How can you tell if tuna has been CO-treated?
The main visual cue is color that looks artificially consistent — uniformly vivid cherry-red that doesn't shift with age, season, or storage time. Genuine untreated Akami (lean Bluefin) is naturally dark red-purple, not bright red. The deeper problem: CO treatment makes it impossible to judge freshness by eye, since the cherry-red color persists for weeks regardless of condition. Japan, EU, Canada, and Singapore ban it. Sashimi DC's Bluefin is never CO-treated.
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.
The mechanism
Oxymyoglobin → Carboxymyoglobin
Untreated tuna
OxyMb (bright red) → MetMb (brown) as oxygen leaves the heme iron and the iron oxidizes from Fe²⁺ to Fe³⁺. Color visibly darkens over days — a reliable freshness indicator.
CO-treated tuna
OxyMb + CO → COMb (cherry red, locked). CO's 50× affinity for heme iron displaces oxygen and prevents oxidation to MetMb. Color stays vivid for 7–21+ days regardless of condition.
Detection: COMb has a characteristic UV-Vis Soret band at 420 nm (vs. 418 nm for OxyMb; 408 nm for MetMb). Second-derivative spectroscopy in the 500–600 nm range clearly distinguishes CO-treated from untreated tuna. Gas chromatography/mass spectrometry (GC/MS) can quantify CO residues directly.
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.
The histamine risk is specific to tuna. In beef and pork, the concern is different: bacterial spoilage produces visible sliminess and off-odors that act as secondary warnings even when color is masked. In tuna, the primary safety hazard is histamine — odorless, tasteless, and accumulating invisibly behind a chemically fixed cherry-red surface.
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.
MHLW Violation Criteria (1997)
A tuna product is classified as CO-treated and in violation of the Food Sanitation Act Article 6 if either of the following conditions applies:
- ①VBN ≥ 200 mg/100g at start of CO treatment, AND the second VBN measurement is clearly lower than the first — indicating CO was applied to a fish already in measurable spoilage, artificially reversing the apparent VBN signal.
- ②VBN ≥ 500 mg/100g at start of CO treatment — regardless of any measurement comparison, a fish at this level of spoilage is unfit for sale.
Source: MHLW Notice, May 21, 1997. 原文 (Japanese)
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.
Japan — Banned 1997
Food Sanitation Act Article 6 · MHLW Notice
Prohibited under the Food Sanitation Act. Enforcement via VBN testing at import. Violation criteria: VBN ≥ 200 mg/100g with CO-induced suppression of the second measurement, or VBN ≥ 500 mg/100g at start of treatment. Applies to tuna, marlin, and bonito.
European Union — Prohibited
Directive 95/2/EC · Regulation 1333/2008
CO is not listed as a permitted food additive. The European Commission's stated basis: "the stable cherry colour can last beyond the microbiological shelf life of the meat, and thus mask spoilage." RASFF alerts on CO-treated tuna imports began in 2003.
Canada & Singapore — Prohibited
Consumer protection basis
Both countries prohibit CO treatment of fish on consumer deception grounds — color treatment does not reflect actual freshness and misleads buyers at point of sale.
United States — Permitted (GRAS)
FDA GRAS · Labeling required, compliance variable
FDA classifies CO treatment as GRAS, approved for retail packaging since 2004. Labeling is required. Compliance with labeling requirements is inconsistently enforced. CO-treated tuna is widely available at US supermarkets and sushi counters, frequently without clear disclosure.
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.
Color guide for untreated Bluefin Tuna Akami: Natural Akami is dark red-purple to deep ruby — similar to a dark garnet. Summer Akami may be slightly brighter; winter Akami (peak December–February) is denser and deeper. It should never be a vivid, uniform cherry-red. If tuna is the same bright color regardless of cut, season, or storage duration, the color has been chemically stabilized.
Sources
- MHLW (厚生省), マグロへの一酸化炭素ガスの使用について, May 21, 1997. mhlw.go.jp — Primary source for Japan's VBN-based violation criteria.
- Prache et al. (2006), "A rapid spectroscopic method to detect the fraudulent treatment of tuna fish with carbon monoxide," Food Chemistry 100(3). DOI: 10.1016/j.foodchem.2006.02.014 — CO–myoglobin binding mechanism and Soret band detection at 420 nm.
- Cano-Sancho et al. (2016), "Carbon Monoxide Residues in Vacuum-Packed Yellowfin Tuna Loins," PMC5076633 — CO residue quantification; freshness indicator independence; EU regulatory context.
- Mancini & Hunt (2018), "Carbon Monoxide in Meat and Fish Packaging: Advantages and Limits," Foods 7(2):12. MDPI open access — Comprehensive mechanism, safety, and regulatory review.
- Hunt, Mancini 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 69(1):FCT45 — CO-MAP in beef; finding that 0.4% CO did not mask spoilage in beef.
- Visciano et al. (2022), "Colorimetric Analysis and Determination of Histamine in Yellowfin Tuna," Foods 11(5):639. PMC8909452 — Histamine accumulation independent of CO treatment; scombroid poisoning risk.
- Cornforth & Hunt (2008), "Low-Oxygen Packaging of Fresh Meat with Carbon Monoxide: Meat Quality, Microbiology, and Safety," AMSA White Paper No. 2. PDF