The deterioration of fish is the change in taste, smell and appearance. Factors that cause spoilage in fish are many. The species of fish, whether they are oily and whether they are hungry at the time they are caught are a few of these factors. For example, flatfish or oily fish break down more quickly. Similarly, fish that are satiated quickly deteriorate. When the fish die, the microorganisms found in the skin, gills and intestines are immediately activated and significant changes in the fish begin. The freshness of the fish can be determined by various chemical, physical and microbiological analyzes.

Trimethylamine oxide (TMAO) is abundant in almost all marine fish, especially white meat fish. Trimethylamine (TMA) determination is an analysis method for early detection of deterioration in fish. Trimethylamine oxide (TMAO) is converted to trimethylamine (TMA) with the effect of bacteria multiplying in fish meat. Trimethylamine (TMA) is found in very small amounts in fresh marine fish, and as the stale of the fish increases, so does the amount of this compound. Briefly, the amount of trimethylamine (TMA) is an indicator of stale and deterioration in fish. This compound gives the characteristic fish odor in the muscle of fish.

The amount of trimethylamine oxide (TMAO) is extremely low because freshwater fish are present in a different aquatic environment. According to the freshness of the fish TMA values ​​are as follows:

• In fresh fish, 50 ppm, percent 5 mg
• In stale fish, 50-100 ppm, 10 mg per cent
• In spoiled fish, over 100 ppm, more than percent 10 mg

Fish have become increasingly preferred in recent years. However, proteins present in fish, nitrogenous compounds and lipids that are not of protein nature are subjected to biochemical reactions after the fish die.

Trimethylamine (TMA) determination in fish is carried out by authorized laboratories within the scope of chemical food analysis. In these studies, standards and test methods published by domestic and foreign organizations are followed.