Bars represent standard error (n = 6).

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Skin-whitening activities of CHE and OLE

As the concentrations of CHE and OLE increased, tyrosinase inhibitory activities proportionally increased (Fig. 4). CHE showed 37.1–52.1% inhibition, while OLE showed 29.8–100.4%, indicating that OL exhibited relatively high overall inhibition activity. In particular, 20 mg·mL− 1 OLE demonstrated 100.4% inhibition, which was comparable to that of the positive control AsA (97.7%).

Fig. 4
figure 4

Analysis of tyrosinase inhibitory activities of cabbage head extract (CHE) and outer leaves extract (OLE). The concentration of positive control ascorbic acid (AsA) was 1 mg mL− 1. Different lowercase letters indicate a significant difference (Tukey’s HSD test, P < 0.05). Bars represent standard error (n = 3).

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Analysis of glucosinolates, vitamins, and Sulforaphane contents in CHE and OLE

Differences in the types and contents of glucosinolates were noticed between CHE and OLE (Table 1). The content of aliphatic glucosinolates was relatively high in OLE. In particular, the levels of glucoerucin, glucoalyssin, gluconapoleiferin, and gluconapin in OLE were higher than those in CHE (trace amounts). Additionally, the contents of indole glucosinolates (4-hydroxyglucobrassicin, neoglucobrassicin, and glucobrassicin) and aromatic glucosinolates (gluconasturtiin) were relatively high in OLE. Total glucosinolate contents were 14.57 and 45.09 µmol g− 1 in CHE and OLE, respectively. Meanwhile, the vitamins identified in cabbage extracts were C and K1, with vitamin C detected only in OLE (0.14 µmol g− 1) (Table 2). Sulforaphane was not detected in CHE but was found in trace amounts in OLE.

Table 1 Analysis of glucosinolate contents in cabbage head extract (CHE) and outer leaves extract (OLE).
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Table 2 Analysis of vitamin and Sulforaphane content in cabbage head extract (CHE) and outer leaves extract (OLE).
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Discussion

