PHYSICAL, CHEMICAL AND MICROBIAL CHANGES DURING CHILLED STORAGE

DEVANAND UCHOI¹, SAGAR C MANDAL², * DEBTANU BARMAN³, VIKASH KUMAR¹, ALUDEE JANE ANDRADE³ & NEN PHANNA³

¹Central Institute of Fisheries Education, Versova, Mumbai-400061, India

²College of Fisheries, Central Agricultural University, Lembucherra, Tripura (W)-799210, India

³Laboratory of Aquaculture & Artemia Reference Center, Ghent University, Belgium

*Corresponding author: debtanu08@gmail.com, Mobile- +32488191632

Introduction:

The chilling of fish is a process by which temperature of a fish is reduced close to but not below freezing point of water (0°C). It delays both biochemical and bacteriological process 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. Quality of chilled products depend mainly on the initial quality of raw fish, 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 (fresh water, brine or seawater). For chilling fish on board of 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 the smaller fishes. As smaller fishes are tendered in nature and their skin 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 damage sooner.

iv. Fat content

Fatty fish start spoiling faster than the lean fish. Their abdomen will become soft and sometimes belly bursting occurs. Due to fat content their muscle are 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 stiffening of the muscle during rigor mortis process. This happens since the protein gets denatured and actin and myosin fail to slide leading no contraction and relaxation.

iii. Eyes, skin, scales

Due to longer chilled storage the eyes structure become concave. The scales will get removed and the skin become bare which is undesirable and not economic.

iv. Weight loss

Weight loss occurs due to 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 moisture contents of both freshwater and marine water species showed a small but erratic, mostly increase 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

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

Used for the assessment of degree of freshness.

iii. NPN content

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

This shows deterioration of flavor.

iv. Alpha amino nitrogen

AAN content of freshwater murrel and brackish water milkfish in unpacked condition 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 decrease in salt-soluble/ myofibrillar protein fraction at the end of ice storage period.

viii. TMAO

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

ix. FFA

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

x. ATP degradation

While rigor mortis process ATP is degraded and hypoxanthine is formed which is bitter in taste. More amount of hypoxanthine indicates higher degree of spoilage.

C. Microbial changes:

Proportion of mesophiles decrease and psychrophiles increases.

Psychrotrophic bacteria belonging to Pseudomonas spp. and Shewanella putrefaciens dominate the spoilage flora of iced stored fish.

Types of ice used for chilling:

Block ice: widely used due to many reasons

Crushed block ice

Flake ice: most preferred and considered the best ice.

Plate ice

Tube ice

Chilled foods:

i. Controlled atmosphere storage

Atmosphere differs from ambient air

Low oxygen and/or high Carbon dioxide

ii. Modified atmosphere storage

Pack flushed with a pre-mixed gas

Differs from CA above in that no correction takes place in the storage period

iii. Modified Atmosphere Packing

The same as Modified Atmosphere Storage above.

Used extensively in the UK retail trade for extension of shelf-life, especially

for the fish trade.

Summary:

Keep fish cool - < 4°C

Use potable water for ice

Maintain chill temperatures

Fig 1. Effect of Hygienic Handling on Quality Score

Fig 2. Effect of Hygienic Handling on Bacterial Number