Toxicological qualities and detoxification trends of fruit by-products for valorization: A review

Overview

Open-access narrative review (CC BY 4.0, De Gruyter) synthesizing toxicological data on by-products (peels, pomace, seeds, kernels, rinds) of the world’s highest-production fruits as of 2022, motivated by the valorization push that increasingly routes these by-products into food ingredients, nutraceuticals, cosmetics, and animal feed. Scope is global; the review pulls from refs [1–142] without restricting to any single jurisdiction, growing region, or analytical method. Hazards examined are heavy metals (As, Cd, Co, Cr, Ni, Pb, Hg), mycotoxins (aflatoxins, OTA, fumonisins, ZEN, T-2, alternaria toxins, tryptoquialanines), anti-nutritional factors (oxalates, phytates, tannins, alkaloids, saponins, HCN, glycosides), toxicant organic compounds (biogenic amines, AMG, phthalates), and pesticide/fungicide residues. Final section reviews thermal, chemical, and non-thermal detoxification methods (high-pressure processing, pulsed electric field, cold plasma, supercritical CO₂, ultrasound, ozone treatment, lyophilization, fermentation).

Key numbers

Hazard quotient ranges cited from upstream literature (ref [2]) for fruit by-products generically (the review does not attribute these to any specific commodity or sample set): As 2.71–11.38, Cd 0.60–3.32, Cr 0.81–3.18, Co 0.03–0.09, Cu 0.09–0.26, Ni 0.08–0.34, Pb 0.83–2.23 (mg/kg basis stated in the review; HQ values above 1.0 indicate concern in cited contexts).

Table 1 (dominant heavy metals in fruit by-products, all mg/kg unless noted):

• Apple peel: Pb <1, Cd <1, Cr 0.57–3.8, Ni <1 (refs [37,40,41]).
• Apricot kernel: Cd 0.1–6, Co 2.7–35.7 (ref [42]).
• Avocado seed: Cr 0.57–2.29, Co 0.00, Pb 0.00 (refs [40,43]).
• Banana peel: As <0.0001, Cd 0.0013–0.18, Cr 1.42–4.04, Co 0.4–47.2, Ni 0.592, Pb 0.0038–0.64 (refs [36,44,45]).
• Blueberry pomace: Cd 0.011, Cr 0.242, Co 0.08, Ni 0.73 (ref [46]).
• Grape peel/skin/pomace: As 0.004, Cd <0.5, Cr 0.18–2.41, Ni <0.5, Pb 0.021–1.11 (refs [34,35,37]).
• Lemon peel by-products: Cd 0.00047–0.25, Cr 1.04, Co 0.038, Ni 0.973–1.24, Pb 0.0188–0.22 (refs [34,36,39]).
• Lime by-products: As ND, Cd 0.003, Co 0.073, Ni 1.678, Pb 0.128 (ref [39]).
• Mango (peel + seed kernel composite): Cd 0.33, Co 28.0 (ref [45]).
• Orange peel: Cd <0.5, Cr 1.04–4.14, Co 0.015, Ni 0.05–2.36, Pb 0.01–1.75 (refs [34,36,37,39,47]).
• Papaya peel + seed: As 0.0287–0.03, Cd 0.0027–0.00685, Cr 0.278–7.36, Co 0.4–219, Ni 0.246–13.2, Pb 0.03–0.044 (refs [45,48,49]).
• Peach: Cr 0.17–1.38, Pb <0.10 (ref [50]).
• Pineapple peel: As <0.0001, Cd 0.0074, Cr 8.77, Co 70.3, Pb 0.0027 (refs [44,45]).
• Plum peel: As 1.2, Cd ND, Ni 2.8, Pb ND (ref [51]). Plum kernel: As <0.1, Cd 0.13, Co 0.2, Ni 1.7, Pb 0.13 (ref [52]).
• Pomelo peel: Cd 1.36 × 10⁻³, Pb 0.0296 (ref [34]).
• Raspberry pomace: Cd 0.0084, Cr 0.116, Co 0.073, Ni 0.762, Pb 0.047 (ref [46]).
• Strawberry pomace: Cd 0.0078, Cr <0.01, Ni 0.00336, Pb 0.0011 (refs [53,54]).
• Mandarin: Cd 0.00062, Pb 0.020 (ref [38]).
• Watermelon peel + seed: Cd 0.008–0.1, Cr 4.65, Pb 0.06–0.09 (refs [40,55]).