Various strategies have been established for reducing agricultural waste worldwide, and among them, the valorization of agricultural waste and byproducts has been proposed as a major strategy for achieving sustainable agriculture12. Approximately ≥ 30% biomass of edible cabbage is removed during the harvest and distribution stages, leading to significant waste disposal13. Although cabbage OL accounts for most of the discarded biomass, the high antioxidant activity and potential value of OL are noteworthy14. Previous studies have analyzed the internal leaf layers of the cabbage head (CH) and reported vitamin C levels of 163 mg L− 1, 209 mg L− 1, and 202 mg L− 1 in the outer layer, central layer, and inner layer of the CH, respectively28. These layers refer specifically to the structured leaf sections that form the compact head of the cabbage. In contrast, our study focuses on the outer leaves (OL) that are naturally discarded during harvesting and processing, which are physiologically different from the CH leaf layers. In our analysis, the vitamin C content of OLE was measured at 0.14 µmol g− 1, which may be attributed to differences in sample origin, leaf exposure to environmental factors, sample preparation, extraction methods, and analytical conditions. In our study, OL showed higher antioxidant contents and activities than did CH, owing to the presence of relatively high levels of phenolic compounds and vitamin C29,30. Vitamin C is synthesized and regulated by UV rays and helps reduce reactive oxygen species. OL is highly exposed to light and synthesizes a high amount of vitamin C, which may explain the relatively high contents of phenolic compounds observed in OLE31. The relationship between light exposure and production of phenolic compounds has been previously demonstrated31,32. Notably, the levels of major antioxidants, such as flavonoids and anthocyanins, increase owing to the activation of phenylalanine ammonia lyase and chalcone synthase33. Additionally, the dark green phenotype of OL suggests high levels of chlorophyll synthesis owing to light exposure. Chlorophyll, abundant in plants, shows strong antioxidant activity and offers numerous nutritional benefits34. Discarded OL can be utilized as a high value-added resource, and has been previously used as various functional food additives in cakes15 and dietary fiber powder17,18. OL can serve as a plant-derived raw material for health-functional foods or cosmetics. The abundant vitamins, glucosinolates, and sulforaphane in Brassica spp. show antioxidant and anti-inflammatory effects35,36. In addition, vitamin C is a powerful antioxidant, which inhibits melanin synthesis, contributing to a skin-whitening effect. Excessive melanin synthesis and tyrosinase activity cause skin issues such as solar lentigo, melasma, and hyperpigmentation. Vitamins and sulforaphane inhibit melanin synthesis and tyrosinase expression, thereby helping in preventing skin problems and supporting skin whitening35,37. Brassica oleraceae38,39 and Brassica juncae40 improve ethosomal delivery, show skin whitening and anti-wrinkle effects, and aid in burn healing. In our study, OLE showed higher anti-inflammatory and skin-whitening activities than did CHE. To further investigate the bioactive components responsible for these effects, we conducted HPLC analysis focusing on glucosinolates and their hydrolysis products. These compounds, particularly sulforaphane and erucin, are known to modulate oxidative stress and regulate key pathways involved in melanin synthesis, such as the ERK and p38 MAPK pathways37. Glucosinolates, secondary metabolites in Brassicaceae plants, are hydrolyzed by myrosinase to form bioactive isothiocyanates (ITCs), including sulforaphane and erucin41,42. These compounds exhibit antioxidant, anti-inflammatory, and skin-protective properties, making them key contributors to the bioactivity of OLE. Sulforaphane has been shown to modulate melanogenesis and oxidative stress responses through multiple signaling pathways. Specifically, it activates the ERK pathway while inhibiting the p38 MAPK pathway, thereby suppressing melanin synthesis43,44. In addition, it enhances cellular antioxidant defenses by upregulating Nrf2-Keap1 signaling, leading to increased phase II detoxifying enzyme expression45. These findings highlight the dual role of sulforaphane in both skin protection and whitening. Similar to sulforaphane, vitamin C (ascorbic acid) also contributes to skin whitening by modulating key enzymatic and antioxidant pathways. It directly inhibits tyrosinase activity, preventing melanin synthesis, while also reducing oxidative stress46. Additionally, it promotes collagen synthesis, enhancing skin elasticity and uniformity47. Together, sulforaphane and vitamin C contribute to the skin-whitening potential of OLE by targeting key oxidative stress and melanogenesis pathways. In addition to sulforaphane, erucin also contributes to the antioxidant potential of OLE. Erucin, a structural analogue of sulforaphane, is derived from glucoerucin hydrolysis or in vivo reduction of sulforaphane48,49,50. These structurally related ITCs are known for their potent antioxidant activity, which may enhance the functional properties of OLE. In this study, the total glucosinolate content in CHE was measured at 14.57 µmol g− 1, whereas OLE contained a significantly higher amount of 45.09 µmol g− 1, approximately three times that of CHE. Previous studies have reported that glucosinolate content varies depending on the sowing period, leaf position, and cabbage cultivar, with average levels ranging from 8.55 to 13.5 µmol g− 1 in cabbage heads51. In contrast, CHE contained 14.57 µmol g− 1, slightly exceeding the upper range of previous findings, while OLE exhibited a remarkably higher concentration (45.09 µmol·g⁻¹), highlighting its potential as a superior source of glucosinolates. These results suggest that, unlike the compactly layered CH, discarded outer leaves serve as a much richer source of glucosinolates, likely due to their greater exposure to environmental stressors such as light and pathogens. Specifically, OLE contained more than 18 times higher levels of glucoerucin, a precursor of erucin, and also contained a trace amount of sulforaphane. Overall, the high content of bioactive compounds in OLE, particularly sulforaphane and vitamin C, suggests its strong potential as a natural ingredient for skin-whitening applications. Therefore, OL from discarded edible cabbage exhibits excellent antioxidant, anti-inflammatory, and skin-whitening activities and holds significant potential as a material for health-functional foods and cosmetics. However, further studies at the cellular and molecular levels are necessary to elucidate the precise bioavailability and efficacy of OLE-derived compounds in cosmetic and pharmaceutical applications.