The distinct “fishy” smell commonly associated with seafood is a natural process that signals the beginning of spoilage. This odor develops due to specific chemical transformations and bacterial activity that occur once a fish is no longer alive. Understanding the underlying science reveals why this smell emerges and how it can be minimized.
The Chemical Culprit
The primary compound responsible for the characteristic fishy odor is trimethylamine (TMA). This volatile molecule has a pungent, ammonia-like scent. TMA itself is not naturally abundant in live fish. Instead, it is formed from a precursor compound called trimethylamine N-oxide (TMAO), which is present in the tissues of many marine fish.
TMAO plays an important role in the biology of saltwater fish. It helps them maintain osmotic balance, regulating water and salt concentrations within their cells in their environment. TMAO also helps protect fish against pressure and cold in deep-sea habitats. Once a fish is caught and dies, conditions change, allowing TMAO to convert into the more odorous TMA.
The Role of Bacteria in Spoilage
The conversion of TMAO to TMA is primarily a biological process driven by specific bacteria. These spoilage bacteria are naturally present on the surface of the fish, particularly on its skin, gills, and in its gut. After the fish dies, its natural defenses cease, and these bacteria begin to multiply rapidly.
Certain types of bacteria, such as Shewanella putrefaciens and Aeromonas spp., possess enzymes that reduce TMAO into TMA. This conversion is a step in the spoilage process. As the bacterial population grows and breaks down TMAO, the concentration of volatile TMA increases, leading to the noticeable fishy smell. This bacterial activity contributes to fish spoilage.
Factors Influencing Odor Intensity
Several factors influence how quickly and intensely fish develops its characteristic odor. Temperature is a significant element, as higher temperatures accelerate bacterial growth and the enzymatic activity that converts TMAO to TMA. This means fish spoils more rapidly and produces a stronger smell when stored at warmer conditions. The length of time since the fish was caught also directly impacts odor development; the longer the storage, the more TMA is produced.
The type of fish also plays a role in odor intensity. Marine fish, like cod and haddock, naturally contain higher levels of TMAO, making them more prone to developing a strong fishy odor as they spoil. Freshwater fish typically have little to no TMAO and develop a milder, less “fishy” smell, though they can sometimes acquire an earthy or muddy odor from environmental compounds. Improper handling and processing, such as delays in chilling or physical damage, can accelerate spoilage and odor formation.
Keeping Fish Fresh and Odor-Free
Identifying fresh fish involves several sensory cues, as truly fresh fish should have a mild, clean scent, often described as oceanic or like mineral water, rather than a strong fishy smell. Fresh fish also typically has clear, plump eyes, bright red gills, and firm flesh that springs back when gently pressed.
Keeping fish at very low temperatures minimizes odor development and maintains freshness. Refrigeration or storing fish directly on ice (close to 32°F or 0°C) significantly slows the growth of spoilage bacteria and their enzymatic activity. Prompt consumption is also advised; cooking or freezing fish as soon as possible after purchase halts the spoilage process. Storing fish in airtight containers prevents odor transfer to other foods. Cleaning surfaces and utensils thoroughly after handling raw fish prevents bacterial spread.