Rainbow sheen on ham and mojama: what it is and whether it's dangerous

Rainbow sheen on ham: neither rot nor relabelling

Why do green, yellow and pink tones appear on slices of cured ham, mojama or canned tuna? The doubt is not trivial: whoever spots it is usually looking at an expensive product they have just paid for. And the immediate suspicion — decomposition, old canned goods relabelled, rock-bottom quality standards — is more intuition than evidence. Iridescence in salted and cured meats is a well-known physical phenomenon, not an automatic sign that the food has gone off.

The pattern described by whoever raises the issue is specific: multicoloured reflections that appear mainly on **products on offer**, but also on cured ham at **€24/kg** versus the cheap kind at 12 and 15. On machine-cut slices, always in the same areas of the cut, visible before emptying the oil from the tin. And the alarm goes off when comparing it with the grimy puddles next to fish stalls, where the rainbow really is diesel or decomposing fat.

What is the iridescence of cured meat and fish

The explanation that best fits the phenomenon points to the **structure of muscle fibres** and how they align when sliced. When the cut misaligns the fibres and there is surface moisture, light diffracts and that iridescent sheen appears. It is an optical effect, not a bacterial culture. It fits with a detail the account itself provides: iridescence is especially noticeable on **pre-sliced and machine-cut pieces**, less so with a home knife.

Some add another layer: **protein denaturation**. The result would be an organoleptically unattractive product — appearance matters — but safe to eat. The distinction between what disgusts and what poisons is the key to this whole matter.

Is it dangerous to eat meat with multicoloured reflections?

There is no indication that the iridescent sheen by itself signals risk. The line most strongly argued is that it is a reflection of light on smooth surfaces and fatty tissue, and that the colour appears or disappears depending on how the light strikes. In mojama, it is simply considered normal.

What should be kept apart is the reflection from actual spoilage: smell, texture, date. A best-before date **more than a year away** does not make a product suspicious, but it does not make it bulletproof either. The rainbow is not the thermometer.

The business of distrust

The hypothesis of mass relabelling of salted products and old canned goods has no support in what was discussed. What does come up is a perception that the phenomenon has become more frequent. And here comes the attention bias: whoever suddenly notices something sees it everywhere, just as someone who buys a car discovers their model is all over the road. Some have been spotting it sporadically for **twenty years** and see no real increase.

The final irony: the consumer pays more for a €24/kg ham and finds a rainbow that the bargain one, at 12, also has. Colour does not distinguish quality. Neither does price.