Meats, fish, and poultry are all perishable foods. Among the three, fish spoil the fastest. Most meats, such as beef and pork, can be stored for 3 to 5 days. Fish, however, have a very short shelf life of just 1 to 2 days. This is one of the reasons why fish contribute to the immense global food waste. In fact, according to the Food and Agriculture Organization of the United (FAO), around 35% of the global harvest is lost or wasted annually.
There are several mechanisms that make fish a highly perishable commodity. Let’s discuss them.
FISH LIVE UNDERWATER
Allow me to say the already obvious: fish live in an environment where the temperature is low—underwater. This is especially true in the deep ocean water where the temperature is just a little bit above freezing. The bad thing about this is that some microorganisms are present in fish that adapt well to low temperatures.
In an earlier post, I mentioned that temperature is one of the factors that affect the growth of microorganisms. Generally, low temperatures slow down the growth of microorganisms. However, certain bacteria called psychrotrophs can survive low temperatures of 44.6°F (7°C) and below, a typical condition underwater. This means if we put a fish in a refrigerator, the bacteria in the fish flesh require lesser time to adapt to the temperature. They would continue to consume and break down the proteins and fats, and produce volatile and undesirable odor molecules. This 1984 study isolated psychrotrophic Pseudomonas fluorescens, Pseudomonas putida and non-fluorescent Pseudomonas sp in raw tuna fish.
Psychrotrophic bacteria is also the reason why cold water (ocean water) fish spoil quicker than warm water (fresh water) fish, as well as other types of meat. Microorganisms and enzymes in living animals for meat such as cattle, pigs, and chickens are used to warmer temperatures. Hence, putting meat in refrigerator temperature at 40°F (4.4°C) would slow things down for the bacteria present.
FISH CONTAIN TRIMETHYLAMINE OXIDE (TMAO)
Fish tissues contain a high concentration of a chemical called trimethylamine oxide (TMAO). TMAO is a piezolytes, whose purpose is to protect the fish from the protein-destabilizing effect of water pressure that may kill the animal. Piezolytes provide the proteins the flexibility they require in order to function properly under great pressure.
TMAO is the most abundant piezolyte in marine animals. This chemical is colorless, odorless, and flavorless. But when the fish dies and its tissues get exposed to air, TMAO is degraded and broken down by bacteria into trimethylamine. Trimethylamine is what gives fish a strong, fishy odor. And considering the soft texture of fish, spoilage microorganisms can easily penetrate and decompose it.
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Majority of ocean water fish contain TMAO. Fresh water fish and land animals do not contain this compound.
FATTY ACIDS IN FISH IS PREDOMINANTLY UNSATURATED
You may have already heard about the term omega-3 fatty acids in many TV commercials. Omega-3 fatty acids are unsaturated fatty acids in fish that help reduce the risk of heart disease. These are fats whose hydrocarbon molecules contain 2 carbons that share several bonds, hence, are not saturated with hydrogen atoms. For this reason, the structures are weak, making them liquid (oil) at oil temperature. Saturated fats, on the other hand, are fats whose hydrocarbon molecules contain hydrogen atom on every carbon. Saturated fats are typically found in meat, and are solid at room temperature.
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While a healthy kind of fat for the human health, unsaturated fats contribute to the short shelf life of fish. This is because unsaturated fats are highly prone to lipid oxidation because of the unstable double bonds, which permit oxygen to react. Oxidation is a complex chemical process where air reacts with food components that results in undesirable quality changes (color, flavor and nutrient content). Oxidation is what happens when a flesh of an apple turn brown after being exposed to oxygen. This is the same reason why sunflower and safflower, and other unsaturated oils have shorter shelf life than saturated oils.
In the case of fish, oxidation results in rancidity that occurs even at refrigeration temperature. Rancidity happens at a faster rate for fish with high oil content. These include herring, salmon, tuna.
Cold water fish contain higher levels of monounsaturated and polyunsaturated fatty acids than freshwater fish.
This review discusses lipid oxidation in fish products in more detail.
FISH HAVE LITTLE GLYCOGEN STORE
Aerobic respiration provides the energy needed for muscular contraction in animals. This causes the breakdown of the glucose to produce 36 molecules of ATP and carbon dioxide. This glucose is kept in the form of glycogen, which is broken down when energy is required.
When an animal dies, its blood flow and aerobic respiration cease, depriving the muscle of oxygen. Glycolysis after death is influenced by a number of variables, including the glycogen reserve before harvest. However, less glycogen is stored in fish than in other animals. They use up their glycogen while they struggle during the catch, which gives them only a little window of time before their ability to metabolize carbohydrates and produce ATP runs out.
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Cells make less ATP from glucose in the absence of oxygen. Glycolysis continues at this point, but glycogen is now transformed into lactic acid. The process continues until the glycogen is depleted or a pH (acidity) of 5.5 is reached. However, higher pH level is typically obtained in fish because of the relatively less glycogen available to produce lactic acid for preservation.
From a microbiological perspective, a low ultimate pH is preferred since it prevents bacteria growth. A high ultimate pH leads to poor microbial growth resistance.
AUTOLYSIS IN FISH IN RAPID
Autolysis (“self-digestion”) or enzymatic spoilage is the destruction of a cell due to the action of its enzymes present. What exactly occurs is the internal breakdown of the structure of the protein and fats because of the complex series of reactions. Phosphorylases, lipases, cathepsins, and gut enzymes are some of the enzymes involved in autolysis. Autolysis of protein starts immediately after rigor and creates a favorable environment for the growth of spoilage microorganisms. All types of meat go through this process, but autolysis occurs at a faster rate in fish.
The action of enzymes affects the flavor and taste of fish. One enzymatic process is the gradual breakdown of ATP into AMP and hypoxanthine. Hypoxanthine is a fish muscle nucleotide that imparts a bitter taste, which is accompanied by the loss of freshness.
Another result of enzymatic actions is belly bursting. This is caused by the weakening of the belly wall as a result of the action of enzymes present in the gut of fish. This may occur several hours after harvest in fish, including sardines and herring. This study found out that dipping fish in 15% sodium chloride (NaCl) solution for 30 minutes prior to freezing is effective in reducing belly bursting.
Now that you know why fish are highly perishable, this post should help you decide if the fish is fresh: These 5 Signs Will Tell You If Fish Is Fresh.
