Carbon Monoxide Treatment
in Tuna — What It Is,
Why Japan Banned It

What CO Treatment Is

Carbon monoxide treatment — sometimes marketed as "filtered wood smoke" or "tasteless smoke" — exposes raw tuna to CO gas before vacuum-packing. The chemistry is simple: CO binds to the iron atom in myoglobin's heme group with approximately 50 times the binding affinity of oxygen. The resulting complex, carboxymyoglobin (COMb), produces a vivid cherry-red color that is extraordinarily stable — resistant to oxidation, and persistent for days or weeks under refrigerated storage.

In untreated fish, myoglobin follows a natural progression. Oxymyoglobin (OxyMb) — the bright-red form present immediately after processing — oxidizes over time to metmyoglobin (MetMb), which is brown. This color shift is a genuine freshness signal: it reflects what is actually happening to the fish's proteins and fat. CO treatment chemically halts this transition. The fish looks the same on day one and day fourteen.

Why It Matters: The Freshness Signal Problem

Eating CO-treated tuna is not directly toxic. CO bound to myoglobin proteins is biologically inert when ingested — breathing CO gas is dangerous; eating it bound to protein is not. The problem is entirely about information.

Color is the primary tool consumers and buyers have to assess fish quality by eye. CO treatment permanently removes that tool. A piece of CO-treated tuna three weeks out of the water looks identical to one processed three days ago. There is no visual difference.

For tuna specifically, this creates a direct food safety risk. Tuna is a scombroid species — unusually high in the amino acid histidine. When tuna is held at unsafe temperatures during any point in the supply chain, bacteria convert histidine to histamine, the toxin responsible for scombroid poisoning. Histamine accumulates at the same rate whether or not CO treatment is applied. It has no visible indicator. CO treatment removes the color signal that might otherwise prompt a consumer or inspector to question the product's freshness.

Critically, TVB-N (total volatile basic nitrogen, 揮発性塩基窒素量) — Japan's primary chemical index of fish freshness — continues to rise in CO-treated tuna exactly as in untreated tuna. CO fixes color, not the underlying biochemistry of spoilage.

Japan's Ban — The MHLW 1997 Criteria

Japan's Ministry of Health and Labour (厚生省, now Ministry of Health, Labour and Welfare — MHLW) issued a formal notice on May 21, 1997, classifying CO-treated tuna as a violation of Article 6 of the Food Sanitation Act (食品衛生法第6条). The notice, titled 「マグロへの一酸化炭素ガスの使用について」 (On the Use of Carbon Monoxide Gas on Tuna), was issued by the Food Chemistry Division and Food Preservation Standards Division of the Environmental Health Bureau (生活衛生局食品化学課／食品保存基準課).

The notice applied to sliced, vacuum-packed tuna (マグロ), marlin (カジキ), and bonito (カツオ/鰹), targeting Southeast Asian exporters and Japanese importers. All such products were to be inspected before export and subjected to self-testing before distribution.

The VBN threshold is significant. For reference, Japan's Food Sanitation Act generally considers fish fresh at VBN < 25–30 mg/100g; borderline at 30–40 mg/100g; and unfit above 40 mg/100g for most species. The MHLW criteria were not set at the freshness boundary — they were set at the point where CO treatment was being used to disguise fish that was already clearly spoiled. The implicit legal position is that any CO treatment of fish constitutes misrepresentation of freshness condition under the Food Sanitation Act, independent of the VBN thresholds, which serve as the measurable enforcement criteria.

Since 1997, CO-treated tuna has remained prohibited under Japanese food law. The rule is enforced at the import stage — tuna entering Japan is subject to VBN testing and, where CO treatment is suspected, spectroscopic detection.

Global Regulatory Landscape

Japan's 1997 ban predates the other major prohibitions. The European Union prohibited CO treatment of fish and meat under Directive 95/2/EC (food additives other than colors and sweeteners), with CO not included on the positive permitted list. The EU Rapid Alert System for Food and Feed (RASFF) began flagging CO-treated tuna imports from Southeast Asia in 2003, and the prohibition was consolidated under Regulation (EC) No 1333/2008. Canada and Singapore also prohibit it.

The United States takes the opposite position. The FDA classifies CO treatment at low concentrations as Generally Recognized as Safe (GRAS), approved first for distribution MAP in 2002 and for retail fresh meat packaging in 2004. US law requires labeling — CO-treated products must disclose the treatment — but compliance is inconsistently enforced, and many retailers carry CO-treated tuna without clear labeling.

CO Treatment Beyond Fish: Beef and Pork

CO treatment is not unique to fish. In the US, beef and pork are widely packaged in Modified Atmosphere Packaging (MAP) — sealed trays flushed with a gas blend typically containing 0.4% CO, 30% CO₂, and 69.6% N₂. The CO binds to beef myoglobin in exactly the same way, locking in a stable bright red color for 20–30 days on the shelf.