Single-paper outlier (peel): Pineapple skin reported at 0.64 mg/100 g Pb (= 6.4 mg/kg) dry weight per refs [13,14] — above the WHO 0.02 mg/100 g (= 0.2 mg/kg) maximum limit for edible parts of crops cited in the review. Six other by-products in the same comparison (orange peel, watermelon rind, banana peel, apple pomace, strawberry pomace, grape pomace) showed Pb below 0.1 µg/L (a unit the review states without converting to mg/kg; not reconciled in the source text).

Banana peel ANFs (M. sinensis, fresh, ref [15]): alkaloid 0.66 g/100 g, oxalate 37.0 g/100 g, phytate 2.78 g/100 g, saponin 6.57 g/100 g. Same paper reported boiled banana fruit extract values higher: alkaloid 1.76 g/100 g, oxalate 40.2 g/100 g, saponin 8.12 g/100 g.

Regulatory limits cited in review (mg/kg unless noted):

• WHO maximum for Pb in edible parts of crops: 0.02 mg/100 g (= 0.2 mg/kg) (ref [12]).
• WHO maximum for As and Pb (monthly intake): 0.21 mg/100 g (ref [12]).
• WHO maximum for Cd: 0.25 mg/100 g (ref [12]).
• WHO maximum for Al: <0.2 mg/100 g (ref [12]).
• EU Regulation 1275/2013 / Ordinance 10/2009 (animal feed): 1 mg/kg Cd, 10 mg/kg Pb (ref [38]).
• European Commission MRL for citrus by-products: As 0.02, Cd 0.02, Hg n/a, Co n/a, Ni n/a, Pb 0.1 mg/kg (ref [56]).
• WHO/FAO RDI thresholds for orange by-product analytes: Cr 0.05–0.2, Ni 1.4, Pb 0.214 mg/day (ref [87]).
• FAO/WHO ADI: Pb 0.21–0.25 mg/day, Cd 0.06–0.07 mg/day (ref [55]).

Mycotoxin highlights (mg/kg, from Table 2 of the review):

• Apple peel/skin/seed/pomace: AOH-3-S 1.4–10.8 × 10⁻³, AME-3-S 1.7–10 × 10⁻³ (refs [57–61]).
• Apricot seeds: AFB1 and AFB2 = 0.0017–22.451 (refs [62–64]).
• Pineapple shell: Fusarium = 250 (refs [70–72]). Pineapple pomace: Aflatoxin B₂ and B₁ = 0.008 × 10⁻³, Aflatoxin G₁ = 0.013–0.033 × 10⁻³, OTA = 0.051 × 10⁻³ (refs [60,72,79]).
• Tangerine peel: tryptoquialanine A = 248.1, tryptoquialanine C = 375.80 (refs [34,60,69,72–75]).
• Watermelon peel: AOH = 0.003–0.017 (refs [60,81,82]).
• Pineapple shell mycotoxins: aflatoxin B₂, aflatoxin G₁, and OTA all measured below detection limit 2.9 × 10⁻³ mg/kg (ref [77]).
• Stępień et al. [78]: pineapple skin infected with mycotoxin mainly Fusarium 250 mg/kg (cited US FDA mycotoxin exposure guideline for total fusarium in cereal foods is 2–4 mg/kg — paper notes the cited value as hazardous without pretreatment).

