Fish, due to its rich bacterial microflora, is a product that, if not properly protected, has a short shelf life and is extremely susceptible to becoming spoiled. Due to the place of origin, they often require long transportation and storage before reaching the consumer.
To prolong the shelf life of fish, many methods have been developed to preserve them. One standard procedure is to subject the raw fish to heat treatment. Due to the wide variety of fish species available, techniques of heat treatment are not the same. Depending on the species, fish may differ significantly in terms of their nutritional value. In the case of oily fish, it is particularly important to protect Essential Fatty Acids (EFAs) from the oxidative effects of heat treatment. EFAs are among the most nutritionally valuable components of fish meat. In addition, marine fish, especially oily ones, are an excellent source of vitamin D, which fortunately, is not sensitive to high temperatures. At the same time, fish are an excellent source of complete, easily digestible protein, needed to build all body tissues.
In the case of protein, heat treatment improves its digestibility. Mild heating, e.g. pasteurization and cooking, increases its availability for digestive enzymes without destructive influence on its structure. Prolonged heating at temperatures of 180-300°C (e.g. baking, frying), on the other hand, breaks down or converts amino acids, making the protein less valuable. Of course it is possible to digest raw fish meat, but it is better to subject it to gentle heat treatment.
Polyunsaturated Fatty Acids (PUFAs) contained in fish fat are very easily negatively affected by heat. At temperatures as low as 100°C (in the presence of oxygen), autooxidation occurs. In practice, such conditions occur most frequently in cold smoking. Heat treatment above 100°C leads to thermal oxidation, which occurs during frying, baking, stewing, grilling or hot smoking. At temperatures above 200°C (e.g. deep frying), some lipids can undergo thermal polymerization and cyclization. During grilling, pyrolysis of fatty acids can occur with the involvement of open flames. Each of the oxidation processes can lead to the formation of peroxides, hydroperoxides and epoxides that exhibit toxic effects. Ultimately, they can damage our cells and disrupt the function of certain enzymes. Whereas polymerization and cyclization, in addition to the loss of beneficial properties of EFAs, may transform them into indigestible compounds. From a toxicological point of view, the most dangerous substances are those formed during pyrolysis, including α-benzopyrene with strong mutagenic and carcinogenic effects.
To preserve the highest biological value of fish lipids, it is best to boil, microwave, pasteurize or sterilize them. Alternatively one can turn to baking or stewing.
Fish (including lean fish), as a group of food products, are an excellent source of B vitamins: vitamin B1 (thiamine), B2 (riboflavin), PP (niacin), B6 (pyridoxine), B12 (cobalamin). Each vitamin reacts differently to heat treatment. The most unstable among B vitamins is thiamine, which disintegrates during prolonged heating, e.g. cooking and baking. Short-term heating, such as pasteurization and sterilization, allows it to maintain its biological properties. Thiamine, as a water-soluble vitamin, passes into water during cooking. But heat treatment in this case also has its advantages, namely it denatures thiaminase, the enzyme that decomposes thiamin. In order to preserve as much of the vitamin B1 as possible, one should choose a treatment that has a short duration and relatively high temperature, and preferably with small amount of water used.
In the case of riboflavin, which is resistant to heating, the biggest problem is its loss during cooking, since it also passes into water. Up to 20% of it can be lost during cooking. Therefore, frying, grilling, smoking, pasteurization and sterilization are the most appropriate procedures.
Pyridoxine exists in several chemical forms that react differently to heating. Most pyridoxine is converted to lisinopyridoxal, which is poorly absorbed by the human body. Therefore, the heat treatment process should be shortened as much as possible to preserve this vitamin. Depending on the temperature and duration of processing, the loss of vitamin B6 can reach up to 50-70%. To preserve as much of the valuable properties of pyridoxine as possible, it is best to preserve fish products by pasteurization or sterilization. Short-term heating of cans can minimize the loss of pyridoxine to several percent.
Like riboflavin, cobalamin is relatively resistant to heat treatment. However, during frying, baking and prolonged microwave heating, which generate large water losses, its loss becomes significant. The best way to preserve it is through pasteurization and sterilization.
When using heat treatments, it is important to consider that each nutrient reacts differently to high temperatures. Some are resistant but pass easily into water or are lost as leakage, while others are quickly broken down or transformed and become biologically inactive or unabsorbable. Among the various types of heat treatments, pasteurization and sterilization appear to be the most beneficial for preserving nutritional value. On one hand, isolation from external conditions limits the influence of oxygen and light; on the other hand, heating is so short that thermal decomposition of vitamins is small and leakage is often even desirable as a component of the product's brine.
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