A key US study (Hunt, Mancini et al., Journal of Food Science, 2004) found that 0.4% CO in beef MAP did not mask microbial spoilage — at this concentration, carboxymyoglobin color did not persist beyond the point where aerobic bacteria reached spoilage levels (≥ log 7 CFU/g). This finding is important context: the "masking" concern for beef is different from the concern for tuna. In tuna, the hazard is histamine accumulation — which is odorless, tasteless, and visually undetectable — not aerobic bacterial spoilage, which eventually produces visible and olfactory cues regardless of color.

The EU, Canada, and Japan have banned CO treatment for beef and pork as well, on consumer deception grounds. A consumer who grows up buying only CO-treated beef and CO-treated tuna learns to associate vivid red with freshness — which is not biologically accurate, and which makes it harder to apply appropriate skepticism at the fish counter.

Sashimi DC: Never CO-Treated

None of Sashimi DC's fish is CO-treated. The Bluefin Tuna arrives from Goto Islands, Nagasaki — sourced under Japanese domestic supply chain standards, where CO treatment is prohibited. The natural color of our Akami is dark red-purple, not cherry-red. If it looks slightly darker than what you are used to seeing at American supermarkets, that is the actual color of fresh, untreated Bluefin Tuna.

This is one of the reasons color is a useful quality signal when buying from Sashimi DC — it remains an honest signal. Vivid cherry-red tuna at a US fish counter is, with very few exceptions, CO-treated. Dark red-purple tuna means the color has not been chemically fixed, and that you are seeing the fish's actual condition.

Salmon Color: Astaxanthin, Not Dye

A question that comes up alongside CO treatment: is farmed salmon dyed? The short answer is no — but the mechanism is worth understanding, because it clarifies exactly where the CO problem ends and normal aquaculture practice begins.

Why salmon flesh is orange or red

Salmon flesh color comes entirely from carotenoid pigments — primarily astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione) and, to a lesser degree, canthaxanthin. Salmon cannot synthesize these pigments themselves; they must obtain them from diet. The pigment is deposited in muscle tissue by binding to the protein alpha-actinin — it is structurally incorporated into the flesh, not a surface coating or a post-harvest chemical fix.

Wild salmon get their carotenoids by eating krill, shrimp, and copepods, which are themselves rich in astaxanthin produced by microalgae (Haematococcus pluvialis). Farmed salmon lack access to krill, so astaxanthin is added to their feed and accumulates over weeks of normal metabolic activity — the same biological process, just with a different dietary source.

Natural vs. synthetic astaxanthin

Two main forms are used in aquaculture:

• Synthetic astaxanthin (from Roche/DSM): the dominant form globally. Same molecular formula (C₄₀H₅₂O₄) as the natural compound, but produced as a racemic mixture of three stereoisomers — (3S,3′S), (3R,3′S), and (3R,3′R) in roughly a 1:2:1 ratio. In free (non-esterified) form, it is highly bioavailable and efficient at pigmenting flesh.
• Natural astaxanthin: from H. pluvialis microalgae, Phaffia rhodozyma yeast, or Calanus copepod oil. Predominantly the (3S,3′S) isomer, matching wild sources. Esterified form requires hydrolysis during digestion, making it somewhat less efficient at flesh pigmentation — but growing in use as consumer preference shifts toward natural ingredients.

Why some salmon look redder, others more orange

Both hues come from the same pigment family. The difference is driven by concentration, pigment composition, and species:

• Concentration: higher total carotenoid content = deeper, more saturated color. The aquaculture industry uses the Roche SalmoFan™ (a color card with 15 chips, numbered 20–34) to grade and manage flesh color. Western markets typically target 26–28; Japanese markets prefer 30–34.
• Canthaxanthin ratio: canthaxanthin absorbs at a slightly shorter wavelength (~470 nm vs. ~480 nm for astaxanthin), producing a more intensely red hue. Wild sockeye's signature deep red comes partly from naturally high canthaxanthin in its crustacean diet. Commercial feeds often blend both pigments to hit target color scores.
• Species: sockeye accumulates up to 40 mg/kg of carotenoids — the benchmark for deep salmon red. Wild Atlantic salmon holds ~5–8 mg/kg and appears more orange-pink. Pink salmon is palest at ~4–6 mg/kg.
• Spawning stage (wild only): as Pacific salmon migrate upriver to spawn, they stop eating and redirect stored carotenoids from muscle to skin and roe. Flesh actually becomes paler near spawn — ocean-caught fish is at peak flesh color because this redistribution has not yet started.

How this differs from CO treatment

The key distinction is when and how the color is established:

• Astaxanthin is deposited in living muscle over weeks while the fish is alive. After harvest it degrades naturally — flesh still browns and fades as the fish ages, so color remains a genuine freshness signal.
• CO is applied to dead fish after harvest, chemically locking myoglobin into a stable cherry-red that does not degrade as the fish spoils. A CO-treated fillet can appear bright red while histamine accumulates to dangerous levels.