ANF outliers (mg/kg):

• Orange peel oxalates: 997.8 (Table 2).
• Watermelon meal: phytate 9,900, tannin 32 × 10⁵, oxalate 2,130 (ref [81]).
• Mango kernel flour ANFs: tannins 1027.4, phytic acid 1149.8, oxalates 213.4, HCN 0.00 (ref [63]).
• Pineapple pomace: HCN 715, alkaloids 161.9.
• Apricot seed AMG (amygdalin): 52,000 mg/kg (ref [62]) — review notes 3.0 mg/kg body weight oral dose did not induce changes in rabbit spermatozoa.

Pesticide outliers (mg/kg):

• Mango peel: chlorpyrifos and methiocarb residues within Codex MRL (refs [65,66]).
• Orange pomace: imidacloprid 162.16, carbendazim 372.1, abamectin 0.261, cypermethrin 495.6, prochloraz 8.11.7 (refs [34,60,69,72–75]).
• Pomelo: total pesticides 2.143 mg/kg, procymidone 0.7 (ref [34]).

Fruit waste percentages (Figure 1b, % of fresh fruit weight discarded as by-product): pineapple peel + crown + core 60%, orange pomace 50%, lemon pomace 50%, mango peel + seed 45%, banana peel 35%, watermelon rind 33%, kiwi seeds 30%, melon peel + seed 30%, papaya peel + seed 29.5%, peach peel + seed 24.8%, grape pomace 20%, cashew-apple bagasse 20%.

Global production 2022, million tonnes (Figure 1a, FAOSTAT): bananas 135.1, watermelons 100.4, apples 95.8, oranges 76.4, mangoes/guavas/mangosteens 59.2, tangerines/mandarins/clementines 44.2, pineapples 29.4, melons 28.6, peaches/nectarines 26.4, pears 24.6, plums/sloes 12.4, papayas 13.8.

Cytotoxicity findings noted:

• Banana peel aqueous extract: no cytotoxicity to CAL 27, Caco-2, or HepG2 at 0.014–10 mg/mL across 0.5/1/2 h treatments; 10 mg/mL polysaccharide-free aqueous extract showed slight proliferation in Caco-2 (ref [105]).
• Pomelo seed oil: IC₅₀ in LO₂ and HepG2 cells 16.31–28.8 mg/mL (non-cytotoxic per the >1 mg/mL criterion stated by the review) (ref [93]).
• Avocado peel/seed (Hass and Fuerte): no cytotoxicity to RAW 264.7 cells at 0.1–100 µg/mL (ref [103]).
• Citrus deliciosa peel EO (mature fruit): IC₅₀ 110 µg/mL against HT-29 cells (ref [94]).

Methods (brief)

Narrative review, not a systematic review. The authors prioritized fruits with global production exceeding 1.2 million tonnes in 2022 per FAOSTAT (ref [22]). No PRISMA-style search strategy, inclusion/exclusion criteria, or risk-of-bias assessment is reported. No pooled estimates or meta-analytic synthesis. All numerical values are extracted from cited primary literature (refs [1–142]) and presented in Tables 1 (heavy metals), 2 (mycotoxins/toxicant organic compounds/ANFs/pesticides), and narrative subsections. Sample sizes, analytical methods, LODs, and matrix-prep conditions are not consistently reported for each entry — many cells in Table 1 give a single concentration or a range without n or method context.

Editorial timing: received 14 October 2024, accepted 24 March 2025. Open-access publication under CC BY 4.0.

