Fish oil freshness: why some omega chews smell

Short answer: Omega chews smell strong when the marine oil in them has oxidised. Oxidation causes the smell and consumes the EPA and DHA at the same time, so a rancid product also delivers less than its label claims. Standard test panels — heavy metals, microbial, pesticides — do not detect it. The three numbers that do are peroxide value (PV), p-anisidine value (AnV) and TOTOX.

If you've ever opened a jar of fish oil chews and been hit by a smell far stronger than you expected, you've met oxidation. It is the most common quality failure in marine oil supplements, and it is almost never tested for.

What is oxidation, and why does it cause a smell?

The fatty acids that make marine oil valuable — EPA and DHA — are polyunsaturated. Those double bonds are exactly what makes them biologically useful, and exactly what makes them fragile. (If the EPA and DHA distinction is new, start with why EPA and DHA matter more than total omega-3.)

Exposed to oxygen, heat or light, they break down. The process produces the aldehydes and ketones responsible for the smell, and it consumes the EPA and DHA in the process.

So a rancid product has two problems, not one. It smells unpleasant, and it contains less of the active than the label claims.

Why doesn't standard testing catch it?

Look at a typical certificate of analysis. Heavy metals, microbial counts, proximate analysis, perhaps a pesticide screen.

None of these detect oxidation. A thoroughly rancid product will pass every one of them. What "third-party tested" actually means covers what else a standard panel leaves out.

Oxidation also develops over time rather than at manufacture. A batch can be tested on the day it's made, pass everything, and degrade substantially over the following eighteen months in a warehouse.

What are PV, AnV and TOTOX?

They are the oil industry's established measures of oxidation. Standard, inexpensive, and rarely published on pet products.

Peroxide Value (PV) measures primary oxidation — the first stage. Expressed as meq/kg of oil.

p-Anisidine Value (AnV) measures secondary oxidation — the breakdown products responsible for smell and taste.

TOTOX combines both into a single figure, calculated as (2 × PV) + AnV.

PV alone can be misleading, because peroxides form and then break down further. An oil can show a low peroxide value while being well past its best. This is why AnV and TOTOX matter.

Widely used limits for marine oil are PV at or below 5, AnV at or below 20, and TOTOX at or below 26.

What makes an oil oxidise faster?

Several things determine whether an oil arrives fresh and stays fresh:

  • Antioxidant system — mixed tocopherols and rosemary extract are natural options; there are synthetic ones too
  • Packaging — an oxygen barrier matters, which is why a sealed jar or a foil-lined pouch outperforms a thin plastic bag
  • Storage temperature — oxidation accelerates with heat, so a garage or a loft is a poor place to keep supplements
  • Time — a 24-month shelf life is a long time for a polyunsaturated oil
  • Encapsulation — microencapsulated oil powder protects the oil in a coating, though it carries a weight and cost penalty

Our own choices here: an 18/12 marine concentrate rather than a cheap unconcentrated oil, mixed tocopherols and rosemary extract as the antioxidant system, and an opaque, resealable jar. The EPA and DHA amounts per serving are on the Skin + Coat Soft Chews page.

How can I tell at home whether a chew has gone off?

By smell. You don't need a laboratory to notice a problem.

Fresh marine oil smells mildly of the sea. Oxidised oil smells sharp, painty, or like old cooking oil. Most people recognise it immediately once they know what they're smelling for.

A useful test: open a new jar and note the smell. Reseal it, leave it in normal storage for six weeks, and smell it again. If it has changed noticeably in that time, the product will be considerably worse by the end of its shelf life.

Dogs will often tell you first. Refusing a chew they previously accepted is worth paying attention to.

Why don't brands publish these numbers?

Partly because customers don't ask, and partly because there is no requirement to.

There's a commercial reason as well. Oxidation limits constrain how cheaply you can source oil and how long you can claim a shelf life. Testing every batch costs money, and publishing the result creates an obligation.

We think it should be normal. It currently isn't.

What to ask

  1. Do you test peroxide value, anisidine value and TOTOX?
  2. Are those tested on every batch or once?
  3. At incoming oil, finished product, or both?
  4. What antioxidant system is used?
  5. Is the packaging an oxygen barrier?

Further reading

These statements have not been evaluated by the Food and Drug Administration. Consult your veterinarian before starting any supplement.

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