Astaxanthin creates an honest color baseline; CO removes color as a signal entirely. A consumer who sees salmon fading or browning at the edges is seeing a normal, informative freshness cue. A consumer who sees tuna remaining uniformly vivid cherry-red for two weeks is seeing a signal that has been chemically overridden.

Sources

• Cornforth, D. P., & Hunt, M. C. (2008). CO MAP Packaging — AMSA White Paper. American Meat Science Association. https://meatscience.org/docs/default-source/publications-resources/white-papers/wp_002_2008_co_map_packaging.pdf — Industry white paper summarizing CO-MAP science, safety data, and regulatory position.
• Dissing, B. S., Nielsen, M. E., Ersbøll, B. K., & Frosch, S. (2011). Multispectral Imaging for Determination of Astaxanthin Concentration in Salmonids. PLOS ONE. https://doi.org/10.1371/journal.pone.0019032 — Validates the mechanism by which astaxanthin binds to muscle protein alpha-actinin in salmon flesh.
• Djenane, D., & Roncalés, P. (2020). Carbon Monoxide in Meat and Fish Packaging: Advantages and Limits. Foods. https://www.mdpi.com/2304-8158/7/2/12 — Comprehensive review of CO-MAP mechanism, CO–myoglobin binding kinetics, regulatory status by country, and the color-stability/freshness decoupling problem.
• Hunt, M.C. et al. (2004). Carbon Monoxide in Modified Atmosphere Packaging Affects Color, Shelf Life, and Microorganisms of Beef Steaks and Ground Beef. Journal of Food Science. https://ift.onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2621.2004.tb17854.x — Foundational red-meat CO-MAP study. Established that 0.4% CO maintains cherry-red color in beef for 28+ days with no deleterious effect on microbial safety — the basis for US GRAS approval that later extended to fish.
• Marrone, R. et al. (2015). Carbon Monoxide Residues in Vacuum-Packed Yellowfin Tuna Loins (Thunnus Albacares). Italian Journal of Food Safety. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5076633/ — Quantifies CO residues in commercially treated tuna loins and models consumer exposure.
• Ministry of Health and Welfare. (1997, May 21). マグロへの一酸化炭素の使用について [Regarding the Use of Carbon Monoxide for Tuna]. Ministry of Health, Labour and Welfare. https://www.mhlw.go.jp/www1/index.html — Primary regulatory notice classifying CO-treated tuna as a violation of Article 6 of Japan's Food Sanitation Act, with the specific VBN 200/500 mg/100g criteria — the guide's central primary source, quoted and linked directly in the body.
• Nakano, T., & Wiegertjes, G. (2020). Properties of Carotenoids in Fish Fitness: A Review. Marine Drugs, 18(11), 568. https://www.mdpi.com/1660-3397/18/11/568 — Comprehensive review of astaxanthin and canthaxanthin in salmonids: biosynthesis, dietary uptake, deposition physiology, antioxidant roles.
• Pais, G.L., Meloni, D. et al. (2022). Colorimetric Analysis and Determination of Histamine in Samples of Yellowfin Tuna. Foods. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8909452/ — Documents the key safety concern: histamine and TVB-N (authentic spoilage markers) increase normally in CO-treated tuna while color remains stable cherry-red, making visual freshness assessment unreliable.
• SeafoodSource. (n.d.). Conning Consumers with CO — SeafoodSource. https://www.seafoodsource.com/features/conning-consumers-with-co — Trade publication investigation into CO labeling practices in US retail sushi.
• Smulevich, G. et al. (2007). A rapid spectroscopic method to detect the fraudulent treatment of tuna fish with carbon monoxide. Food Chemistry. https://www.sciencedirect.com/science/article/abs/pii/S0308814606002135 — Demonstrates reflectance spectroscopy as an objective method to distinguish carboxymyoglobin (CO-treated) from oxymyoglobin (fresh) in tuna — the Soret absorption band at 420 nm shifts predictably.
• Van Rooyen, L.A., Allen, P., & O'Connor, D.I. (2017). The application of carbon monoxide in meat packaging needs to be re-evaluated within the EU: An overview. Meat Science. https://www.sciencedirect.com/science/article/abs/pii/S0309174017302097 — Reviews the European re-evaluation process and confirms the EU ban on CO-MAP for meat and fish, citing consumer deception (color masking freshness) as the primary ground — not food safety per se.
• Zhao, W., Guo, Y.-C., Huai, M.-Y., Li, L., Man, C., Pelletier, W., Wei, H.-L., Yao, R., & Niu, J. (2022). Comparison of Retention Rates of Synthetic and Natural Astaxanthin in Feeds and Their Effects on Pigmentation, Growth, and Health in Rainbow Trout. Antioxidants, 11(12), 2473. https://www.mdpi.com/2076-3921/11/12/2473 — Controlled trial comparing synthetic (free-form racemic: 3S,3'S / 3R,3'S / 3R,3'R at 1:2:1) vs. natural astaxanthin from H. pluvialis (esterified, predominantly 3S,3'S) in rainbow trout.

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