Limitations explicit to this review’s evidence quality:

1. Tertiary synthesis. All concentrations are second-hand from cited primary studies. Disagreements between primary sources are noted in Table 1 (e.g., banana peel Cd 0.0013–0.18 mg/kg spans two orders of magnitude across refs [36,44,45]) but not arbitrated.
2. Units are inconsistent across the review body and tables. mg/kg, µg/L, mg/100 g, g/100 g, and dry-weight versus wet-weight bases are used in adjacent sentences without harmonization. The pineapple Pb 0.64 mg/100 g vs other by-products “below 0.1 µg/L” contrast is the clearest example.
3. Speciation absent. Heavy metal entries are reported as total element (As, Hg, Cr) without iAs/tAs, MeHg/tHg, or Cr-VI/Cr-III breakdown. Toxicologically critical for As, Hg, and Cr.
4. Hazard quotient ranges (As 2.71–11.38, etc.) are cited from a single upstream review (ref [2]) without identifying the underlying contamination concentrations, exposure scenarios, body-weight assumptions, or consumption rates used to compute them.
5. Pineapple peel Pb outlier (0.64 mg/100 g dry weight) is presented without sample n, geographic origin, growing-region soil background, or replication context. A single dry-weight measurement reported as a hard maximum should not be over-weighted in any downstream synthesis.
6. No regulatory-event documentation. The review cites WHO and EC limits as comparators but does not document any enforcement actions, recalls, or jurisdictional MRL exceedances tied to by-product valorization.
7. Suspected source-printing errors preserved verbatim in Key numbers (the review is the authoritative record; we do not silently correct):
   • Watermelon meal tannin “32 × 10⁵ mg/kg” (Table 2, ref [81]) implies 3,200 g/kg, which exceeds physical possibility (>100% of dry mass). Most likely intended 32 × 10² (= 3,200 mg/kg) or similar; do not propagate as a quantitative input without re-checking ref [81].
   • Orange pomace “Prochloraz = 8.11.7 mg/kg” (Table 2, refs [34,60,69,72–75]) is a malformed numeric (extra decimal). Do not propagate as a quantitative input without re-checking the cited refs.

Implications

For certification: Pineapple peel, mango peel/kernel, orange peel, and watermelon rind concentrate metals (notably Cd, Cr, Co, Pb) and ANFs relative to whole-fruit edible flesh. Programs certifying derivative ingredients sourced from these by-products (fruit-fiber concentrates, peel-derived pectin, seed-meal flours, pomace press-cakes used as functional ingredients) cannot rely on whole-fruit contamination profiles. This review does not provide enough primary-data structure to set thresholds directly, but it establishes that by-products as a functional ingredient class warrant separate analytical treatment from the edible fraction.

For app modeling: Insufficient sample-size, LOD, and method-context disclosure to support ppb-level exposure modeling for any specific by-product ingredient. Use as a routing pointer to the primary studies cited in Tables 1 and 2 rather than as a quantitative input. The review’s chief contribution to app modeling is signaling which fruit by-products are studied at all — many tropical fruits (papaya, pomelo, cherimoya) and stone fruits beyond apricot have only one or two primary citations.

For courses and synthesis pages: Useful as the entry point for the fruit-by-product valorization domain. Sections 7 (“Current trends on fruit by-product toxicant reduction”) and Figure 4 catalog the detoxification methods (HPP, PEF, cold plasma, supercritical CO₂, ultrasound, lyophilization, fermentation) that are increasingly relevant to ingredient suppliers — material for an ingredient-handling course module.

Verification notes

2026-05-20 (merge-enhance pass). Pre-existing page dated 2026-05-14 carried defects from earlier ingest:

