Fish suddenly smells of ammonia: spoilage or a cover-up?

Ammonia smell in fish: spoilage or fraudulent practices?

A consumer bought a kilo of fresh fish (white cod-like fish) at €12/kg at a traditional market. After cooking and eating 300 grams without any problem, he stored the rest in the fridge. Four hours later, when he went to freeze it, the fish gave off a strong ammonia smell. He also experienced stomach noises and a persistent bad taste in his mouth. How is it possible for that smell to appear in such a short time? Is it accelerated spoilage, or some chemical treatment to mask deterioration?

The science behind ammonia

The ammonia smell in fish comes from the bacterial breakdown of urea and other nitrogen compounds. Species such as ray and shark naturally accumulate urea to regulate osmotic pressure, and after death, bacteria convert it into ammonia. But in this case, the fish was a white fillet similar to cod, which does not usually have high urea levels. The most plausible explanation is that the fish was already in the early stages of spoilage at the time of purchase. When cooked, the heat releases aromas and can mask the smell, but as it cools, the volatile compounds become evident. The four hours in the fridge were enough for residual bacterial activity to produce the characteristic smell.

Traditional practices: urine as a preservative

Several comments in the discussion mention the use of urine to preserve fish, especially in street vending. One user claimed that in Málaga, sellers of chanquetes (tiny fried fish) urinate in the buckets to prevent the catch from spoiling during hours in the sun. Although it sounds like an urban legend, there are historical and documentary references. The urea in urine acts as a weak preservative as it breaks down into ammonium, but it does not eliminate the risk of poisoning. Others point to sulphite baths or other additives that temporarily hide poor condition. The fact that the fish smelled fine when cooked and then stank suggests some chemical treatment was used that fades with heat or time.

Health risk and recommendations

The consumer who started the thread confirmed having digestive symptoms (noises, discomfort, an ammonia taste), indicating that the fish was no longer safe. An ammonia smell is a clear sign that spoilage has advanced. Although some cultures eat fermented fish (such as Korean hongeo-hoe), this is not that case. The sensible thing is to throw the product away. To avoid being fooled, it is best to buy fish from trusted establishments, check the smell when raw (it should smell of the sea, not ammonia) and not be taken in by excessively low prices or overly firm textures. Freezing at -18°C for 48 hours kills parasites, but does not stop chemical spoilage.

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The question that remains: how much supposedly fresh fish reaches the table with preservation tricks that mask its true condition? The discussion suggests the practice is more common than admitted, especially in unregulated channels. The consumer, armed with data and scepticism, is their own best defence.

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