Spoilage of Fish during Chilled Storage

 Introduction:

The chilling of fish is a process by which the temperature of a fish is reduced close to but not below the freezing point of water (0°C). It delays both biochemical and bacteriological processes in fish and consequently prolongs the storage of fish and fish products. 

So long as the chill temperature is maintained, deteriorative changes in fish remain retarded. The quality of chilled products depends mainly on the initial quality of raw fish, the method, and duration of chilling and efficient storage. The main merit of the method is that it provides the maximum possibility of preserving the natural nutritional and functional properties of the fish. 

Fish may be chilled by ice or by a homogeneous coolant such as cold air or cold liquid (freshwater, brine, or seawater). For chilling fish on board a fishing vessel, ice, CSW, CFW, RSW, or cold water is used.

Chilling is an effective way of reducing spoilage if done quickly and handled carefully & hygienically. The objective is to cool fish as quickly as possible, to as low a temperature as possible, without freezing. 

Chilling cannot prevent spoilage, but the colder the fish the greater the reduction in bacterial and enzyme activity.

Chilling leads to various changes according to the initial condition of the fish. Some of the general factors affecting the chilling process are as follows:

Factors affecting the changes are:

i. Species (Marine/ freshwater)

Marine fish are more prone to spoilage during chilling due to a high content of TMA which is converted into TMAO by the bacteria. In the case of freshwater fish, there is an antimicrobial covering on the skin which is absent in marine fishes. Moreover, marine shrimp are more quickly damaged due to high amino acid content leading to softening of the muscle.

ii. Size

It is found that larger fish get spoiled slower than smaller fish. As smaller fishes are tendered in nature and their skin is not that tough, they easily get bruised and hence accelerate spoilage.

iii. Method of capture and handling

The quality of chilled fish largely depends on the method of capture and handling. How carefully and properly they are handled will be reflected during the storage life. If the fish is undergoing physical deformities from the beginning itself, then it will get damaged sooner.

iv. Fat content

Fatty fish start spoiling faster than lean fish. Their abdomen will become soft and sometimes belly bursting occurs. Due to fat content, their muscle is more soft and susceptible to digestive enzymes.

v. Type of ice

If the edge of the ice is sharp and rough then physical damage may occur during chilling. It may puncture the skin and give way to microbes to enter into the flesh. So the storage life will be affected.

A. Physical changes:

i. Color

While chilling there is a color change of the fish. They become discolored from natural color. The pigments may get oxidized or affected by some other factors.

ii. Texture

The texture becomes harder due to the stiffening of the muscle during the rigor mortis process. This happens since the protein gets denatured and actin and myosin fail to slide leading to no contraction and relaxation.

iii. Eyes, skin, scales

Due to longer chilled storage, the eye's structure becomes concave. The scales will get removed and the skin becomes bare which is undesirable and not economical.

iv. Weight loss

Weight loss occurs due to the leaching of amino acids (shrimp), WSP, and vitamins from the fish body when the ice melts.

B. Biochemical changes:

i. Moisture content

During iced storage of moisture contents of both freshwater and marine water, species showed a small but erratic, mostly increased fluctuation. With fillets of ghol whether wrapped or unwrapped, cultured Rohu, and a few species of major carps, moisture content showed a minor increase at the beginning followed by a decrease.

ii. TVB content (total volatile basic nitrogen)

Least concerned with Freshwater fish. Marine water fish show an increase after 3 days or more.

Used for the assessment of the degree of freshness.

iii. NPN content (nonprotein nitrogen)

Both Freshwater and Marine water show a gradual decrease during ice storage.

This shows the deterioration of flavor.

iv. Alpha-amino nitrogen

AAN content of freshwater mussels and brackish water milkfish in unpacked conditions showed a gradual decrease during the period of ice storage.

v. Total nitrogen/ Protein

In the limited number of cases where data are available, total nitrogen/protein content in most of the cases decreased substantially during iced storage. This decrease may be mainly due to the loss of soluble protein components.

vi. Water-soluble and sarcoplasmic protein content

Leach out when the ice is melting and hence decrease.

vii. Salt-soluble protein content

Most of the species of tropical fish examined showed a decrease in salt-soluble/ myofibrillar protein fraction at the end of the ice storage period.

viii. TMAO (trimethyl amino oxide)

As TMA is degraded into TMAO by the bacteria its quantity increases indicating spoilage condition.

ix. FFA (free fatty acids)

Due to lipid oxidation, the FFA increases after a longer period of ice storage.

x. ATP degradation (adenosine triphosphate)

During the rigor mortis process ATP is degraded and hypoxanthine is formed which is a bitter taste.