• raw_handle: manual-fetch-kimi replaced with MFK_toxicological-qualities-and-detoxification-trends- per current MFK_ handle convention.
• raw_path was truncated to ...fruit by-pro.pdf; corrected to the full PDF filename ...fruit by-products_ A review.pdf.
• raw_sha256 was missing; added (8f66d00ee3ae954d5e379415834e79667cf20b21a02edb6e95752154c8d69e36).
• access_url was missing; added DOI link.
• metals was [tAs, Pb, Cd, Cr, Ni, tHg]; Co added — the paper’s Table 1 reports Co for apricot kernels, avocado seeds, banana peel, blueberry pomace, lemon, lime, mango, orange, papaya, pineapple, plum, raspberry, watermelon, with single-paper outliers up to 219 mg/kg (papaya, ref [45,48,49]).
• ingredients previously listed invented sub-variant slugs (banana-peel, apple-pomace, orange-peel, grape-pomace, mango-peel, pineapple-peel, watermelon-rind, strawberry-pomace) that do not exist in the wiki’s ingredient taxonomy and would route to nothing. Replaced with the seventeen valid whole-fruit parent slugs that match the species discussed: banana, apple, oranges, grapes, mango, pineapple, watermelon, avocado, peach, plums, blueberries, strawberries, raspberries, grapefruit, clementines-tangerines, pear, fruit. The by-product framing is preserved via matrices and the body.
• matrices retained as [fruit-by-products, fruit-peels, fruit-pomace, fruit-seeds] — these accurately describe the review’s subject (by-product fractions, not fresh fruit) and the existing routing audit treats this set as advisory-only.
• products: [] and jurisdictions: [] retained — review article with no specific commercial product framing and global scope.
• Key numbers expanded from ~5 lines to full Table 1 commodity-by-commodity breakdown (all 19 commodities with their per-fraction metal concentrations and refs), plus Table 2 mycotoxin/ANF/pesticide outliers, regulatory-limit citations, global production 2022 in million tonnes from Figure 1a, by-product waste percentages from Figure 1b, and the four cytotoxicity findings reported in Sections 3–6.
• Methods section expanded to flag six explicit evidence-quality limitations: tertiary synthesis nature, unit inconsistency, absent speciation, hazard-quotient citation gap, single-measurement Pb outlier risk, and absence of regulatory-event documentation. The original page omitted these.
• Legacy “Wiki pages updated on ingest” section removed (replaced by the new schema’s expectation that downstream propagation happens during synthesis, not ingest).
• ingest_method and created_by fields added per current frontmatter schema.

No primary-data values were silently altered. The review’s pineapple peel Pb outlier (0.64 mg/100 g dry weight) and the contrasting “below 0.1 µg/L” framing for the other six by-products are preserved verbatim as they appear in the source on p.2 even though the unit basis is inconsistent — the review’s own framing is the authoritative record.

Audit subagent (2026-05-20) — REVISE verdict; three ⚠️ findings, all applied:

1. Pineapple shell vs pomace mycotoxin attribution — the original “Pineapple shell/pomace” collapsed row was split per Table 2 p.7 into “Pineapple shell: Fusarium = 250” (refs [70–72]) and “Pineapple pomace: Aflatoxin B₂ and B₁ = 0.008 × 10⁻³, Aflatoxin G₁ = 0.013–0.033 × 10⁻³, OTA = 0.051 × 10⁻³” (refs [60,72,79]). The aflatoxin label was also corrected from “B₁” to the source’s “B₂ and B₁”.
2. Watermelon meal tannin (32 × 10⁵ mg/kg) and orange pomace Prochloraz (8.11.7 mg/kg) added as bullet 7 in the limitations list, explicitly flagged as suspected source-printing errors preserved verbatim. The review is the authoritative record; we do not silently correct primary-source typesetting.
3. Matrices vocabulary clarification: matrices: [fruit-by-products, fruit-peels, fruit-pomace, fruit-seeds] are descriptive of the review’s actual subject (by-product fractions, not fresh fruit) and are treated as advisory-only by the routing audit. They are not enumerated in the taxonomy snapshot’s controlled-vocabulary list, but they remain in frontmatter because removing them would mis-describe the source.

Update history

The five most recent substantive edits to this page, classified major (evidence or structure moved), correction (a published value or statement was wrong and has been fixed), or minor (narrative rewritten without changing the underlying evidence). Each description is derived from what the edit did to this page; the linked commit is the authoritative record, routine regeneration passes are excluded, and the full version history lives in git. When DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.